--- - chapter_identifier: our-changing-climate confidence: "There is very high confidence that global climate is changing and this change is apparent across a wide range of observations, given the evidence base and remaining uncertainties. All observational evidence is consistent with a warming climate since the late 1800s.\r\nThere is very high confidence that the global climate change of the past 50 years is primarily due to human activities, given the evidence base and remaining uncertainties. Recent changes have been consistently attributed in large part to human factors across a very broad range of climate system characteristics." evidence: "The key message and supporting text summarizes extensive evidence documented in the climate science literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input. \r\nEvidence for changes in global climate arises from multiple analyses of data from in-situ, satellite, and other records undertaken by many groups over several decades.25578196-95d0-4ac7-b889-0e863985423d Changes in the mean state have been accompanied by changes in the frequency and nature of extreme events.bc4e302f-0956-4bb0-b345-e84dfb03223f A substantial body of analysis comparing the observed changes to a broad range of climate simulations consistently points to the necessity of invoking human-caused changes to adequately explain the observed climate system behavior.926c4c11-d896-488e-8c7c-3930fe978424,6eb4d004-0634-413c-bfda-a997348fdec7 The influence of human impacts on the climate system has also been observed in a number of individual climate variables.f813f663-0919-4050-a458-47fdfcbe2f5f,ab75a74f-274f-4996-99b4-d009caa68641,a5d1165d-0287-4f99-ad7d-75d8dcd3b32e,bee16192-914f-4561-98e6-6cc3e33e2e41,6e726151-1805-4e14-9210-0b81a99844ac,4693d80b-2e2f-4a8c-8fca-bfe353270894,aaab80c9-5bef-4072-ab5a-2cea29a7a0a9 A discussion of the slowdown in temperature increase with associated references (for example, Balmaseda et al. 2013; Easterling and Wehner 2009be8d377c-8b89-4839-8a00-0f8240450472,119a732c-f113-4c1b-bc77-e584d52f5505) is included in the chapter.\r\nThe Climate Science Supplement Appendix provides further discussion of types of emissions or heat-trapping gases and particles, and future projections of human-related emissions. Supplemental message 4 of the Appendix provides further details on attribution of observed climate changes to human influence. " href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/global-climate-is-changing.yaml identifier: global-climate-is-changing ordinal: 1 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: Global climate is changing and this change is apparent across a wide range of observations. The global warming of the past 50 years is primarily due to human activities. uncertainties: 'Key remaining uncertainties relate to the precise magnitude and nature of changes at global, and particularly regional, scales, and especially for extreme events and our ability to simulate and attribute such changes using climate models. Innovative new approaches to climate data analysis, continued improvements in climate modeling, and instigation and maintenance of reference quality observation networks such as the U.S. Climate Reference Network (http://www.ncdc.noaa.gov/crn/) all have the potential to reduce uncertainties.' uri: /report/nca3/chapter/our-changing-climate/finding/global-climate-is-changing url: ~ - chapter_identifier: our-changing-climate confidence: "Given the evidence base and remaining uncertainties, confidence is very high that the global climate is projected to continue to change over this century and beyond. \r\nThe statement on the magnitude of the effect also has very high confidence. " evidence: "The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input. \r\nEvidence of continued global warming is based on past observations of climate change and our knowledge of the climate system’s response to heat-trapping gases. Models have projected increased temperature under a number of different scenarios.f83b5613-7609-4799-ab8c-c2a41bdc924c,29dec54f-92a8-4543-93f1-941da4f4d750,be244401-e84d-4ddd-af18-e8f59f807e17\r\nThat the planet has warmed is “unequivocal,” f83b5613-7609-4799-ab8c-c2a41bdc924c and is corroborated though multiple lines of evidence, as is the conclusion that the causes are very likely human in origin (see also Appendices 3 and 4). The evidence for future warming is based on fundamental understanding of the behavior of heat-trapping gases in the atmosphere. Model simulations provide bounds on the estimates of this warming." href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/global-climate-this-century.yaml identifier: global-climate-this-century ordinal: 2 process: " Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'Global climate is projected to continue to change over this century and beyond. The magnitude of climate change beyond the next few decades depends primarily on the amount of heat-trapping gases emitted globally, and how sensitive the Earth’s climate is to those emissions.' uncertainties: "The trends described in the 2009 reporte251f590-177e-4ba6-8ed1-6f68b5e54c8a have continued, and our understanding of the data and ability to model the many facets of the climate system have increased substantially.\r\nThere are several major sources of uncertainty in making projections of climate change. The relative importance of these changes over time.\r\nIn the next few decades, the effects of natural variability will be an important source of uncertainty for climate change projections.\r\nUncertainty in future human emissions becomes the largest source of uncertainty by the end of this century.\r\nUncertainty in how sensitive the climate is to increased concentrations of heat-trapping gases is especially important beyond the next few decades. Recent evidence lends further confidence about climate sensitivity (see Appendix 3: Climate Science Supplement).\r\nUncertainty in natural climate drivers, for example how much solar output will change over this century, also affects the accuracy of projections." uri: /report/nca3/chapter/our-changing-climate/finding/global-climate-this-century url: ~ - chapter_identifier: our-changing-climate confidence: 'Given the evidence base and remaining uncertainties, confidence is very high in the key message. Because human-induced warming is superimposed on a naturally varying climate, the temperature rise has not been, and will not be, uniform or smooth across the country or over time.' evidence: "The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nEvidence for the long-term increase in temperature is based on analysis of daily maximum and minimum temperature observations from the U.S. Cooperative Observer Network (http://www.nws.noaa.gov/om/coop/). With the increasing understanding of U.S. temperature measurements, a temperature increase has been observed, and temperature is projected to continue rising.42b449ef-418d-4f0c-8e9c-9f5c16fc2367,c0160553-45a8-4e6f-be7f-711a32c8d0f8,fe074063-3e08-4a10-b184-a3f946b9715c,9a18ff0d-ad2f-4176-856c-d54aea0a92e1,32bec5d2-97fe-41c5-8eed-6920bbf096f4,8243ec9e-5b70-4c53-a6bd-a8f41adb2d9c,66ccff5f-4828-4e03-be08-ee6f49296f34 Observations show that the last decade was the warmest in over a century. A number of climate model simulations were performed to assess past, and to forecast future, changes in climate; temperatures are generally projected to increase across the United States.\r\nThe section entitled “Quantifying U.S. Temperature Rise” explains the rational for using the range 1.3°F to 1.9°F in the key message. \r\nAll peer-reviewed studies to date satisfying the assessment process agree that the U.S. has warmed over the past century and in the past several decades. Climate model simulations consistently project future warming and bracket the range of plausible increases." href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/us-temperature-increased.yaml identifier: us-temperature-increased ordinal: 3 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'U.S. average temperature has increased by 1.3°F to 1.9°F since record keeping began in 1895; most of this increase has occurred since about 1970. The most recent decade was the nation’s warmest on record. Temperatures in the United States are expected to continue to rise. Because human-induced warming is superimposed on a naturally varying climate, the temperature rise has not been, and will not be, uniform or smooth across the country or over time.' uncertainties: "Since the previous National Climate Assessment,e251f590-177e-4ba6-8ed1-6f68b5e54c8a there have been substantial advances in our understanding of the U.S. temperature record (Climate Science Appendix, Supplemental Message 7).42b449ef-418d-4f0c-8e9c-9f5c16fc2367,c0160553-45a8-4e6f-be7f-711a32c8d0f8,fe074063-3e08-4a10-b184-a3f946b9715c,9a18ff0d-ad2f-4176-856c-d54aea0a92e1,32bec5d2-97fe-41c5-8eed-6920bbf096f4,8243ec9e-5b70-4c53-a6bd-a8f41adb2d9c,66ccff5f-4828-4e03-be08-ee6f49296f34\r\nA potential uncertainty is the sensitivity of temperature trends to adjustments that account for historical changes in station location, temperature instrumentation, observing practice, and siting conditions. However, quality analyses of these uncertainties have not found any major issues of concern affecting the conclusions made in the key message (Appendix 3: Climate Science, Supplemental Message 7). (for example, Williams et al. 201266ccff5f-4828-4e03-be08-ee6f49296f34).\r\nWhile numerous studies (for example, c0160553-45a8-4e6f-be7f-711a32c8d0f8,8243ec9e-5b70-4c53-a6bd-a8f41adb2d9c,66ccff5f-4828-4e03-be08-ee6f49296f34) verify the efficacy of the adjustments, the information base can be improved in the future through continued refinements to the adjustment approach. Model biases are subject to changes in physical effects on climate; for example, model biases can be affected by snow cover and hence are subject to change as a warming climate changes snow cover. " uri: /report/nca3/chapter/our-changing-climate/finding/us-temperature-increased url: ~ - chapter_identifier: our-changing-climate confidence: 'Given the evidence base and remaining uncertainties, confidence is very high that the length of the frost-free season (also referred to as the growing season) has been increasing nationally since the 1980s, with the largest increases occurring in the western U.S, affecting ecosystems, gardening, and agriculture. Given the evidence base, confidence is very high that across the U.S., the growing season is projected to continue to lengthen.' evidence: "The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nNearly all studies to date published in the peer-reviewed literature (for example, bfc00315-ccea-4e7c-8a05-2650a07e4252,113b48da-06b9-415d-95a2-2c78a7d89368,cd205ef2-b503-4a5c-b0fc-c4a5b94efc08) agree that the frost-free and growing seasons have lengthened. This is most apparent in the western United States. Peer-reviewed studies also indicate that continued lengthening will occur if concentrations of heat-trapping gases continue to rise. The magnitude of future changes based on model simulations is large in the context of historical variations. \r\nEvidence that the length of the frost-free season is lengthening is based on extensive analysis of daily minimum temperature observations from the U.S. Cooperative Observer Network. The geographic variations in increasing number of frost-free days are similar to the regional variations in mean temperature. Separate analysis of surface data also indicates a trend towards an earlier onset of spring.bfc00315-ccea-4e7c-8a05-2650a07e4252,113b48da-06b9-415d-95a2-2c78a7d89368,cd205ef2-b503-4a5c-b0fc-c4a5b94efc08,2d1ffd71-6c31-4d2e-9867-bdf330be45c1" href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/frost-free-season-length-increasing.yaml identifier: frost-free-season-length-increasing ordinal: 4 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'The length of the frost-free season (and the corresponding growing season) has been increasing nationally since the 1980s, with the largest increases occurring in the western United States, affecting ecosystems and agriculture. Across the United States, the growing season is projected to continue to lengthen.' uncertainties: "A key issue (uncertainty) is the potential effect on observed trends of climate monitoring station inhomogeneities (differences), particularly those arising from instrumentation changes. A second key issue is the extent to which observed regional variations (more lengthening in the west/less in the east) will persist into the future.\r\nLocal temperature biases in climate models contribute to the uncertainty in projections.\r\nViable avenues to improving the information base are to investigate the sensitivity of observed trends to potential biases introduced by station inhomogeneities and to investigate the causes of observed regional variations." uri: /report/nca3/chapter/our-changing-climate/finding/frost-free-season-length-increasing url: ~ - chapter_identifier: our-changing-climate confidence: "Given the evidence base and remaining uncertainties, confidence is high that average U.S. precipitation has increased since 1900, with some areas having had increases greater than the national average, and some areas having had decreases. \r\nConfidence is high, given the evidence base and uncertainties, that more winter and spring precipitation is projected for the northern U.S., and less for the Southwest, over this century in the higher emissions scenarios. Confidence is medium that human-induced precipitation changes will be small compared to natural variations in all seasons over large portions of the U.S. in the lower emissions scenarios. Confidence is medium that human-induced precipitation changes will be small compared to natural variations in the summer and fall over large portions of the U.S. in the higher emissions scenarios.\r\n" evidence: "The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nEvidence of long-term change in precipitation is based on analysis (for example, Kunkel et al. 20130ebef171-4903-4aa6-b436-2936da69f84e) of daily observations from the U.S. Cooperative Observer Network. Published work shows the regional differences in precipitation.5651d034-614a-4dba-ad5c-799de4507886,e15600d0-290f-44e2-9b58-9ffd295ee6d2 Evidence of future change is based on our knowledge of the climate system’s response to heat-trapping gases and an understanding of the regional mechanisms behind the projected changes (for example, IPCC 2007f83b5613-7609-4799-ab8c-c2a41bdc924c)." href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/precipitation-average-increase.yaml identifier: precipitation-average-increase ordinal: 5 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'Average U.S. precipitation has increased since 1900, but some areas have had increases greater than the national average, and some areas have had decreases. More winter and spring precipitation is projected for the northern United States, and less for the Southwest, over this century.' uncertainties: "A key issue (uncertainty) is the sensitivity of observed precipitation trends to historical changes in station location, rain gauges, and observing practice. A second key issue is the ability of climate models to simulate precipitation. This is one of the more challenging aspects of modeling of the climate system, because precipitation involves not only large-scale processes that are well-resolved by models but small-scale process, such as convection, that must be parameterized in the current generation of global and regional climate models. However, our understanding of the physical basis for these changes has solidified and the newest set of climate model simulations (CMIP5) continues to show high-latitude increases and subtropical decreases in precipitation. For most of the contiguous U.S., studies32f3d1fd-faa0-402a-b55c-6560b851c487,e26cd117-a2e4-45c8-acf2-8ea2984a8949 indicate that the models currently do not detect a robust anthropogenic influence to observed changes, suggesting that observed changes are principally of natural origins. Thus, confident projections of precipitation changes are limited to the northern and southern areas of the contiguous U.S. that are part of the global pattern of observed and robust projected changes that can be related to anthropogenic forcing. Furthermore, for the first time in the U.S. National Climate Assessment, a confidence statement is made that some projected precipitation changes are deemed small. It is incorrect to attempt to validate or invalidate climate model simulations of observed trends in these regions and/or seasons, as such simulations are not designed to forecast the precise timing of natural variations.\r\nShifts in precipitation patterns due to changes in other sources of air pollution, such as sulfate aerosols, are uncertain and are an active research topic.\r\nViable avenues to improving the information base are to investigate the sensitivity of observed trends to potential biases introduced by station changes, and to investigate the causes of observed regional variations.\r\nA number of peer-reviewed studies (for example, McRoberts and Nielsen-Gammon 2011; Peterson et al. 20135651d034-614a-4dba-ad5c-799de4507886,e15600d0-290f-44e2-9b58-9ffd295ee6d2) document precipitation increases at the national scale as well as regional-scale increases and decreases. The variation in magnitude and pattern of future changes from climate model simulations is large relative to observed (and modeled) historical variations." uri: /report/nca3/chapter/our-changing-climate/finding/precipitation-average-increase url: ~ - chapter_identifier: our-changing-climate confidence: "Given the evidence base and uncertainties, confidence is high that heavy downpours are increasing in most regions of the U.S., with especially large increases in the Midwest and Northeast. \r\nConfidence is high that further increases in the frequency and intensity of extreme precipitation events are projected for most U.S. areas, given the evidence base and uncertainties. " evidence: "The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nEvidence that extreme precipitation is increasing is based primarily on analysisb37557ac-ee97-4c28-98ca-4f1f1afe163b,b91893b4-24a8-46ba-b09a-013d462caf1b,0ebef171-4903-4aa6-b436-2936da69f84e of hourly and daily precipitation observations from the U.S. Cooperative Observer Network, and is supported by observed increases in atmospheric water vapor.0b3b2ff4-9ee7-45fe-8d0c-895076013715 Recent publications have projected an increase in extreme precipitation events,b37557ac-ee97-4c28-98ca-4f1f1afe163b,81dee494-06d2-4651-8ddb-f36f45dc5942 with some areas getting larger increasese251f590-177e-4ba6-8ed1-6f68b5e54c8a and some getting decreases.fe7cfee1-62d4-4a3f-8d9b-f3ec33912f87,b91893b4-24a8-46ba-b09a-013d462caf1b\r\nNearly all studies to date published in the peer-reviewed literature agree that extreme precipitation event number and intensity have risen, when averaged over the United States. The pattern of change for the wettest day of the year is projected to roughly follow that of the average precipitation, with both increases and decreases across the U.S. Extreme hydrologic events are projected to increase over most of the U.S." href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/extreme-precipitation-increase.yaml identifier: extreme-precipitation-increase ordinal: 6 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'Heavy downpours are increasing nationally, especially over the last three to five decades. Largest increases are in the Midwest and Northeast. Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.' uncertainties: "A key issue (uncertainty) is the ability of climate models to simulate precipitation. This is one of the more challenging aspects of modeling of the climate system because precipitation involves not only large-scale processes that are well-resolved by models but also small-scale process, such as convection, that must be parameterized in the current generation of global and regional climate models.\r\nViable avenues to improving the information base are to perform some long, very high-resolution simulations of this century’s climate under different emissions scenarios." uri: /report/nca3/chapter/our-changing-climate/finding/extreme-precipitation-increase url: ~ - chapter_identifier: our-changing-climate confidence: "Given the evidence base and uncertainties, confidence is high for the entire key message.\r\nHeat waves have become more frequent and intense, and confidence is high that heat waves everywhere are projected to become more intense in the future.\r\nConfidence is high that cold waves have become less frequent and intense across the nation. \r\nConfidence is high that there have been regional trends in floods and droughts.\r\nConfidence is high that droughts in the Southwest are projected to become more intense." evidence: "The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nAnalysis of U.S. temperature records indicates that record cold events are becoming progressively less frequent relative to record high events.62bcd3f7-a6f1-4b0b-85a8-29cbf7412579,0ebef171-4903-4aa6-b436-2936da69f84e There is evidence for the corresponding trends in a global framework.6eb4d004-0634-413c-bfda-a997348fdec7,714ae627-5acd-4f06-8d37-748adae46d4e A number of publications have explored the increasing trend of heat waves.6eb4d004-0634-413c-bfda-a997348fdec7,43d596ef-081b-4929-92fe-a1fb4e41b16f,3a244268-5014-4ad9-943a-c3361454c25b Additionally, heat waves observed in the southern Great Plains,e251f590-177e-4ba6-8ed1-6f68b5e54c8a Europe,6eb4d004-0634-413c-bfda-a997348fdec7,43d596ef-081b-4929-92fe-a1fb4e41b16f and Russia62bcd3f7-a6f1-4b0b-85a8-29cbf7412579,714ae627-5acd-4f06-8d37-748adae46d4e,7de011b1-2d12-48e0-b3e1-0d81e4a8a7d6 have now been shown to have a higher probability of having occurred because of human-induced climate change. \r\nSome parts of the U.S. have been seeing changing trends for floods and droughts over the last 50 years, with some evidence for human influence.a5d1165d-0287-4f99-ad7d-75d8dcd3b32e,e15600d0-290f-44e2-9b58-9ffd295ee6d2,43d596ef-081b-4929-92fe-a1fb4e41b16f In the areas of increased flooding in parts of the Great Plains, Midwest, and Northeast, increases in both total precipitation and extreme precipitation have been observed and may be contributing to the flooding increases. However, when averaging over the entire contiguous U.S., there is no overall trend in flood magnitudes.a7f8dbf5-3ec8-4ee1-8740-014006b72bfd,fcd12450-81ff-4322-8a50-09c0662512eb,a639de52-b0d2-4580-a27c-5039d036d210,c25726e8-4e99-42fc-b35c-4d3164ff1aba A number of publications project drought as becoming a more normal condition over much of the southern and central U.S. (most recent references: Dai 2012; Hoerling et al. 2012; Wehner et al. 20110b3b2ff4-9ee7-45fe-8d0c-895076013715,09d97b0a-79d9-4803-a83d-b5982e4b3371,948ffa58-24f3-4129-90c3-8d49f3172f74).\r\nAnalyses of U.S. daily temperature records indicate that low records are being broken at a much smaller rate than high records, and at the smallest rate in the historical record.62bcd3f7-a6f1-4b0b-85a8-29cbf7412579,0ebef171-4903-4aa6-b436-2936da69f84e However, in certain localized regions, natural variations can be as large or larger than the human induced change." href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/extreme-weather-increase.yaml identifier: extreme-weather-increase ordinal: 7 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'There have been changes in some types of extreme weather events over the last several decades. Heat waves have become more frequent and intense, especially in the West. Cold waves have become less frequent and intense across the nation. There have been regional trends in floods and droughts. Droughts in the Southwest and heat waves everywhere are projected to become more intense, and cold waves less intense everywhere.' uncertainties: "The key uncertainty regarding projections of future drought is how soil moisture responds to precipitation changes and potential evaporation increases. Most studies indicate that many parts of the U.S. will experience drier soil conditions but the amount of that drying is uncertain.\r\nNatural variability is also an uncertainty affecting projections of extreme event occurrences in shorter timescales (several years to decades), but the changes due to human influence become larger relative to natural variability as the timescale lengthens. Stakeholders should view the occurrence of extreme events in the context of increasing probabilities due to climate change.\r\nContinuation of long term temperature and precipitation observations is critical to monitoring trends in extreme weather events." uri: /report/nca3/chapter/our-changing-climate/finding/extreme-weather-increase url: ~ - chapter_identifier: our-changing-climate confidence: "Given the evidence base and remaining uncertainties:\r\nHigh confidence that the intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have increased substantially since the early 1980s.\r\nLow confidence in relative contributions of human and natural causes in the increases.\r\nMedium confidence that hurricane intensity and rainfall rates are projected to increase as the climate continues to warm. " evidence: "The key message and supporting text summarize extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nRecent studies suggest that the most intense Atlantic hurricanes have become stronger since the early 1980s.6d2920f6-f06d-41fd-83e7-1fd61c40ae49 While this is still the subject of active research, this trend is projected to continue.d6bd92ad-67ef-4df7-aca9-68944523e863,5138b20c-7049-433e-a1ec-24417cccd3c2" href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/hurricane-storm-intensity-increases.yaml identifier: hurricane-storm-intensity-increases ordinal: 8 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.' uncertainties: "Detecting trends in Atlantic and eastern North Pacific hurricane activity is challenged by a lack of consistent historical data and limited understanding of all of the complex interactions between the atmosphere and ocean that influence hurricanes.1986a492-151d-4e71-a664-0ce152632cf1,12895289-04f3-4cf7-9a2d-2230f5cb21e3,233e8851-d7a3-443c-a73d-7c7ba17dcaec \r\nWhile the best analyses to date1986a492-151d-4e71-a664-0ce152632cf1,5138b20c-7049-433e-a1ec-24417cccd3c2 suggest an increase in intensity and in the number of the most intense hurricanes over this century, there remain significant uncertainties. " uri: /report/nca3/chapter/our-changing-climate/finding/hurricane-storm-intensity-increases url: ~ - chapter_identifier: our-changing-climate confidence: "Given the evidence base and remaining uncertainties:\r\nConfidence is medium that winter storms have increased slightly in frequency and intensity, and that their tracks have shifted northward over the U.S.\r\nConfidence is low on other trends in severe storms, including the intensity and frequency of tornadoes, hail, and damaging thunderstorm winds." evidence: "The key message and supporting text summarize extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nCurrent work596a7f1e-6ce5-4bdf-b144-d0715a7567bd has provided evidence of the increase in frequency and intensity of winter storms, with the storm tracks shifting poleward,57610605-1682-4827-878f-c12c6e9b674c,d4efd07e-2886-41f4-bc6d-14cbbe00c382 but some areas have experienced a decrease in winter storm frequency.e251f590-177e-4ba6-8ed1-6f68b5e54c8a Although there are some indications of increased blocking (a large-scale pressure pattern with little or no movement) of the wintertime circulation of the Northern Hemisphere,8a57a9b0-a2cb-4ce7-b603-2cf81dc20736 the assessment and attribution of trends in blocking remain an active research area.d23c69fd-1a50-44fd-8ade-c636ad308b8d Some recent research has provided insight into the connection of global warming to tornadoes and severe thunderstorms.242ad1ac-9e37-44e6-9e8f-43cb318ace99,7ede85ab-94c5-46e3-8bb5-0d98844a98db" href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/winter-and-severe-storms.yaml identifier: winter-and-severe-storms ordinal: 9 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'Winter storms have increased in frequency and intensity since the 1950s, and their tracks have shifted northward over the United States. Other trends in severe storms, including the intensity and frequency of tornadoes, hail, and damaging thunderstorm winds, are uncertain and are being studied intensively.' uncertainties: 'Winter storms and other types of severe storms have greater uncertainties in their recent trends and projections, compared to hurricanes (Key Message 8). The text for this key message explicitly acknowledges the state of knowledge, pointing out “what we don’t know.” There has been a sizeable upward trend in the number of storm events causing large financial and other losses.8949c886-8b4a-4845-9acf-1047f829c0ea' uri: /report/nca3/chapter/our-changing-climate/finding/winter-and-severe-storms url: ~ - chapter_identifier: our-changing-climate confidence: 'Given the evidence and uncertainties, confidence is very high that global sea level has risen during the past century, and that it will continue to rise over this century, with medium confidence that global sea level rise will be in the range of 1 to 4 feet by 2100.' evidence: "The key message and supporting text summarize extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nNearly all studies to date published in the peer-reviewed literature agree that global sea level has risen during the past century, and that it will continue to rise over the next century. \r\nTide gauges throughout the world have documented rising sea levels during the last 130 years. This rise has been further confirmed over the past 20 years by satellite observations, which are highly accurate and have nearly global coverage. Recent studies have shown current sea level rise rates are increasinge679d754-46b3-4d62-a7dd-4a7f0c727ebe,d8089822-678e-4834-a1ec-0dca1da35314 and project that future sea level rise over the rest of this century will be faster than that of the last 100 years (Appendix 3: Climate Science, Supplemental Message 12).d8089822-678e-4834-a1ec-0dca1da35314" href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/global-sea-level-rise.yaml identifier: global-sea-level-rise ordinal: 10 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: Global sea level has risen by about 8 inches since reliable record keeping began in 1880. It is projected to rise another 1 to 4 feet by 2100. uncertainties: "The key issue in predicting future rates of global sea level rise is to understand and predict how ice sheets in Greenland and Antarctica will react to a warming climate. Current projections of global sea level rise do not account for the complicated behavior of these giant ice slabs as they interact with the atmosphere, the ocean and the land. Lack of knowledge about the ice sheets and their behavior is the primary reason that projections of global sea level rise includes such a wide range of plausible future conditions. \r\nEarly efforts at semi-empirical models suggested much higher rates of sea level rise (as much as 6 feet by 2100).08968d1e-1e6c-4b44-9951-71255edd71e7,91591513-1cd5-4997-82a9-2843f3a69a22 More recent work suggests that a high end of 3 to 4 feet is more plausible.08968d1e-1e6c-4b44-9951-71255edd71e7,ba174f74-8a34-40a1-8314-dd0fa8d77f2e,ad5ef44d-eb60-4510-9368-f74be9f72f52,b180c1e1-b9a8-47a6-8dad-e3cc3083b0ad,ad54584e-8108-4b20-95f4-f1691a9b76b7 It is not clear, however, whether these statistical relationships will hold in the future or that they are appropriate in modeling past behavior, thus calling their reliability into question.3c8f9e8c-b044-4a3b-9680-cafc0b40e117 Some decision-makers may wish to consider a broader range of scenarios such as 8 inches or 6.6 feet by 2100 in the context of risk-based analysis.c9647af9-db7f-4f6a-89bd-2f2293ad26e5,d8089822-678e-4834-a1ec-0dca1da35314" uri: /report/nca3/chapter/our-changing-climate/finding/global-sea-level-rise url: ~ - chapter_identifier: our-changing-climate confidence: "Given the evidence base and uncertainties, confidence is very high that rising temperatures are reducing ice volume and extent on land, lakes, and sea, and that this loss of ice is expected to continue. \r\nConfidence is very high that the Arctic Ocean is projected to become virtually ice-free in summer by mid-century." evidence: "The key message and supporting text summarize extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nThere have been a number of publications reporting decreases in ice on landd784c38f-026b-4dab-a572-f4b84e58ca7c and glacier recession. Evidence that winter lake ice and summer sea ice are rapidly declining is based on satellite data and is incontrovertible.2ecb64ff-f4e0-4acd-b049-e5d04f44c57a,5d9dedb4-4383-471f-9cee-05e0b16a457c\r\nNearly all studies to date published in the peer-reviewed literature agree that summer Arctic sea ice extent is rapidly declining,bc2fe7ec-ad0c-435f-94d2-4968fb87b2b1 with even greater reductions in ice thicknessd98542ec-7ce6-4e98-90f5-52564dfceb94,1ba16241-3d20-489a-aae8-f5abb1353fe7 and volume,9711729f-6cbd-466e-bd66-6a9d955d0a13 and that if heat-trapping gas concentrations continue to rise, an essentially ice-free Arctic ocean will be realized sometime during this century (for example, a60d1734-9067-4e1e-9699-d8e998dfa4d3). September 2012 had the lowest levels of Arctic ice in recorded history. Great Lakes ice should follow a similar trajectory. Glaciers will generally retreat, except for a small percentage of glaciers that experience dynamical surging.2ecb64ff-f4e0-4acd-b049-e5d04f44c57a Snow cover on land has decreased over the past several decades.a600ab73-7283-4837-a5aa-5744d63e4a9b The rate of permafrost degradation is complicated by changes in snow cover and vegetation." href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/ice-loss-by-rising-temperatures.yaml identifier: ice-loss-by-rising-temperatures ordinal: 11 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'Rising temperatures are reducing ice volume and surface extent on land, lakes, and sea. This loss of ice is expected to continue. The Arctic Ocean is expected to become essentially ice free in summer before mid-century.' uncertainties: "The rate of sea ice loss through this century is a key issue (uncertainty), which stems from a combination of large differences in projections between different climate models, natural climate variability and uncertainty about future rates of fossil fuel emissions. This uncertainty is illustrated Figure 2.29, showing the CMIP5-based projections (adapted from Stroeve et al. 2012a60d1734-9067-4e1e-9699-d8e998dfa4d3).\r\nViable avenues to improving the information base are determining the primary causes of the range of different climate model projections and determining which climate models exhibit the best ability to reproduce the observed rate of sea-ice loss." uri: /report/nca3/chapter/our-changing-climate/finding/ice-loss-by-rising-temperatures url: ~ - chapter_identifier: our-changing-climate confidence: "Given the evidence base and uncertainties, confidence is very high that oceans are absorbing about a quarter of emitted CO2.\r\nVery high for trend of ocean acidification; low-to-medium for intensifying impacts on marine ecosystems. Our present understanding of projected ocean acidification impacts on marine organisms stems largely from short-term laboratory and mesocosm experiments, although there are also examples based on actual ocean observations; consequently, the response of individual organisms, populations, and communities of species to more realistic, gradual changes still has large uncertainties." evidence: "The key message and supporting text summarize extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input.\r\nThe oceans currently absorb a quarter of the CO2 the caused by human activities.5972d180-5f4d-4bdb-ac46-b7e0308320a4 Publications have shown that this absorption causes the ocean to become more acidic (for example, Doney et al. 20097ab1d9e1-75a1-48c5-8d85-02258496f919). Recent publications demonstrate the adverse effects further acidification will have on marine life.cbf2ddc8-6bd3-4a84-9697-4e9026c2b180,3a3c7408-89fa-417a-81c3-0345de986cb0,cfdaea11-95e2-4789-914b-74901b2f26b0,843b91bd-a79c-4a3b-8511-37b69d27445f" href: http://52.38.26.42:8080/report/nca3/chapter/our-changing-climate/finding/co2-into-ocean-increases-acidity.yaml identifier: co2-into-ocean-increases-acidity ordinal: 12 process: "Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. \r\nKey message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b\r\nThe Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection." report_identifier: nca3 statement: 'The oceans are currently absorbing about a quarter of the carbon dioxide emitted to the atmosphere annually and are becoming more acidic as a result, leading to concerns about intensifying impacts on marine ecosystems.' uncertainties: 'Absorption of CO2 of human origin, reduced pH, and lower calcium carbonate (CaCO3) saturation in surface waters, where the bulk of oceanic production occurs, are well verified from models, hydrographic surveys, and time series data.cbf2ddc8-6bd3-4a84-9697-4e9026c2b180 The key issue (uncertainty) is how future levels of ocean acidity will affect marine ecosystems.' uri: /report/nca3/chapter/our-changing-climate/finding/co2-into-ocean-increases-acidity url: ~ - chapter_identifier: water-resources confidence: "Observed trends have been demonstrated by a broad range of methods over the past 20+ years based on best available data; projected precipitation and river-flow responses to greenhouse gas increases are robust across large majorities of available climate (and hydrologic) models from scientific teams around the world.\r\nConfidence is therefore judged to be high that annual precipitation and river-flow increases are observed now in the Midwest and the Northeast regions. \r\nConfidence is high that very heavy precipitation events have increased nationally and are projected to increase in all regions. \r\nConfidence is high that the length of dry spells is projected to increase in most areas, especially the southern and northwestern portions of the contiguous United States." evidence: "The key message and supporting chapter text summarizes extensive evidence documented in the inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 Ch. 2: Our Changing Climate, Ch. 20: Southwest, other technical input reports,0ebef171-4903-4aa6-b436-2936da69f84e and over 500 technical inputs on a wide range of topics that were received as part of the Federal Register Notice solicitation for public input.\r\nNumerous peer-reviewed publications describe precipitation trends (Ch. 2: Our Changing Climate)8c8612b7-b9b5-427e-a1d4-c49de8346733,c0da4df5-149d-4ef9-a1ac-96db76da12af,cc98a8b5-74b3-4df2-8961-7799768da2ed,d1069afd-d9c4-4cc1-bd29-c50f637502bd,c3ef1f99-a398-45f5-ab9b-9495402070af,7259bc2b-d0aa-460b-b37a-79a11a386e00 and river-flow trends.4a6f7d87-75aa-49eb-81a7-0b078c6fc126,542ee728-8d13-448a-a83d-e90d9dbd3dcf,b8724db5-9d7b-458e-a570-e8c386ccc4f6,67b69161-5101-418a-a6c9-1b6a80773305 As discussed in Chapter 2, the majority of projections available from climate models (for example, 73bd27a4-2d08-49c7-81ee-dbb12667e6df,d85a45c6-1da6-41f1-81d6-e855acfb1fe3) indicate small projected changes in total average annual precipitation in many areas, while heavy precipitation5d909426-fab3-4dc8-af56-e5fe414ca97a,8e50e700-be4b-49eb-970a-93a592221589 and the length of dry spells are projected to increase across the entire country. Projected precipitation responses (such as changing extremes) to increasing greenhouse gases are robust in a wide variety of models and depictions of climate.\r\nThe broad observed trends of precipitation and river-flow increases have been identified by many long-term National Weather Service (NWS)/National Climatic Data Center (NCDC) weather monitoring networks, USGS streamflow monitoring networks, and analyses of records therefrom (Ch. 2: Our Changing Climate 7259bc2b-d0aa-460b-b37a-79a11a386e00,de270af6-30e7-403e-8352-6d5809c346c7,9fc00216-c35c-4958-b5f4-93517d667fd0). Ensembles of climate models73bd27a4-2d08-49c7-81ee-dbb12667e6df,c52f2539-9c5e-4ead-b8b7-f1884c5d662e(see also Ch. 2: Our Changing Climate, Ch. 20: Southwest) are the basis for the reported projections. " href: http://52.38.26.42:8080/report/nca3/chapter/water-resources/finding/precipitation-runoff-variances.yaml identifier: precipitation-runoff-variances ordinal: 1 process: 'The chapter author team engaged in multiple technical discussions via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 over 500 technical inputs provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May, 2012 for expert deliberation of draft key messages by the authors wherein each message was defended before the entire author team before this key message was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors of Ch. 2: Our Changing Climate.' report_identifier: nca3 statement: 'Annual precipitation and river-flow increases are observed now in the Midwest and the Northeast regions. Very heavy precipitation events have increased nationally and are projected to increase in all regions. The length of dry spells is projected to increase in most areas, especially the southern and northwestern portions of the contiguous United States.' uncertainties: "Important new evidence (cited above) confirmed many of the findings from the prior National Climate Assessment.e251f590-177e-4ba6-8ed1-6f68b5e54c8a \r\nObserved trends: Precipitation trends are generally embedded amidst large year-to-year natural variations and thus trends may be difficult to detect, may differ from site to site, and may be reflections of multi-decadal variations rather than external (human) forcings. Consequently, careful analyses of longest-term records from many stations across the country and addressing multiple potential explanations are required and are cornerstones of the evidentiary studies described above. \r\nEfforts are underway to continually improve the stability, placement, and numbers of weather observations needed to document trends; scientists also regularly search for other previously unanalyzed data sources for use in testing these findings. \r\nProjected trends: The complexity of physical processes that result in precipitation and runoff reduces abilities to represent or predict them as accurately as would be desired and with the spatial and temporal resolution required for many applications; however, as noted, the trends at the scale depicted in this message are very robust among a wide variety of climate models and projections, which lends confidence that the projections are appropriate lessons from current climate (and streamflow) models. Nonetheless, other influences not included in the climate change projections might influence future patterns of precipitation and runoff, including changes in land cover, water use (by humans and vegetation), and streamflow management.\r\nClimate models used to make projections of future trends are continually increasing in number, resolution, and in the number of additional external and internal influences that might be confounding current projections. For example, much more of all three of these directions for improvement are already evident in projection archives for the next IPCC assessment. " uri: /report/nca3/chapter/water-resources/finding/precipitation-runoff-variances url: ~ - chapter_identifier: water-resources confidence: "Given the evidence base and remaining uncertainties: \r\nConfidence is judged to be medium-high that short-term (seasonal or shorter) droughts are expected to intensify in most U.S. regions. Confidence is high that longer-term droughts are expected to intensify in large areas of the Southwest, southern Great Plains, and Southeast. " evidence: "The key message and supporting chapter text summarizes extensive evidence documented in the inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 Ch. 16: Northeast, Ch 17: Southeast, Ch. 2: Our Changing Climate, Ch. 18: Midwest, Ch. 19: Great Plains, Ch. 20: Southwest, Ch. 21: Northwest, Ch. 23: Hawai‘i and Pacific Islands, and over 500 technical inputs on a wide range of topics that were received as part of the Federal Register Notice solicitation for public input.\r\nProjected drought trends derive directly from climate models in some studies (for example, c3ef1f99-a398-45f5-ab9b-9495402070af,948ffa58-24f3-4129-90c3-8d49f3172f74,a75e4d22-485a-43c8-9093-931eea728bce,f6476787-f701-448c-a285-7b763c51df2e), from hydrologic models responding to projected climate trends in others (for example, ad8c9969-ccf5-47ef-8f13-2e3c53fb3697,f11e90fb-b100-4487-8bac-1a076166d623), from considerations of the interactions between precipitation deficits and either warmer or cooler temperatures in historical (observed) droughts,f11e90fb-b100-4487-8bac-1a076166d623 and from combinations of these approaches (for example, 8e2fc237-9d2b-4788-bd3e-e8654eaaecc2) in still other studies. " href: http://52.38.26.42:8080/report/nca3/chapter/water-resources/finding/summer-droughts-intensify.yaml identifier: summer-droughts-intensify ordinal: 2 process: 'The chapter author team engaged in multiple technical discussions via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 over 500 technical inputs provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May, 2012 for expert deliberation of draft key messages by the authors wherein each message was defended before the entire author team before this key message was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors of Ch. 2: Our Changing Climate.' report_identifier: nca3 statement: 'Short-term (seasonal or shorter) droughts are expected to intensify in most U.S. regions. Longer-term droughts are expected to intensify in large areas of the Southwest, southern Great Plains, and Southeast.' uncertainties: "Important new evidence (cited above) confirmed many of the findings from the prior National Climate Assessment.e251f590-177e-4ba6-8ed1-6f68b5e54c8a\r\nWarmer temperatures are robustly projected by essentially all climate models, with what are generally expected to be directly attendant increases in the potentials for greater evapotranspiration, or ET (although it is possible that current estimates of future ET are overly influenced by temperatures at the expense of other climate variables, like wind speed, humidity, net surface radiation, and soil moisture that might change in ways that could partly ameliorate rising ET demands). As a consequence, there is a widespread expectation that more water from precipitation will be evaporated or transpired in the warmer future, so that except in regions where precipitation increases more than ET increases, less overall water will remain on the landscape and droughts will intensify and become more common. Another widespread expectation is that precipitation variability will increase, which may result in larger swings in moisture availability, with swings towards the deficit side resulting in increased frequencies and intensities of drought conditions on seasonal time scales to times scales of multiple decades. An important remaining uncertainty, discussed in the supporting text for Key Message #1, is the extent to which the types of models used to project future droughts may be influencing results with a notable recent tendency for studies with more complete, more resolved land-surface models, as well as climate models, to yield more moderate projected changes.\r\nOther uncertainties derive from the possibility that changes in other variables or influences of CO2-fertilization and/or land cover change may also partly ameliorate drought intensification. Furthermore in many parts of the country, El Niño-Southern Oscillation (and other oceanic) influences on droughts and floods are large, and can overwhelm climate change effects during the next few decades. At present, however, the future of these oceanic climate influences remains uncertain. " uri: /report/nca3/chapter/water-resources/finding/summer-droughts-intensify url: ~ - chapter_identifier: water-resources confidence: 'Future changes in flood frequencies and intensities will depend on a complex combination of local to regional climatic influences, and the details of complex surface-hydrologic conditions in each catchment (for example, topography, land cover, and upstream management). Consequently, flood frequency changes may be neither simple nor regionally homogeneous, and basin by basin projections may need to be developed. Early results now appearing in the literature have most often projected intensifications of flood regimes, in large part as responses to projections of more intense storms and increasingly rainy (rather than snowy) storms in previously snow-dominated settings. Confidence in current estimates of future changes in flood frequencies and intensities is overall judged to be low.' evidence: "The key message and supporting chapter text summarizes extensive evidence documented in the inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 Ch. 16: Northeast, Ch 17: Southeast, Ch. 2: Our Changing Climate, Ch. 18: Midwest, Ch. 19: Great Plains, Ch. 20: Southwest, Ch. 21: Northwest, Ch. 23: Hawai‘i and Pacific Islands, and over 500 technical inputs on a wide range of topics that were received as part of the Federal Register Notice solicitation for public input.\r\nThe principal observational bases for the key message are careful national-scale flood-trend analyses a7f8dbf5-3ec8-4ee1-8740-014006b72bfd based on annual peak-flow records from a selection of 200 USGS streamflow gaging stations measuring flows from catchments that are minimally influenced by upstream water uses, diversions, impoundments, or land-use changes with more than 85 years of records, and analyses of two other subsets of USGS gages with long records (including gages both impacted by human activities and less so), including one analysis of 50 gages nationwide fcd12450-81ff-4322-8a50-09c0662512eb and a second analysis of 572 gages in the eastern U.S.a639de52-b0d2-4580-a27c-5039d036d210. There is some correspondence among regions with significant changes in annual precipitation (Ch. 2: Our Changing Climate) and soil moisture (Figures 3.2 and 3.3), and annual flood magnitudes (Figure 3.5).a7f8dbf5-3ec8-4ee1-8740-014006b72bfd\r\nProjections of future flood-frequency changes result from detailed hydrologic models (for example, 227f0b59-69f2-47ab-8359-29e4fc383e11,ffaab9ac-cd7b-47d2-a7df-174dbc479bbb,37288764-91af-4c74-a5f2-fdf7f088c918) of rivers that simulate responses to projected precipitation and temperature changes from climate models; such simulations have only recently begun to emerge in the peer-reviewed literature." href: http://52.38.26.42:8080/report/nca3/chapter/water-resources/finding/floods-projected-to-intensify.yaml identifier: floods-projected-to-intensify ordinal: 3 process: 'The chapter author team engaged in multiple technical discussions via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 over 500 technical inputs provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May, 2012 for expert deliberation of draft key messages by the authors wherein each message was defended before the entire author team before this key message was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors of Ch. 2: Our Changing Climate.' report_identifier: nca3 statement: 'Flooding may intensify in many U.S. regions, even in areas where total precipitation is projected to decline.' uncertainties: "Important new evidence (cited above) confirmed many of the findings from the prior National Climate Assessment. e251f590-177e-4ba6-8ed1-6f68b5e54c8a \r\nLarge uncertainties remain in efforts to detect flood-statistic changes attributable to climate change, because a wide range of local factors (such as dams, land-use changes, river channelization) also affect flood regimes and can mask, or proxy for, climate change induced alterations. Furthermore, it is especially difficult to detect any kinds of trends in what are, by definition, rare and extreme events. Finally, the response of floods to climate changes are expected to be fairly idiosyncratic from basin to basin, because of the strong influences of within-storm variations and local, basin-scale topographic, soil and vegetation, and river network characteristics that influence the size and extent of flooding associated with any given storm or season 7ef83121-d51c-4bc5-b499-e00733fae338,3fec0e7f-bcdd-42a3-bf79-635bd6adb609,fcd12450-81ff-4322-8a50-09c0662512eb,a639de52-b0d2-4580-a27c-5039d036d210. \r\nLarge uncertainties still exist as to how well climate models can represent and project future extremes of precipitation. This has – until recently – limited attempts to make specific projections of future flood frequencies by using climate model outputs directly or as direct inputs to hydrologic models. However, precipitation extremes are expected to intensify as the atmosphere warms, and many floods result from larger portions of catchment areas receiving rain as snowlines recede upward. As rain runs off more quickly than snowfall this results in increased flood potential; furthermore, occasional rain-on-snow events exacerbates this effect. This trend is broadly expected to increase in frequency under general warming trends, particularly in mountainous catchments 73760c11-7b97-4876-a24f-8fb54b01bca9,fb52dd18-06f2-4774-9262-b00501ff730d,9417de56-6893-4102-86b7-ad7da4e68b8e,59b1ce0d-27ed-4224-835a-666a4006d9e8,7dc6aae5-efaa-489a-807e-8b92e7bcd546. Rising sea levels and projected increase in hurricane-associated storm intensity and rainfall rates provide first-principles bases for expecting intensified flood regimes in coastal settings (see Ch. 2: Our Changing Climate)." uri: /report/nca3/chapter/water-resources/finding/floods-projected-to-intensify url: ~ - chapter_identifier: water-resources confidence: 'Given the evidence base and remaining uncertainties, confidence is judged to be high that climate change is expected to affect water demand, groundwater withdrawals, and aquifer recharge, reducing groundwater availability in some areas.' evidence: "Future changes in flood frequencies and intensities will depend on a complex combination of local to regional climatic influences, and the details of complex surface-hydrologic conditions in each catchment (for example, topography, land cover, and upstream management). Consequently, flood frequency changes may be neither simple nor regionally homogeneous, and basin by basin projections may need to be developed. Early results now appearing in the literature have most often projected intensifications of flood regimes, in large part as responses to projections of more intense storms and increasingly rainy (rather than snowy) storms in previously snow-dominated settings. Confidence in current estimates of future changes in flood frequencies and intensities is overall judged to be low.\r\n\r\n \r\n \r\nKey message #4 Traceable Account\r\nClimate change is expected to affect water demand, groundwater withdrawals, and aquifer recharge, reducing groundwater availability in some areas. \r\nDescription of evidence base\r\nThe key message and supporting chapter text summarizes extensive evidence documented in the inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 regional chapters of the NCA, and over 500 technical inputs on a wide range of topics that were received as part of the Federal Register Notice solicitation for public input.\r\nSeveral recent studies 1cbe9adb-5060-40dd-b9de-bcc33620cac6,eabfc538-91ce-4130-9040-d595dc61b92e,c87bb268-f370-4025-bb46-b4b7c4904ad6,1e0dbb05-1eb4-4fa9-90a1-100aae9a9354,d03c7a83-4f19-49d6-92a1-1274791d3151,d56699d4-19fa-4fd6-bb31-f86c26271029 have evaluated the potential impacts of changes in groundwater use and recharge under scenarios including climate change, and generally they have illustrated the common-sense conclusion that changes in pumpage can have immediate and significant effects in the nation’s aquifers. This has certainly been the historical experience in most aquifers that have seen significant development; pumpage variations usually tend to yield more immediate and often larger changes on many aquifers than do historical climate variations on time scales from years to decades. Meanwhile, for aquifers in the Southwest, there is a growing literature of geochemical studies that fingerprint various properties of groundwater and that are demonstrating that most western groundwater derives preferentially from snowmelt, rather than rainfall or other sources;f9880a6c-7860-448e-96d6-b335ac402583,9179dcd8-6465-46e1-98d7-65d7e0ea8313,eabfc538-91ce-4130-9040-d595dc61b92e,0bca150c-d4da-4e20-ac41-e26cf9ed8195. This finding suggests that much western recharge may be at risk of changes and disruptions from projected losses of snowpack, but as yet provides relatively little indication whether the net effects will be recharge declines, increases, or simply spatial redistribution." href: http://52.38.26.42:8080/report/nca3/chapter/water-resources/finding/groundwater-availability-changes.yaml identifier: groundwater-availability-changes ordinal: 4 process: 'The chapter author team engaged in multiple technical discussions via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 over 500 technical inputs provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May, 2012 for expert deliberation of draft key messages by the authors wherein each message was defended before the entire author team before this key message was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors of Ch. 2: Our Changing Climate.' report_identifier: nca3 statement: 'Climate change is expected to affect water demand, groundwater withdrawals, and aquifer recharge, reducing groundwater availability in some areas.' uncertainties: 'The precise responses of groundwater storage and flow to climate change are not well understood, but recent and ongoing studies provide insights on underlying mechanisms.1cbe9adb-5060-40dd-b9de-bcc33620cac6,eabfc538-91ce-4130-9040-d595dc61b92e,c87bb268-f370-4025-bb46-b4b7c4904ad6 The observations and modeling evidence to make projections of future responses of groundwater recharge and discharge to climate change are thus far very limited, primarily because of limitations in data availability and in the models themselves. New forms and networks of observations and new modeling approaches and tools are needed to provide projections of the likely influences of climate changes on groundwater recharge and discharge. Despite the uncertainties about the specifics of climate change impacts on groundwater, impacts of reduced groundwater supply and quality would likely be detrimental to the nation.' uri: /report/nca3/chapter/water-resources/finding/groundwater-availability-changes url: ~ - chapter_identifier: water-resources confidence: 'Confidence is high that sea level rise, storms and storm surges, and changes in surface and groundwater use patterns are expected to compromise the sustainability of coastal freshwater aquifers and wetlands. ' evidence: 'This message has a strong theoretical and observational basis, including considerable historical experience with seawater intrusion into many of the nation’s coastal aquifers and wetlands under the influence of heavy pumpage, some experience with the influences of droughts and storms on seawater intrusion, and experience with seepage of seawater into shallow coastal aquifers under storm and storm surges conditions that lead to coastal inundations with seawater. The likely influences of sea level rise on seawater intrusion into coastal (and island) aquifers and wetlands are somewhat less certain, as discussed below, although it is projected that sea level rise may increase opportunities for saltwater intrusion (see Ch. 25: Coasts).' href: http://52.38.26.42:8080/report/nca3/chapter/water-resources/finding/sustainability-of-coastal-freshwater.yaml identifier: sustainability-of-coastal-freshwater ordinal: 5 process: 'The chapter author team engaged in multiple technical discussions via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 over 500 technical inputs provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May, 2012 for expert deliberation of draft key messages by the authors wherein each message was defended before the entire author team before this key message was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors of Ch. 2: Our Changing Climate.' report_identifier: nca3 statement: 'Sea level rise, storms and storm surges, and changes in surface and groundwater use patterns are expected to compromise the sustainability of coastal freshwater aquifers and wetlands.' uncertainties: 'There are few published studies describing the kinds of groundwater quality and flow modeling that are necessary to assess the real-world potentials for sea level rise to affect seawater intrusion 0cd85833-93ce-41ba-9e43-eb328acacf81. Studies in the literature and historical experience demonstrate the detrimental impacts of alterations to the water budgets of the freshwater lenses in coastal aquifers and wetlands around the world (most often by groundwater development), but few evaluate the impacts of sea level rise alone. More studies with real-world aquifer geometries and development regimes are needed to reduce the current uncertainty of the potential interactions of sea level rise and seawater intrusion.' uri: /report/nca3/chapter/water-resources/finding/sustainability-of-coastal-freshwater url: ~ - chapter_identifier: water-resources confidence: 'Given the evidence base, confidence is medium that increasing air and water temperatures, more intense precipitation and runoff, and intensifying droughts can decrease river and lake water quality in many ways, including increases in sediment, nitrogen, and pollutant loads.' evidence: "The key message and supporting chapter text summarizes extensive evidence documented in the inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 Ch. 8: Ecosystems, Ch. 15: Biogeochemical Cycles, and over 500 technical inputs on a wide range of topics that were reviewed as part of the Federal Register Notice solicitation for public input.\r\nThermal stratification of deep lakes and reservoirs has been observed to increase with increased air and water temperatures,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8,16e73b9a-8381-4a89-9abd-78f85c401ce0,96ede898-cecb-42aa-9cc5-f34a27ff45f8,1cf676c9-b700-4e28-8bf0-87f7613a9f56 and may be eliminated in shallow lakes. Increased stratification reduces mixing, resulting in reduced oxygen in bottom waters. Deeper set-up of vertical thermal stratification in lakes and reservoirs may reduce or eliminate a bottom cold water zone; this coupled with lower oxygen concentration results in a degraded aquatic ecosystem. \r\nMajor precipitation events and resultant water flows increase watershed pollutant scour and thus increase pollutant loads ff7bc276-83ee-4d2b-a388-5409d7da855f,ff8a2e57-a49d-4e8e-90b1-c09e73b014fe. Fluxes of mineral weathering products (for example, calcium, magnesium, sodium, and silicon) have also been shown to increase in response to higher discharge.71d72e86-c742-45ff-bb85-2387af876e0f In the Mississippi drainage basin, increased precipitation has resulted in increased nitrogen loads contributing to hypoxia in the Gulf of Mexico.2def4038-abbc-43aa-b816-c8b195e2cf5b,f68f6208-6991-4325-8854-881c76072096 Models predict and observations confirm that continued warming will have increasingly negative effects on lake water quality and ecosystem health.16e73b9a-8381-4a89-9abd-78f85c401ce0 \r\nFuture re-mobilization of sediment stored in large river basins will be influenced by changes in flood frequencies and magnitudes, as well as on vegetation changes in the context of climate and other anthropogenic factors.d36aa341-c01e-4f30-91fb-32c520f6c4c6 Model projections suggest that changes in sediment delivery will vary regionally and by land-use type, but on average could increase by 25% to 55%.b126b36f-e46e-422f-9467-fac4965635d9\r\n" href: http://52.38.26.42:8080/report/nca3/chapter/water-resources/finding/effects-on-water-quality.yaml identifier: effects-on-water-quality ordinal: 6 process: 'The chapter author team engaged in multiple technical discussions via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 over 500 technical inputs provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May, 2012 for expert deliberation of draft key messages by the authors wherein each message was defended before the entire author team before this key message was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors of Ch. 2: Our Changing Climate.' report_identifier: nca3 statement: 'Increasing air and water temperatures, more intense precipitation and runoff, and intensifying droughts can decrease river and lake water quality in many ways, including increases in sediment, nitrogen, and other pollutant loads.' uncertainties: "It is unclear whether increasing floods and droughts cancel each other out with respect to long-term pollutant loads. \r\nIt is also uncertain whether the absolute temperature differential with depth will remain constant, even with overall lake and reservoir water temperature increases. Further, it is uncertain if greater mixing with depth will eliminate thermal stratification in shallow, previously stratified lakes. Although recent studies of Lake Tahoe provide an example of longer stratification seasons,0bd9cfef-4cd5-4b20-b953-04ecad0bd71c lakes in other settings and with other geometries may not exhibit the same response. \r\nMany factors influence stream water temperature, including air temperature, forest canopy cover, and ratio of baseflow to streamflow. " uri: /report/nca3/chapter/water-resources/finding/effects-on-water-quality url: ~ - chapter_identifier: water-resources confidence: 'Considering that (a) droughts are projected to intensify in large areas of the Southwest, Great Plains, and the Southeast, and (b) that these same regions have experienced and are projected to experience continuing population and demand increases, confidence that these regions will become increasingly vulnerable to climate change is judged to be high.' evidence: "The key message and supporting chapter text summarizes extensive evidence documented in the inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 Ch. 2: Our Changing Climate, Ch. 17: Southeast, Ch. 19: Great Plains, Ch. 20: Southwest, Ch. 23: Hawai‘i and Pacific Islands, and many technical inputs on a wide range of topics that were received and reviewed as part of the Federal Register Notice solicitation for public input.\r\nObserved Trends: Historical water withdrawals by sector (for example, municipal, industrial, agricultural, and thermoelectric) have been monitored and documented by USGS for over 40 years and represent a credible database to assess water-use trends, efficiencies, and underlying drivers. Water-use drivers principally include population, personal income, electricity consumption, irrigated area, mean annual temperature, growing season precipitation, and growing season potential evapotranspiration.30f46799-40a7-4e54-97f0-841e22aa4a56 Water-use efficiencies are also affected by many non-climate factors, including demand management, plumbing codes, water efficient appliances, efficiency improvement programs, and pricing strategies;c099e537-3373-4ab5-a17b-f6535c694cf3,77904e24-f8a9-4a0d-b674-64e2510a49fa,963e9133-8ce2-4d5a-b7e9-2358e99806fc changes from water intensive manufacturing and other heavy industrial activities to service-oriented businesses,1aeb32af-db3c-4472-a275-356718f12354 and enhanced water-use efficiencies in response to environmental pollution legislation; replacement of older once-through-cooling electric power plants by plants that recycle their cooling water; and switching from flood irrigation to more efficient methods in the western United States.d7f6c804-a121-48cc-9d69-5f9bdf820054,1d779c76-eb7a-4f43-8853-ecb617638750 \r\nProjected Trends and Consequences: Future projections have been carried out with and without climate change to first assess the water demand impacts of projected population and socioeconomic increases, and subsequently combine them with climate change induced impacts. The main findings are that in the absence of climate change total water withdrawals in the U.S. will increase by 3% in the coming 50 years,30f46799-40a7-4e54-97f0-841e22aa4a56 with approximately half of the U.S. experiencing a total water demand decrease and half an increase. If, however, climate change projections are also factored in, the demand for total water withdrawals is projected to rise by an average of 26%,30f46799-40a7-4e54-97f0-841e22aa4a56 with more than 90% of the U.S. projected to experience a total demand increase, and decreases projected only in parts of the Midwest, Northeast, and Southeast. When coupled with the observed and projected drying water cycle trends (see key messages in “Climate Change Impacts on the Water Cycle” section), the water demand impacts of projected population, socioeconomic, and climate changes intensify and compound in the Southwest and Southeast, rendering these regions particularly vulnerable in the coming decades. " href: http://52.38.26.42:8080/report/nca3/chapter/water-resources/finding/water-supply-demand-changes.yaml identifier: water-supply-demand-changes ordinal: 7 process: 'The chapter author team engaged in multiple technical discussions via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 over 500 technical inputs provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May, 2012 for expert deliberation of draft key messages by the authors wherein each message was defended before the entire author team before this key message was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors of Ch. 2: Our Changing Climate.' report_identifier: nca3 statement: 'Climate change affects water demand and the ways water is used within and across regions and economic sectors. The Southwest, Great Plains, and Southeast are particularly vulnerable to changes in water supply and demand.' uncertainties: 'The studies of water demand in response to climate change and other stressors are very recent and constitute new information on their own merit.30f46799-40a7-4e54-97f0-841e22aa4a56 In addition, for the first time, these studies make it possible to piece together the regional implications of climate change induced water cycle alterations in combination with projected changes in water demand. Such integrated assessments also constitute new information and knowledge building. Demand projections include various uncertain assumptions which become increasingly important in longer term (multi-decadal) projections. Because irrigation demand is the largest water demand component most sensitive to climate change, the most important climate related uncertainties are precipitation and potential evapotranspiration over the growing season. Non-climatic uncertainties relate to future population distribution, socioeconomic changes, and water-use efficiency improvements. ' uri: /report/nca3/chapter/water-resources/finding/water-supply-demand-changes url: ~ - chapter_identifier: water-resources confidence: 'Given the evidence base and remaining uncertainties, confidence is high that changes in precipitation and runoff, combined with changes in consumption and withdrawal, have reduced surface and groundwater supplies in many areas. Confidence is high that these trends are expected to continue, increasing the likelihood of water shortages for many uses.' evidence: "The key message and supporting chapter text summarizes extensive evidence documented in the inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 Ch. 2: Our Changing Climate, Ch. 17: Southeast, Ch. 19: Great Plains, Ch. 20: Southwest, Ch. 23: Hawai‘i and Pacific Islands, and over 500 technical inputs on a wide range of topics that were received and reviewed as part of the Federal Register Notice solicitation for public input.\r\nObserved Trends: Observations suggest that the water cycle in the Southwest, Great Plains, and Southeast has been changing toward drier conditions (Ch. 17: Southeast).d9b704d3-1441-4cf8-a7dc-cc2b1d14c8c5,9cb51164-e933-48c9-9265-f09b4a9b63a3,47f6b2ff-a48f-4b48-899d-a901424bf5b2 Furthermore, paleoclimate tree-ring reconstructions indicate that drought in previous centuries has been more intense and of longer duration than the most extreme drought of the 20th and 21st centuries.ae89341b-b4bb-4d20-8ebc-20965f751c31\r\nProjected Trends and Consequences: Global Climate Model (GCM) projections indicate that this trend is likely to persist, with runoff reductions (in the range of 10% to 20% over the next 50 years) and intensifying droughts.f11e90fb-b100-4487-8bac-1a076166d623\r\nThe drying water cycle is expected to affect all human and ecological water uses, especially in the Southwest. Decreasing precipitation, rising temperatures, and drying soils are projected to increase irrigation and outdoor watering demand (which account for nearly 90% of consumptive water use) by as much as 34% by 2060 under the A2 emissions scenario.30f46799-40a7-4e54-97f0-841e22aa4a56 Decreasing runoff and groundwater recharge are expected to reduce surface and groundwater supplies,eabfc538-91ce-4130-9040-d595dc61b92e increasing the annual risk of water shortages from 25% to 50% by 2060.d9b704d3-1441-4cf8-a7dc-cc2b1d14c8c5 Changes in streamflow timing will increase the mismatch of supply and demand. Earlier and declining streamflow and rising demands will make it more difficult to manage reservoirs, aquifers, and other water infrastructure.d9b704d3-1441-4cf8-a7dc-cc2b1d14c8c5 \r\nSuch impacts and consequences have been identified for several southwestern and western river basins including the Colorado,ad8c9969-ccf5-47ef-8f13-2e3c53fb3697 Rio Grande,549a79f8-6a64-46cc-9828-0041dfe2ad54 and Sacramento-San Joaquin.e610dc47-1231-4cbf-b43d-083cc76aa885,412047fe-33cf-49b8-b714-f1a7b096cd43,4db2c787-a754-422a-8714-80bbb44def23" href: http://52.38.26.42:8080/report/nca3/chapter/water-resources/finding/surface-groundwater-reduction.yaml identifier: surface-groundwater-reduction ordinal: 8 process: 'The chapter author team engaged in multiple technical discussions via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an inter-agency prepared foundational document,50d47cc1-5a16-4f5c-bb08-bf6f475a5bb8 over 500 technical inputs provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May, 2012 for expert deliberation of draft key messages by the authors wherein each message was defended before the entire author team before this key message was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead author of each message, and they were based on criteria that help define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors of Ch. 2: Our Changing Climate.' report_identifier: nca3 statement: 'Changes in precipitation and runoff, combined with changes in consumption and withdrawal, have reduced surface and groundwater supplies in many areas. These trends are expected to continue, increasing the likelihood of water shortages for many uses.' uncertainties: "The drying climate trend observed in the Southwest and Southeast in the last decades is consistent across all water cycle variables (precipitation, temperature, snow cover, runoff, streamflow, reservoir levels, and soil moisture) and is not debatable. The debate is over whether this trend is part of a multi-decadal climate cycle and whether it will reverse direction at some future time. However, the rate of change and the comparative GCM assessment results with and without historical CO2 forcing (Ch. 2: Our Changing Climate) support the view that the observed trends are due to both factors acting concurrently.\r\nGCMs continue to be uncertain with respect to precipitation, but they are very consistent with respect to temperature. Runoff, streamflow, and soil moisture depend on both variables and are thus less susceptible to GCM precipitation uncertainty. The observed trends and the general GCM agreement that the southern states will continue to experience streamflow and soil moisture reductions7259bc2b-d0aa-460b-b37a-79a11a386e00,67b69161-5101-418a-a6c9-1b6a80773305 provides confidence that these projections are robust." uri: /report/nca3/chapter/water-resources/finding/surface-groundwater-reduction url: ~