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.

This finding is from chapter 2 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: 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. Key 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 The 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.

Description of evidence base: 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. Nearly 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. Tide 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

New information and remaining 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. Early 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

Assessment of confidence based on evidence: 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.

References :

• Exploring high-end scenarios for local sea level rise to develop flood protection strategies for a low-lying delta—the Netherlands as an example (ad54584e)
• Sensitivity of 21st century sea level to ocean-induced thinning of Pine Island Glacier, Antarctica (b180c1e1)
• Climate related sea-level variations over the past two millennia (e679d754)
• Sea level projections to AD2500 with a new generation of climate change scenarios (08968d1e)
• Comparing climate projections to observations up to 2011 (ba174f74)
• Calibrated prediction of Pine Island Glacier retreat during the 21st and 22nd centuries with a coupled flowline model (ad5ef44d)
• Global sea level linked to global temperature (91591513)
• Global Sea Level Rise Scenarios for the United States National Climate Assessment. NOAA Tech Memo OAR CPO-1 (d8089822)
• Coastal Impacts, Adaptation and Vulnerabilities: A Technical Input to the 2013 National Climate Assessment (c9647af9)
• Twentieth-Century Global-Mean Sea Level Rise: Is the Whole Greater than the Sum of the Parts? (3c8f9e8c)
• Monitoring and Understanding Trends in Extreme Storms: State of Knowledge (b37557ac)
• Monitoring and Understanding Changes in Extremes: Extratropical Storms, Winds, and Waves (596a7f1e)
• CMIP5 Climate Model Analyses: Climate Extremes in the United States (b91893b4)
• Monitoring and Understanding Changes in Heat Waves, Cold Waves, Floods, and Droughts in the United States: State of Knowledge (e15600d0)