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.

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. There 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 Nearly 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.

New information and remaining 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). Viable 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.

Assessment of confidence based on evidence: 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. Confidence is very high that the Arctic Ocean is projected to become virtually ice-free in summer by mid-century.

References :

• Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations (a60d1734)
• Relationships between Recent Pan-Arctic Snow Cover and Hydroclimate Trends (a600ab73)
• CryoSat-2 estimates of Arctic sea ice thickness and volume (9711729f)
• Arctic Climate Issues 2011: Changes in Arctic Snow, Water, Ice and Permafrost. SWIPA 2011 Overview Report (2ecb64ff)
• webpage Arctic Sea Ice Reaches Lowest Extent for the Year and the Satellite Record (bc2fe7ec)
• Temporal and Spatial Variability of Great Lakes Ice Cover, 1973–2010 (5d9dedb4)
• Decline in Arctic sea ice thickness from submarine and ICESat records: 1958-2008 (d98542ec)
• Melting trends over the Greenland ice sheet (1958–2009) from spaceborne microwave data and regional climate models (d784c38f)
• Distribution and trends in Arctic sea ice age through spring 2011 (1ba16241)
• 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)
• Monitoring and Understanding Trends in Extreme Storms: State of Knowledge (b37557ac)
• Monitoring and Understanding Changes in Extremes: Extratropical Storms, Winds, and Waves (596a7f1e)