finding 11.3 : key-finding-11-3

Arctic land and sea ice loss observed in the last three decades continues, in some cases accelerating (very high confidence). It is virtually certain that Alaska glaciers have lost mass over the last 50 years, with each year since 1984 showing an annual average ice mass less than the previous year. Based on gravitational data from satellites, average ice mass loss from Greenland was −269 Gt per year between April 2002 and April 2016, accelerating in recent years (high confidence). Since the early 1980s, annual average arctic sea ice has decreased in extent between 3.5% and 4.1% per decade, become thinner by between 4.3 and 7.5 feet, and began melting at least 15 more days each year. September sea ice extent has decreased between 10.7% and 15.9% per decade (very high confidence). Arctic-wide ice loss is expected to continue through the 21st century, very likely resulting in nearly sea ice-free late summers by the 2040s (very high confidence).

This finding is from chapter 11 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Arctic land and sea ice loss observed in the last three decades continues, in some cases accelerating. A diverse range of observational evidence from multiple data sources and independent analysis techniques provide consistent evidence of substantial declines in arctic sea ice extent, thickness, and volume since at least 1979, mountain glacier melt over the last 50 years, and accelerating mass loss from Greenland. An array of different models and independent analyses indicate that future declines in ice across the Arctic are expected resulting in late summers in the Arctic becoming ice free by the 2040s.

Description of evidence base: The Key Finding is supported by observational evidence from multiple ground-based and satellite-based observational techniques (including passive microwave, laser and radar altimetry, and gravimetry) analyzed by independent groups using different techniques reaching similar conclusions.a9664a29-554f-4680-a6cd-9264b475d17b 3d339c60-bdf6-44f9-900d-249676925b4f 25342659-e41d-47c9-b230-c48cbac4361c 6b65bead-0bf2-443b-8cc7-540607984b3c aaf023b5-1ca5-455c-b203-8d5db9d33d7e 566e80d8-e05c-4be1-a90c-788f328629bc 94117a50-acc5-4dbf-8029-368aa3fc9680 08047702-47b0-4401-ab44-a0f46a16efe5 Additionally, the U.S. Geological Survey repeat photography database shows the glacier retreat for many Alaskan glaciers (Figure 11.4: Muir Glacier). Several independent model analysis studies using a wide array of climate models and different analysis techniques indicate that sea ice loss will continue across the Arctic, very likely resulting in late summers becoming nearly ice-free by the 2040s.b3bbc7b5-067e-4c23-8d9b-59faee21e58e 6e730a84-66a2-4e74-96cb-c9e6824cf185 df11db8a-acca-4f76-ab80-b1bf7b1ee9f7

New information and remaining uncertainties: Key uncertainties remain in the quantification and modeling of key physical processes that contribute to the acceleration of land and sea ice melting. Climate models are unable to capture the rapid pace of observed sea and land ice melt over the last 15 years; a major factor is our inability to quantify and accurately model the physical processes driving the accelerated melting. The interactions between atmospheric circulation, ice dynamics and thermodynamics, clouds, and specifically the influence on the surface energy budget are key uncertainties. Mechanisms controlling marine-terminating glacier dynamics—specifically the roles of atmospheric warming, seawater intrusions under floating ice shelves, and the penetration of surface meltwater to the glacier bed—are key uncertainties in projecting Greenland Ice Sheet melt.

Assessment of confidence based on evidence: There is very high confidence that arctic sea and land ice melt is accelerating and mountain glacier ice mass is declining given the multiple observational sources and analysis techniques documented in the peer-reviewed climate science literature.

It is very likely that accelerating arctic land and sea ice melt impacts the United States. Accelerating Arctic Ocean sea ice melt increases coastal erosion in Alaska and makes Alaskan fisheries more susceptible to ocean acidification by changing Arctic Ocean chemistry. Greenland Ice Sheet and Alaska mountain glacier melt drives sea level rise threatening coastal communities in the United States and worldwide, influencing marine ecology, and potentially altering the thermohaline circulation.

This finding was derived from figure -.2: Confidence / Likelihood

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