| Summary: | Final Draft 1. Annual average near-surface air temperatures across Alaska and the Arctic have increased over the last 50 years at a rate more than twice as fast as the global average temperature. (Very high confidence) 2. Rising Alaskan permafrost temperatures are causing permafrost to thaw and become more discontinuous; this process releases additional CO2 and methane, resulting in an amplifying feedback and additional warming (high confidence). The overall magnitude of the permafrost–carbon feedback is uncertain; however, it is clear that these emissions have the potential to complicate the ability to meet policy goals for the reduction of greenhouse gas concentrations. 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). 4. It is virtually certain that human activities have contributed to Arctic surface temperature warming, sea ice loss since 1979, glacier mass loss, and northern hemisphere snow extent decline observed across the Arctic (very high confidence). Human activities have likely contributed to more than half of the observed Arctic surface temperature rise and September sea ice decline since 1979 (high ...
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