evidence: In the Arctic, annual average temperatures have increased more than twice as fast as the global average, accompanied by thawing permafrost and loss of sea ice and glacier mass. Arctic-wide glacial and sea ice loss is expected to continue; by mid-century, it is very likely that the Arctic will be nearly free of sea ice in late summer. Permafrost is expected to continue to thaw over the coming century as well, and the carbon dioxide and methane released from thawing permafrost has the potential to amplify human-induced warming, possibly significantly. [...] Some of the most rapid observed changes are occurring in Alaska and across the Arctic. Over the last 50 years, for example, annual average air temperatures across Alaska and the Arctic have increased more than twice as fast as the global average temperature.117,118,119,120,121,122 As surface temperatures increase, permafrost—previously permanently frozen ground—is thawing and becoming more discontinuous.123 This triggers another self-reinforcing cycle, the permafrost–carbon feedback, where carbon previously stored in solid form is released from the ground as carbon dioxide and methane (a greenhouse gas 35 times more powerful than CO2, on a mass basis, over a 100-year time horizon), resulting in additional warming.25,122 The overall magnitude of the permafrost–carbon feedback is uncertain, but it is very likely that it is already amplifying carbon emissions and human-induced warming and will continue to do so.124,125,126 Permafrost emissions imply an even greater decrease in emissions from human activities would be required to hold global temperature below a given amount of warming, such as the levels discussed in Box 2.4.