The Arctic is particularly vulnerable to rising temperatures, since so much of it is covered in ice and snow that begin to melt as temperatures cross the freezing point. The more the Arctic warms, the more snow and ice melts, exposing the darker land and ocean underneath. This darker surface absorbs more of the sun’s energy than the reflective ice and snow, amplifying the original warming in a self-reinforcing cycle, or positive feedback.
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.
Most arctic glaciers are losing ice rapidly, and in some cases, the rate of loss is accelerating.127,128,129,130 This contributes to sea level rise and changes in local salinity that can in turn affect local ocean circulation. In Alaska, annual average glacier ice mass for each year since 1984 has been less than the year before, and glacial ice mass is declining in both the northern and southern regions around the Gulf of Alaska.131 Dramatic changes have occurred across the Greenland ice sheet as well, particularly at its edges. From 2002 to 2016, ice mass was lost at an average rate of 270 billion tons per year on average, or about 0.1% per decade, a rate that has increased in recent years.131 The effects of warmer air and ocean temperatures on the melting ice sheet can be amplified by other factors, including dynamical feedbacks (faster sliding, greater calving, and increased melting for the part of the ice that is underwater), near-surface ocean warming, and regional ocean and atmospheric circulation changes.132,133,134,135
Finally, much of the Arctic region is ocean that is covered by sea ice, and like land ice, sea ice is also melting (Figure 2.7).122 Since the early 1980s, annual average arctic sea ice extent has decreased by 3.5%–4.1% per decade.127,136 The annual minimum sea ice extent, which occurs in September of each year, has decreased at an even greater rate of 11%–16% per decade.137 Remaining ice is also, on average, becoming thinner (Figure 2.7), as less ice survives to subsequent years, and average ice age declines.137 The sea ice melt season—defined as the number of days between spring melt onset and fall freeze-up—has lengthened across the Arctic by at least five days per decade since 1979.
Melting sea ice does not contribute to sea level rise, but it does have other climate effects. First, sea ice loss contributes to a positive feedback, or self-reinforcing cycle, through changing the albedo or reflectivity of the Arctic’s surface. As sea ice, which is relatively reflective, is replaced by darker ocean, more solar radiation is absorbed by the ocean surface. This contributes to a greater rise in Arctic air temperature compared to the global average and affects formation of ice the next winter. Ice loss also acts to freshen the Arctic Ocean, affecting the temperature of the ocean surface layer and how surface heat is distributed through the ocean mixed layer. This also affects ice formation in subsequent seasons, as well as regional wind patterns, clouds, and ocean temperatures. And finally, sea ice loss also impacts key marine ecosystems and species that depend on the ice, from the polar bear to the ring seal,138,139,140 and the Alaska coastline becomes more vulnerable to erosion when it is not shielded from storms and waves by sea ice.141
It is virtually certain that human activities have contributed to arctic surface temperature warming, sea ice loss, and glacier mass loss.122,142,143,144,145,146,147,148 Observed trends in temperature and arctic-wide land and sea ice loss are expected to continue through the 21st century. It is very likely that by mid-century the Arctic Ocean will be almost entirely free of sea ice by late summer for the first time in about 2 million years.26,149As climate models have tended to under-predict recent sea ice loss,143 it is possible this will happen before mid-century.