State of the Climate in 2021 - The Arctic

Disruptive environmental change in the Arctic continued in 2021. While few indicators were at record levels, the ongoing trends provide a stark illustration of an Arctic that is a very different place than the Arctic of the twentieth century. Air and ocean temperatures in the Arctic are intimately linked with sea ice and are directly connected to the biological productivity of the region. Terrestrial snow cover, or the lack thereof, plays an important role in modulating air temperatures and the hydrologic cycle. During the winter, lower latitude drivers such as the El Niño-Southern Oscillation, the Madden-Julian Oscillation, and the evolution of the stratospheric polar vortex affect regional conditions and sub-seasonal variability. These processes add to the complexity of annually assessing the state of the Arctic, despite numerous examples of observed broadscale directional change across the region. For the Arctic (poleward of 60°N) as a whole, 2021 was the coolest year since 2013. Yet, 2021 was still the 13th-warmest year on record (since 1900), highlighting the dominance of the strong warming trend in recent decades. Within the Arctic, both the European (0°–90°E) and Asian (90°E–180°) sectors each experienced spring (April–June) temperatures among their highest 10% among all springs, while the Greenland-Iceland region experienced mean temperatures in the highest 10th percentile during all seasons, except spring. Collectively, these contributed substantially to the annual temperature anomaly for the entire Arctic. Spring and summer air temperatures are linked to the strong increase in tundra vegetation productivity that emerged in the late 1990s, a phenomenon known as “the greening of the Arctic.” The overall trend in circumpolar “greening” is strongly positive but recent years have seen the emergence of increased regional variability, such as strong greening on the Alaska North Slope, but “browning” in parts of northeast Asia. In 2021, the circumpolar greening index was the second highest since observations commenced in 2000, just 2.7% lower than in 2020. Evidence for increasing Arctic precipitation (liquid and frozen) comes from the intensifying hydrologic cycle and long-term trend of increasing river discharge (Holmes et al. 2021), but has not been previously been reported in the State of the Climate reports. Today, advances in reanalysis now allow for regionally reliable year-round precipitation estimates (Barrett et al. 2020; Wang et al. 2019). In 2021, total precipitation was modestly higher than the 1991–2020 climatology, but on average consisted of a considerably lower percentage of snowfall, relative to the 30-year average. Timing of the seasonal transition of the predominant phase of precipitation, the terrestrial snow cover establishment in autumn and melt in spring, has profound effects on air temperatures. Similarly, the snowpack mass at the end of the accumulation season drives ecosystem and hydrologic responses during and beyond the melt season. Snow accumulation during the 2020/21 winter was near-normal across the Eurasian Arctic and above normal across the North American Arctic. Despite the absence of a significant negative trend in snow mass, spring snow extent has been persistently below normal for the last 15 years due to earlier snow melt.  The seasonal maximum sea ice extent for the Arctic is typically reached in March. In 2021, the March extent was the ninth lowest since 1979. Spring melt was rapid in the Laptev Sea, resulting in record low ice extent for May and June in this region, and the East Greenland Sea was nearly ice-free during much of the summer. This early loss of sea ice contributed to August 2021 mean SSTs that were 1° to 3.5°C above the 1982–2010 average in the Kara, Laptev, and East Greenland Seas. In contrast, cloudy and cool weather, combined with more unusually high concentrations of multi-year ice for recent years in the Chukchi and Beaufort Seas, resulted in the 12th-lowest September mean extent in the 43-year record. Also, northern Barents Sea, Baffin Bay, and Chukchi Sea were marked by anomalously low SSTs in August 2021, up to 1°C lower than the 1982–2010 mean. Yet, near the end of the melt season in September 2021, the amount of multiyear ice remaining in the Arctic was still the second lowest on record, indicating the Arctic’s sustained transition to a younger, thinner ice cover. Changes in sea ice seasonality and warming ocean ecosystems allow for expanded Arctic maritime activity, increasing pollution in the region. This takes the form of conventional “trash” and potentially toxic materials, and of increased ocean noise levels, with potential impacts to marine species, especially marine mammals for whom underwater sounds can disrupt communication that is critical to their normal activities.  Mass changes on the Greenland Ice Sheet and other Arctic glaciers and ice caps that make up the Arctic year-round terrestrial cryosphere are quite sensitive to summer temperatures. Although Greenland Ice Sheet mass loss in the 2020/21 season was about half of the 2000–21 average, the ice sheet has now lost mass every year since 1998. Extreme events during summer 2021 included a widespread melt event on 14 August, the latest on record, which produced for the first time on record (since 1989) rain at Summit Station (3216 m a.s.l.). Outside of Greenland, observations of monitored Arctic glaciers and ice caps from 2020 and 2021 show regional and inter-annual variations in mass change, with a continuing trend of significant ice loss throughout the Arctic, especially in Alaska and Arctic Canada.  Permafrost refers to ground materials that remain at or below 0°C for at least two consecutive years and underlies extensive regions of the high-latitude landscape. Permafrost temperatures continue to increase across the Arctic. Greater increases in permafrost temperature are generally observed in colder permafrost at higher latitudes, where the largest increase in air temperature has been observed. Permafrost temperatures in 2021 were generally higher than those observed in 2020 and the highest on record at many monitoring sites. However, some recent, slight cooling occurred at a few sites as well. During the polar night, the very cold stratospheric polar vortex facilitates ozone depletion through chemical reactions that are inactive at temperatures higher than −78°C, while strong anomalies in stratospheric temperatures and winds can descend to the lower stratosphere where they persist for many weeks, affecting both the stratospheric ozone layer and the jet stream. Early stratospheric polar vortex formation in November 2020, which was conducive to ozone depletion, was cut short by a major Sudden Stratospheric Warming event in January 2021. Another result was that the average Arctic total ozone columns (ozone amounts integrated from Earth’s surface to the top of the atmosphere) in March 2021 were close to normal, and spring UV index values were generally within two standard deviations of the 2005–20 mean. (This chapter includes a focus on glaciers and ice caps outside Greenland, section 5f, which alternates yearly with a section on Arctic river discharge, as the scales of regular observation for both of these climate components are best suited for reporting every two years.)