State of the Climate in 2019 - The Arctic

Mean annual Arctic surface air temperatures (SAT) over land have increased more than twice as fast as the global mean since the mid-1980s. Observations from 2019 continue to highlight prolonged changes across key and connected features of the Arctic’s land, ice, ocean, and atmosphere. Through these connections, the changing Arctic environment has a magnified impact on ecosystems and societies on regional and global scales. Central to observed changes throughout the Arctic system is the persistent and pronounced increase in Arctic SAT, which in 2019 was the second highest in the 120-year observational record. In particular, the southward displacement of the polar vortex over North America—a repeat of conditions from 2018—brought record-high SATs to Alaska and northwest Canada. These conditions contributed to the second winter in a row when sea ice coverage in the Bering Sea was by far the lowest in observed or reconstructed records dating back to 1850 (Richter-Menge et al. 2019; Walsh et al. 2019). More generally, warming air temperatures are linked to the continued Arctic-wide decline in the extent and thickness of the sea ice cover. In March 2019, when the ice reached its maximum extent for the year, thin, first-year ice predominated at ~77%, compared to about 55% in the 1980s. This transformation toward thinner and more mobile ice makes the sea ice cover more vulnerable to melting out in summer and, therefore, diminishes the ice cover’s role in cooling the Arctic region by reflecting incoming solar radiation back to space. In September 2019, the minimum sea ice extent at the end of summer was tied with 2007 and 2016 for the second smallest in the 41-year satellite record. The declining trend in the extent of the sea ice cover is driving changes in sea surface temperatures (SSTs) in the Arctic Ocean and adjacent seas, largely caused by direct solar heating of exposed, ice-free—and thus darker (i.e., lower albedo)—Arctic waters. A warmer ocean, in turn, melts more sea ice. This feedback, known as the ice–albedo feedback, contributes to the continued and accelerated warming of the Arctic region. August mean SSTs show significant warming for 1982–2019 in most regions of the Arctic Ocean that are ice-free in August. On a regional scale, the Chukchi and Bering Seas continue to exhibit larger warming trends in August than the Arctic-wide August mean, with the Chukchi Sea mean SSTs in August 2019 being the second highest on record. Increased ocean temperatures and reduced sea ice in the Bering Sea are leading to shifts in fish distributions within some of the most valuable fisheries in the world. On the Bering Sea shelf, the summer distribution of fishes living on the seafloor is closely tied to the extent of the cold pool (bottom water temperatures < 2°C), which forms during autumn freeze-up when cold dense water sinks to the seafloor where it persists throughout the following summer. As this cold pool was considerably reduced during summers 2018 and 2019 in association with the record low winter sea ice coverage, southern fish species expanded northward. As a result, larger and more abundant boreal (southern Bering Sea) species, as opposed to smaller and less abundant Arctic species, dominated a large portion of the shelf in 2018 and 2019. These shifts in populations present challenges for the management of commercial and subsistence fisheries alike, while illuminating the potential for further cascading changes to the ecosystem. On land, the increasing SATs are causing a decrease in the extent of the Arctic spring snow cover, an increase in the overall amount of Arctic vegetation, and the warming and thawing of perennially-frozen ground, known as permafrost. These components of the Arctic environment interconnect to influence hydrology, surface stability, wildlife, infrastructure, and the livelihoods of Indigenous Peoples. Permafrost thaw also promotes the release of carbon dioxide and methane from soils to the atmosphere through the microbial conversion of permafrost carbon that has accumulated over hundreds to thousands of years. New evidence suggests that the increasing release of these sequestered greenhouse gases may be shifting permafrost soils from being a net carbon sink to being a net carbon source, thereby further accelerating global climate warming. Land-based ice across the Arctic is similarly responding to the persistent rise of SATs. Melt across the Greenland ice sheet (GrIS) is contributing to global average sea level rise at a current rate of about 0.7 mm yr−1. During the 2019 melt season, the extent and magnitude of ice loss over the GrIS rivaled 2012, the previous year of record ice loss. Observations from 2018 and 2019 reveal a continuing trend of significant ice loss from glaciers and ice caps across the Arctic, especially in Alaska and Arctic Canada. The Arctic-wide mass loss from glaciers and ice caps outside of Greenland is estimated to contribute approximately 0.4 mm yr−1 to global sea level rise, which, if normalized by area, represents more melt water per area than the GrIS. Long-term observing in the Arctic has revealed a region undergoing sustained and often rapid change. Yet, throughout this chapter, observations are also often marked by regional differences (e.g., continental-scale differences in snow cover and terrestrial greening), indicating a complex and variable system, tied in part to its global connections via the ocean and atmosphere. The Arctic plays a critical role in regulating global climate, primarily through the reflective properties of sea ice, land ice, and snow. As these features diminish in extent, the Arctic will increasingly exert its influence on the rest of Earth in other ways, too. Through global sea level rise, the release of permafrost carbon, and its role in steering global weather patterns, the Arctic is vitally connected to people worldwide. (This chapter includes a focus on glaciers and ice caps outside Greenland, section f, 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.)