Review ENVIRONMENTAL SCIENCES
The polar regions in a 2°C warmer world
Science Advances 04 Dec 2019:
Vol. 5, no. 12, eaaw9883
Over the past decade, the Arctic has warmed by 0.75°C, far outpacing the global average, while Antarctic temperatures have remained comparatively stable. As Earth approaches 2°C warming, the Arctic and Antarctic may reach 4°C and 2°C mean annual warming, and 7°C and 3°C winter warming, respectively. Expected consequences of increased Arctic warming include ongoing loss of land and sea ice, threats to wildlife and traditional human livelihoods, increased methane emissions, and extreme weather at lower latitudes. With low biodiversity, Antarctic ecosystems may be vulnerable to state shifts and species invasions. Land ice loss in both regions will contribute substantially to global sea level rise, with up to 3 m rise possible if certain thresholds are crossed. Mitigation efforts can slow or reduce warming, but without them northern high latitude warming may accelerate in the next two to four decades. International cooperation will be crucial to foreseeing and adapting to expected changes.
Earth has warmed by approximately 0.8°C since the late 19th century, while the Arctic has warmed by 2° to 3°C over the same period (Fig. 1A) (1). Conversely, the Antarctic has experienced more pronounced interannual and decadal variation in mean annual temperature anomalies than the Arctic, with no obvious upward trend in the last two decades (Fig. 1A). Spatially, observed warming has been markedly heterogeneous in both regions during the more recent instrumental satellite record (since 1986), with both warming and spatial variability in warming having increased more for the Arctic than the Antarctic over the past 13 years (Fig. 1B) (2, 3). Therefore, despite similarities in defining characteristics such as pronounced seasonality and the year-round presence of ice and snow, these two regions may face different futures in response to ongoing warming.
Fig. 1 Temperature trends and variability for the Arctic and Antarctic regions.
(A) Annual mean anomalies of the combined Land-Ocean Temperature Index (L-OTI) for the Arctic (64°N to 90°N), Antarctic (64°S to 90°S), and globe between 1880 and 2018 (zonal data bins defined by data acquired at https://data.giss.nasa.gov relative to the mean period 1951–1980). Temperature anomalies for the Arctic during each of the four IPYs, the first of which was based in the Arctic, are highlighted in purple. (B) Annual [January to December (J-D)] mean temperature change (°C) in the Northern (left) and Southern (right) hemispheres for 1986–2005 (upper) and 1986–2018 (lower) relative to the mean period of 1951–1980. Generated from the NASA/Goddard Institute for Space Studies (GISS) online plotting tool (2); the GISS analysis is based on updated Global Historical Climatology Network v3/SCAR (2, 3) and updates to Analysis (v3).
Having arrived at the 10th anniversary of the Fourth International Polar Year (IPY), a milestone that intensified focus on observed and expected changes in the polar regions, we review key environmental and ecological impacts of warming over the past decade. We also review ancillary effects of polar warming at lower latitudes, for which evidence has mounted recently. Over the past decade alone, the Arctic has warmed by 0.75°C relative to the mean for 1951–1980, while the Antarctic has remained comparatively stable (2009–present; Fig. 1A). Our emphasis is on consideration of consequences for atmospheric, cryospheric, and biospheric changes in the polar regions, as Earth continues to approach 2°C global mean warming (Table 1). Hence, we first consider the expected magnitude and pace of warming in the Arctic and Antarctic under two carbon emissions futures: Representative Concentration Pathway (RCP) 8.5 and RCP4.5 scenarios. We then outline potential consequences of such warming on the basis of recent observed changes in both regions. While our retrospective assessments of warming to date (Fig. 1) refer to temperature anomalies relative to the period covered by the instrumental record (1880–2018) (2) and a baseline mean period (1951–1980), our projections of expected warming are presented relative to the Intergovernmental Panel on Climate Change (IPCC) standard baseline period (1981–2005) (4).
The most recent generation of general circulation models in the Coupled Model Intercomparison Project Phase 5 (CMIP5) indicates that the Arctic is expected to continue to warm much more rapidly than lower latitudes, even under the moderate carbon mitigation trajectory characterized by the RCP4.5 scenario. The Arctic is expected to achieve an additional 2°C annual mean warming above the 1981–2005 baseline approximately 25 to 50 years before the globe as a whole under the business-as-usual (RCP8.5) and moderate mitigation (RCP4.5) scenarios, respectively (Fig. 2, A and B). The Antarctic, in contrast, is expected to lag slightly a 2°C global mean warming under the business-as-usual scenario (Fig. 2C) but reach 2°C annual mean warming slightly earlier than the globe under the moderate mitigation scenario (Fig. 2D). Under both scenarios, Antarctic warming is expected to outpace global mean warming only during austral late autumn and winter months (Fig. 2, C and D).
Fig. 2 Approximate year by which the 2°C warming threshold is reached for the Arctic and Antarctic compared to the globe as a whole.
Expected time to 2°C warming above the 1981–2005 mean under RCP8.5 (red) and RCP4.5 (blue) for the globe (open circles) compared to the Arctic [solid circles; (A and B)] and Antarctic [solid circles; (C and D)]. Means of 36 CMIP5 ensemble runs by Overland et al. (1) are shown. In (B) and (D), symbols positioned at year 2100 indicate that 2°C warming could be at 2100 or later.
The Arctic may experience as much as 4°C mean annual warming and 7°C warming in late boreal autumn, when a 2°C global mean warming above the 1981–2005 mean is reached, regardless of which RCP scenario is considered (Fig. 3, solid circles) (1). Particularly notable is the 13°C Arctic warming projected for boreal late autumn months by the end of the 21st century under a business-as-usual scenario (RCP8.5) (1). Annual mean warming in the Antarctic is expected to reach approximately 2°C under both scenarios, with slightly greater warming possible under RCP8.5 during the austral autumn and early winter (Fig. 3, open circles). Hence, mitigation of carbon emissions with a target of constraining global annual mean warming to 2°C may not constrain the annual mean warming in the Arctic or Antarctic to below 2°C. However, mitigation of carbon emissions can delay the crossing of the 2°C annual mean warming threshold for the Arctic, as suggested by the difference in time to annual mean 2°C warming between the RCP4.5 and RCP8.5 scenarios in Fig. 2.
Fig. 3 Greater warming likely in the Arctic and Antarctic with 2°C global warming.
Expected magnitude of monthly and mean annual warming above the 1981–2005 mean in the Arctic (solid circles) and Antarctic (open circles) with 2°C global warming under RCP8.5 (red) and RCP4.5 (blue) according to 36 CMIP5 ensemble runs by Overland et al. (1).
Recognizing the urgency of the magnitude and pace of ongoing and expected future warming in the polar regions, we present below a series of eight urgent considerations spurred by developments over the past decade. These are followed by a brief, concluding overview of international agreements in the Arctic and Antarctic as exemplars for cooperative scientific and political engagement that is likely necessary for addressing the complexities of expected climate-related changes in the polar regions. Our objectives are to catalyze consideration of potential consequences of a 2°C warmer world for the polar regions and to thereby inform policy considerations of these consequences. A key emergent feature of this synthesis is that direct comparisons of ongoing and expected changes in the Arctic and Antarctic are rendered difficult by the relative inaccessibility and data scarcity of the Antarctic compared to the Arctic. This disparity is especially evident in our capacity to anticipate expected changes to terrestrial ecosystems in the Antarctic. We stress that this synthesis is not intended as a comprehensive review of recent and growing emphases in polar research, some notable examples of which include arctic ozone dynamics (5, 6), Southern Ocean heat uptake from the atmosphere (7), and associations between Southern Ocean warming and ice sheet dynamics on land (8).