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The 2019/2020 summer of Antarctic heatwaves

2020; Wiley; Volume: 26; Issue: 6 Linguagem: Inglês

10.1111/gcb.15083

1365-2486

Sharon A. Robinson, Andrew Klekociuk, Diana H. King, Marisol Pizarro Rojas, Gustavo E. Zúñiga, Dana M. Bergstrom,

Arctic and Antarctic ice dynamics

This summer, a heatwave across Antarctica saw temperatures soar above average. Temperatures above zero are especially significant because they accelerate ice melt. Casey Station had its highest temperature ever, reaching a maximum of 9.2°C and minimum of 2.5°C. The highest temperature in Antarctica was 20.75°C on 9 February. Here we discuss the biological implications of such extreme events. Extreme events associated with global change are predicted to increase in frequency and become more impactful. As we come to the end of the hottest decade ever recorded, recent weather events have once again been unprecedented. Significant attention has rightly focused on drought and heatwaves resulting in unprecedented fires in Australia (Nolan et al., 2020), delays to Indian monsoonal rains followed by flooding and the extreme winter warming in the Northern Hemisphere high latitudes (https://www.washingtonpost.com/weather/2020/02/10/record-arctic-oscillation/). However, the Antarctic summer has also broken records. In the past, much of East Antarctica has been spared from rapid climate warming due in part to ozone depletion, which cools surface temperatures slightly and enhances the strength of the westerly wind jets which shield Antarctica from more northerly warming air (Bornman et al., 2019; Robinson & Erickson, 2015). North of this wind jet, the Antarctic Peninsula and subantarctic islands have shown rapid warming, similar in magnitude to the Arctic. Much concern has focused on the warming oceans which are melting Antarctica from below (https://earthobservatory.nasa.gov/images/146289/pine-island-glaciers-newest-iceberg/). In late 2019, stratospheric warming led to an early breakup of the ozone hole (Lewis, 2019) and Antarctic temperature records started to break (Figure 1a). In what we believe is a first, we report a heatwave event at Casey Station, East Antarctica (Figure 1b) in January, to add to the record high temperatures reported for Antarctica in February. Heatwaves are rarely reported in Antarctica, but elsewhere are often classified as three consecutive days with both extreme maximum and minimum temperatures. Using this classification, Casey experienced a heatwave between 23 and 26 January with minimum temperatures above zero and maximum temperatures above 7.5°C. Casey also recorded its highest maximum temperature ever (9.2°C) on 24 January followed by its highest minimum (2.5°C) the following morning (http://www.bom.gov.au/climate/averages/tables/cw_300017_All.shtml/). This means the record minimum was 0.2°C higher than the mean maximum temperature for Casey for January (2.3°C, 31 year record), while the maximum temperature that day was 6.9°C higher. The World Meteorological Organization (https://public.wmo.int/en/media/news/new-record-antarctic-continent-reported/) reported a new record maximum temperature of 18.4°C (6 February 2020) for Antarctica, at the Argentine research base, Esperanza (northern tip, Antarctic Peninsula; Figure 1b). This was almost 1°C hotter than the former record of 17.5°C (24 March 2015). Three days later, this record was eclipsed by a report of 20.75°C by Brazilian scientists at Marambio Base on Seymour Island, off the eastern side of the Antarctic Peninsula (https://www.theguardian.com/world/2020/feb/13/antarctic-temperature-rises-above-20c-first-time-record/). If verified, this beats the pre-2020 record by 3°C. The February average daily temperature exceeded the long-term means over the preceding years of record by + 2.0°C (+1.2 SDs; 70 year record) for Esperanza and + 2.4°C (+1.4 SDs; 49 year record) for Marambio (based on data from https://legacy.bas.ac.uk/met/READER). Such positive anomalous temperatures around Antarctica will have impacted biological systems across the continent. Most life exists in small ice-free oases, which collectively cover only 0.44% of the continent's landmass (Brooks, Jabour, Hoff, & Bergstrom, 2019), and largely depend on melting snow and ice for their water supply (Robinson, Wasley, & Tobin, 2003). The warm summer conditions will have accelerated melting of snowbanks, as has been documented for Eagle Island (see Figure 1c) and observed by us this summer near Escudero Station on King George Island and in the Vestfold Hills in East Antarctica. Although it is too early for full reports, this warm summer will have impacted Antarctic biology in numerous ways, probably leading to long-term disruptions at ecosystem, community and population scales. A similar anomalous year in the Dry Valleys during summer 2001–2002 resulted in heat waves, melting of glaciers and flash flooding (Ball, Barrett, Gooseff, Virginia, & Wall, 2011; Barrett et al., 2008; Bergstrom, Woehler, Klekociuk, Pook, & Massom, 2018). This ended a decade of drought and, over the next 13 years, resulted in shifts in populations of soil nematodes to favour those preferring wetter conditions (such as Eudorylaimus sp.; Gooseff et al., 2017). In addition, there was increased productivity of Nostoc spp, within streams, but no change in the biomass of another cyanobacterial mat species, Phormidium (Gooseff et al., 2017). Warming can result in both positive and negative impacts. Melt water flooding can provide additional water to these desert ecosystems with positive effects for many organisms in terms of increased growth and reproduction (Barrett et al., 2008; Clarke, Robinson, Hua, Ayre, & Fink, 2012; Gooseff et al., 2017). We observed localized flooding appearing to benefit Vestfold Hills' moss banks which were previously reported to be very drought-stressed (Robinson et al., 2018). Prior to the flood event, most mosses were grey and moribund, but 1 month later many moss shoots were green. This site also experienced rare rain events (22 January 2020). Given the generally cold environment, in many cases, the warmer temperatures will also be positive for the flora (mosses, lichens and two vascular plants), microbes and invertebrates that live in Antarctica's ice-free areas. However, excessive flooding can also be negative, dislodging plants as well as altering community composition of both invertebrates and microbial mats (Barrett et al., 2008; Gooseff et al., 2017). If ice melts completely, early in the season, then ecosystems will suffer drought for the rest of the season (Robinson et al., 2018). Elevated temperatures might also lead to heat stress in certain organisms. Mosses and lichens in Antarctica are often dark in colour allowing the absorption of maximum solar radiation and the creation of warm microclimates for optimum metabolism (Bramley-Alves, King, Robinson, & Miller, 2014). This is effective when air temperatures are just above zero, warming the moss between 10 and 30°C, but this strategy could easily lead to heat stress once air temperatures exceed 10°. On King George Island, our measurements show that in January 2019 moss surface temperatures only exceeded 14°C for 3% of the time, but in 2020 this increased fourfold (to 12% of the time.) Based on our experience from previous anomalous hot summers in Antarctica we can expect a multitude of biological impacts to be reported in coming years, illustrating how climate change is impacting even the most remote areas of the planet. Fieldwork in Antarctica was supported by the Australian Antarctic Division (DMB) and the Instituto Antártico Chileno (GEZ, MPR). NCEP/NCAR Reanalysis 1 data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their web site at https://www.esrl.noaa.gov/psd/. This work is a contribution to Australian Research Council Discovery projects DP180100113 and DP200100223, Australian Antarctic Science projects 4516 (SAR, DHK, DMB) and 4387 (ARK) and INACH project RT_14-17 (GEZ, MPR, SAR).