One predicted consequence of global warming is an increased frequency of extreme weatherevents, such as heat waves, droughts, or heavy rainfalls. In parts of the Arctic, extreme warmspells and heavy rain-on-snow (ROS) events in winter are already more frequent. How theseweather events impact snow-pack and permafrost characteristics is rarely documentedempirically, and the implications for wildlife and society are hence far from understood. Here wecharacterize and document the effects of an extreme warm spell and ROS event that occurred inHigh Arctic Svalbard in January–February 2012, during the polar night. In this normally coldsemi-desert environment, we recorded above-zero temperatures (up to 7 °C) across the entirearchipelago and record-breaking precipitation, with up to 98 mm rainfall in one day (returnperiod of >500 years prior to this event) and 272 mm over the two-week long warm spell. Theseprecipitation amounts are equivalent to 25 and 70% respectively of the mean annual totalprecipitation. The extreme event caused significant increase in permafrost temperatures down toat least 5 m depth, induced slush avalanches with resultant damage to infrastructure, and left asignificant ground-ice cover (∼5–20 cm thick basal ice). The ground-ice not only affectedinhabitants by closing roads and airports as well as reducing mobility and thereby tourismincome, but it also led to high starvation-induced mortality in all monitored populations of thewild reindeer by blocking access to the winter food source. Based on empirical-statisticaldownscaling of global climate models run under the moderate RCP4.5 emission scenario, wepredict strong future warming with average mid-winter temperatures even approaching 0 °C,suggesting increased frequency of ROS. This will have far-reaching implications for Arcticecosystems and societies through the changes in snow-pack and permafrost properties.
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