MANY people fear that the rapid disappearance of Arctic sea ice spells doom for polar bears. The effect of global warming on another famous northern species, the reindeer, is, however, less cut and dried. Until recently, researchers thought reindeer benefit, rather than suffer, from climate change. The lichens, grasses and shrubs they eat grow better in warmer summers, and their populations have been rising. But Åshild Ønvik Pedersen and Jean-Charles Gallet of the Norwegian Polar Institute (NPI) in Tromsø, who have been investigating the matter in detail, argue that the benefits of warmer summers may soon be nullified by the countervailing consequences of warmer and wetter winters.
The reindeer around Ny Ålesund, a former mining town in south-western Svalbard that has now become an Arctic research centre, have been a subject of study for almost four decades. In particular, researchers from the NPI have, since 2000, been looking at the effects on the deer of a phenomenon called “rain-on-snow”. This happens when it rains during warm spells in the long Arctic winter. As the temperature swings back to normal, which can be as low as -30°C in this part of the world, rainwater that has percolated through the snow freezes, forming a thick crust of ice that seals off the tundra below. To examine the consequences, the NPI’s researchers have been collecting data regularly about the depth of the snow and ice at hundreds of points across Svalbard.
In the past half century the number of rainy winter days per year on the archipelago has more than doubled, with a concomitant increase in the amount of icebound tundra. That is bad news for reindeer. They survive the winter by foraging under the snow, which they clear away with their hooves. Unfortunately, they are unable to stamp through surface ice to get at the food underneath if that ice is more than a couple of centimetres thick. Dr Ønvik Pedersen and Dr Gallet have found that the greater the average thickness of the ice in their study area is, the fewer are the animals that survive the winter and the fewer the calves born the following spring. Sometimes the effect is catastrophic. In the aftermath of a particularly severe incident in 1993, for example, the reindeer population of Svalbard fell by nearly 70%.
Moreover, even when the animals do not die, they suffer. Another research team, led by Steve Albon of the James Hutton Institute in Aberdeen, Britain, has been weighing them regularly. Dr Albon’s team has found, to no one’s surprise, that reindeer lose a lot of weight—as much as 20%—after bad icing events. More surprisingly, and more worryingly, the team has also found a long-term decline in the animals’ weight. Adult female reindeer in Svalbard weigh today, on average, 12% less than their counterparts did in 1994. That has brought many of them below 50kg, which experience suggests is a threshold beyond which their reproductive success and the survival of their calves decline sharply.
Nor is it just reindeer that are affected by rain-on-snow events. Dr Ønvik Pedersen’s team has found that at least three other species—ptarmigans (a type of grouse), sibling voles and Arctic foxes—are similarly hit. The consequences of rain-on-snow events could thus, she argues, cascade through the Arctic ecosystem.
Correlating the snow- and ice-depth data with information from surveys about the animals themselves (including their whereabouts, body weights, winter survival rate and reproduction) permits the researchers to track the situation and to develop models of how the local ecosystem works. In particular, they are looking for tipping points beyond which rain-on-snow events could have irreversible effects on the Arctic food web.
In the shorter term their approach may help prevent incidents such as that which happened four years ago on the Yamal peninsula in Siberia. Unlike reindeer in Svalbard, which are wild, Yamal’s are herded by local people. These herders lost 61,000 of their beasts to starvation after rainstorms deluged the region in November 2013. If herders knew which places would be hit badly on such occasions, they could then take their animals to less threatened areas—or, were that not possible, call up mobile slaughterhouses to kill the deer humanely, before they lost weight, thus minimising financial losses. Applying Dr Ønvik Pedersen’s methods to regions where herders operate would assist that.
This article appeared in the Science & technology section of the print edition under the headline "Dreaming of a White Christmas"