Is Climate Change Icing Out Rudolph?

Thanks to global warming, arctic winters increasingly alternate between mild and freezing so the tundra is covered in ice instead of snow, preventing reindeer from digging for food — but longer autumns may help them survive, for now

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Thanks to climate change, temperatures are rising around the globe, but they are rising fastest at the Poles. Temperature records have shown that in the Arctic, temperatures are rising three times faster than the global annual average. But as climate changes, it’s not just the average temperature that increases, but climactic variability increases too, and this is creating many unanticipated environmental problems.

One pervasive problem is the increasing fluctuations between cold and warm temperatures. These temperature variations melt snow and ice and then refreeze the resulting slush into a thick sheet of ice when temperatures drop again. Ice storms are also increasingly frequent: known by climate scientists as rain-on-snow (ROS) events, these ROS events cover the landscape in thick layers of ice.

Researchers are working to better understand the effects of climate change in the Arctic by studying reindeer. Svalbard reindeer, Rangifer platyrhynchus, are the smallest in the world — half the size of mainland reindeer. These tiny reindeer dwell on Svalbard — formerly known as Spitzbergen — an island archipelago located in the Arctic Ocean between mainland Norway and the North Pole. One of the world’s northernmost areas inhabited by people, Svalbard is known for its extreme environment featuring short summers and harsh winters, its rugged and remote terrain filled with glaciers and frozen tundra that test the survival skills of all its specially adapted creatures, especially those of its tiny isolated population of reindeer.

How do Svalbard reindeer survive such extremely cold winters? For example, how do they find enough to eat?

“In a normal Svalbard winter, the ground is covered with snow and the animals can dig for food”, said the study’s lead author, wildlife biologist Leif Egil Loe, a professor of Environmental Science and Nature Management at the Norwegian University of Life Sciences.

But they cannot dig through a thick layer of ice to gain access to ground-level vegetation. Without access to sufficient food, reindeer can die of starvation, sometimes in large numbers. How do the diminutive Svalbard reindeer survive the winter when ice cover is increasingly common?

Professor Loe and an international team of Arctic biologists and climate modelers set out to find some answers. On Spitsbergen, the largest island in the Svalbard archipelago, scientists have tracked the resident herd of roughly 850 reindeer for 25 years. They also recorded annual population numbers. Additionally, during this time period, Professor Loe and his collaborators caught and weighed about 1000 female Svalbard reindeer. Then in 2009, they added GPS tracking collars to 65 reindeer.

The researchers analyzed 25 years of their collected data to investigate what effects ROS events have on reindeer survival.

Because there are almost no predation pressures on Svalbard, ROS events control reindeer population size on Svalbard by limiting access to their food plants. As a result, reindeer population numbers can grow for a few years until until ROS events occur, resulting in a large population crash due to starvation. But something abruptly changed in the last ten years: the reindeer population has grown continuously despite icy winters. Why?

“These [ROS] icing events on vegetation tend to get a lot of attention, including from some of our previous work,” Professor Loe said in a statement. “It wasn’t until we started to look at the pattern of snow in autumn that we realized how strong and important the ‘autumn effect’ was.”

The autumn effect refers to the warmer and longer Arctic autumns that allow reindeer longer access to the plants they eat. This, combined with warmer summers, can lead to a doubling of overall plant biomass, which then provides plenty of nutritious food in autumn to the reindeer as they prepare for winter. Are longer and warmer autumns sufficient to improve reindeer survival, despite generating shorter and harsher winters?

“Reindeer are able to feed up and store more fat prior to the autumn”, Professor Loe explained, “and if the snow comes late, they can start to use their fat reserves later.”

Indeed, Professor Loe and his collaborators found that reindeer with greater body masses have better odds of surviving winter, and that body masses increased with warmer autumn temperatures because snow cover was delayed.

“In the years and areas where the snow arrived late, the animals were on average five kilograms and ten percent heavier at the end of winter”, Professor Loe noted.

This relationship between food access and winter survival more than compensated for extremely harsh winters, the researchers found.

“Warmer autumns more than offset the negative effects of icy winters,” Professor Loe elaborated, noting that a delay in autumn snows led to roughly a 20% growth in population. In fact, increasingly warmer autumns are “enough to counteract all but the most extreme icing events.”

The autumn effect sounds like good news for Svalbard’s tiny reindeer, but real life is, of course, more complicated than this.

“In some cases, the climate change card is pulled too easily”, Professor Loe pointed out. “There are lots of important underlying factors.”

For example, the herd’s age structure and population density, as well as delayed effects from the previous year, can affect population changes. Additionally, the warmer temperatures created by climate change make the reindeer uncomfortable.

“On very warm summer days, they are less active. They seek out cooler ground and snow patches to rest on”, Professor Loe explained. “There are both positive and negative effects of continued warming.”

Further, as climate changes, it’s not just the average temperature increases that are troubling, but the subtle and complicated effects upon many environmental factors, so each season experiences a different combination of effects. For example, as we are now experiencing across the United States and much of Europe, the climate’s rapidly increasing fluctuations between growing extremes should worry us. For Svalbard’s reindeer, such severe climactic volatility increases the probability of a bad ‘combination year’ of early autumn snows followed by multiple ice storms, which means the Svalbard reindeer population can still suddenly crash.

Source:

Leif Egil Loe, Glen E. Liston, Gabriel Pigeon, Kristin Barker, Nir Horvitz, Audun Stien, Mads Forchhammer, Wayne Marcus Getz, Robert Justin Irvine, Aline Lee, Lars K. Movik, Atle Mysterud, Åshild Ø. Pedersen, Adele K. Reinking, Erik Ropstad, Liv Monica Trondrud, Torkild Tveraa, Vebjørn Veiberg, Brage B. Hansen, and Steve D. Albon (2021). The neglected season: Warmer autumns counteract harsher winters and promote population growth in Arctic reindeer, Global Change Biology 27:993–1002 | doi:10.1111/gcb.15458

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