Weird new winters: From ‘false springs’ to freezing temperatures

Winter is frozen, cold, dark, snowy, less productive, variable, deadly, understudied and changing. These nine fundamental principles of winter affect everything that happens in nature during the three months when all species experience the shortest days and lowest temperatures. The cold reality is that many individuals alive in autumn are dead by spring.

Winter is surprisingly hard to define because it varies so much in space and in time. Winter is experienced differently depending on where you are located. The darkest days do not coincide with the coldest temperatures, typically the coldest temperatures lag two to eight weeks behind the date of the winter solstice.

This lag effect is due to the thermal inertia of the sea and land, it takes a long time to cool down. Thermal inertia of the limestone rocks near the soil surface in the Burren is why cows are moved up into “winterage” pastures in the uplands during the winter to take advantage of continued grass growth.

While we might consider winter to be a universal concept, only about 30 per cent of the world’s land mass experiences a frigid winter; that is the higher latitude temperate, boreal and polar ecosystems. But for that 30 per cent winter has a profound effect on all of the processes of life.

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Cold temperatures slow down the chemical processes in organisms. Biological membranes can lose fluidity and fats become more solid. Organisms that live through cold winters must adapt to these new extreme conditions and can evolve metabolisms that enable them to resist the chemical changes, escape through migration or hibernate their way through the worst of the winter weather.

Hibernating black bears even reabsorb their own urine to prevent dehydration. The adaptations to winter can be so hard-wired into organisms that some plant seeds cannot germinate unless they have been exposed to a certain number of days of cold temperatures.

When water freezes inside or outside an organism it has major impacts. Some organisms have “antifreeze” running through their veins that mean they can remain unfrozen at extreme temperatures. Other organisms expend energy to remain warm despite freezing conditions in the outside world. Ice floating on the surface of water prevents gas exchange between water and the atmosphere and increases the reflectance of light back into the atmosphere, cooling the climate.

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There are whole “sub-nivean” ecosystems that exist in the thermal refuge between the land and its blanket of snow. Snow cover buffers invertebrates and small mammals from the worst of the cold air temperatures. Larger mammals can also find shelter from the elements in snow dens. Smaller organisms can travel fast over the snow and some plant species disperse their seeds in winter and benefit from additional dispersal of seeds through wind blow over smooth snow surfaces.

Lack of light in winter can lead to blunting of the body clock rhythms that control sleep cycles and even immune systems, particularly for mammals that overwinter in dark burrows. It is no surprise that ecosystems are less productive in winter, plants and animals hunker down until conditions become more conducive for photosynthesis, hunting and decomposition.

Winter is fascinating but still understudied, as ecologists focus their activities and experiments on the more charismatic and fast-moving seasons of spring, summer and autumn. This is a problem as our understanding of the consequences of climate-change induced “winter weirding” is incomplete. Extreme winter weather is becoming more common, increasing the variability of winter and its transition into spring. Unusual warm periods in winter can initiate “false spring” events where animals and plants emerge too early and are killed by a subsequent freezing event.

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Novel winter conditions are increasingly combining with other stressors such as warmer summers, ocean acidification, pollutants and invasive species. Pollutants may have a stronger effect in winter when animals are already depleted in energy or fat stores. Some insecticides are more toxic to honey bees at low winter temperatures.

The adaptations of organisms to the cold, dark, snowy days may not work in the weird new winters that they are exposed to. Improving our ecological understanding of winter will be essential for assessing how ecosystems and the services they provide to humans will change under different climate change scenarios.

Yvonne Buckley is an ecologist and professor of zoology at Trinity College Dublin

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