Snow has been falling all week long in Copenhagen – winter has come properly. From a comfortable office an ecologist can easily contemplate the effect of the seasons on the natural world – then also easily spend the later part of that afternoon drinking gløgg while the storm continues outside!

I’ve been thinking about what winter means at high latitudes. Recently I flew over a region further north than Denmark, and the view out the window was one of ice lit by a pale winter sun. An inhospitable place for any organism.

But what about in the past? The earth has been in a greenhouse climate for most of the last five hundred million years, where ‘greenhouse’ is defined as the absence of ice caps. That means that for long intervals, the North and South poles have been warm and wet. We have evidence from 100 million years ago (during the Mesozoic) in Antarctica, that large swaths of land were covered in rainforests! And about five million years ago, before the last major incursion of polar ice, Antarctica would have been covered in beech (Fagus sp.) forests, like the one you see below from modern-day Sweden.

The presence of these forests raises a tricky question – what about the polar winters? At high latitudes, several months of the year pass in complete darkness. What is life like for a plant or an animal when the world is always dark, when there are no dawns? We think that many animals would have migrated during the winters (for many millions of years, Antarctica was connected by land to other lower-latitude land masses) while some would have hibernated or made do as modern polar animals do.

But plants face a different challenge – they cannot move when the darkness comes, yet they depend on the sun for energy to power carbon acquisition via photosynthesis. What happens to a lush rainforest during a multi-month interval of darkness? The answer is that the plants slowly use up stores of carbon in their roots and stems, and come closer and closer to dying of starvation. Respiration, the transformation of stored carbon into energy, is a necessary process for all organisms, and plants are no exception.

One strategy is for plants to become deciduous, dropping their leaves or killing off stems in the winter. This strategy reduces the total rate of respiration and so decreases mortality (more details). However, the fossil record shows that not all polar species were deciduous – and in fact, many were evergreen! So how did these plants survive?

The answer is still controversial, but may depend on some nuances of plant physiology. Mesozoic atmospheres had much higher levels of carbon dioxide in the atmosphere. This CO2 boost has several potential consequences: 1) increasing rates of carbon acquisition during summer months because of greater availability of CO2 for photosynthetic enzymes, 2) decreasing rates of carbon acquisition because sustained high summer light levels interact with high CO2 to inhibit the efficiency of photosynthetic enzymes, 3) decreased winter respiration rates, because higher CO2 directly inhibits mitochondrial enzyme activity, and 4) higher winter respiration rates, because higher CO2 can increase demand for metabolic products produced by respiration. You may note that these four hypotheses predict completely conflicting outcomes for the overall carbon balance of the plant. Fortunately a recent study tested these different possibilities by growing a variety of modern species in environments with elevated CO2 and simulated polar light regimes for several years. They chose species that were close relatives of the ones that would have lived in polar regions in the Mesozoic.

To make a long story short, they found that overall rates of respiration decrease (yes to 3, no to 4) and that for long summers, overall carbon acquisition does not change (no to 1, no to 2). Of course, the results depend closely on which species are being studied – but overall, it looks like high CO2 levels reduced the rate of carbon usage in winter, making the polar winter challenge less severe than it would be today. The BBC has a good summary of this work.

But step back from the details – imagine being in a Mesozoic Antarctic rainforest in the dead of winter – walk for weeks and you would still be in utter darkness, though the animals would be gone and the trees would still have leaves – a world waiting for change.