The seasons are turning. In Britain, with an oceanic climate and the mellowing influence of the Gulf Stream, the days become progressively cooler, early morning frosts soon thaw in the autumn sunshine and the descent into winter is a gradual affair. I have just returned from a trip to Yellowstone and Grand Teton national parks in the US, where summer was just about to become winter, with an abruptness that surprises even a well-traveled Briton.
Humans in the central US and Britain generally respond to the onset of the cold with similar changes in behavior, spending more time indoors and digging out woolly jumpers from the bottom drawer. Other animals react by finding a suitable den in which to slumber through the months of cold.
The taxi driver who took me to Jackson Hole Airport told me that it would be snowing within two weeks. The grizzly bears of Grand Teton would already be looking for somewhere to hibernate. Many birds, being among the most mobile of vertebrates, take to the wing and fly south to find warmth. Some North American and northern European humans do the same, overwintering in Mediterranean refugia.
We are enjoying a relatively long and stable period of pleasant weather at the moment. By that, I do not mean the last few years or decades. I mean the last 12,000 years or so. For about 2.6 million years, the Earth has experienced a fluctuating climate, swinging between icy glaciations and warm interglacials. It is an uneven division, with glaciations lasting about 100,000 years and interglacials just 10,000 to 15,000 years.
So just imagine the impact on plants and animals of these switches from long, wintry glaciations to brief, summery interglacials. The differences in climate are extreme. At the peak of the last ice age, just 20,000 years ago, vast ice sheets descended over the northern parts of Europe and North America. For any animals adapted to a temperate climate, glaciations posed something of a challenge. Hibernation and woolly jumpers would not be enough to ride out 100,000 years of winter.
Those warm-loving animals (and humans) cleared out of the north and sought refuge in the south. However, do not imagine bands, packs and herds streaming southwards. Most populations simply went extinct in northern latitudes, leaving their southern cousins to carry the torch for their species. Pollen records show the same thing happening to plants.
Even for those populations of plants or animals living in a habitable area — or refugium — in the south, survival right through a glaciation is far from guaranteed. The refugium may just be too small to support a viable population, especially for large-bodied animals or carnivores. Often the last glimpse of extinct ice-age species in the fossil record comes from relic populations clinging on in southern refugia, just before their species’ light is extinguished. Neanderthals are often thought of as being “cold-adapted.” They might have been a bit stockier than us, but they were not by any means an Arctic species. They existed in Europe for about 200,000 years, but like all the other temperate-adapted animals, they cleared out of northern Europe during glaciations and hunkered down in southern refugia. We see them for the last time in the southernmost tip of the Iberian peninsula, living in caves on the Rock of Gibraltar, about 28,000 years ago. And then they were gone.
Refugia are not just places where extinction is more likely, though. They are also places where new species are born. Fragmented, isolated populations are likely to evolve in different directions. For warm-loving species, the south may provide an important refugium during a glaciation, but a few small populations may also be able to hang on in pockets of suitable habitat in the north. Some time in the last 300,000 years, a population of brown bears clung on through a long glaciation in an isolated northern refugium — perhaps in Ireland. Those bears gradually adapted to the icy north. They became a new species — of white, polar bears.
Our own species may have appeared in a similar way, in a glacial refugium. It is thought that modern humans (Homo sapiens) and Neanderthals (Homo neanderthalensis) evolved from the same ancestral species: Homo heidelbergensis. It is likely that these ancient humans spread across Africa, Europe and Asia during an interglacial, then the population became fragmented during a glaciation. A later interglacial provided modern humans with the opportunity to expand out of Africa and across the globe. At the peak of the last glaciation, the modern human population was hit hard, but we survived.
So here we are, in an interglacial. It is sobering to remember that this is just a brief interlude. Some time fairly soon, we will probably begin the descent into a long glacial winter again. When that happens, all the temperate-adapted animals that range widely across the northern hemisphere will clear out of the far north. Ice sheets will grow down over North America and northern Europe again. Our civilizations have grown up in this unseasonably stable (and already overlong) warm interglacial. We have grown a huge global population in this favorable climate. It will be even more difficult to support such a massive number of people when the world becomes colder and drier again.
However, for those cold-adapted animals which are currently hanging out in Arctic refugia, things will look up. If polar bears manage to cling on, there will be much more room for them in the next glaciation.
Alice Roberts is professor of public engagement in science at the University of Birmingham.