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.