The Rainforest and the Ice Ages

Have you ever wondered how plants, animals, and people survived the last ice age?



In real life, Earth was more or less a greenhouse for millions of years after the nonavian dinosaurs went away 66 million years ago and mammals came into their own.

Then things began chilling down.

It played out over tens of millions of years, haltingly, with cooler periods alternating with warmer ones. Sometimes the changes were quick, but only in geologic terms — something that takes three hundred thousand years to happen is amazingly fast for Earth but an unimagineably long event for us.

Then, during the Pliocene epoch, Earth went through a complete transition into cycling ice ages, with the North Pole freezing up permanently and the ice field on Antarctica expanding. Interglacial warmer periods still come and go — we’re in one now — but Earth’s climate has been set to “icehouse” ever since late Pliocene times.

Like bosses. (Image: Armin S. Kowalski, CC BY-SA 2.0)

H. sapiens arrived late to the game, in the Pleistocene, but we have survived at least one glacial period just like most other modern life.

As it began, ice sheets didn’t move very fast but they kept coming on, a few inches per year, until, at the last glacial maximum some 20,000 years ago, most of North America and northern Europe was buried to a depth of up to a mile.

Our own ancestors faced the same choices as plants and animals:

  • Adapt
  • Go extinct
  • Move

Unfortunately, early people did not keep journals, and the cave paintings they left behind do not tell us much about their life decisions.

But we’re proof that they didn’t go extinct. How did they survive?

Scientists are still trying to understand what happened, and the development of new technology has provided some tantalizing clues.

The basic survival technique, of course, was having babies. As all these lives played out, human genes and bones stored up surprisingly detailed isotopic and genetic information.

We can now read a little of this record.

It suggests that they moved south to Spain and other peninsulas around the Mediterranean coast, where it’s warmer.

As we’ve seen, some trees did this, too.

Yes, real continental ice sheets move so slowly that even trees can get out of the way.


You might think that tropical life could shrug off anything short of a “Snowball Earth” scenario.

While that’s not exactly true — there are many important direct and indirect effects ice ages have on all parts of the world — you would not be wrong.

But Sundaland turns into dry land pretty much every time the planet even thinks about doing another big glaciation.


For one thing, the seas around what are now Indonesia and Borneo are very shallow, since this whole region is part of Asia’s continental shelf.

For another, glaciers are made out of frozen water — snow, rain, ice — that doesn’t return to the sea as it’s doing in today’s water cycle.

Get a continent-sized ice sheet (several million square miles) going, and global sea levels will drop dramatically.

By some estimates, by the Last Glacial Maximum the world’s oceans and seas were almost 400 feet below their current levels.

This gave almost twice as much land to Sundaland’s plants and animals at a time when European and North American life forms had their ranges drastically reduced.

The rainforest and its residents exploited that new resource to the limit. Eventually, sea level began to rise again as the ice age ended.

And most of this was covered by rainforest, per Cannon et al, in the source list.

In western Eurasia, when the last ice age ended some 12,000 years ago, plants, animals, and people spread out, following the retreating ice.

But think about it. Just as the Earth was warming up enough for us to invent agriculture and start domesticating livestock, rising sea level was forcing Sundaland’s plants and animals to shelter on the high elevations that we now call islands.

These hardcore survivalists are holed up on the lower Malay Peninsula, which is also a part of Sundaland. (Image: Lexe-I, CC BY 2.0)

It’s exactly the opposite ice-age experience from our own. The rich diversity we see on Sumatra today is actually life in hiding.

All those plants and animals will expand out over Sundaland if and when the next ice age comes.

An interesting point raised by Cannon et al. (see source list) is that the lifespan of many rainforest tree species is a couple centuries, at least.

On its own time scale, rainforest has covered Sundaland, not just Sumatra, Borneo, and other islands, for most of the last million years.

While tigers and other animals show evolutionary adaptations to this intermittent isolation on islands (such as forming new species, like the Sumatran tiger or another big cat we will meet soon, the Sunda clouded leopard), the rainforest maintains its diversity, biding its time until it can once again spread out across Sundaland.

Sadly, we are decimating that forest during this population bottleneck.

If we could understand exactly how Sundaland’s rainforest has survived such huge natural changes (from dry land to islands and back again), we might be able to develop better ways to protect it now, while it’s being stressed artificially during a time of refuge.

Featured image: Inga Vitola, CC BY 2.0


Cannon, C. H., Morley, R. J., & Bush, A. B. (2009). The current refugial rainforests of Sundaland are unrepresentative of their biogeographic past and highly vulnerable to disturbance. Proceedings of the National Academy of Sciences, 106(27), 11188-11193.

De Bruyn, M.; Stelbrink, B.; Morley, R. J.; Hall, R.; and others. 2014. Borneo and Indochina are major evolutionary hotspots for Southeast Asian biodiversity. Systematic Biology, 63(6): 879-901.

Francis, J. E.; Marenssi, S.; Levy, R.; Hambrey, M.; and others. 2009. From greenhouse to icehouse – the Eocene/Oligocene in Antarctica, in Developments in Earth and Environmental Sciences, Vol. 8, eds. Florindo, F., and Siegert, M., 311–372. Elsevier.

Hewitt, G. 2000. The genetic legacy of the Quaternary ice ages. Nature, 405(6789): 907

Kitchener, A. C., and Yamaguchi, N. 2010. What is a tiger? Biogeography, morphology, and taxonomy, in Tigers of the World (pp. 53-84). William Andrew Publishing.

Lyle, M.; Barron, J.; Bralower, T. J.; Huber, M.; and others. 2008. Pacific Ocean and Cenozoic evolution of climate. Reviews of Geophysics, 46(2).

Prothero, D. R. 2006. After the Dinosaurs: The Age of Mammals. Bloomington and Indianapolis: Indiana University Press. Retrieved from

Zachos, J.; Pagani, M.; Sloan, L.; Thomas, E.; and Billups, K. 2001. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science, 292: 686–693.

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