Boosted by pollution (Image: Ben Gunsberger/Workbook Stock/Getty)

Good news for climate modellers. After 25 years of trying, they have succeeded in simulating the hot climates of Earth’s prehistory. So why has it taken so long? Not because of the models themselves, but because the modellers made a false assumption: that aerosol pollution in the distant past was as bad as it is today.

At various times in Earth’s history, the planet was much hotter than it is now. Rocks and fossils laid down in these hothouse stretches show that the temperature difference between the poles and the equator was much smaller than it is today.

Climate models have long failed to simulate these warm times, says modeller Paul Valdes of the University of Bristol, UK. Specifically, they don’t heat the poles enough, often falling as much as 15 °C short. The models can get the poles right if modellers inject more greenhouse gases into the simulated atmosphere, but then the tropics overheat.


Trying to fix this cold-pole problem, Jeff Kiehl of the National Center for Atmospheric Research in Boulder, Colorado, simulated the climate of 55 million years ago. At that time Earth briefly experienced the hottest climate it has seen since the time of the dinosaurs, an event called the Palaeocene-Eocene thermal maximum. The average global temperature then was 35 °C – the average today is just 15 °C.

Clouds busted

Kiehl wondered whether the cold-pole problem came from the assumptions used to run the models. Most of these are derived from studying the modern atmosphere, which is heavily polluted with aerosols such as airborne soot. Water droplets condense around aerosols, so their presence affects the ways clouds form and behave – with significant consequences for the climate as a whole.

But there were no humans 55 million years ago, so the atmosphere was probably a lot cleaner. To find out how clean, Kiehl went back to a 2001 study that counted atmospheric water droplets in different regions of Earth. Polluted areas had up to 400 droplets per cubic centimetre, while less polluted ones had as few as 50.

Kiehl ran his model of the ancient climate with clean skies, and found that the cold-pole problem largely disappeared. With clouds forming in unpolluted air, the poles warmed up much more than the tropics, giving a climate within a few degrees of the one that actually existed.

The resulting model is a very good match for the Palaeocene-Eocene thermal maximum, says Paul Pearson of Cardiff University in the UK, who uses fossil animals to study past climates. Pearson says Kiehl’s model is the first to reproduce the temperature distribution revealed by the fossils.

“It’s reassuring,” Kiehl says. “If this is the explanation, there isn’t anything drastically wrong with our climate models.”

Kiehl presented his work on Tuesday at a Royal Society meeting on warm climates of the past in London.