A Two-Million-Year History of the Temperature of the Earth

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Core samples from the bottom of the world’s oceans suggest a new take on ice ages of the past, and a sneak peak at the millennium ahead.

By Nathan Collins

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(Photo: Ian Waldie/Getty Images)

A reconstruction of two million years of global average surface temperatures and a comparison with greenhouse gas levels over a similar timeframe suggests that doubling greenhouse gas emissions would warm the planet by about nine degrees—another sign the centuries to come are going to be warmer than they’ve been in a very long time.

“This result suggests that stabilization at today’s greenhouse gas levels may already commit Earth to an eventual total warming of 5 degrees Celsius …over the next few millennia as ice sheets, vegetation and atmospheric dust continue to respond to global warming,” writes Carolyn Snyder, a Stanford University climate scientist.

Figuring out how hot it was several million years ago is both less and more difficult than one might imagine. It’s relatively easy to take a sample and determine its age and the temperature at the corresponding time. For example, researchers can use radiocarbon dating to date sea sediment cores, then examine them for signs of phytoplankton activity, the mix of fossilized organisms, and so on to get clues to the temperature at the time those creatures lived.

Even if greenhouse gas concentrations somehow didn’t get any higher, the Earth is likely to get about five degrees Celsius warmer over the next several millennia.

The tricky part, Snyder points out, is that sea sediment samples from two million years ago are easier to come by than land-based samples. That means reconstructing global temperatures requires a certain amount of interpolation and modeling, for which Snyder used models incorporated in the Paleoclimate Model Intercomparison Project. Similar to the Climate Model Intercomparison Project, which is focused on the recent past and the future, PMIP uses an ensemble of simulations to better understand climate change over hundreds of thousands of years or more.

Based on 59 sea sediment samples from previously published studies (and their aforementioned modeling), Snyder concludes that temperatures gradually declined by 0.34 degrees Celsius per 100,000 years between two and 1.2 million years ago, for a net change of 2.72 degrees. After that, temperatures pretty much leveled off. To be sure, the planet went in and out of ice ages every 100,000 years or so, but in the geological long term, not much changed.

That result stands out for two reasons. First, long-term global cooling most likely stopped well before the mid-Pleistocene transition, when the ice-age cycle switched from about 41,000 to 100,000 years long. If Snyder’s estimates are correct, then global cooling alone was not enough to trigger the transition on its own, as some had thought.

More important for the present day, Snyder reports that temperatures are closely correlated with the 800,000 years’ worth of greenhouse gas data from Antarctic ice core samples and, in turn, with global warming. Based on that finding, Snyder calculates that, even if greenhouse gas concentrations somehow didn’t get any higher, the Earth is likely to grow about five degrees Celsius warmer over the next several millennia—a number comparable to other researchers’ estimates, but much faster than anything that’s happened over the last two million years.

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