REM photograph of a planktic foraminifera. (Photo by Hannes Grobe)

One key concept that climate scientists and policymakers use to forecast future global warming — and estimate how much we should reduce greenhouse-gas emissions — is known as “climate sensitivity.” In a nutshell, it’s the amount of warming that will occur each time carbon dioxide concentrations in the atmosphere double. The United Nations’s Intergovernmental Panel on Climate Change’s best estimate right now for this quantity is roughly 1.5 to 4.5 degrees Celsius for a doubling of carbon dioxide.

Right now, humans are well on pace to at least double carbon dioxide concentrations from preindustrial levels by the middle of the century.

But given how complex the climate system is, how do we know that the IPCC’s sensitivity estimate holds true? There’s a lot at stake — if the scientists are overestimating the climate sensitivity then global warming might be less worrying. No wonder that climate “skeptics” have often cast doubt on the matter.

To study climate sensitivity, researchers rely on basic physics and chemistry. But they also look at past climates to see how they responded when carbon dioxide levels changed.

It’s in this context that a new piece of evidence — just published in the journal Nature — backs up the IPCC. And it comes, of all places, from a set of tiny microorganisms, preserved in ocean sediments, whose shells hold chemical fingerprints of past carbon dioxide concentrations going back millions of years. An analysis of these carbon dioxide fingerprints, in conjunction with other climate records stretching back millions of years, shows that Earth’s sensitivity to carbon dioxide has long fallen in that familiar 1.5 to 4.5 degree range.

Studying past climates has long been of interest to climate scientists. They’re especially interested in the time period known as the Pliocene epoch (5.3 to 2.6 million years ago) because a significant portion of it (3.3 to 3 million years ago) was a few degrees Celsius warmer than today. That makes it a good candidate for understanding how climate might look in the near-term future.

We have a good sense of what the Earth’s carbon dioxide levels looked like as far back as 800,000 years ago, thanks to ice core records. But ice core records stop well before the Pliocene. So instead, the researchers, led by Miguel Angel Martinez-Boti of Britain’s National Oceanography Centre in Southampton, turned to one-celled, ocean-dwelling shelled microbes known as foraminifera.

One aspect of these protists’ chemistry — the ratio of isotopes of the chemical element boron in their shells — is affected by concentrations of carbon dioxide in seawater. And from those, we can actually infer what air concentrations were, too.

So, the researchers dated shells that had been drilled from multiple sites at the bottom of the ocean, and then tried to estimate airborne carbon dioxide levels at the time.

The analysis revealed that in the Pliocene, carbon dioxide levels varied a fair amount, but they reached as high as 450 parts per million, similar to what we might encounter in the near future (they’re now pushing 400 ppm). Based on this information, the researchers could calculate climate sensitivity figures.

The result? A number that fell somewhere in that 1.5 to 4.5 degree range for the Pliocene, the researchers found.

Then researchers did the same calculation for the late Pleistocene — a period (2.6 million to 12,000 years ago) which was marked by the planet’s most recent ice ages — based on a range of carbon dioxide records including ice cores. Temperatures actually changed twice as much in the late Pleistocene as in the Pliocene for a given carbon dioxide change, the researchers found.

Why? That’s because the planet was also experiencing large fluctuations in the size of its ice sheets, which reflect sunlight back to space. The melting of ice as the planet warmed would have amplified the warming due to carbon dioxide concentration increases alone. When researchers subtracted out the ice sheets’ contribution, though, they found that carbon dioxide’s effect on warming was about the same as it was in the Pliocene.

This study thus gives us another reason to think that the IPCC’s climate sensitivity range — at least for the foreseeable future — is on solid footing. In other words, the Earth’s climate is as responsive to rising carbon dioxide concentrations as we’ve long suspected. Whether that would still hold true if we triple or even quadruple carbon dioxide concentrations is another matter.

But for now, it all amounts to another reason to trust climate scientists.”This suggests that the research community has a sound understanding of what the climate will be like as we move toward a Pliocene-like warmer future caused by human greenhouse gas emissions,” said study co-author Gavin Foster of the University of Southampton.