LONDON: 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.
