From Ars Technica: How sensitive is the climate to added CO2?

As recent events have shown, even the World Bank is trying to understand the trajectory of future climate changes. Although there are a number of ways of doing this, many organizations rely on a measure called the climate sensitivity. It’s a bit rough, but it’s simple: it provides a value for the temperature increase we’d expect given a doubling of CO2.

Currently, the Intergovernmental Panel on Climate Change places this value between 2 and 4.5°C, with a most likely value of about 3°C. But a variety of studies have come up with measurements spread around that range, and nailing down the likely upper limit has been a challenge. Now, a large group of researchers has gone through millions of years of data on the Earth’s past, incorporating information from a number of past studies. In the end, the group decided that the IPCC estimates are more or less on target.

Adding more carbon dioxide to the atmosphere doesn’t drive temperatures in a linear manner. You can think of this in terms of the infrared photons they absorb: each one can only be absorbed once, and the more CO2 molecules you add, the more likely it is that an existing one would have absorbed that photon anyway. As a result, each doubling of carbon dioxide concentrations are expected to have roughly an equivalent impact.

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from Ars Technica

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