Major study rules out super-high and low climate sensitivity to CO₂

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One of the most important numbers in climate science is 3C. This isn't about a projection of future warming or the impacts that come with it, though. It's about how much warming you get if you double the amount of greenhouse gases in the atmosphere. That value can be made more general as a metric known as climate sensitivity," which describes how much warming you get for a given amount of emissions. If the number is small, we can burn a lot of fossil fuels with minimal consequences. If the number is extremely high, emissions are extraordinarily dangerous.

This number is commonly defined against a doubling of the concentration of CO₂ in the air, in part because CO₂'s effect is logarithmic and each doubling is roughly equivalent. Calculations of this value go back to the turn of the 20th century, when the Swedish scientist Svante Arrhenius came up with numbers in the 4-6C range. But a major milestone was reached in 1979, when a group of scientists released a climate report that included this value. The scientists wrote, We estimate the most probable global warming for a doubling of CO₂ to be near 3C with a probable error of 1.5C."

Despite all the scientific progress since then, that answer (1.5-4.5C) has held up. The 2007 Intergovernmental Panel on Climate Change report tightened it up a bit to 2.0-4.5C, but then a handful of studies released just before their 2013 report caused confusion that led to a return to the old 1.5-4.5C range.

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