One of the key measures of the impact of atmospheric carbon dioxide is called the climate sensitivity, which provides an estimate of how much the planet will warm in response to a doubling of the CO₂ concentration. This figure has been estimated using a variety of methods, producing a range of values; the Intergovernmental Panel on Climate Change estimates that the most likely value is 3 Kelvin, but recognizes there's a reasonable chance it could range anywhere from 2.4-4.5K. A new study that uses a climate model to evaluate the peak of the last glacial period, however, suggests that the IPCC's figure might be a bit high, and that very high values are overwhelmingly unlikely.

Glacial periods are triggered by small changes in the Earth's orbit. These aren't enough by themselves to alter the global climate, but they set off a drop in atmospheric CO₂ and an expansion of ice, which reflects sunlight back to space. These feedbacks help the Earth enter a deep chill during glacial periods.

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