Sensitivity

Climate sensitivity is a measure of how much the Earth's climate will cool or warm after a change in the climate system, for instance, how much it will warm for doubling in carbon dioxide (CO2) concentrations. In technical terms, climate sensitivity is the average change in the Earth's surface temperature in response to changes in radiative forcing, the difference between incoming and outgoing energy on Earth. Climate sensitivity is a key measure in climate science, and a focus area for climate scientists, who want to understand the ultimate consequences of anthroprogenic climate change.
The Earth's surface warms as a direct consequence of increased atmospheric CO2, as well as increased concentrations of other greenhouse gases such as nitrous oxide and methane. Increasing temperatures have secondary effects on the climate system, such as an increase in atmospheric water vapour, which is itself also a greenhouse gas. Because scientists do not know exactly how strong these climate feedbacks are, it is difficult to precisely predict the amount of warming that will result from a given increase in greenhouse gas concentrations. If climate sensitivity turns out to be on the high side of scientific estimates, the Paris Agreement goal of limiting global warming to below 2 °C (3.6 °F) will be difficult to achieve.The two primary types of climate sensitivity are the shorter-term "transient climate response", the increase in global average temperature that is expected to have occurred at a time when the atmospheric CO2 concentration has doubled; and "equilibrium climate sensitivity", the higher long-term increase in global average temperature expected to occur after the effects of a doubled CO2 concentration have had time to reach a steady state. Climate sensitivity is typically estimated in three ways; using direct observations of temperature and levels of greenhouse gases taken during the industrial age; using indirectly estimated temperature and other measurements from the Earth's more distant past; and modelling the various aspects of the climate system with computers.

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