Climate sensitivity

Climate sensitivity measures how much global mean surface temperature changes in response to a change in climate forcing. It is commonly expressed as the warming after atmospheric carbon dioxide doubles and the climate reaches equilibrium.

Climate feedback: A process that amplifies or reduces a climate response to an initial change. Feedbacks turn the initial warming from doubled carbon dioxide into a larger or smaller temperature response.
Radiative forcing: A change in Earth’s energy balance caused by an external influence, measured in watts per square meter. Sensitivity relates a specified forcing change to the resulting global temperature change.
Intergovernmental Panel on Climate Change: A United Nations body that assesses scientific knowledge about climate change. Its assessments synthesize evidence and report ranges for equilibrium climate sensitivity.
Earth’s energy imbalance: The difference between incoming solar energy and outgoing energy at Earth’s top of atmosphere. Observed ocean heat uptake helps constrain how much warming remains unrealized.
Planck response: The stabilizing increase in outgoing infrared radiation as a planet warms. This basic radiative response limits warming even before other climate feedbacks are included.
Equilibrium climate sensitivity: The long-term global warming after atmospheric carbon dioxide doubles, once the climate system reaches equilibrium. This is the standard measure of sensitivity defined by a doubled-carbon-dioxide experiment.
Energy budget method: A method for estimating climate sensitivity from observed warming, forcing, and changes in Earth’s energy uptake. It uses measurements of the historical climate response to constrain sensitivity.
Effective climate sensitivity: An estimate of equilibrium sensitivity inferred from a climate system that may not be fully equilibrated. Its value can shift as feedbacks evolve over time in observations and models.
Water vapor feedback: The amplification of warming as warmer air holds more water vapor, a greenhouse gas. Increasing atmospheric water vapor is a major positive feedback in climate sensitivity.
Transient climate response: The global warming at the time carbon dioxide doubles during a sustained 1% annual increase. Unlike equilibrium sensitivity, it measures warming before the oceans have fully adjusted.

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