Climate Feedbacks and Sensitivity

The processes that amplify or damp the climate's response to a forcing, and the resulting equilibrium warming expected from doubling carbon dioxide.

Definition

A climate feedback is a process that changes in response to warming or cooling in a way that amplifies or damps the original change, and climate sensitivity is the equilibrium global temperature change produced by a doubling of atmospheric carbon dioxide.

Scope

This topic covers the feedback processes that govern how strongly the climate responds to a radiative perturbation: the water vapor feedback, the lapse-rate feedback, the surface-albedo feedback from melting snow and ice, and the cloud feedbacks that remain the most uncertain. It treats how these combine into the equilibrium climate sensitivity and the transient climate response, and the multiple lines of evidence, from process studies, the instrumental record, and paleoclimate, used to constrain these quantities.

Core questions

• Which feedbacks amplify warming and which damp it?
• Why is the cloud feedback the dominant source of uncertainty?
• What is the best estimate and range of equilibrium climate sensitivity?
• How do multiple lines of evidence constrain sensitivity?

Key theories

Water vapor and lapse-rate feedbacks

A warmer atmosphere holds more water vapor, a powerful greenhouse gas that strongly amplifies warming, while changes in the vertical temperature profile partially offset this through the lapse-rate feedback.

Multiple-lines-of-evidence constraint on sensitivity

Combining feedback process understanding, the historical warming record, and paleoclimate evidence narrows the likely equilibrium climate sensitivity and rules out very low values.

Mechanisms

An initial forcing warms the surface, triggering feedbacks: rising water vapor and retreating snow and ice amplify the warming, while increased thermal emission and certain lapse-rate and cloud responses can damp it. The net feedback determines how much additional warming is needed to restore energy balance, so a strongly positive net feedback yields a high climate sensitivity.

Clinical relevance

Climate sensitivity sets how much warming results from a given emission of greenhouse gases, so reducing its uncertainty directly sharpens projections used for setting emission targets and planning adaptation.

Evidence & guidelines

The IPCC Sixth Assessment Report, drawing on the multiple-lines-of-evidence synthesis, assesses equilibrium climate sensitivity with a best estimate near 3 degrees Celsius and a likely range of about 2.5 to 4 degrees Celsius per carbon dioxide doubling.

History

The 1979 Charney report first framed the modern range of climate sensitivity at roughly 1.5 to 4.5 degrees Celsius, a range that persisted for decades; recent work combining process understanding, the historical record, and paleoclimate has at last narrowed it, mainly by improving the treatment of cloud feedbacks.

Debates

Magnitude and sign of cloud feedbacks

Whether low-cloud changes amplify or damp warming, and by how much, is the leading reason climate sensitivity remained uncertain, though recent observations and high-resolution modeling favor a net amplifying effect.

Key figures

• Jule Charney
• Steven Sherwood
• Syukuro Manabe
• Isaac Held

Seminal works

• charney1979
• sherwood2020

Frequently asked questions

What is equilibrium climate sensitivity?

It is the global average warming reached, after the system fully adjusts, in response to a doubling of atmospheric carbon dioxide, assessed as likely about 2.5 to 4 degrees Celsius.

Why are clouds so important for climate sensitivity?

Clouds both reflect sunlight and trap heat, and small changes in their amount, height, or brightness can substantially amplify or damp warming, making them the largest uncertainty.