Climate sensitivity

Climate sensitivity is the amount of global warming that results from a given change in radiative forcing, most commonly expressed as the warming caused by a doubling of atmospheric carbon dioxide. It is one of the most important numbers in climate science, because it links the emissions humanity chooses to how much the planet warms.

The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (2021) gives a best estimate of 3 °C of eventual warming for a doubling of carbon dioxide. Its likely range is 2.5–4 °C and its very likely range 2–5 °C, the narrowest range in the history of the assessments.

Scientists use several related quantities:

• Equilibrium climate sensitivity (ECS): the long-term warming of global mean surface temperature after carbon dioxide is doubled from pre-industrial levels and the climate has fully adjusted, apart from the slowest components such as ice sheets. It takes centuries to reach because the oceans absorb heat slowly.
• Transient climate response (TCR): the warming at the moment of doubling when carbon dioxide rises by 1% per year, roughly 70 years. It is lower than ECS because the deep ocean is still catching up. The IPCC best estimate is 1.8 °C (likely 1.4–2.2 °C).
• Transient climate response to cumulative emissions (TCRE): the warming per trillion tonnes of carbon emitted. It underlies the carbon budget concept, because warming scales almost linearly with total cumulative emissions.
• Earth system sensitivity: the very long-term response including slow feedbacks such as ice-sheet retreat and vegetation shifts. It is generally larger than ECS.

Doubling carbon dioxide traps about 4 watts per square meter of extra energy (the IPCC estimate is 3.9 W/m²). If nothing else changed, Earth would warm by roughly 1.2 °C before radiating that extra energy back to space, a stabilizing effect called the Planck response.

The actual warming is larger because the climate system responds in ways that amplify or dampen the initial change (see climate feedback loops):

• Water vapor feedback (amplifying): A warmer atmosphere holds about 7% more water vapor per degree (the Clausius-Clapeyron relation), and water vapor is itself a powerful greenhouse gas. This is the largest single feedback.
• Lapse-rate feedback (dampening): In the tropics, the upper troposphere warms more than the surface and radiates heat to space more efficiently, partly offsetting the water vapor feedback.
• Surface albedo feedback (amplifying): Melting snow and sea ice expose darker surfaces that absorb more sunlight (see ice-albedo feedback and albedo).
• Cloud feedback (likely amplifying): Clouds both reflect sunlight and trap heat, and their response to warming has long been the largest source of uncertainty. Improved observations and process studies led the IPCC to conclude that the net cloud feedback is likely positive, mainly because low clouds over tropical oceans thin as the climate warms.

No single method can pin down climate sensitivity, so scientists combine several independent lines of evidence:

1. Process understanding: physical studies of each feedback, informed by observations and high-resolution models.
2. The historical record: warming since the 1800s, compared with the forcing from greenhouse gases and cooling aerosols and with the heat taken up by the ocean (see ocean heat content).
3. Paleoclimate: past climates such as the Last Glacial Maximum, when carbon dioxide was low and temperatures several degrees colder, and warm periods such as the Pliocene, when carbon dioxide was similar to today. These are studied in paleoclimatology.
4. Climate models: comprehensive Earth system models, used to understand feedbacks rather than as the sole basis for the estimate.

A major 2020 assessment led by the World Climate Research Programme combined these lines of evidence. It ruled out very low values (below about 2 °C) more firmly than before and made very high values less likely, which fed into the narrower IPCC range.

Svante Arrhenius made the first quantitative estimate in 1896, calculating about 5–6 °C of warming for a doubling of carbon dioxide. In 1979 the U.S. National Academy of Sciences' Charney report concluded that a doubling would cause 1.5–4.5 °C of warming, with 3 °C most likely. That range stayed largely unchanged through five IPCC assessments until the Sixth Assessment Report narrowed it.

Some of the newest generation of climate models (CMIP6) have equilibrium sensitivities above 5 °C. Because these "hot models" warm more than the historical record and paleoclimate evidence support, the IPCC weighted them less in its assessment rather than adopting their values directly.

Climate sensitivity determines how much global warming follows from any emissions pathway, and therefore how quickly emissions must fall to meet the goals of the Paris Agreement. A value at the high end of the range would bring higher risks of crossing tipping points in the climate system, faster sea level rise, and more severe extremes. A value at the low end would buy time but not remove the need to reach net-zero carbon dioxide emissions to stabilize temperature.