Chapter 01 · Climate & transitionClimate & Greenhouse Gas Emissions
Climate sensitivity
Definition
Climate sensitivity quantifies how strongly the global climate responds to rising greenhouse gas concentrations. Equilibrium climate sensitivity (ECS) is the long-term warming once the climate system fully adjusts to doubled CO2; the IPCC Sixth Assessment Report assesses a best estimate of 3°C, a likely range of 2.5–4°C, and a very likely range of 2–5°C.
References
AR6 ECS best estimate 3°C, likely 2.5–4°C, very likely 2–5°C
AR6 narrowing of the ECS range versus AR5; TCRE metric
ECS/TCR/ESS distinctions and estimation methods
Overview
What it means
It converts emissions into expected warming, so it anchors carbon budgets, mitigation targets and scenario projections.
How it is used
Estimated from climate models, the instrumental record and palaeoclimate evidence; the related transient climate response (TCR) measures warming at the moment of doubling under a gradual increase.
Why it matters
The remaining uncertainty — especially the upper tail — directly affects how quickly emissions must fall to meet Paris Agreement goals. - **Note:** Values above 5°C cannot yet be ruled out; the very-likely upper bound carries only medium confidence in IPCC terminology, and recent high-sensitivity studies (e. g. Hansen et al. 2023) remain contested.