Climate sensitivity: how Earth's temperature responds to greenhouse forcing

Key concepts and numbers

Doubled CO2 produces an increase in downward radiative forcing at the top of the atmosphere of roughly 3.7 watts per square metre (often rounded to about 4 W/m²). That forcing by itself does not determine the final temperature change — the climate system's feedbacks amplify or reduce the initial effect. A widely cited central estimate for ECS is near 3 °C per CO2 doubling, though studies and assessment reports give a range of plausible values. Many older and widely used assessments report credible ranges that span roughly 1.5–4.5 °C; more recent work has refined estimates but uncertainty remains. For summaries of these assessments and data sources, consult technical summaries and review articles.

Feedbacks that control sensitivity

• Water vapor and lapse rate: Warmer air holds more moisture, and water vapor is a strong greenhouse gas; interactions with vertical temperature structure alter warming.
• Surface albedo: Melting snow and ice reduce reflectivity, letting the surface absorb more solar energy.
• Clouds: Changes in cloud amount, altitude and type can either amplify or damp warming and are a major source of uncertainty.
• Carbon cycle interactions: Warming can release or store greenhouse gases from soils and oceans, modifying long‑term concentrations.

These feedbacks operate on different time scales. Some, like water vapor adjustments, act quickly; others, such as ice sheet responses or slow carbon cycle feedbacks, play out over decades to millennia.

Variants and how they are used

ECS and TCR are the two standard formal definitions used in scientific and policy contexts. ECS represents the equilibrium (long‑term) response, while TCR measures warming at the time CO2 reaches doubling under a scenario of gradually increasing CO2 (often used to characterize near‑term climate commitment). Observational analyses, paleoclimate records, and climate models each constrain these quantities in complementary ways; see observational constraints, paleoclimate evidence, and modeling studies for methods and examples.

Importance for policy and projections

Climate sensitivity is a central parameter for estimating future warming under different emissions pathways and for calculating remaining carbon budgets consistent with temperature targets. Higher sensitivity means smaller allowable cumulative CO2 emissions to avoid a given amount of warming. Because the exact value is uncertain, sensitivity contributes materially to the range of projected outcomes used by planners and negotiators. For accessible summaries and policy‑oriented explanations, see further reading.

Notable facts: sensitivity is not a fixed universal constant — it can depend on the background climate state and on the pattern of forcing — and research continues to refine estimates and reduce uncertainty through improved observations, process understanding and model development.