Equilibrium climate sensitivity to doubled atmospheric CO₂ is likely between 1.5°C and 4.5°C.

Climate sensitivity asks a specific question: if the amount of carbon dioxide in the atmosphere doubled, how much warmer would Earth eventually become after the oceans and other slow parts of the climate system had adjusted?

That quantity is called equilibrium climate sensitivity.

The answer is not simply the warming produced by CO₂ itself. Warming triggers feedbacks. More water vapor strengthens the greenhouse effect, melting ice exposes darker surfaces that absorb more sunlight, and changes in clouds can either amplify or reduce warming. Because these feedbacks are difficult to constrain precisely, the uncertainty remained broad for decades.

A range similar to 1.5–4.5°C appeared in the 1979 Charney report and persisted through several generations of climate assessments, with a central estimate near 3°C.

By the IPCC’s Sixth Assessment Report, several independent lines of evidence had become more informative. Researchers combined observed modern warming and Earth’s energy imbalance with evidence from ancient climates and better physical estimates of feedbacks.

AR6 assessed a likely range of 2.5–4.0°C, with a best estimate of 3°C. A wider very likely range of 2–5°C reflects the fact that values outside the likely range are still possible.

The major change was therefore a narrowing of uncertainty, not a dramatic movement of the central estimate.