Equilibrium climate sensitivity
The Charney report's range stood for forty years. Combining process understanding, historical warming and paleoclimate narrowed it: the IPCC's 2021 assessment gives a likely range of 2.5 to 4 °C. Cloud feedbacks remain the biggest source of uncertainty, and they are what separates a manageable future from a severe one.
Posed 1896, Svante Arrhenius; 1979 Charney report: 1.5–4.5 °C.
What would settle it
A narrower observed range
Sources
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