What is Earth's equilibrium climate sensitivity?
In plain words
Doubling CO2 will warm Earth by somewhere around 2.5 to 4 degrees once the oceans catch up. Narrowing that range has proved hard for decades.
Precise statement
Equilibrium climate sensitivity (ECS) is the equilibrium global-mean surface air temperature change for doubled CO2 (forcing about $4\mathrm{e}3\,\mathrm{erg}/(\mathrm{s}\,\mathrm{cm}^{2})$). The 2020 multi-evidence assessment gives a 66 percent range of about $2.6\ \text{to}\ 3.9\,\mathrm{K}$ and the 2021 IPCC assessment a likely range of $2.5\ \text{to}\ 4\,\mathrm{K}$. An answer is a 66 percent range narrower than $1\,\mathrm{K}$ consistent with process, historical and paleoclimate evidence.
What would settle it
Process-level constraints on cloud feedbacks combined with paleoclimate and historical energy-budget estimates that agree within a sub-kelvin range.
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