The influence of internal variability on Earth's energy balance framework and implications for estimating climate sensitivity

Abstract. Our climate is constrained by the balance between solar energy absorbed by the Earth and terrestrial energy radiated to

space. This energy balance has been widely used to infer equilibrium climate sensitivity (ECS) from observations of

20th-century warming. Such estimates yield lower values than other methods, and these have been influential in pushing down

the consensus ECS range in recent assessments. Here we test the method using a 100-member ensemble of the Max Planck Institute Earth System Model

(MPI-ESM1.1) simulations of the period 1850–2005 with known forcing. We calculate ECS in each ensemble member using

energy balance, yielding values ranging from 2.1 to 3.9 K. The spread in the ensemble is related to the central

assumption in the energy budget framework: that global average surface temperature anomalies are indicative of anomalies

in outgoing energy (either of terrestrial origin or reflected solar energy). We find that

this assumption is not well supported

over the historical temperature record in the model ensemble or more recent satellite observations. We find that framing

energy balance in terms of 500 hPa tropical temperature better describes the planet's energy balance.