What is the effect of unresolved internal climate variability on climate sensitivity estimates?

R. Olson, R. Sriver, W. Chang, M. Haran, N. M. Urban, and K. Keller

Journal of Geophysical Research, D: Atmospheres (27 May 2013)

DOI: 10.1002/jgrd.50390

Many studies have attempted to estimate the equilibrium climate sensitivity (CS) to the doubling of CO2 concentrations. One common methodology is to compare versions of Earth models of intermediate complexity (EMICs) to spatially and/or temporally averaged historical observations. Despite the persistent efforts, CS remains uncertain. It is, thus far, unclear what is driving this uncertainty. Moreover, the effects of the internal climate variability on the CS estimates obtained using this method have not received thorough attention in the literature. Using a statistical approximator (“emulator”) of an EMIC, we show in an observation system simulation study that unresolved internal climate variability appears to be a key driver of CS uncertainty (as measured by the 68

keywords: climate sensitivity; internal climate variability; uncertainty; Gaussian process; 1616 Climate variability; 1622 Earth system modeling; 3275 Uncertainty quantification

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