Journal
GEOPHYSICAL RESEARCH LETTERS
Volume 42, Issue 12, Pages 4973-4980Publisher
AMER GEOPHYSICAL UNION
DOI: 10.1002/2015GL064240
Keywords
climate sensitivity; nonlinear feedbacks; observational estimates; GCMs; long tail
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Funding
- NSF part of the Mathematics and Climate Research Network [DMS-0940261]
- Direct For Mathematical & Physical Scien
- Division Of Mathematical Sciences [0940261] Funding Source: National Science Foundation
- Directorate For Geosciences
- Div Atmospheric & Geospace Sciences [0933936] Funding Source: National Science Foundation
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The long-term warming from an anthropogenic increase in atmospheric CO2 is often assumed to be proportional to the forcing associated with that increase. This paper examines this linear approximation using a zero-dimensional energy balance model with a temperature-dependent feedback, with parameter values drawn from physical arguments and general circulation models. For a positive feedback temperature dependence, warming increases Earth's sensitivity, while greater sensitivity makes Earth warm more. These effects can feed on each other, greatly amplifying warming. As a result, for reasonable values of feedback temperature dependence and preindustrial feedback, Earth can jump to a warmer state under only one or two CO2 doublings. The linear approximation breaks down in the long tail of high climate sensitivity commonly seen in observational studies. Understanding feedback temperature dependence is therefore essential for inferring the risk of high warming from modern observations. Studies that assume linearity likely underestimate the risk of high warming.
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