Editors' Highlight
The fate of climate feedbacks as temperatures warm
Small disturbances to climate can be quantified in terms of climate sensitivity, a marker that combines how changes in water vapor, cloud cover, reflectance, and other properties affect ground temperatures and vice versa. To better understand how climate feedbacks respond to a full spectrum of changes, Colman and McAvaney (2009) coupled an atmospheric general circulation model with a mixed layer ocean model and subjected it to a range of between 1/16 and 32 times the current CO2 concentration in the atmosphere. They found that as climate warms, climate sensitivity weakens, although not linearly. Further, the feedback whereby solar heat absorption by the planet changes surface cover to allow more solar radiation to be absorbed also weakens. However, some feedbacks strengthen, such as the one by which increased humidity from higher temperatures fosters higher atmospheric water vapor concentrations. The authors concluded that such studies can provide the scientific community with a benchmark for investigating what physical processes appear important in determining feedback strengths and how these systematically vary across a broad range of climate changes.
Climate feedbacks under a very broad range of forcing
Abstract
An atmospheric general circulation model, coupled to a mixed layer ocean, is subjected to a broad range of forcing away from the current climate between 1/16 to 32 times current CO2 in halving/doubling steps. As climate warms climate sensitivity weakens (although not monotonically), albedo feedback weakens (driving much of the sensitivity weakening), water vapour feedback strengthens (at a rate slightly larger than it would if relative humidity remained unchanged), and lapse rate feedback increases (negatively); this latter change essentially offsetting the water vapour increases. Longwave cloud feedbacks are relatively stable (moderate and positive) across the full range; shortwave cloud feedback remains relatively weak, apart from under the coldest climates. Cloud optical property related components (from total water content, water/ice fraction and cloud thickness) remain remarkably stable. Cloud ‘amount’ feedbacks show the greatest trends: weakening as temperatures increase. Although cloud feedbacks show an overall consistency of features in different latitudes, precise patterns of changes differ substantially for different baseline climates.
(Received 7 October 2008; accepted 2 December 2008; published 3 January 2009.)
Citation: (2009), Climate feedbacks under a very broad range of forcing, Geophys. Res. Lett., 36, L01702, doi:10.1029/2008GL036268.
Link to abstract: http://www.agu.org/pubs/crossref/2009/2008GL036268.shtml
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