Climate Feedbacks

Guest writer Brian Soden of the Division of Meteorology and Physical Oceanography at the Rosenstiel School of Marine & Atmospheric Science at Miami University (yeah, that's usually abbreviated) explains about what climate feedbacks are and how they are factored into what we know about climate change and how that is modeled.

Current model estimates of the climate sensitivity, defined as the equilibrated change in global-mean surface temperature resulting from a doubling of CO2, range from 2.6 to 4.1 K, consistent with observational constraints (see previous article). This range in climate sensitivity is attributable to differences in the strength of 'radiative feedbacks' between models and is one of the reasons why projections of future climate change are less certain than policy makers would like.

Although radiative forcings and radiative feedbacks both influence the climate by altering the radiative fluxes at the top of the atmosphere, it is important to distinguish between the two. A radiative forcing results from changes that are external to the climate system and may be either natural or anthropogenic in origin. For example, anthropogenic emissions of CO2, changes in solar flux, and the reflection of sunlight from volcanic aerosols are all examples of radiative forcings. A radiative forcing initiates a change in climate that is distinct from the system's internal variability. A radiative feedback, on the other hand, arises from the response of the climate to either external forcing or internal variability. These responses can either amplify (a positive feedback) or dampen (a negative feedback) the initial perturbation. The exact boundary between a feedback and a forcing depends on what is considered to be part of the 'system' and can sometimes be a little fuzzy. This discussion addresses just the feedbacks associated with the atmospheric physical system, but other, less well understood, feedbacks (changes in land vegetation, biogeochemical processes, and atmospheric chemical feedbacks - see the NRC 2003 report), while potentially important, are not part of the generally understood definition of 'climate sensitivity'.