An Assessment of Radiative Forcing, Radiative Adjustments, and Radiative Feedbacks in Coupled Ocean Atmosphere Models

Monday, May 12, 2014 - 07:00
Add to Calendar

This study provides a comprehensive assessment of the direct radaitive forcing from quadrupling CO2, the rapid radiative adjustments to this forcing, and the radiative feedbacks which develop in response to the resulting surface warming in an ensemble of coupled ocean-atmosphere models under 2 different forcing scenarios. We show that differences in the effective climate sensitivity between models are dominated by uncertainties in the radiative feedback. Uncertainties in the direct radiative forcing and fast radiative adjustments are comparable in magnitude, but less than one-third of that due to the radiative feedbacks, while uncertainties in the rate of ocean heat uptake are a minor contributor to the intermodel difference in global temperature response on multi-decadal time scales. In the troposphere, uncertainties in the fast radiative adjustments from temperature, water vapor and clouds are comparable in magnitude, although the spread in temperature and water vapor adjustments partially offset each other so that their sum reduces the spread by ~50%. Similar conclusions apply to the troposphere feedbacks. In the stratosphere, feedbacks in these models are virtually non-existent, although uncertainties in the stratospheric adjustments are significant. The spatial distributions of all tropospheric adjustments tend to oppose their corresponding feedbacks, implying that a significant fraction of the adjustment may be an artifact of the methodology and are not independent of the surface driven feedbacks. We further show that the classification of the tropospheric responses as either an adjustment to the forcing or as a feedback upon the surface warming has little direct impact on the magnitudes of the forcing, feedbacks, or transient climate response.