To understand the response of the climate system to perturbations it’s necessary to know the magnitude of the perturbation, or forcing. Feedback studies, in which the response is assumed to scale linearly with the perturbation, further require the ability to disentangle the responses that depend directly on the nature of the perturbation from those that scale linearly with forcing to obtain the so-called “effective radiative forcing” or ERF.

This talk will focus on uncertainties in estimates of present-day ERF, which has four sources: 1) errors in the fast radiation codes used to estimate the sensitivity of flux to changes in atmospheric composition on global scales; 2) uncertainty in historical distributions of temperature, humidity, and clouds; 3) uncertainty in “rapid adjustments”, i.e. the response of the climate system to particular changes in composition, and 4) uncertainty in the changes in composition themselves, primarily related to aerosols. I’ll describe the current state of understanding of each of these sources of uncertainty and explain sets of experiments to be undertaken as part of CMIP6 that will help refine the knowledge of ERF in nature as well as in the models participating in CMIP6. I’ll pay special attention to the forcing of atmospheric absorption, for which the uncertainty is especially large, and explain the implications for relationships between temperature and hydrologic sensitivity.