An alternative to radiative forcing for estimating the relative importance of climate change mechanisms

Radiative forcing is widely used to measure the relative efficacy of climate change mechanisms. Earlier general circulation model (GCM) experiments showed that the global‐mean radiative forcing could be used to predict, with useful accuracy, the consequent global‐mean surface temperature change regardless of whether the forcing was due to, for example, changes in greenhouse gases or solar output. More recent experiments indicate that for changes in absorbing aerosols and ozone, the predictive ability of radiative forcing is much worse. Building on a suggestion from Hansen and co‐workers, we propose an alternative, the “adjusted troposphere and stratosphere forcing”. We present GCM calculations showing that it is a significantly more reliable predictor of this GCM's surface temperature change than radiative forcing. It is a candidate to supplement radiative forcing as a metric for comparing different mechanisms and provides a framework for understanding the circumstances in which radiative forcing is less reliable.