Atmosphere Response Time Scales Estimated from AOGCM Experiments

Abstract

he global-mean surface air temperature response of the climate system to a specific radiative forcing shows characteristic time scales. Identifying these time scales and their corresponding amplitudes (climate sensitivity) allows one to approximate the response to arbitrary radiative forcings. The authors estimate these time scales for a set of atmosphere?ocean general circulation models (AOGCMs) based on relatively short integrations of 100?300 yr for some idealized forcings. Two modes can be clearly distinguished but a large spread in time scales and climate sensitivities exists among the AOGCMs. The analysis herein also shows that different factors influence the mode estimates. The value and uncertainty of the smallest time scale estimate is significantly lower when based on step scenarios than gradual scenarios; the uncertainty on the climate sensitivity of the slow mode can only be reduced significantly by performing longer AOGCM simulations; and scenarios with only a monotonically increasing forcing do not easily permit the climate sensitivity and the response time for the slow mode to be disentangled. Finally, climate sensitivities can be estimated more accurately than response times.