The magnitude-timescale relationship of surface temperature feedbacks in climate models

Abstract

Abstract. Because of the fundamental role feedbacks play in determining the characteristics of climate it is important we are able to specify both the magnitude and response timescale of the feedbacks we are interested in. This paper employs three different climate models driven to equilibrium with a 4 × CO2 forcing to analyze the magnitude and timescales of surface temperature feedbacks. These models are a global energy balance model, an intermediate complexity climate model and a general circulation model. Rather than split surface temperature feedback into characteristic physical processes, this paper adopts a linear systems approach to split feedback according to their time constants and corresponding feedback amplitudes. The analysis reveals that there is a dominant net negative feedback realised during the first year. However, this is partially attenuated by a spectrum of positive feedbacks for time constants in the range 10 to 1000 years. This attenuation was composed of two discrete phases which are attributed to the effects of ''diffusive – mixed layer'' and ''circulatory – deep ocean'' ocean heat equilibration processes. The diffusive equilibration was associated with time constants on the decadal timescale and accounted for approximately 75 to 80 % of the overall ocean heat equilibration feedback, whilst the circulatory feedback operated on a centennial timescale and accounted for the remaining 20 to 25 % of the response. It is important to quantify these decadal and centennial feedback processes to understand the range of climate model projections on these longer timescales.