Evaporation-Limited Tropical Temperatures as a Constraint on Climate Sensitivity

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Abstract
Studies of paleoclimate and modern observations indicate that evaporative effects limit thermal response in equatorial regions. We develop a latitude-resolved, steady-state energy balance model which incorporates the effect of an evaporative constraint on the variation of equatorial temperature with solar luminosity. For a diffusive model of surface heat transport the constraint requires the diffusion coefficient to vary with insolation. We find that the movement of the iceline with insolation is four times larger than in standard energy balance models with a constant thermal diffusion coefficient. This is a consequence of the global energy balance which forces temperature changes to occur at high latitudes when they are evaporatively buffered at the equator. Nonlinear temperature-ice albedo feedback at high latitudes then amplifies the response leading to greater sensitivity in the vicinity of current climate.