An explanation for the difference between twentieth and twenty-first century land-sea warming ratio in climate models

A land-sea surface warming ratio (or phi) that exceeds unity is a robust feature of both observed and modelled climate change. Interestingly, though climate models have differing values for phi, it remains almost time-invariant for a wide range of twenty-first century climate transient warming scenarios, while varying in simulations of the twentieth century. Here, we present an explanation for time-invariant land-sea warming ratio that applies if three conditions on radiative forcing are met: first, spatial variations in the climate forcing must be sufficiently small that the lower free troposphere warms evenly over land and ocean; second, the temperature response must not be large enough to change the global circulation to zeroth order; third, the temperature response must not be large enough to modify the boundary layer amplification mechanisms that contribute to making phi exceed unity. Projected temperature changes over this century are too small to breach the latter two conditions. Hence, the mechanism appears to show why both twenty-first century and time-invariant CO2 forcing lead to similar values of phi in climate models despite the presence of transient ocean heat uptake, whereas twentieth century forcing-which has a significant spatially confined anthropogenic tropospheric aerosol component that breaches the first condition-leads to modelled values of phi that vary widely amongst models and in time. Our results suggest an explanation for the behaviour of phi when climate is forced by other regionally confined forcing scenarios such as geo-engineered changes to oceanic clouds. Our results show how land-sea contrasts in surface and boundary layer characteristics act in tandem to produce the land-sea surface warming contrast.