Idealized model for changes in equilibrium temperature, mixed layer depth and boundary layer cloud over land in a doubled CO2 climate
An idealized equilibrium model for the undisturbed partly-cloudy boundary layer (BL) is used as a framework to explore the coupling of the energy, water and carbon cycles over land in mid-latitudes and show the sensitivity to the clear-sky shortwave flux, the mid-tropospheric temperature, moisture, CO2 and subsidence. The changes in the surface fluxes, the BL equilibrium and cloud cover are shown for a warmer, doubled CO2 climate. Reduced stomatal conductance in a simple vegetation model amplifies the background 2K ocean temperature rise to an (unrealistically large) 6 K increase in near-surface temperature over land, with a corresponding drop of near-surface relative humidity of about 19%, and a rise of cloud-base of about 70 hPa. Cloud changes depend strongly on changes of mean subsidence; but evaporative fraction (EF) decreases. EF is almost uniquely related to mixed layer (ML) depth, independent of background forcing climate,. This suggests that it might be possible to infer EF for heterogeneous landscapes from ML depth. The asymmetry of increased evaporation over the oceans and reduced transpiration over land increases in a warmer doubled CO2 climate.
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Plain English Discussion
As CO2, a greenhouse gas, increases in the atmosphere, many things change. Less heat radiation (infrared) can escape to space and this warms the earth’s surface. Evaporation goes up over the warmer oceans and the greenhouse effect of water vapor triples the warming produced by CO2. Over land, the increase of CO2 also means that plants evaporate less water as they take up CO2 to grow: so with less evaporation, land surface temperatures go up more than over the ocean and cloud cover may fall. This paper constructs a simple model to put all these processes together to estimate how much surface temperature and cloud cover will change over land if CO2 doubles.
Betts, A. K. and J. C. Chiu (2010), Idealized model for changes in equilibrium temperature, mixed layer depth and boundary layer cloud over land in a doubled CO2 climate. J. Geophys. Res.,115, D19108, doi:10.1029/2009JD012888