Increasing CO2 suppresses boundary-layer clouds in temperate climates

Cumulus clouds in the atmospheric boundary layer play a key role in the hydrologic cycle, in the onset of severe weather by thunderstorms, and in modulating the Earth's reflectivity and climate. How these clouds respond to climate change, in particular over land, and how they interact with the carbon cycle is poorly understood. It is expected that as atmospheric CO2 concentrations rise, the effectiveness of photosynthesis by vegetation will increase. Since this is associated with reduced evapotranspiration, the sensible heat flux increases, which contributes to the surface temperature rise. Using a soil-water-atmosphere-plant model we show that increasing CO2 causes a reduction of boundary layer cloud formation in middle latitudes. This could be partly counteracted by a growth in biomass and greater ability of a warmer atmosphere to take up water. Model results are evaluated with a comprehensive observational data set taken at Cabauw (The Netherlands). We find that current climate conditions are optimal for boundary layer cloud formation over land, while in projections of future conditions, cloudiness will be increasingly suppressed. Our results emphasize the intricate connection between biological and physical aspects of the climate system.