More carbon dioxide leads to lower cloud formation

The warmer the air, the more water can evaporate: a simple relationship familiar to us from everyday life. Researchers from Germany and the Netherlands have now established that this is not always the case: although an increase in the greenhouse gas CO2 makes the climate warmer, it also allows less water to evaporate.

Plants, with their billions of tiny leaf pores, are the cause of this apparent contradiction. They influence the gas and moisture content of the air around them. Using new calculations of an atmospheric model, the researchers found that this sets in motion a cascade of processes, finally resulting in global warming.

''We wanted to know how the foreseeable rise in CO2 would affect cloud formation in temperate climate zones and what part the vegetation plays in this,'' says Jordi Vilà-Guerau de Arellano from the University of Wageningen in the Netherlands.

Working with colleagues from the Max Planck Institutes for Chemistry and Meteorology, the geophysicists made use of, for the first time, a computer model that takes account of the soil, water cycle, atmosphere and growth processes of plants. The model results highlight how local and daily variable processes, through turbulence, can influence the atmosphere on larger scales.

The scientists simulated three scenarios for their analysis: a doubling of the CO2 in the atmosphere from the current 0.038 per cent to 0.075 per cent, an increase in the average global temperature by two degrees Celsius and a combination of both. The calculations represent the conditions expected for the year 2100 and compared to 2003 values based on scenarios from the Intergovernmental Panel on Climate Change (IPCC).

The researchers established that some land-vegetation-atmosphere exchange processes respond more strongly to increasing CO2 and climate change than others.