Forests make heatwaves initially warmer

During heatwaves forests reduce their evaporation. This causes the atmosphere to warm up even more. During extremely long periods of heat, however, this reduction enables the forests to continue their evaporation for longer, so the net effect is ultimately one of cooling in relation to the surroundings. This emerged from research by Ryan Teuling (Wageningen University, part of Wageningen UR) and colleagues just published in Nature Geoscience.
Ryan Teuling worked on the investigation in collaboration with climate researchers from a number of European countries. The study was prompted by recent heatwaves in Europe, which have raised interest in questions about the influence of land use on temperatures and climate. Up to now, scientists had assumed that a lack of precipitation during heatwaves automatically led to a reduction in evaporation. That reduction was thought to be less for forests, because trees, with their deeper root systems, have more water available to them. Examination of the precise role of land use, however, has been largely neglected up to now.
The study found large differences in evaporation strategies during heat waves. Grasslands evaporate more at higher temperatures and stop only when no more water is available. Forests, in contrast, respond to higher temperatures by evaporating less, which leaves more water at their disposal. During brief heatwaves, therefore, the greatest warming is found above forests, but during prolonged heat waves the increased evaporation of grasslands ends up causing a shortage of water. This can lead to exceptionally high temperatures, such as those measured in France in the summer of 2003. This mechanism might also offer an explanation for the unusually high temperatures near Moscow this summer. In these types of extreme situations, forests in fact have a cooling effect on the climate.
The research was done on the basis of observations made above forests and grasslands in Europe by an extensive network of flux towers, including those in Cabauw (KNMI) and Loobos (Alterra Wageningen UR). For areas without towers, satellite data were used. Head researcher Ryan Teuling recently received the prestigious VENI innovation grant from NWO to further investigate the role of forests in the regional climate system.

Full bibliographic information

Contrasting response of European forest and grassland energy exchange to heatwaves
Adriaan J. Teuling et al. Nature Geoscience
Published online: 5 September 2010 | doi:10.1038/ngeo950
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