Forests make heatwaves ‘initially warmer’


During heatwaves forests reduce their evaporation, causing the atmosphere to warm up even more, say researchers.

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, explained a team of scientists led by Ryan Teuling from Wageningen University in the Netherlands.

Writing in the journal Nature Geoscience, Dr 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 had 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, the researchers suggest.

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. For areas without towers, satellite data were used.

Source: Wageningen University press release

Date: 06/09/2010

Human activity ‘triggers rise in Borneo forest fires’


Severe fires in Indonesia – responsible for some of the worst air quality conditions worldwide – are linked not only to drought, but also to changes in land use and population density, according to a new study in Nature Geoscience.

“During the late 1970s, Indonesian Borneo changed from being highly fire-resistant to highly fire-prone during drought years, marking the period when one of the world’s great tropical forests became one of the world’s largest sources of pollution,” said lead researcher Robert Field, a PhD student of atmospheric physics at the University of Toronto, Canada.

“Ultimately, this abrupt transition can be attributed to rapid increases in deforestation and population growth,” he explained.

“The resulting occurrences of haze currently rank among the world’s worst air pollution episodes, and are a singularly large source of greenhouse gas emissions.”

Sumatra has suffered from large fires since at least the 1960s, but Indonesian Borneo seems to have been resistant to large fires, even in dry years, until population density and deforestation increased substantially and land use changed from small-scale subsistence agriculture to large-scale industrial agriculture and agro-forestry.

“We’ve had a good understanding of fire events since the mid 1990s, but little before this due to the absence of fire data from satellites,” said Mr Field.

“However, one of the major impacts of large-scale fires is a reduction in visibility due to the smoke produced.

“Visibility is recorded several times a day at airports in the region, and these records proved to be an excellent indicator of severe fire activity.

“We were able to piece together visibility observations back to the 1960s, and hence develop a longer term record of the fires.”

Having a long-term record of the fires allowed the scientists to better understand their causes.

“Using weather records, we were able to estimate the specific rainfall level below which large fires have occurred in the previous two decades,” Mr Field added.

“In turn, we found that the rainfall over Indonesia was influenced equally by the Indian Ocean Dipole and the El Nino Southern Oscillation phenomena.”

Mr Field concluded: “Hopefully, this information can be used to better anticipate and prevent future haze disasters in Indonesia.”

He said that there was a direct link between the increased prevalence of severe fires and haze disasters and the man-made change in land use.

“The visibility record also showed, quite strikingly, the impact of human settlement on a previously pristine tropical forest.

“This should give pause to further agro-forestry expansion in Indonesia, particularly for oil palm as a source of biofuel.”

Source: EurekAlert

Date: 22/02/2009

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