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

Teak trees offer clues to drought history


A group of scientists are developing more accurate drought and harvest forecasts for Indonesia using tree rings, historic rice production figures and sea surface temperature data, the Reuters news agency reports.

Indonesia is one of the world’s most populous nations and a major producer of rice, cocoa, coffee and tobacco.

But the country is regularly at risk of drought caused by the El Nino phenomenon, which causes the eastern Pacific ocean to heat up, resulting inĀ  wet weather moving toward the east and leaving drier weather in west around South-East Asia and Australia.

US scientist Rosanne D’Arrigo and colleague Robert Wilson are working on simplified statistical models that can predict drought ahead of the main September-December rice planting season, and how severe the drought might be.

The models focus on Java, one of the world’s most densely populated islands with 120 million people.

“We’re trying to develop simple, predictive model of drought and crop productivity on Java,” said Dr D’Arrigo of the Lamont-Doherty Earth Observatory.

“There are complex models out there but you need to have a local type of analysis and something simple for local people to use .”

She was speaking to Reuters from Dalat, southern Vietnam, where she was presenting her team’s work at a climate change conference this week.

A key part of the model is using sea surface temperature data from the tropical Pacific and from the Indian Ocean.

A separate phenomenon called the Indian Ocean Dipole can also cause drought in Australia and affect rainfall in Indonesia.

Other data, such as sea-level pressure and wind indexes, are also used.

The data are examined several months before the usual onset of the monsoon to try to accurately predict likely rainfall patterns over Indonesia.

Dr D’Arrigo said she also found good agreement between the sea surface temperature model, a local drought index in Java and government data on crop productivity.

This suggested “we could estimate not only the coming drought condition but also the kind of crop season you would expect to have,” she said, adding she was also looking at a predictive model for the onset of the monsoon.

Her team also looked at tree rings from old teak trees in Java and Sulawesi island to build up a chronology of past droughts and found a very strong correlation with El Nino.

“Indonesia is kind of unique in the sense that it’s probably the area where you have the greatest ‘ground-zero’ climate signal related to El Nino,” she explained.

The oldest teak tree ring records came from the 16th Century, she said, but added it had been hard work finding the remaining centuries-old teak trees.

“It takes fair a bit of research. You have to do a bit of detective work to find the few remaining last stands that haven’t been cut for furniture.”

Source: Reuters

Date: 18/02/2009

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