Global warming may be bad for asthma sufferers

By Alister Doyle, Environment Correspondent

OSLO (Reuters) - Global warming may be bad for asthma sufferers because of longer plant growing seasons and signs that weeds scattering vast amounts of pollen are conquering new territory, experts say.

But higher temperatures might bring benefits for some sufferers because house mites and viruses that thrive in winter in centrally heated homes will not flourish if people do not need to use their heat systems.

By Spring, pollen has been in the air for months in the northern hemisphere even in countries where snows bring a winter respite from coughing and wheezing for allergics.

In south Sweden, for instance, hazel trees have been flowering since December.

"In the United States the incidence of asthma is up nearly four times since 1980," said Paul Epstein, Associate Director of the Center for Health and the Global Environment at Harvard Medical School.

"No one has really been looking at the aerobiology dimension (such as pollen). But I think it helps account for it," he said. Other triggers range from mites and dust to viruses and food.

And any warming may make things worse, he told Reuters.

A draft U.N. climate report due for release on April 6 says that plant growing seasons have become longer because of a warming trend blamed on human burning of fossil fuels.

It says that the world's agricultural potential is likely to rise, especially in temperate countries, if temperatures rise by up to 3 Celsius (5.4 Fahrenheit). Above that level, farm potential will fall in all regions.

"The pollen season will probably last almost all around the year" even in southern Scandinavia, said Aslog Dahl, of the Botaniska Analysgruppen at Sweden's Gothenburg University. Hazel trees have flowered three times before New Year in the region in modern times, all in the past six years.

BEECH, BIRCH

Long-term effects on human health could be mixed.

Big pollen producers such as American ragweed were getting a foothold in southern Scandinavia. But tree species such as beech, which do not flower every year, were growing better and might push annually flowering birch forests north.

Epstein ran a study showing that ragweed, for instance, produced 60 percent more pollen if grown with double the normal concentration of the greenhouse gas carbon dioxide. At the same time, the stalks grew just 10 percent more.

"Warming is touted as good for agriculture, but weeds may be reacting disproportionately fast," he said. "This is an issue with great importance for human health and agricultural yields."