March 5 (Bloomberg) -- Arctic tundra may burn more frequently as the warming climate allows flammable vegetation to creep northward, U.S. scientists said.

An analysis of Arctic sediments from thousands of years ago showed bigger shrubs were more common -- as were fires that destroyed them, researchers led by Philip Higuera at Seattle's University of Washington found. With plant life again increasing, fires could become more frequent, Higuera said.

``There is evidence of increasing shrub biomass in modern tundra ecosystems, and we expect temperatures to continue to increase and overall moisture levels to decrease,'' Higuera said in a statement. ``Combine these two factors, and it suggests a greater potential for fires. The sediment cores indicate that it's happened before.''

The findings may add another ``feedback loop'' to forecasts of climate change, in which warmer temperatures trigger changes that then cause further heating. More Arctic fires could further fuel global warming as carbon locked up in tundra soils is released into the atmosphere, the researchers said.

Greenhouse gases such as carbon dioxide trap the sun's heat in the atmosphere, raising temperatures, a phenomenon that United Nations scientists warned last year could increase sea levels and drought and alter patterns of disease. The UN panel forecast that global temperatures will warm by 1.1 to 6.4 degrees Celsius (2 to 12 degrees Fahrenheit) by 2100.

Fire Frequency

The researchers' findings are published today in the journal Public Library of Science One. The scientists analyzed sediment cores from four Alaskan lakes to determine the frequency of fires both now and in the past. From 14,000 to 9,000 years ago, a typical patch of tundra burned about once every 144 years, a rate exceeding present fire frequency, Higuera said.

``This was a surprise: modern tundra burns so infrequently that we don't really have a good idea of how often tundra can burn,'' Higuera said. ``Best estimates for the most flammable tundra regions are that it burns once every 250-plus years.''

While the past historical sediment records of Arctic tundra provides a useful guide to what may happen in the future, they are still ``imperfect analogs,'' the researchers said. The historical record showed one dominant shrub, called betula. Now three species are spreading in the tundra: betula, and two less flammable plants, alnus and salix, the researchers said.

To contact the reporter on this story: Alex Morales in London at amorales2@bloomberg.net.

Last Updated: March 5, 2008 07:13 EST