* Longer burning season means fires burn deeper into soil
* Deep fires release ancient stored carbon into atmosphere
* Interior Alaska goes from carbon sink to carbon emitter
By Deborah Zabarenko, Environment Correspondent
WASHINGTON, Dec 5 (Reuters) - Climate change is fanning longer- and deeper-burning fires in interior Alaska, changing the area from a carbon sink -- where planet-warming gases are stored naturally in the soil -- to a carbon emitter, scientists reported on Sunday.
The shift has occurred within the last 10 years and is due in large part to a longer burning season, according to a study published in Nature Geosciences.
The research was released at the start of a second week of international climate change negotiations in Cancun, Mexico.
When fires burn late into the season, past the end of July in northern latitudes, they don't just burn what's on the surface, but go deep into the soil where plant matter and other biomass have accumulated over thousands of years, said Merritt Turetsky, the study's lead author.
At some locations in interior Alaska, this accumulated biomass is as much as 26 feet (8 metres) deep, and is where climate-warming carbon has been stored. When it ignites, the carbon is emitted into the atmosphere.
Until about the year 2000, this part of Alaska stored more carbon than was emitted by industry and other sources in the area, but the last 10 years have seen an abrupt shift as long-stored soil carbon has been released by fire.
"Even though these systems have burned in the past, there was enough productivity, enough carbon being fixed (stored) by vegetation, that even when you took into account these fire emissions, they were still a small net sink of carbon," Turetsky said by telephone from the University of Guelph in Ontario, Canada.
"In the past 10 years there was this unprecedented amount of carbon released as a consequence of burning, and that changed the system from a small net sink to a net carbon source," she said.
These deep burns occur more often now because the fire season is longer and the late summer fires are fueled by dry ground and warmer air; these conditions are part of a warming environment. About half these fires are started by people, the other half by lightning.
Decades' worth of accumulated biomass, called peat, can burn in minutes, Turetsky said.
To get an idea of how much carbon these fires are putting into the atmosphere, study co-author Eric Kasischke of the University of Maryland offered a vivid comparison.
In the last decade, Kasischke said in an email, the biggest fire year in Alaska was 2004, when more than 56.7 million tons (51.5 teragrams or 51.5 billion kg) of carbon was emitted over 90 days, more than was released by all domestic U.S. airline flights for the whole year. That is also nearly as much carbon as was released by U.S. electricity generating plants for the same 90-day period. (Editing by Stacey Joyce and Eric Walsh)