More frequent forest fires have turned Alaska’s Yukon Flats into a net exporter of carbon to the atmosphere, according to a new analysis. This is worrisome, researchers said, because Arctic and sub-Arctic boreal forests like those of the Yukon Flats contain roughly one-third of the Earth’s terrestrial carbon stores.
The research is reported in the journal Nature Climate Change.
Alaska fire records go back only to 1939, and researchers often assume that present-day fire activity mirrors that of the ancient past. The scientists on the new study instead used actual fire data from a previous study in which they analyzed charcoal fragments preserved in lake sediments in the Yukon Flats. In that study, they found that fire frequency in a 2,000-kilometer swath of the Yukon Flats is higher today than at any time in the last 10,000 years.
For the new analysis, the research team plugged their data into a computer model of carbon cycling for the study area developed by co-authors Hélène Genet, A. David McGuire and colleagues of the University of Alaska Fairbanks and U.S. Geological Survey.
“Having these data allowed us to simulate not only recent decades, but the entire past millennium of carbon cycling,” said lead author Ryan Kelly, a former graduate student at the University of Illinois and a former visiting student with McGuire at UAF.
The new findings challenge studies that assume that recent fire activity reflects the norm over thousands of years. Those assumptions would lead scientists to conclude that the region has been a net carbon sink in recent decades, the researchers said.
“An important outcome for the modeling community is that this study shows the importance of representing the impact of disturbance on the dynamics of carbon stocks in ecosystem models,” said Genet, ecosystem modeler at the UAF Institute of Arctic Biology. “If you look at soil carbon today you’re looking at hundreds to thousands of years of carbon accumulation that have been affected by the history of fire in those soils.”
With these new fire data the ecosystem model can more accurately reflect that history of carbon accumulation, storage and release, the researchers said. “It’s a predictive model of ecosystem dynamics, driven by climate and disturbance,” said McGuire. “The outputs of the model tell us what is happening in the real world, and we interpret those results with respect to how they are relevant to the real world, to inform people involved in making decisions related to climate policy.”
The Arctic has seen rapid climate change in recent decades and is projected to warm more than twice the global average during the 21st century.
“Most studies of carbon cycling in boreal forests have been motivated by the fact that there’s just an enormous amount of carbon in these high-latitude ecosystems,” Hu said. “Up to 30 percent of the earth’s terrestrial carbon is in that system. And, simultaneously, this region is warming up faster than any other parts of the world.”
Increasing numbers of fires are unbalancing the cycle of carbon capture and release, the researchers report. More carbon dioxide in the atmosphere could enhance plant growth, but it also contributes to further climate warming in the higher latitudes, said Kelly, who is now a data scientist and modeler for Neptune and Company, Inc.
“Such warming would likely be attended by increased wildfire activity, which would more than cancel out plants’ carbon uptake and lead to a net increase in atmospheric carbon dioxide,” he said.
Using fire data from the past millennium, rather than assuming fires occurred at the same rate as in recent decades, creates a starkly different picture of the carbon cycle in the Yukon Flats, the researchers said.
“The effects of forest fires on the carbon cycle are very dramatic. Fires explain about 80 percent of the change in carbon storage over the past millennium, and a large amount of carbon has been lost from this ecosystem because of increasing forest fires,” Hu said. “This area has burned more than any other place in the boreal forests of North America. We chose the area for this study because we thought it could be an early indicator of the future.”
The researchers see a troubling trend in which climate warming increases the number of fires, which release more carbon to the atmosphere and enhance warming.
“Boreal forests contain vast carbon stocks that make them inherently big players in the global carbon cycle,” Kelly said. “And the main way that this stored carbon is eventually released is through fire.”
Hélène Genet, research associate, Institute of Arctic Biology, University of Alaska Fairbanks, firstname.lastname@example.org, 907-474-5660
A. David McGuire, professor, Institute of Arctic Biology, University of Alaska Fairbanks; assistant leader, Alaska Cooperative Fish and Wildlife Research Unit, U.S. Geological Survey, email@example.com, 907-474-6242
Feng Sheng Hu, professor, University of Illinois, firstname.lastname@example.org, 217-244-2982