气候变化加剧黄石公园火灾频率

An increase in wildfires due to climate change could rapidly and profoundly alter the Greater Yellowstone Ecosystem, according to a new study authored by environmental engineering and geography Professor Anthony Westerling of the University of California, Merced. The study by Westerling and his colleagues — which will be published online this week in the Proceedings of the National Academy of Sciences — suggests that the expected rising temperatures caused by climate change could increase the frequency of large wildfires in Yellowstone to an unprecedented（空前的） level.

The projected increase in fires would likely cause a major shift in the Greater Yellowstone Ecosystem, with fewer dense forests and more open woodland, grass and shrub（灌木） vegetation. The change could happen by 2050, Westerling posits, with forests becoming younger, the mix of tree species changing and some forests failing to regenerate after repeated fires. This would affect the region's wildlife, hydrology（水文学） , carbon storage and aesthetics（美学） .

"What surprised us about our results was the speed and scale of the projected changes in fire in Greater Yellowstone," Westerling said. "We expected fire to increase with increased temperatures, but we did not expect it to increase so much or so quickly. We were also surprised by how consistent the changes were across different climate projections."

For their study, the researchers compiled climate data from 1972 to 1999 and examined it in relation to the occurrence and size of large wildfires in the northern Rocky Mountains over the same time period. Using the resulting statistical patterns, Westerling and his coauthors projected how climate change would affect Greater Yellowstone fires through the year 2099.

"Large, severe fires are normal for this ecosystem. It has burned this way about every 100 to 300 years, for thousands of years," said coauthor Monica Turner, the Eugene P. Odum Professor of Ecology at the University of Wisconsin, Madison. "But if the current relationship between climate and large fires holds true, a warming climate will drive more frequent large fires in the Greater Yellowstone Ecosystem in the future."

In the researchers' simulations, years with no large fires — very common in the recent past — become extremely rare by 2050 and are all but eliminated after 2050. The projections show that after 2050, the average annual area burned is about 100,000 hectares（公顷） , or nearly 400 square miles. By 2075, the average yearly burn exceeds that of the historic season of 1988, when fires claimed more than 1,200 square miles.

Westerling cautioned, though, that the models used in the study will not work once the increase in fires creates a fundamental change in the ecosystem. As the landscape changes, the relationships between climate and fire would change as well.

"The biggest challenge for us is to understand what can happen when the ecosystem is transformed," Westerling said. "Our projections also depend on the climate models we are using — for example, if projections for winter snow pack or summer rainfall were to increase significantly, that would change our results."

Westerling and his coauthors said warming alone is likely to lead to a decline in suitable habitat for tree species currently found in greater Yellowstone, and the projected increase in frequency and severity of wildfires could accelerate that process to a tipping point at which the trees may no longer regenerate. This could cause some forested areas to be converted to woodland or non-forest, and similar changes might be expected in other subalpine（亚高山带的） or boreal forests.

"The climatic conditions projected for the second half of this century are similar to what we see in areas of the west today that have different forest types from Yellowstone's," Westerling said. "We don't know how fast those species will migrate in response to climate change, though, so the immediate response of the ecosystem is hard to predict. Because of its pristine（原始的） condition, Yellowstone provides an unparalleled natural laboratory to monitor and learn from fire and ecosystem responses to climate change."