Warming West is ground zero for wildfires
by Jane Kay, San Francisco Chronicle Environment Writer
July 21, 2008
California has been hit by 2,000 fires this year, and climate scientists are predicting that the situation will worsen as temperatures rise.
The American West has been warming dramatically during the past 60 years at a rate surpassed only by Alaska. This year has been particularly dry for California, with less snowfall, earlier snowmelt and lower summer river flows.
Some of the state's top scientists say the changing water picture is caused by humans producing greenhouse gases, and the U.N. Intergovernmental Panel on Climate Change predicts more intense and longer droughts with warmer spring and summer temperatures in the West.
That, scientists say, leads to increases in the length of the fire seasons, number of fires, time needed to put out the fire and size of the burned area.
"The snow melts sooner, the dry season gets longer and rivers crest earlier. That gives more of a chance for drying out and therefore a likelihood of more fires," said Tim Barnett, a climatologist at the Scripps Institution of Oceanography at UC San Diego who led research on the effects of greenhouse gases on the changing hydrology in the western United States.
"If you look at where we will be in 20 or 30 years, we'll have serious problems," he said.
Scientists are quick to caution against blaming one fire or heat wave on global warming. But, Barnett said, "At the minimum, you're getting a glimpse of your future. Do you like it? I think not."
Research by teams of scientists at Scripps, Lawrence Livermore National Laboratory, the U.S. Geological Survey and other institutions have probed associations between land-use patterns and climate change because of increases in fires during the past 25 years.
The picture is complex, particularly in Northern California, they say.
With warmer and earlier springs, moisture has been uneven, and winter precipitation in some parts tends to come as rain, not snow. At the same time, logging and mining have changed the character of forests, and the practice of preventing low-burning fires in past decades may have made the forests more susceptible to wildfires, experts say.
But taking all of the factors into consideration, including weather patterns shown in tree rings over past centuries, they conclude that the intensity of fires is linked most closely to the rising temperatures, less snowpack, earlier snowmelt and a longer, drier fire season.
The peak time of melting snow is already about 10 to 15 days earlier in different parts of the West. Scientists have projected a speed-up of 25 to 35 days earlier by the end of this century. A study just released by Purdue University found that at the end of the century, the snowmelt could come 70 days earlier. The effect of the lost snow, and increased heat from solar radiation absorbed in the exposed ground and vegetation, would raise temperatures more than have previously been expected.
In the western United States, temperatures for the past five years have risen an average 1.7 degrees when compared with the 20th century. California's average temperatures between 2003 and 2007 rose 1.1 degrees above the past century's. That is slightly more than the 1 degree rise for the planet as a whole. The Colorado River Basin, Arizona, Montana, Utah and Wyoming have had temperatures rise more than 2 degrees in the past five years compared with the past century.
Turning up the heat
The West has had more frequent and severe heat waves, with the number of extremely hot days increasing by up to four days per decade since 1950, according to research supported by the Rocky Mountain Climate Organization, a coalition of 17 local governments, businesses, nonprofits and Colorado's largest water provider.
The West has warmed more than east of the Rocky Mountains.
Drought is now more common in the West, while east of the Rockies it is noticeably wetter in general, said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research, operated by a nonprofit consortium of research universities. Trenberth attributes the difference between West and East to basic climate conditions, but also to the nature of changes in atmospheric circulation.
"All of this indeed promotes wildfire risk, and the 'dry lightning' is disastrous, especially in areas where trees are damaged such as by bark beetle," said Trenberth, a lead author of the center's 2007 scientific assessment of climate change.
This year followed the trend.
"We had very dry conditions in April, May and a bit of June," said Scott Stephens, associate professor of fire science in UC Berkeley's department of environmental science, policy and management. "This year, we had almost zero rain. When the dry lighting strikes came through, we had 1,000 fires in one weekend, June 20, one of the highest we've ever experienced," something not seen in at least 50 years, he said.
So far, more than 900,000 acres have burned, destroying about 100 houses and threatening thousands more. People were evacuated in Big Sur, the Sierra foothills and Butte County, communities around Santa Cruz and other spots in Northern California.
"We're going to have more surprises like this," Stephens said.
Duration of burns increases
Since 1980, U.S. wildfires have burned an average of 8,500 square miles per year, a jump from the 1920-1980 average of 5,000 square miles per year.
In the past three decades, the wildfire season in the western United States has increased by 78 days, according to work led by Anthony Westerling, formerly at Scripps, now at UC Merced. Roughly half that increase was due to earlier ignitions, and half to later control. Burn duration of fires greater than 1,000 acres has increased from 7.5 to 37.1 days in response to a spring-summer warming.
People on the fire lines see that the wildfire intensity and size have changed and question whether global warming is to blame, Stephens said. "They know that the temperatures are increasing, and the snow is leaving earlier. One thing is certain: Weather and fire are tied together. They know that better than anybody."
The U.S. Forest Service has a study in progress that examines the severity of forest fires in the Sierra Nevada. Hugh Safford, regional ecologist, and analyst Jay Miller led a team investigating about 200 fires that occurred between 1984 and 2007 in the Sierra Nevada.
The researchers found that fires had increased in severity beginning in the 1980s and continued until today. By analyzing state and federal data, they also showed that fire frequency, total burned area and average fire size have also increased during the same period.
Rising temperatures play a part, they said. But at the same time, they found that increases in forest density because of 70 years of fire suppression are also to blame.
The study concluded that "in light of recent alarming projections for increased temperatures and fire-season length by the end of the century," it is time to rethink the current policy of suppressing fires and, under the proper circumstances, let more fires burn to reduce problem fuels.
Wildfires and climate by the numbers
Climate scientists predict a continuing trend of rising temperatures in the West. The warmer the spring, the earlier the snowmelt, the drier the summer, the longer the fire season and the higher the frequency of big fires, they say. Multiyear droughts degrade trees' abilities to generate defensive chemicals, increasing their susceptibility to insects. Higher winter temperatures allow a larger fraction of overwintering larvae to survive. Spruce budworm in Alaska, mountain pine beetle in British Columbia and tent caterpillar in Alberta are providing dead, desiccated fuels for large wildfires. The greatest increases in forest fires are in the northern Rocky Mountains, followed by the Sierra Nevada, southern Cascades and Coast ranges of Northern California and southern Oregon.
-- Since 1980, U.S. wildfires have burned an average of 8,500 square miles a year, a jump from the 1920 to 1980 average of 5,000 square miles a year.
-- The forested area that burned in the western United States from 1987 to 2003 is 6.7 times the area burned from 1970 to 1986.
-- In the past three decades, the wildfire season in the western United States has increased by 78 days. Roughly half that increase was due to earlier ignitions and half to later control. Burn duration of fires of more than 1,000 acres has increased from 7.5 to 37.1 days in response to a spring-summer warming.
Sources: Intergovernmental Panel on Climate Change, Scripps Institution of Oceanography