August 29, 2013
Wildfires Could Become More Frequent Due To Changing Climate
April Flowers for redOrbit.com - Your Universe Online
The Western US, where firefighters are currently battling dozens of fires in at least 11 states, has received bad news from a new study led by environmental scientists at the Harvard School of Engineering and Applied Sciences (SEAS).
The study, published online and in an upcoming print issue of Atmospheric Environment, suggests wildfire seasons by 2050 will be about three weeks longer, up to twice as smoky, and will burn a wider area in the western states. The research team based their results on a set of internationally recognized climate scenarios, decades of historical meteorological data and records of past fire activity.
Scientists are becoming aware that in the coming years gradual climate change may contribute to increases in significant, disruptive events like severe storms and floods.
Loretta J. Mickley, a senior research fellow in atmospheric chemistry at Harvard SEAS, is taking this up a notch, looking at secondary effects like forest fires and air quality that rely heavily on meteorological factors.
“We weren’t altogether certain what we would find when we started this project,” Mickley says. “In the future atmosphere we expect warmer temperatures, which are conducive to fires, but it’s not apparent what the rainfall or relative humidity will do. Warmer air can hold more water vapor, for instance, but what does this mean for fires?
“It turns out that, for the western United States, the biggest driver for fires in the future is temperature, and that result appears robust across models," Mickley adds. "When you get a large temperature increase over time, as we are seeing, and little change in rainfall, fires will increase in size.”
Months of analysis were needed to reach that conclusion with statistical confidence. This was because, at the local level, wildfires are very difficult to predict.
“Wildfires are triggered by one set of influences—mainly human activity and lightning—but they grow and spread according to a completely different range of influences that are heavily dependent on the weather," says Xu Yue, who was a postdoctoral fellow at Harvard SEAS and is now at Yale University. "Of course, when all the factors come together just right—whoosh, there’s a big fire.”
The team examined records of past weather conditions and wildfires, finding that main factors influencing the spread of fires vary from region to region. For example, in the Rocky Mountain Forest the best predictor of wildfire area in a given year is the amount of moisture in the forest floor, which depends on the temperature, rainfall and relative humidity that season. In contrast, in the Great Basin region, the area burned is influenced by the relative humidity in the previous year, which promotes fuel growth.
Mathematical models, created by Yue, closely link these types of variables — seasonal temperatures, relative humidity, the amount of dry fuel and so forth — with the observed wildfire outcomes for six "ecoregions" in the West.
After developing these models using historical data, the team inputted data based on the conclusions of the fourth Intergovernmental Panel on Climate Change (IPCC), which use socioeconomic scenarios to predict possible future atmospheric and climatological conditions. The team followed the A1B scenario. This scenario considers the climatological effect of a fast-growing global economy relying on a mixture of fossil fuels and renewable energy sources. The Harvard team ran the IPCC’s climate data for the year 2050 through their fire prediction models, which allowed the team to calculate the area burned for each ecoregion at midcentury.
THE FUTURE OF WILDFIRES
Compared to present-day conditions, the models suggest the following for 2050 in the western US:
* The month of August could see an increase of the area burned by 65 percent in the Pacific Northwest. In the Eastern Rocky Mountains / Great Plains regions the area burned could almost double, and in the Rocky Mountain Forest region it might quadruple.
* Large fires could increase by a factor of 2-3.
* The models predict an earlier start date for the fire season – late April instead of mid-May. The end date could also change, becoming later – mid-October instead of early October.
The models also suggest air quality will suffer as a result of these larger, longer-lasting wildfires. Organic and black carbon particles make up the smoke from wildfires that can impede visibility and cause respiratory problems.
The research team used data from two sources: the US Forest Service records of the amount of fuel (biomass) available across the entire United States and a set of databases known as the Landscape Fire and Resource Management Planning Tools, which tracks specific types of vegetation for each square kilometer of land. The team combined this information with known emission factors for combustion to predict that smoke will increase 20–100 percent by the 2050s, depending on the region and the type of particle.
The key finding of the study is its reliance on an ensemble of climate models, rather than just one or two. In the science of climate change, one of the greatest uncertainties is the sensitivity of surface temperatures to rising levels of greenhouse gases.
“Our use of a multi-model ensemble increases confidence in our results,” says principal investigator Jennifer A. Logan, a recently retired Senior Research Fellow at Harvard SEAS.
In central and southern California, the fire prediction model developed by the team performed the least well. The rugged topography results in a patchwork of ecoregions, each with a different fire response to changing meteorology. The research team has been investigating the unusual factors at play in California. They expect to release their findings shortly.
The ultimate goal of this project was to understand how air quality could be affected by climate change, given that smoke from wildfires is a major source of particulate matter in the atmosphere.
In the past 40 years, air quality has improved over much of the US as a result of government efforts to regulate emissions. Increasing wildfires, however, may erase some of that progress.
“I think what people need to realize is that embedded in those curves showing the tiny temperature increases year after year are more extreme events that can be quite serious,” she says. "It doesn't bode well."