Climate change appears to be extending wildfire seasons in many places around the world. At the same time, it is expanding the areas experiencing these extended seasons. And it is increasing the areas where the occurrence of longer fire seasons is rising.
Those are the conclusions a team of scientists has drawn from an analysis of daily fire-weather conditions worldwide covering a 35-year span, from 1979 to 2013.
The results reinforce the idea that the most effective approaches for reducing the risk of large wildfires in a warming climate likely will come from controlling available fuel and, where possible, from tighter controls on ignition sources, notes Matt Jolly, an ecologist with the US Forest Service's Fire Science Laboratory in Missoula, Mont.
The study comes at a time when the US is on track to experience once of its most damaging fire seasons for the past 10 years, according to data gathered at the National Interagency Fire Center in Boise, Idaho.
In California, where the fast-moving Rocky Fire has caused 12,000 people to be evacuated, Democratic Gov. Jerry Brown declared a state of emergency and called out the National Guard to help with recovery. The fire, which had destroyed six homes as of Monday morning and threatened thousands of others, was one of 21 that 8,000 firefighters were battling in the parched state. Washington and Oregon also were dealing with wildfires as the West Coast grapples with severe to exceptional drought.
Nationwide, through July 29, just under 36,000 wildfires had torched some 5.7 million acres in the US. That is second only to 2011's 6.1 million acres for the same period and 2 million acres above the 2005 to 2014 average for the period. Only three other years during the period had comparable or larger burn areas by the end of July. Two of those years, 2006 and 2007, topped 9 million acres burned, while 2011 came in at nearly 9 million acres.
Wildfires need the right weather conditions, fuel, and something hot enough to light them. Of these three factors, “weather is the thing that we’re not going to have control over. So we have to focus on the other two aspects if we’re going to adapt,” says Dr. Jolly, who led the team of US and Australian researchers who conducted the analysis. The study does not look at trends in fires themselves, but focuses on long-term trends in weather conditions that raise the risk that fires will start and spread.
At the same time, the study lays a foundation for gauging the relative importance of factors that determine wildfire risk, he suggests. In some cases, weather during fire seasons may hold the most sway over the risks while in other cases human activities – from using fire to clear farmland to a carelessly tossed match – may dominate.
Researchers need to get a better handle on “all of the complex factors that drive fire globally because fire has the potential to be a positive feedback,” he says, releasing additional heat-trapping carbon dioxide into the atmosphere.
Fires themselves release the carbon dioxide stored in the vegetation they burn. And large fires can lead to a change in the types of vegetation that returns, reducing the landscape's ability to act as a natural storage depot for CO2 compared with its previous state.
An increase in the frequency or magnitude of large wildfires – or both – would reinforce global warming already attributed to the CO2 humans release as they burn fossil fuels.
The new fire-weather analysis, which appears in the current issue of Nature Communications, rests on measures of overall fire-weather hazards.
For weather information, the researchers turned to three sets of global data from the US and Britain, which treat Earth’s surface as a grid of squares covering between 30 and 80 square miles each.
From each of the three sets, the scientists plucked maximum temperature, minimum humidity, precipitation, rain-free days, and winds. They used that information to calculate three indexes of fire-weather hazards, developed by the US, Canada, and Australia. Each index was calculated from each data source for each vegetation-covered square on the planet for every day during the 35-year study period.
During that time, the average length of fire seasons around the world grew by 18.4 percent – about four days. The change seems small, Jolly acknowledges. But the global averages mask some dramatic differences at regional levels.
Across North America’s conifer forests, for example, the average fire season has expanded by nine days, with some areas seeing the season grow by up to five weeks.
Similarly, tropical and subtropical forests, grasslands, and savannahs in South America have seen their fire seasons grow by an average of 33 days during the study period.
While Africa as a whole also has seen its fire season lengthen, largely in the eastern half of the continent, much of western African has watched its fire season shrink, the researchers found. This came about as severe-drought conditions that prevailed in the 1980s gradually eased.
Australia is the odd continent out: It shows no trend in the average length of its fire season. That’s largely because the continent’s weather patterns are heavily influenced by the comings and goings of El Niño in the tropical Pacific and another natural, multiyear pattern of climate variability in the Indian Ocean.
Wildfire seasons not only have grown longer, these longer seasons also have expanded their reach, the researchers found.
Averaged around the world, the area of vegetated landscapes affected by longer wildfire seasons grew by 108 percent.
For the US, the researchers highlight the 2012 fire season, during which 47.7 percent of the plant-covered areas of the country experienced longer-than-normal fires seasons – an expanse the team terms “unprecedented.”
The team also noted that compared with the first half of the study period, 53.4 percent of the Earth's plant-covered area saw an increase in the frequency of longer fire-weather seasons, while 34.6 percent saw a decline.
Since 1979, only six years have brought long fire-weather seasons that have covered more than 20 percent of Earth's vegetated surfaces, the team notes. All six occurred between 2005 and 2013, falling within the warmest 15-year period on record.
Now that the researchers have described trends so far in the weather piece of the global wildfire puzzle, the next step is to compare the information with wildfire activity gleaned from satellite data “to see where weather has the most influence on fire activity and possibly humans have the most influence,” Jolly says.