Climate

Across The Globe, Wildfire Season Is Lasting Longer

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With 35 active large fires currently burning up and down the West Coast — and with dry, hot conditions sparking an unprecedented number of fires throughout Western Canada — the 2015 wildfire season has started strong, and shows no sign of slowing down.

Now, a new report out in Nature Communications has a some more bad news for the West, and wildfire-prone regions around the world: In the last 35 years, wildfire season has gotten longer, and the global area affected by wildfire has doubled.

Though several studies have looked at the relationship between climate change and regional wildfire patterns, scientists lacked a comprehensive assessment of how climate change might be influencing wildfire seasons on a global scale. Using a combination of fire danger indices and surface weather data, a group of American and Australian scientists looked at how “fire weather” — weather conditions that are especially conducive to fire — has changed around the world over the last three and a half decades.

They found that as global temperatures have increased (by about .2 degrees Celsius per decade since 1979), the length of wildfire season has also increased by 18.7 percent around the world. Across nearly a quarter of the world’s vegetated areas, the length of fire season increased. Only 10 percent of vegetated areas saw a decrease in the length of fire season — Australia was the only vegetated continent that did not exhibit a significant increase in both fire season length and affected area.

Over the last several decades, the report notes, the United States has seen a particularly marked increase in the frequency and duration of large wildfires, especially in the Northern Rocky Mountains. The report links this increase to earlier snowmelt, which creates drier conditions earlier in the summer. In general, the report found, areas with the greatest changes in local weather are the most likely to see changes in their wildfire season:

Our results extend these findings by demonstrating that areas with the most significant change in fire weather season length occur where not only temperature but also changes in humidity, length of rain-free intervals and wind speeds are most pronounced. In 2012, for example, longer-than-normal fire weather seasons across an unprecedented 47.4% of the vegetated area of the US culminated in a near-record setting ~3.8 MHa of burned area.

The tropical and subtropical forests of South America have also experienced what the report refers to as a “tremendous fire weather season length changes,” with a median 33 day increase over the last 35 years.

The average length of fire season, the report notes, does not perfectly equate with fire activity — even if the conditions are right for fires to occur, wildfires still need some sort of ignition spark and ample fuel. But the researchers warn that “if these fire weather changes are coupled with ignition sources and available fuel, they could markedly impact global ecosystems, societies, economies and climate.”

An increase in fire activity could impact everything from public health to the economy. When fires burn, they emit smoke that can travel hundreds of miles, impacting air quality and exposing residents in places removed from the direct dangers of wildfire to harmful particles that can exacerbate existing health conditions, especially in the very young and very old. Fighting wildfires is also expensive, costing the U.S. government an average of $1.13 billion a year in the last decade. As climate change exacerbates wildfires, one study estimates that fighting wildfires could cost as much as $62.5 billion annually by 2050.

An increase in wildfires can also lead to an increase in climate change. As wildfires last longer, and cover a greater area, more trees burn, releasing carbon into the atmosphere and turning some forests from carbon sinks into carbon sources. And as places like Alaska experience longer wildfire seasons, carbon-rich permafrost could melt more quickly, releasing even more carbon into the atmosphere.