Even as wildfire season rages – particularly in Alaska and Canada, where over 11 million combined acres have burned so far this year – a new scientific study says that fire risks across the Earth are rising, in tandem with a changing climate.
It will come as no surprise to most researchers that wildfires are influenced by climatic factors – they are generally favored by hot and dry conditions. “Climate is a well-known driver of wildfire behavior,” notes a recent study of the factors behind increased expenditures on combating wildfires in the west.
But the new research — published Tuesday in Nature Communications, and dubbed a work of “global pyrogeography” by its authors — shows that a changing climate is indeed likely altering wildfires, and doing so on a global scale. The study finds that the seasons of the year featuring fire-friendly conditions – so-called “fire weather” – are lengthening, and moreover, that this has been happening on every continent that features wildfires except for Australia.
“We’re able to develop this metric of fire weather season length that allows us to examine changes in a common way across the whole globe,” says Matt Jolly, lead author of the new study and a researcher with the U.S. Forest Service’s Fire Sciences Laboratory in Missoula, Montana. “So that’s helped us, for the first time, really to explore these long term trends and the changes and variability of fire seasons across the planet.”
Jolly conducted the research with a team of U.S. and Australian researchers across multiple institutions, including Nevada’s Desert Research Institute and the University of Tasmania.
More specifically, the research found that from 1979 to 2013, “fire weather season length increased by 18.7 %.” And it’s not just longer season lengths – the research also found that “the amount of area each year that’s impacted by a season that’s longer than normal was also increasing,” says Jolly. “So globally, each year we’re seeing more areas that are pushing into these unusually long seasons.”
One thing that’s new about the research is that this finding applies to the entire world. According to the paper, a “comprehensive global assessment of the interactions of recent climatic changes that lead to an expansion or contraction of fire seasons” has, until now, been “lacking.”
The number of days of the year featuring fire-friendly conditions in a given place is not, to be sure, the same thing as the number of fires that develop, or the number of acres that burn. You still need factors like lightning strikes (or careless humans) to light fires and, of course, you need available fuels – trees, shrubs, grasses – to burn.
Nonetheless, the study asserts, fire season length and actual fire activity are (not surprisingly) closely related.
South America was particularly strongly affected by the trends detected, the study found. The continent’s “tropical and subtropical forests, grasslands and savannas have experienced tremendous fire weather season length changes, with a median increase of 33 days over the last 35 years,” the paper reported.
The researchers also took another step – relating these longer fire seasons to estimates of the amount of carbon being taken up by land surfaces, and the plants that grown on them, in different regions of the globe.
Plants pull in carbon dioxide in the process of photosynthesis – meaning that they partly offset human greenhouse gas emissions. Thus, planting and restoring forests is a vital component of addressing global warming.
Fires, though, combust plants and release their carbon back into the atmosphere. Thus, there’s a danger that if wildfire activity increases on a global scale, the overall ability of plants to store carbon may decrease – especially if plant life does not grow back as quickly after intense fires burn through, or is replaced by different, smaller or less carbon sequestering plants.
Sure enough, the study found that as fire weather season length increased, the amount of carbon uptake by land surfaces and their vegetation tended to decrease – the two were correlated. “When average fire weather seasons are longer-than-normal or when long seasons impacted more global burnable area, net global terrestrial carbon uptake is reduced,” the paper stated.
That’s not to say that more fires are the sole reason for less carbon uptake by vegetation – a third factor, like drought, could simultaneously stunt plant growth and also fan fire activity, Jolly explains — but the two do seem to be related. “In some of the areas we saw a pretty strong coupling between our metrics of fire weather season length and these connections to the land carbon uptake,” says Jolly.
Indeed, the researchers used a weighted phrase in climate science circles to describe what’s going on – “positive feedback,” a self-reinforcing process (sometimes also called a vicious cycle).
“We observed an overall lengthening of the number of days each year that wildfires may burn across more than a quarter of the Earth’s vegetated surface and these fire weather changes could manifest themselves as a positive feedback to global atmospheric carbon accumulations if all the requirements for wildfires are present,” the study noted.
While the trend towards longer fire weather seasons was observed globally (save for in Australia), not every different type of ecosystem manifested it. Rather surprisingly, the globe’s northern or “boreal” forests did not show an increasing trend in the study – save, that is, for the forests of Alaska, currently on course for a possible record wildfire year.
“Alaska is the one place, the one boreal region where we saw consistent, significant trends in the season length,” says Jolly.