A structure burns in Lower Lake, Calif., on in August 2016. (Kent Porter/Press Democrat via AP)

Around the world, the amount of land being burned in wildfires is declining — and human activity is largely the cause, scientists say. According to a new study, out Thursday in the journal Science, global burned area has declined by nearly a quarter in the past two decades. The surprising decrease occurred largely as agriculture has expanded and intensified throughout the world, taking over many of the natural areas where wildfires commonly occur.

While many recent studies have suggested that climate change will cause longer and more intense fire seasons in certain parts of the world — recent research has suggested that it’s been making wildfires in the western United States more intense for decades — the new study indicates that human land-use changes also have a significant impact on when and where fires occur. In many places, the influence of human land use is simply “just stronger than the climate signal,” said the new study’s lead author, Niels Andela, a research scientist at NASA’s Goddard Space Flight Center.

This work highlights how humans can shape global fire regimes,” David Bowman, a global wildfire expert and professor of environmental-change biology at the University of Tasmania who was not involved with the research, said in an email. “Realistically factoring in humans into global climate change and global carbon dynamic projections has always been difficult, and this work demonstrates that this difficulty must be more thoroughly addressed to create plausible scenarios of Earth system change.”  

The researchers came to their conclusions by analyzing satellite data on burned area across the planet between 1998 and 2015. During this period, they found that burned area declined about 24 percent — an “enormous area,” according to Andela — with much of the decrease occurring in the world’s grasslands and savannas.

The researchers also used a model to investigate the effect of factors such as precipitation and human activities on the global fire patterns. They found that precipitation had little influence on the long-term decline in global burned area — but human activity, and particularly agriculture, was a strong driver.

In some places, particularly tropical forest landscapes, the researchers found that agricultural activity was actually associated with an increase in fires, probably as a result of agricultural waste burning or deforestation to make room for cropland. But these increases were outweighed by the areas where agriculture was associated with a reduction in burned area, mainly in grassland and savanna landscapes, where there’s less biomass available to burn and where fire may be less necessary as a land-clearing or management tool.

The researchers found that fire reductions were particularly pronounced in places with greater socioeconomic development and higher investment in their pastures and croplands. Wealthier areas may be less inclined to risk their crops, livestock, homes and infrastructure by using fire as a management tool, the researchers suggest, and the people living in these places also may be less likely to accept the poor air quality caused by smoke from wildfires. In fact, they may begin to employ fire suppression tactics to prevent natural wildfires from occurring.

These global fire reductions may come with pros and cons, the researchers note. For one thing, fire is a natural element of many ecosystems’ life cycles, helping to recycle nutrients, regulate competition among types of plants and make space for new growth. So suppressing wildfires can actually have a negative impact on a landscape’s vegetation and biodiversity.

On the other hand, the researchers note that a reduction in fire also comes with a decline in the carbon emissions it produces, which could be helpful in the fight against climate change. That said, there’s the potential that the carbon released by the land conversion and agricultural expansion driving these declines may actually offset the reduction in fire emissions. A great deal more research will be necessary before scientists can say for sure how all of these changes are affecting the global carbon cycle, Andela said.

And Bowman, the University of Tasmania scientist, also pointed out that this study “should not be construed as suggesting the global wildfire crisis is being resolved.” Even while burned area is declining on a global scale, human management techniques — such as starting fires where they don’t belong and suppressing them where they might otherwise naturally occur — may actually be priming many landscapes for more-extreme wildfires in the future. Fire suppression, in particular, can sometimes lead to an overabundance of dry fuel in certain areas, and these unburned spaces “may eventually burn ferociously under hotter and drier conditions expected by climate change,” Bowman suggested. 

Andela also added that “overall I wouldn’t describe our findings as being a positive thing for the Earth system or global ecosystems.” The spread of agriculture and the decrease in natural burning, particularly in grassland ecosystems worldwide, is indicative of the profound changes taking place in these natural landscapes, he said.

“The disappearance of fire from those ecosystems really symbolizes how we are using these last wildernesses of the Earth,” he said.