The number of bush and grass fires across the globe has declined by almost a quarter in less than two decades – but that’s not necessarily all good news.
Using data from several satellites gathered over the past 18 years, researchers at NASA’s Goddard Space Flight Center, in the US state of Maryland, found the total area burnt each year has dropped by about 24%.
In a paper published in the journal Science, lead researcher Niels Andela and colleagues note that the largest decreases in annual fire activity have been around forest margins in South America, the Eurasian steppes and the African savannahs.
The researchers suggest the drop – in total, accounting for 700,000 square kilometres – is being driven largely by an expansion of agriculture and, with it, the establishment of permanent settlements and road.
“When land use intensifies on savannahs, fire is used less and less as a tool,” Andela says. “As soon as people invest in houses, crops and livestock, they don’t want these fires close by anymore. The way of doing agriculture changes, the practices change, and fire slowly disappears from the grassland landscape.”
These disappearing blazes are a cause for concern. Grassland ecology is significantly dependent on periodic wildfires, which serve to thin out dense plant populations, return minerals to the soil and, for some species, stimulate germination.
In Africa, especially, fewer fires on the savannahs will enable trees and shrubs to gain territory historically occupied by grasses. This could have severe effects on the resources available to grazing species such as zebra and gnu – and, consequently, on peak predators such as lions.
Looking at the data from another perspective, however, Andela and his team note that the drop in emissions from the savannah, steppe and forest regions pretty much balances out the boost in emissions from a recorded rise in fire numbers and severity in the colder latitudes due to global warming.
“Climate change has increased fire risk in many regions, but satellite-burned-area data show that human activity has effectively counterbalanced that climate risk, especially across the global tropics,” explains co-author Doug Morton.
Although the reduction in large burns in the tropics presents long-term challenges for the health of grassland ecosystems, it also brings some benefits.
NASA’s data shows that in the most affected areas, air quality has improved significantly, thanks to a drop in carbon monoxide levels.
The boosted concentration of trees and shrubs has also improved the regions’ abilities to act as carbon sinks – lifting the capacity of world’s vegetation to absorb carbon emissions by an estimated 7%.
The global fire data – which is available on an open source platform here – presents a complex picture of cause and effect, which Andela’s team suggest should be factored in to climate-change models.