Wildfire is a natural hazard – and a natural part of the grassland ecosystem. Against expectation, the world’s fires are burning low.
LONDON, 6 July, 2017 – The explosive growth in human population has damped down the risk of wildfire. Although researchers have predicted more intense forest fires with global warming, on a global scale the reverse has happened.
The total area burned by wildfire – grassland, scrub or forest fires – each year declined by 24% between 1998 and 2015, according to a new analysis of decades of satellite data. And this may not be good news.
Although fire threatens life, destroys property and introduces a wider health hazard in the form of soot and smoke particles, it is and has always been an integral part of the natural management of grassland and savanna ecosystems.
Researchers from the US, Europe and China report in the journal Science that, according to readings from two Nasa satellites, the total area of the Earth’s surface that caught alight has fallen by 1.2 million square kilometres. This is equivalent to the area of South Africa. The decreases were most dramatic in Central and South America, and in northern Africa.
“A billion and a half more people have been added to the planet over the past 20 years, livestock has doubled in many places, and wide-open areas once kept open by fire are now being farmed,” said James Randerson, an earth system scientist at the University of California, Irvine.
“Our fire data are a sensitive indicator of the intense pressure humans are placing on these important ecosystems.”
For thousands of years fire has kept the African grasslands open for nomad cattle herds, and home to elephants, lions, zebras and other animals that are already, for a mix of reasons, disappearing with the wilderness.
But as human populations have expanded, more people have turned to permanent agriculture in what was once savanna, and by 2015 savanna fires in Africa had fallen by 700,000 square kilometres. This is an area the size of Texas.
“When land use intensifies on savannas, fire is used less and less as a tool,” said Niels Andela of the Nasa Goddard Spaceflight Center in Maryland, in the US, who led the research.
“As soon as people invest in houses, crops and livestock, they don’t want these fires close by any more. The way of doing agriculture changes, the practices change, and fire slowly disappears from the grassland landscape.”
The finding is unexpected: some researchers had predicted fiercer wildfires with global warming, and certainly greater hazard in parts of the United States. Others had taken the longer view, and argued that, overall, wildfire incidence may have declined.
But neither side had directly factored in the role humans have played as global population soared, and more than 36% of the total land surface of the planet became devoted to agriculture, in a shift so dramatic that some scientists want to declare a new geological epoch, called the Anthropocene.
There have been hints, too, that natural grasslands were changing, either with climate or because of human action: one study examined the tree population in the drylands and grasslands of the planet and identified an undiscovered forest roughly the size of the entire European Union.
“We’ve seen a substantial global decline over the satellite record, and the loss of fire has some really important implications for the Earth system”
Fire is a mechanism that stops woodland from colonising the savanna, which accounts for roughly one fifth of the terrestrial habitat. That too might have been an indicator that fire was in decline.
But this decline has not been uniform. The researchers found that, although the incidence of fire worldwide had declined, the terrain masked by smoke and cinders had certainly increased in Canada and the US West, and in some parts of China, India and Brazil.
“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.
“We’ve seen a substantial global decline over the satellite record, and the loss of fire has some really important implications for the Earth system,” said Douglas Morton, another Goddard Spaceflight Center scientist.
“The loss of fire from agricultural landscapes has a big impact on communities and ecosystems. Looking ahead, models that account for changes in fire activity from human management will help us understand the feedbacks from fewer fires on vegetation, air quality and climate.” – Climate News Network