Wildfires are spreading to fuel-abundant regions of the world that used to be less prone to burning, according to a new analysis of 20 years of data by the Guardian.

While the overall area of annual burn in the world has remained relatively static in this period, the research indicates a shifting regional fire pattern that is affecting more forests and fewer grasslands.

In recent years, fires have devastated areas of California, Australia, Siberia and the Pantanal that used to be relatively unaffected. In Africa, by contrast, there has been a reduction of savannah fires.

Experts believe the changing fire patterns are driven by human factors: global heating, which is creating more tinderbox conditions in forests, and land conversion, which is turning grasslands into farm fields, conurbations and roads.

The causes and consequences are still being studied, but scientists are concerned this shift will put more carbon dioxide into the atmosphere from forests, while eroding the unique bio-vitality of grasslands, which are better adapted to fire.

“Since the early 2000s, we are seeing a decline in grassland fires, which dominate the global numbers. At the same time, there is an increase in some high-fuel systems such as the western US that several studies have connected to climate change,” Niels Andela, an expert in remote sensing at Cardiff University, said. “This trend is not yet visible everywhere, but it is likely to become more apparent in other parts of the world.”

Drying has made Australian forests more vulnerable

In Australia last year the fire season was exceptional because of where the fires were rather than the number of square kilometres burned.

The area affected was actually down in 2019 – but the smoke cloud was three times bigger than anything seen before. Scientists described it as “a new benchmark on the magnitude of stratospheric perturbations”.

Most years, large fires are left to spread across the sparsely populated northern and western regions. But in the 2019-20 fire season, fires hit the south-east and consumed forests that were not used to blazes on this scale.

Dr Pep Canadell, chief research scientist in the Climate Science Centre at the Commonwealth Scientific and Industrial Research Organisation in Australia, blamed the hottest temperatures in over 100 years, a two-year drought in many parts of south-east Australia that dried out the forest and provided fuel for the fire. He warned such trends were likely to continue.

“There is no question that climate change was a very significant factor in the extreme fire activity of the last season. We have always had droughts and heatwaves leading to extreme fire weather conditions, but our background long-term temperature trend is now 1C over the pre-industrial level, with much hotter and longer heatwaves than before, and consistent with the temperature elevation, our droughts are now hotter leading to drier fuels more able to burn quickly.”

Californian fires spreading northwards

Like Australia, California’s fires are spreading to new locations as their size and frequency hit record levels.

John Abatzoglou, associate professor at the University of California, Merced, said: “In forested systems, including those that maintain fire suppression responses such as the US and Canada, we have seen an increase in burned area and a number of very large fires over the past several decades.”

He said part of the blame can be attributed to a warming, drying climate that makes potential fuels vulnerable to fire.

“The confluence of increased fuel in a warmer and drier climate has certainly contributed to the very large increase in forested burned area in parts of the western US.”

Dr Matthew W Jones, senior research associate at the University of East Anglia’s Tyndall Centre for Climate Change Research, said: “There has been an astonishing eightfold increase in the area burned by forest wildfires in California over the past 20 years. Changes on this scale are not only seen in California, but western US forests more generally.

“Water is becoming scarcer and forests are drying out more regularly throughout the spring, summer and autumn. Forests are essentially becoming tinderboxes of rich fuels, primed to burn more often as a consequence of climate change.”

Drying pattern repeated in southern Europe

Climate change-related drying has also contributed to increased fire activity in southern European countries such as Portugal.

According to Dr Jacquelyn Chase, professor of geography and planning at California State University: “There is consensus among fire researchers that climate change is extending the dry season and contributing to megafires, although the vegetation and demographic changes in rural areas are also important factors in severity and location of fires there.

“The Mediterranean climate has always produced fires in the dry season but the size of these has become clearly associated with recent changes.”

Rainforests such as the Amazon also hit – for different reasons

The Brazilian Amazon has faced more fires recently, and although these are almost entirely human-triggered, experts say that climate change is still playing a part in their severity.

Dr Erika Berenguer, senior research associate at the universities of Oxford and Lancaster, said that all types of fires in the Amazon are human-caused and related to agriculture, falling under three categories: deforestation (with fires used to clear felled trees), pasture (with fires used to keep cattle pasture clear and fertile) and subsistence (with fires used to refresh land plots).

In each case, fires are escaping into the forest understorey with increasing frequency – and the forest itself has been made drier and more vulnerable because of climate change.

She said that “climate change has made the Amazon rainforest become drier in some places and a lot hotter. This makes it harder for the forest to act as a buffer to fire, instead making them more vulnerable.”

Maps based on annual cumulations of burned area data from the Global Fire Emissions Database. With thanks to Michael Humber, assistant research professor at the department of geographical sciences at University of Maryland, who facilitated the Guardian’s analysis of the fires data. Nasa and the School of Earth and Environmental Sciences at Cardiff University also contributed to the Guardian’s analysis of the burned area data.