Exceeding the fire threshold can quickly make fires dangerous

Global climate change has already increased the risk of fire and is likely to fuel even more change, as ever-faster feedback loops have disastrous consequences for both biodiversity and human populations. But accurately forecasting the risks and impact of wildfires and wildfires worldwide is still a work in progress.

In a new study, a team of Yale scientists and colleagues from South Africa, Gabon and the United States set more than 1,000 prescribed fires in grassy savannas, an ecosystem where more than 80% of the world’s firework activity takes place. Using the results of the experimental fires, they tested a model that will help climate scientists more accurately predict when and where changes in the expected frequency and intensity of fires will occur, and how they will affect global climate change.

They report the results on June 20 in the journal Proceedings of the National Academy of Science†

“Areas like the American West and African savannas can suddenly switch from a non-combustible state to a state where everything is burning, or vice versa,” said senior author Carla Staver, an associate professor of ecology and evolutionary biology at Yale. “Predicting when that threshold will be crossed is crucial to understanding the impact of fires now and in the future.”

The Yale team led by Anabelle Cardoso, a former postdoctoral associate in Staver’s lab and now at the University at Buffalo, has set fires in Kruger National Park, in South Africa, and other savannas in Africa and the United States. . They then measured variables such as grassy fuel biomass, moisture levels, air temperature and humidity, as well as seasonality variables like rainfall.

Their findings, they report, indicate that fire spread is analogous to infectious disease transmission and can be modeled similarly to how public health officials predict disease outbreaks. Like infectious diseases, fires require a “source of ignition” (one who initially contract the disease), a minimum of fuel to burn (enough people in the population to be susceptible to contamination), and favorable environmental conditions to spread rapidly ( a disease that is highly contagious and a susceptible population that does not attempt to minimize transmission).

“And just like a person previously infected, an area that has been burned gains ‘immunity’ to future fires until enough fuel grows back,” Staver said. “Climate change affects this immunity because some places burn more and others stop burning. In both cases, biodiversity and ecosystem function are at risk.”

Fires thrive when moisture levels are low, temperatures are high and humidity is moderate to low. All of these conditions can be made worse by: climate change, the authors say. When environmental conditions reach a certain threshold in terms of available fuel and drought, the risks of severe fires and dangerous fires can increase rapidly.

“Thresholds are like switches. Once it’s flipped, everything changes quickly. It’s not gradual,” Cardoso said. “Fire risk doesn’t go from ‘low’ to ‘dangerous’ in small steps. Rather, it can go from ‘low’ to ‘all-on’ with no warning signs.”

Land managers with experience in managing fires intuitively understand these fire thresholds and how quickly fire conditions can change from safe to dangerous. However, many models used by scientists to predict the current and future global effects of fires do not fully take into account these thresholds and how much carbon is released during these fire events, which could make it difficult to accurately predict future fire risks, he said. the authors. say.

Intriguingly, the effects of global change — especially drought — and increased livestock grazing have reduced the amount of fuel available for fires in some African savannas. However, other parts of the world, including the American West, are at much greater risk of catastrophic fires as fuels dry out more.

“The switch can work in either direction,” Staver said.