Burned Area Increasingly Explained by Climate Change

Climate change is increasingly recognized as a key driver of wildfire activity. But the seemingly simple question, "How much has climate change altered burned area?" has not been answered before. Here, we provide a first quantification of this.
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Climate change is increasingly recognised as a key driver of wildfire activity, with hotter and drier conditions leading to more frequent and intense fires. But answering the seemingly simple question, "How much has climate change altered burned area?" is far from straightforward. Fires are complex events, heavily influenced by human actions such as land management, ignition, and suppression. Observing trends is challenging due to uncertainties in fire observations and modelling them is even tougher. Fires are shaped not only by climate but also by fixed effects like land use, which has decreased burned area in many regions. This paper tackles these challenges head-on and, for the first time, provides a global and regional estimate of how much climate change has impacted burned area—shedding light on a critical, yet elusive, climate question.

Modelling fire activity is no trivial task. First, Dynamic Global Vegetation Models (DGVMs) are required- sophisticated tools that strive to represent everything we see on land. These models take in data about the weather, the atmosphere, CO2 concentrations (which also affects plant growth) and human activity – like land use, and population density - and simulate how vegetation grows, the carbon cycles through the ecosystems, and how the land and people interact with the atmosphere and biosphere. But that’s just the first step. To understand fire activity, these models must be linked to a ‘fire model’, a model that predicts how much vegetation burns based on the same inputs + the vegetation and soil characteristics modelled by these DGVMs.

Fires are modelled by considering three crucial elements: ignitions, spread and suppression.  Ignitions can come from natural sources, like lightning or human activity. Lightning is provided as an input, while human ignitions are estimated based on population. Once a fire starts, its spread is shaped by the amount of fuel (vegetation amount and dryness) to burn and, in some models, wind. Suppression is often influenced by population and socio-economical factors like GDP. And, of course, fires don’t just stop at burning— they reshape the ecosystems and alter the carbon cycle, all of which feed back into the complex simulations of the DGVMs (Figure 1).

Schematic of how a fire model works.

Figure 1: Schematic of how a fire model works. First (step 1), a DGVM receives weather, land use and population data and uses this to model vegetation. Then (step 2), this vegetation data, along with the weather, land use and population data are supplied to the fire module, which additionally receives information on natural lightning. The fire module calculates how much of the vegetation burns and returns this to the DGVM (step 3).

The skill of a fire model thus hinges on several factors, from the accuracy of the weather and lightning it receives, the performance of the underlying DGVM, through to the fire model’s ability to simulate ignitions, spread and suppression. To test the model’s performance, we compare its predictions with satellite observations of burned area - though these observations also come with inaccuracies. This long chain of potential uncertainties means pinpointing why a fire model is not doing a good job in a specific instance is no easy task. As a result, the fire models we have today are far from perfect. Each of them has its own strengths and weaknesses; one might excel in tropical grasslands, while another might be better suited at simulating fires in boreal forests.

In this study, we made use of seven different DGVM-fire models. Each model was run twice for the period 1901 to 2019. In the first round of simulations, we fed the models with real-world, observed data on weather, CO2 levels, human activity, land use change and lightning. These simulations represent the planet as it evolved over the last 120 years, we call these the “factual simulations”. For the second set of simulations, we also gave the same observed human activity and lightning, but with a version of the weather stripped of the influence of climate change. Along with keeping atmospheric CO2 levels low, this essentially recreates how the weather could have looked without human greenhouse gas emissions and climate change. This set of simulations represents the planet as it could have evolved over the last 120 years without climate change, we call these the “counterfactual simulations”.

By comparing these two sets of simulations, we can estimate how much burning is affected due to climate change according to each fire model. However, not all fire models are equally good in each region. Therefore, we compare the factual simulations of each model to the satellite-based observations burned area. From this, we calculate a relative weight for each model for each region. The closer a model matches the observed burned area, the higher its weight will be, meaning it plays a bigger role in the final regional estimate of climate-driven fire activity.

The top row of Figure 2 shows climate change's impact on each region between 2003 and 2019. Globally, we find a 16% increase in burned area due to climate change, with particularly sharp increases in Australia, California and Siberia. Secondly, we also compare the first 20 years of the counterfactual with the last 20 years of the counterfactual. Since the counterfactual assumes a stable climate, any difference between these two periods reflects changes driven by direct human activity such as land use changes and population changes. When we compare these two periods, we find a general decline in burned area (middle row of Figure 2), suggesting that land use changes and population growth is driving a decrease in burned area. Finally, when we compare the first 20 years of the counterfactual to the last 20 years of the factual simulations, we capture the combined effect of climate change and human activity (bottom row of Figure 2). This shows a mixed pattern, where some regions have experienced increased burning and others experiencing less.

Changes in burned area due to different factors.

Figure 2: Changes in burned area due to different factors. On the top row, the effect of climate change is visible, which is mostly causing an increase in burned area. In the middle row the effect of human activity (direct human forcings) is shown, which is predominantly negative. On the bottom row, we can see the combined affect from climate change and human activity on burned area between the periods 1901-1920 and 2003-2019.

In a world where both climate change and human activity are reshaping landscapes, this study offers the clearest global picture yet of how climate change is altering fire patterns. While we see that land management has reduced fire in many areas, climate change is driving increases in others, leading to a complex, region-specific story. As we face a future with rising temperatures, understanding these trends will be critical for developing strategies to protect ecosystems, mitigate fire risk, and adapt to the challenges ahead.

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Climate Change
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Climate Sciences > Climate Change
Climate-Change Impacts
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Climate Sciences > Climate Change > Climate-Change Impacts
Fire Ecology
Life Sciences > Biological Sciences > Ecology > Fire Ecology