Vapor Pressure Deficit Emerges as Key To Global Wildfire Threats To Forests

Forests around the world are burning at an alarming rate, and a new study published in Nature Communications reveals that rising atmospheric dryness is a major driver of this crisis. The research, led by scientists from Australia, Spain, and other institutions, shows how a climate metric called vapor pressure deficit (VPD)—a measure of the atmosphere’s “thirst” for moisture—is reshaping wildfire risks in every type of forest, from tropical rainforests to frozen boreal zones.

By combining satellite data, climate models, and population projections, the study warns of devastating consequences—including massive carbon losses, health emergencies from wildfire smoke, and irreversible damage to ecosystems—if greenhouse gas emissions continue unchecked.

What Is Vapor Pressure Deficit (VPD)?

Vapor pressure deficit (VPD) is a critical climate metric calculated using temperature and humidity. It measures the difference between the maximum moisture the air can hold (saturation vapor pressure) and the actual moisture present (actual vapor pressure).

When VPD is high, the air becomes extremely dry, pulling moisture from plants, soil, and dead vegetation. This process turns forests into kindling by reducing the moisture content of leaves, twigs, and other fuels. The study identifies VPD as the most reliable predictor of wildfires, outperforming traditional factors like temperature or rainfall.

For example, dead leaves and twigs become flammable when their moisture content drops to 10%, a threshold directly linked to VPD levels.

To understand global fire risks, the researchers analyzed 17 years of data (2003–2020) from NASA’s MODIS (Moderate Resolution Imaging Spectroradiometer) satellite system, which tracks daily burned areas, and the ERA5 climate reanalysis dataset, a high-resolution global climate model that provides hourly VPD values.

This approach allowed them to pinpoint exact VPD thresholds for different forest types. Tropical forests, such as the Amazon, ignite when VPD exceeds 2.7 kilopascals (kPa), a unit of pressure used to measure atmospheric dryness.

Mediterranean woodlands, like those in Southern Europe, burn at 2.3 kPa, while boreal forests in colder regions, such as Canada’s taiga, catch fire at just 1.3 kPa. These thresholds reflect how each ecosystem has adapted to its native climate—humid tropical forests require much drier air to burn compared to boreal forests, where cold, dry conditions are normal.

Climate Models Predict Rising Global Fire Threats

The research team used three climate models to project how VPD will change under different emissions scenarios. Climate models are computer-based tools that simulate Earth’s climate system to predict future conditions.

The models chosen—GFDL-CM3, CNRM-CM5, and ACCESS1.0—were selected for their accuracy in simulating regional climate patterns. GFDL-CM3, developed by the Geophysical Fluid Dynamics Laboratory in the U.S., projects the most warming and drying.

CNRM-CM5, from France’s National Centre for Meteorological Research, offers conservative warming estimates, while Australia’s ACCESS1.0 provides moderate projections but predicts drier conditions overall. The study focused on two emissions scenarios:

1. RCP4.5: A moderate scenario where greenhouse gas emissions stabilize by 2100 due to climate policies.
2. RCP8.5: A high-emissions “business-as-usual” scenario with no significant emission reductions, leading to severe warming.

To ensure accuracy, the scientists adjusted the models using historical climate data from 1981–2000. They then calculated future fire risks by comparing projected VPD levels to the thresholds identified for each forest type.

The results showed a strong link between VPD and fire activity, with models correctly predicting fire days 73% of the time (a metric known as the true positive rate). For instance, in Mediterranean regions, where fires are already common, the number of high-risk days could increase by 30–50 per year by 2100 under the worst-case scenario.

Global Surge in Fire-Prone Days Revealed

Under the high-emissions scenario (RCP8.5), the study projects dramatic increases in days where VPD exceeds critical thresholds. Tropical forests, including the Amazon and Congo Basin, face the sharpest rises, with up to 150 additional fire-prone days per year by 2100.

Boreal forests in Canada and Siberia could see 45–60 extra days of high fire risk annually. Even temperate forests in regions like the eastern United States and East Asia, which are not traditionally fire-prone, may experience 30–40 additional risky days each year.

These changes are not uniform. Mediterranean forests, already adapted to dry summers, will face longer fire seasons rather than entirely new risks. In contrast, tropical forests—which rarely experience high VPD today—will undergo radical shifts.

For example, the Amazon’s VPD threshold of 2.7 kPa is currently exceeded about 30 days a year. By 2100, this could jump to 180 days, turning the rainforest into a tinderbox during much of the year.

Carbon Sinks at Risk from Escalating Wildfires

Forests play a critical role in absorbing carbon dioxide, acting as carbon sinks that offset human emissions. However, rising VPD threatens to transform them into carbon sources, releasing more CO₂ than they absorb.

The Amazon rainforest, which currently absorbs 5% of global CO₂ emissions annually, is nearing a tipping point—a critical threshold where irreversible ecological collapse becomes likely.

The study estimates that prolonged high VPD, combined with deforestation, could release 123 billion tonnes of carbon from the Amazon—equivalent to a decade of global fossil fuel emissions.

Similarly, boreal forests store vast amounts of carbon in permafrost (permanently frozen soil). Increased fires in these regions could thaw this frozen soil, releasing methane, a greenhouse gas 28 times more potent than CO₂ over a 100-year period.

The researchers also calculated “tonne-days” of carbon exposure, a metric that combines forest biomass with the frequency of high-risk days. For example, under the GFDL-CM3 model, the Amazon faces 11,200 tonne-days of exposure annually by 2100, meaning every tonne of carbon in the forest is exposed to fire risk for 11,200 days.

The Congo Basin, another critical carbon sink, faces 9,600 tonne-days. These numbers highlight the urgent need to protect carbon-rich ecosystems from escalating fire risks.

Wildfire Smoke Health Risks to Millions of Human

Wildfire smoke contains PM2.5—fine particulate matter smaller than 2.5 micrometers in diameter. These particles penetrate deep into the lungs and bloodstream, causing respiratory diseases, heart attacks, and premature death.

The study maps how rising fire activity will expose billions of people to dangerous smoke levels. South Asia, home to over 1.8 billion people, could see 112 million person-days of smoke exposure annually by 2100 (one person-day = one person exposed for one day). East Africa, another hotspot, may face 96 million person-days.

Case studies from recent wildfires illustrate the stakes. Australia’s 2019–2020 fires caused 429 deaths, 3,230 hospitalizations, and $1.5 billion in health costs, according to the study.

In Canada, annual wildfire smoke costs roughly $330 million in healthcare expenses. In regions like South Asia and East Africa, where healthcare systems are already strained, increased smoke exposure could overwhelm hospitals and clinics.

Regional Impact of High-Risk Fire Zones

In tropical South America, the Amazon rainforest faces existential threats. By 2100, high VPD conditions could persist for half the year, pushing the ecosystem toward a savanna-like state.

This shift would disrupt rainfall patterns across the continent, affecting agriculture and water supplies for millions. Biodiversity would also suffer, with an estimated 10% of global species at risk of extinction.

Boreal forests in the Northern Hemisphere are equally vulnerable. Indigenous communities in Canada and Siberia, who rely on these ecosystems for food and cultural practices, face displacement and loss of traditional livelihoods. Wildlife, including caribou and migratory birds, would lose critical habitats to more frequent and intense fires.

Mediterranean regions, such as Southern Europe and California, are no strangers to wildfires. However, the study projects an extra month of extreme fire weather annually, stretching firefighting resources and increasing the likelihood of megafires—fires that burn more than 100,000 acres.

In temperate zones, including the eastern United States and East Asia, urban sprawl into forested areas raises the risk of human casualties and property damage.

Why VPD Matters More in Escalating Wildfire Threats? Policy Solutions 

The study emphasizes that VPD’s influence on wildfires stems from its direct impact on fuel moisture. Dead vegetation ignites more easily when VPD is high, and live plants become stressed, shedding leaves and needles that add to ground fuel.

Additionally, dry air creates “ember storms”, carrying burning debris kilometers ahead of fires and enabling rapid spread. The models used in the study demonstrated strong predictive power, with an average AUC (Area Under the Curve) score of 0.85.

AUC is a metric used to evaluate the accuracy of predictive models, where 1.0 represents perfect accuracy and 0.5 represents random chance. A score of 0.85 indicates excellent reliability, making VPD a valuable tool for fire risk assessments.

To avoid the worst outcomes, the study stresses the need for aggressive emissions reductions. Meeting the Paris Agreement’s goal of limiting warming to 1.5°C—a global pact to combat climate change—could halve fire risks compared to the high-emissions scenario.

Protecting carbon sinks like the Amazon and Congo Basin is equally critical, requiring international efforts to curb deforestation (the clearing of forests for agriculture or logging) and promote sustainable land use.

Adaptation measures are also essential. Controlled burns—intentionally setting small fires to reduce fuel loads—can prevent larger blazes in Mediterranean and boreal zones.

Firebreaks (gaps in vegetation) and early warning systems can protect communities in Australia and California. In regions like South Asia and East Africa, investments in healthcare infrastructure are needed to address rising smoke-related illnesses.

Global cooperation is key. Funding for forest conservation, estimated at $130 billion annually, must be scaled up, and policies to prevent deforestation—such as Brazil’s Amazon protections and Indonesia’s peatland bans (laws against draining carbon-rich peat swamps)—need stricter enforcement.

If emissions continue unchecked, the study warns of catastrophic feedback loops. Forests could emit more carbon than they absorb, accelerating climate change and displacing 200 million people by 2050 due to fires and drought. Economic losses from wildfire damage could reach $23 trillion by 2100, crippling global economies.

Conclusion: A Crossroads for Humanity and Nature

Wildfires are no longer isolated disasters but symptoms of a planet pushed to its limits. This study underscores that rising VPD—a direct consequence of fossil fuel emissions—will redefine ecosystems, economies, and public health. While some impacts are now unavoidable, humanity still has a choice: act decisively to cut emissions and invest in resilience, or face a future where forests and communities alike are consumed by flames. The time to choose is running out.