Forest fires today are burning nearly twice as many trees as they did just two decades ago, according to a study from the University of Maryland’s Global Land Analysis and Discovery laboratory.

The researchers found a typical forest fire season burns 7.4 million more acres than in 2001. Forest fires accounted for a quarter of tree loss globally in the last 20 years, according to a summation of the data produced by the World Resources Institute.

In the U.S. this year alone, several large California wildfires have burned nearly 200,000 acres and killed at least four people, according to data from CalFire. One notable blaze threatened the country’s oldest trees in Yosemite National Park, while the largest fire on the California-Oregon border killed at least four and burned more than 60,000 acres.

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In Europe, large wildfires have affected at least a dozen countries, burning over 1.4 million acres of land, according to reporting from Reuters. Large fires darkened skies in Portugal and France this summer, fed by a dry summer and temperatures that pushed above the century mark.

Wildfires in the largely untamed wilderness of Russia’s Siberian and Far East regions have scorched upward of 7.9 million acres of forest so far this year, according to the Moscow Times, blanketing several towns in toxic wildfire smoke. Parts of China are currently battling numerous wildfires in the midst of the country’s worst heat wave since 1961.

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Warming temperatures from climate change are an important driver of worsening wildfire conditions globally. As the atmosphere becomes warmer, typically lush forests dry out and become more vulnerable to fires.

Dried-out forests can act like tinderboxes, allowing fires to spiral out of control. Vast blazes release even more carbon-dioxide emissions into the atmosphere, contributing to further warming of the planet. The World Resources Institute refers to this cycle as the fire-climate feedback loop, and little can be done to slow it outside of dramatically lowering greenhouse gas emissions.

A changing climate has caused boreal forests to ignite like never before. About 70% of all fire-driven tree loss over the past 20 years has occurred in these forests, which are in northern areas of the planet and warming at higher rates than other parts of the globe.

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In 2021 alone, 16.4 million acres of tree cover were lost in boreal forests, compared with just 3.9 million acres lost in tropical forests such as the Amazon, according to UMD’s GLAD laboratory. In both cases, though, the loss of these trees and the thawing of permafrost threatens to release ancient stores of carbon, turning vast forests from climate-healthy carbon sinks into accidental polluters.

In tropical forests, agriculture and growing deforestation have increased the risk of wildfires while also making the forests less resilient to blazes. The expansion of industry and agriculture into these previously untouched parts of the globe means that most fires in tropical rainforests are sparked by people, as opposed to being ignited naturally by lightning strikes.

While the analysis shows fire-related tree loss in Brazil spiked in 2016 and has shrunk since, the number of trees lost to wildfires in the past five years is still many times higher than it was in the early years of the 21st century.

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The United Nations Intergovernmental Panel on Climate Change’s 2022 report found that each increment of additional warming will lead to more devastation and death from a variety of climate hazards, meaning that keeping temperatures even a tenth of a degree Celsius cooler could have a substantial impact.

For boreal forests, keeping warming under 3.6 degrees Fahrenheit is critical. Scientists with the intergovernmental panel say some of the worst-case warming scenarios would lead to 15 years of greenhouse gas emissions being released from the massive stores of carbon in these regions, something that could be curbed if temperatures are kept below the threshold of 3.6 degrees Fahrenheit.

Humans can also change how they interact with forests — ending deforestation and limiting agricultural techniques such as slash and burn can help improve forest resilience, especially in the tropics. When conditions are hot and dry, experts say people should also avoid activity that can spark fires near forests, as even a small blaze can quickly grow out of control.