Triple Threat of Climate Change to US Forests: Fire, Drought, and Pests

906

Forests are one of Earth’s most powerful tools for fighting climate change. They absorb billions of tons of carbon dioxide (CO₂) from the atmosphere every year, acting as natural carbon sinks—ecosystems that absorb more carbon than they release. This process is critical because CO₂ is the primary greenhouse gas driving global warming.

However, this vital role is under severe threat as climate change fuels wildfires, droughts, and insect outbreaks. A major study published in 2022 in Ecology Letters by a team of scientists led by William Anderegg reveals just how vulnerable U.S. forests are to these growing risks. The research combines decades of forest health data, satellite observations, and climate models to map current and future threats.

The Growing Threats to Forests

To understand the risks, it’s important to start with how forests store carbon. Trees absorb carbon dioxide during photosynthesis, converting it into organic matter like wood, leaves, and roots. This carbon remains stored as long as the tree is alive or the wood is preserved. Healthy forests can keep carbon locked away for centuries.

But when trees die—whether from fire, drought, or insects—they release this carbon back into the atmosphere through decomposition or combustion. Climate change is making all three of these threats worse. Rising temperatures dry out forests, creating ideal conditions for wildfires.

Droughts weaken trees, making them more likely to die from disease or insect attacks. At the same time, warmer winters allow insects like bark beetles to survive and multiply, leading to larger outbreaks.

The study by Anderegg and his team is the first to map these interconnected risks across the entire United States. They analyzed data from 1984 to 2018 to understand historical patterns and used climate models to project risks up to the year 2100.

Their results show that wildfires, droughts, and insects are not just isolated problems—they reinforce each other in ways that could devastate forests and undermine global efforts to combat climate change.

US Forest Study Methodology

The researchers used a combination of high-tech tools and ground-based observations to build their models. For wildfires, they relied on satellite data from the Monitoring Trends in Burn Severity (MTBS) program, a U.S. government initiative that tracks the size and severity of fires across the country.

They combined this with climate data, such as temperature, rainfall, and drought measurements, to identify patterns. A key finding was that dry, hot conditions significantly increase the likelihood of large fires. For example, in the western U.S., a 1°C rise in summer temperatures was linked to a 50% increase in burned area.

To study tree mortality, the team turned to the U.S. Forest Inventory and Analysis (FIA) program, a nationwide network of forest plots monitored by the USDA Forest Service. This program tracks tree growth, death, and health in over 100,000 locations.

The researchers focused on two types of tree death: deaths caused directly by climate stress (like drought) and deaths caused by insects. By comparing tree health data with climate records, they found that droughts and heatwaves were responsible for up to 3% of annual tree loss in vulnerable regions like the Southwest.

Insect outbreaks, meanwhile, were often tied to warmer winters, which allow pests like bark beetles to survive and reproduce faster. The team then used these historical patterns to predict future risks under three climate scenarios:

1. Moderate emissions cuts (SSP2-4.5): Global temperatures rise by 2–3°C by 2100.
2. High emissions (SSP3-7.0): Temperatures rise by 3–4°C.
3. Very high emissions (SSP5-8.5): Temperatures rise by 4–5°C, representing “business as usual” with no major policy changes.

These scenarios, part of the Shared Socioeconomic Pathways (SSPs) developed by climate scientists, help predict how societies and ecosystems might respond to different levels of warming.

Rising Wildfire Risks in US

Wildfires are perhaps the most visible threat to forests. The study found that fires have already become larger and more frequent due to climate change. Between 1984 and 2018, the average annual burned area in the U.S. increased by 40%, with the worst fires occurring in the western states.

For example, California’s 2018 Camp Fire burned over 150,000 acres and destroyed 18,000 buildings, becoming the state’s deadliest wildfire on record.

Looking ahead, the projections are alarming. Under the high-emissions scenario (SSP5-8.5), wildfires could burn 14 times more land by 2100 compared to historical levels. Even with moderate emissions cuts, burned area is expected to quadruple.

This means that by the end of the century, regions like the Rocky Mountains and Pacific Northwest could see 10–15% of their forests burn every year. The southeastern U.S., which has historically been less fire-prone, may also face increased risks as hotter, drier summers create conditions similar to those in California today.

One reason for this surge is the lengthening of fire seasons. In the western U.S., the fire season has already grown by 2–3 months since the 1980s. By 2100, some areas could face year-round fire risk, leaving little time for forests to recover between blazes.

Drought Impact on US Forests

While fires make headlines, drought is a slower but equally deadly threat. Trees rely on water not just for growth but also for basic functions like transporting nutrients. Prolonged droughts weaken trees, making them vulnerable to disease and pests.

The study found that drought-driven tree mortality has increased sharply in the southwestern U.S. over the past two decades. For example, in Arizona and New Mexico, 5–10% of piñon pines died during a severe drought in the early 2000s.

The researchers project that drought risks will intensify as the climate warms. Under the high-emissions scenario, annual tree mortality from drought could rise by 80% by 2100. Some species are more vulnerable than others.

Piñon pines, which have shallow roots and low drought tolerance, face extinction in parts of their current range. By contrast, junipers, which are better adapted to dry conditions, may survive but at the cost of reduced biodiversity.

Droughts also interact with other threats. For instance, dry conditions make forests more flammable, increasing wildfire risks. At the same time, trees stressed by drought produce less resin, a sticky substance that helps repel insects. This creates a vicious cycle where droughts and pests work together to kill trees.

Forest Insect Outbreaks Climate Change’s Unseen Army

Insects are among climate change’s most destructive allies. Bark beetles, in particular, have devastated millions of acres of forests in recent decades. These tiny pests bore into tree bark, disrupting the flow of nutrients and eventually killing the tree.

Warmer winters are a key driver of beetle outbreaks. Cold temperatures once kept beetle populations in check, but milder winters now allow them to survive and reproduce year-round.

The study highlights the mountain pine beetle as a major threat. Between 2000 and 2020, this beetle killed 30–50% of mature pine trees in the Rocky Mountains. By 2100, under the high-emissions scenario, beetle-driven mortality could increase by 70%, spreading into higher elevations and new regions.

For example, whitebark pines—a keystone species (a species critical to maintaining ecosystem balance) in alpine ecosystems—are now under attack as beetles move into previously inhospitable areas.

Insect outbreaks also have economic consequences. The timber industry in states like Colorado and Oregon has suffered billions of dollars in losses due to beetle-killed trees. Dead forests also pose wildfire risks, as dry, dead wood burns more easily than living trees.

Regional US Forest Climate Risks

The study provides a detailed look at how different parts of the U.S. will be affected:

1. Western U.S.
This region faces the most severe risks. California, already grappling with megafires (fires that burn more than 100,000 acres), could see 30–40% of its forests burn each decade by 2100.

Droughts in the Colorado River Basin threaten water supplies for 40 million people and could kill 20–30% of the region’s trees. Meanwhile, bark beetles are decimating iconic species like the lodgepole pine.

2. Southeastern U.S.
While historically humid, the Southeast is not immune to climate threats. Rising temperatures and erratic rainfall could make forests in Georgia and Florida more prone to fires. Loblolly pine plantations, a major source of timber and carbon storage, are at particular risk.

3. Midwest and Northeast
These regions will face new challenges. Maple and birch forests, which are vital to the maple syrup industry, could suffer from heatwaves and pests like the emerald ash borer. Cold winters currently limit insect activity, but by 2050, warmer temperatures may allow pests to thrive.

4. Alaska and Boreal Forests
Alaska’s spruce forests are under siege from bark beetles, which have already killed 40% of mature trees in some areas. Thawing permafrost and longer fire seasons could turn boreal forests—which store vast amounts of carbon—from sinks into sources of emissions.

Carbon Offset Policy Challenges: What Can Be Done?

Forests are central to many climate policies, including carbon offset programs. These programs allow companies to “offset” their emissions by paying to protect or plant forests. However, the study reveals a major flaw: most programs assume forests will store carbon for 100 years or more, ignoring the growing risks of fires, droughts, and pests.

For example, California’s forest offset program requires projects to set aside a buffer pool—a reserve of carbon credits—to cover potential losses from disturbances.

But the study found that current buffers underestimate risks by 50–70% because they don’t account for regional differences. A forest in fire-prone California faces far higher risks than one in Maine, yet both are treated the same.

To fix this, the researchers recommend dynamic buffer pools that adjust based on location-specific risks. They also call for better monitoring using satellite data to track forest health in real time.

Without these changes, carbon offset programs could become ineffective, undermining global climate goals. The study is not all doom and gloom. It outlines several strategies to protect forests:

1. Reduce Emissions
Slashing greenhouse gas emissions remains the most effective solution. Limiting warming to 1.5°C (the Paris Agreement target) could cut fire risks by 30–50% compared to high-emissions scenarios.

2. Improve Forest Management

• Prescribed burns: Controlled fires reduce flammable undergrowth, lowering wildfire intensity.
• Thinning: Removing small trees and dead wood helps forests withstand droughts and pests.
• Assisted migration: Planting tree species better suited to future climates, like drought-resistant junipers.

3. Rethink Carbon Markets
Policymakers must update carbon offset rules to reflect climate risks. This includes larger buffer pools for high-risk areas and insurance schemes to cover catastrophic losses.

4. Protect Communities
Investing in firebreaks, early warning systems, and pest detection technologies can save lives and livelihoods.

While comprehensive, the study has limitations. For example, it assumes forests will remain static, ignoring potential shifts in tree species as climates change. It also doesn’t account for CO₂ fertilization—the phenomenon where higher atmospheric CO₂ levels boost plant growth by enhancing photosynthesis.

Some studies suggest this could partially offset drought stress, though the effect is debated. Additionally, the models don’t fully capture how fires, droughts, and insects might interact to amplify losses. For instance, a forest recovering from a fire may be more vulnerable to pests, creating a cascade of damage.

Conclusion: A Race Against Time

The message from this research is clear: climate change is transforming U.S. forests faster than they can adapt. Without urgent action, wildfires, droughts, and insects could turn these vital ecosystems from climate solutions into climate liabilities. The study provides a roadmap for policymakers, emphasizing the need for emission cuts, smarter forest management, and reforms to carbon markets.

But the clock is ticking. Every year of delayed action increases the risks. Protecting forests isn’t just about saving trees—it’s about safeguarding our planet’s future.