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ABOUT THE SCIENCE: Tree Extinction Due to Human Induced Environmental Stress

Long-term field observations, remote-sensing data, and new climate-biosphere models now converge on a disturbing conclusion: Earth’s forests are undergoing rapid, nonlinear decline driven by a cascading series of human-induced stressors. The interacting effects of pollution, drought, extreme weather, pest outbreaks, wildfire acceleration, and climate feedback loops have pushed multiple forest biomes into sink-to-source transitions, where forests emit more carbon than they absorb.

What began in 2001 as a study of visible canopy loss has evolved into documentation of a global systemic collapse. Satellite evidence confirms that large forest regions–including the African tropical moist broadleaf biome–have already shifted from net carbon sinks to net sources in a period of only seven years (Mensah et al. 2025). Similar transitions are now observed in boreal forests, peatlands, and other major carbon reservoirs.

These processes are not isolated. They are coupled, mutually reinforcing feedback loops capable of accelerating tree mortality on timescales far faster than traditional models predicted.

Pollution remains the most significant driver of global tree decline–and the most underestimated. Because pollution affects air, water, soil chemistry, and atmospheric chemistry simultaneously, its effects manifest through multiple pathways.

At the center of the problem is tropospheric ozone, a toxic oxidant produced by combustion byproducts (NO2, VOCs, methane). Ground-level ozone:

Field and global datasets show that ozone pollution is responsible for a substantial portion of current forest mortality. A 2024 tropical forest analysis found that human-derived ozone has reduced net primary productivity (NPP) by ~17% since 2000, significantly weakening the tropical carbon sink.

Further reading:

Ozone interacts with other pollutants–including nitrogen deposition, particulate matter, and acidifying compounds–to accelerate canopy loss and soil nutrient depletion. Thermal pollution (heat from combustion and urban surfaces) additionally increases ozone formation rates.

Recent decades have experienced unprecedented drought frequency and severity. Lower water tables, heat waves, and multi-year moisture deficits weaken root systems and diminish trees’ ability to withstand pests and disease.

Conversely, excessive rainfall–often more acidic and chemically reactive–damages leaves, alters soil pH, and dissolves essential micronutrients. Acid fog and cloudwater have been documented causing widespread leaf necrosis.

Both extremes–too little and too much water–are now more common due to climate change’s amplification of the hydrological cycle.

Further reading:

Tree mortality from insects such as gypsy moths and borers has long been understood, but recent collapses in insect biodiversity (~80% declines) and changes in soil invertebrates are novel phenomena linked to warming and acidification.

A proliferation of invasive insects and plants–including ailanthus, Asian longhorn beetle, emerald ash borer, and persistent non-native earthworms–has destabilized forest ecosystems.

Warmer winters dramatically reduce larval mortality. USDA data:

These lethal cold thresholds are now rarely reached in many northern regions.

A Nature study shows that insects contribute to ~29% of global deadwood carbon emissions, releasing ~10.9 Gt of carbon annually, comparable to or exceeding fossil-fuel emissions.

Examples:

Pollution, drought, heat, and pests each contribute to mortality–but it is the feedback between them that drives runaway decline.

The Tree Extinctions scientific warning states that one-third of global tree species are now threatened with extinction, risking ecosystem collapse.

Warming has intensified wildfire seasons globally. Highlights:

Hotter temperatures → more fires → fewer forests → more carbon emissions → hotter temperatures.

By 2070, ~2 billion people may live in Saharan-like heat zones (PNAS).

Human activities–pollution, fossil combustion, land use, and climate alteration–are driving an accelerating cycle of tree mortality. Tropospheric ozone, previously underestimated in its global effect, now appears to be one of the dominant controls on forest health and productivity. When combined with drought, pests, invasive species, and wildfires, the result is a self-reinforcing, exponential decline in global forest stability.

Tree mortality accelerates global warming; warming accelerates further tree mortality.

Immediate mitigation of fossil-fuel emissions, ozone precursors, and land-use drivers is essential if Earth’s forests–and the ecosystems and climate stability they support–are to survive the 21st century.

* Our probabilistic, ensemble-based climate model — which incorporates complex socio-economic and ecological feedback loops within a dynamic, nonlinear system — projects that global temperatures are becoming unsustainable this century. This far exceeds earlier estimates of a 4°C rise over the next thousand years, highlighting a dramatic acceleration in global warming. We are now entering a phase of compound, cascading collapse, where climate, ecological, and societal systems destabilize through interlinked, self-reinforcing feedback loops.

\What Can I Do?

From the album “Dense“