A new study from the Australian National University has delivered a stark revelation: the country’s once-reliable tropical rainforests long considered a cornerstone of the planet’s carbon balance have crossed a critical threshold. For the first time, the woody biomass of these forests has shifted from absorbing carbon dioxide to releasing it back into the atmosphere, signaling a profound breakdown in one of Earth’s most important climate buffers.

When the Carbon Sink Turns Against Us?

For decades, tropical rainforests have acted as the planet’s lungs, pulling vast amounts of carbon dioxide from the air through growth that exceeds decay. But the new research reveals that this balance has now tipped. Rising temperatures, prolonged dry spells, and repeated drought events, all intensified by human-driven climate change are driving higher tree mortality and slower regrowth.

“The sink is under threat,” warned lead author Hannah Carle, from the Australian National University. “The expected boost to plant growth from elevated CO₂ simply can’t compensate for the mounting heat and water stress.”

When trees die and decompose, they release the carbon they stored back into the atmosphere, reversing decades of sequestration. This marks the first documented instance of a tropical rainforest biome becoming a net carbon source, challenging the global climate models that have long treated rainforests as stable, self-sustaining carbon sinks.

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Cyclones and Heat Stress Compound the Damage

While heat and drought erode rainforest resilience from within, increasingly powerful tropical cyclones are accelerating the collapse from above. Extreme winds not only fell large trees but also tear open canopies, exposing surviving vegetation to sunlight, heat, and desiccation. The new study found that such disturbances suppress the forest’s ability to store carbon, with impacts lasting years.

“Cyclones are a natural part of rainforest life,” said co-author Francesca Pellicciotti. “But today’s supercharged storms are pushing ecosystems beyond recovery thresholds.”

As global sea surface temperatures rise, storm intensity is expected to increase further, extending their reach southward into areas once considered relatively sheltered. For Australia’s rainforests, that means more frequent structural damage, slower regrowth, and a compounding loss of carbon storage capacity.

Climate Accounting Faces a Wake-Up Call

The consequences of this shift reach far beyond the wet tropics of Queensland. Many national climate strategies and corporate carbon offset schemes depend on the assumption that tropical forests will continue to offset a predictable share of human emissions. If these ecosystems are instead emitting carbon, that undermines global carbon budgets and shortens the timeline for cutting fossil fuel pollution. With fewer trees to regulate moisture and cool their surroundings, regional climates may also worsen. Reduced canopy cover means less evapotranspiration, hotter air, and higher fire risk setting up a self-reinforcing cycle of heat, dryness, and degradation.

“Forests are no longer just passive victims of climate change,” the researchers note. “They are now active participants in amplifying it.”

The Evidence: Long-Term Data, Unprecedented Clarity

What gives this study particular weight is the strength of its data. Researchers analyzed decades-long monitoring records from 62 rainforest plots across northern Australia, offering one of the most detailed looks yet at the interplay between climate variables and forest health. By tracking biomass growth, mortality, and decomposition rates over time, the scientists could clearly distinguish short-term weather fluctuations from structural, climate-driven change. This continuous record shows that the transition from sink to source isn’t a temporary anomaly, it’s an enduring transformation driven by long-term climate trends.

Models Overpromise, Forests Underdeliver

For years, global climate models have relied on the theory of CO₂ fertilization, assuming that higher atmospheric carbon concentrations would enhance photosynthesis and accelerate forest growth. But the new data undercuts that assumption. When trees face simultaneous stress from heat, drought, and storm damage, any fertilization benefit vanishes. The authors warn that existing Earth system models likely overestimate tropical carbon uptake, leading to inflated projections of forest-based climate mitigation potential. Updating these models to incorporate observed mortality rates, drought sensitivity, and cyclone impacts is now critical to keeping climate forecasts credible.

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Reinforcing Forests, Rethinking Policy

Despite the grim findings, the researchers emphasize that tropical forests remain central to climate stability but their value now depends on active protection and adaptation. That means:

Safeguarding intact forests from logging and fragmentation.
Connecting landscapes to allow species migration and ecosystem recovery.
Reducing fire risk through better land management at forest edges.
Restoring diversity, using drought-tolerant and structurally varied species to withstand future climate extremes.

Above all, they stress that the root cause global greenhouse gas emissions must be tackled. “No amount of restoration or adaptation can fully offset the continued warming that drives mortality,” said Carle. “The only durable solution is cutting emissions at their source.”

A Sobering Benchmark for the Planet’s Forests

Australia’s rainforests have served as a bellwether for tropical ecosystems under stress. Their shift from carbon sink to source is not just a local concern; it’s a global climate warning. If one of the world’s wettest, most biodiverse forest systems can flip under current warming, other tropical regions may not be far behind. The findings underscore a hard truth for policymakers and investors alike: carbon sinks are not guaranteed. They are contingent, fragile allies that depend on the climate stability we are rapidly eroding. As the study concludes, the forests’ message is clear: the planet’s green safety net is fraying. To keep it intact, humanity must act faster cutting emissions, updating models, and investing in resilience before more of Earth’s natural carbon guardians turn from shield to source.

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