Why Australia’s rainforests just stopped absorbing carbon

Australia’s rainforests have long been counted among the planet’s most reliable carbon sinks. Now, a new study shows the opposite is true.

The region’s woody biomass – the trunks and branches that store carbon for decades – has become a net source of carbon dioxide to the atmosphere.

This is the first documented case of a tropical rainforest system tipping this way, and it’s a warning sign for climate policy built on the assumption that forests will keep quietly soaking up our emissions.

Heat and dryness kill forests

Tropical rainforests usually pull more carbon out of the air than they release, thanks to a steady balance of growth outpacing decay. That balance has now broken.

Researchers tie the change to rising temperatures, drier air, and more frequent droughts – precisely the kinds of stress linked to human-driven climate change.

The result is higher tree mortality. When those trees die and decompose, their stored carbon goes back to the atmosphere, and new growth isn’t keeping up.

As lead author Hannah Carle from the Australian National University puts it, “the sink is under threat.” The expected boost to plant growth from elevated CO₂ hasn’t compensated for the mounting heat and water stress.

Stronger storms weaken carbon sinks

Storms are part of rainforest life, but stronger cyclones are tipping the scales. When extreme winds rip through a canopy, they don’t just fell trees. They open the forest to sun and desiccation, stress surviving trees, and slow recovery.

The study finds cyclones suppress the carbon sink capacity of woody biomass. As warming seas supercharge storm intensity and extend impacts farther south, more forests are in the line of fire.

Forest failures tighten carbon timelines

A sink turning into a source reverberates far beyond Queensland’s wet tropics. Many national emissions plans assume tropical forests will keep offsetting a predictable share of CO₂.

If a major tropical system can flip under today’s warming, those accounting assumptions get shakier, and the timeline to cut fossil fuel pollution tightens.

Regionally, a weakened sink can also feed back on climate. With fewer leaves and rougher canopies, forests transpire less and amplify heat and dryness.

In addition, they become more fire-prone – pressures that, in turn, increase mortality. Biodiversity takes a hit as old-growth structures collapse and habitats fragment.

Evidence shows lasting carbon change

One reason this finding carries weight is the quality and duration of the monitoring. The rainforest plots at the center of the analysis have been measured with unusual consistency, producing a high-resolution record of forest health through time.

That continuity lets scientists separate short weather swings from structural, climate-driven change. In a field where broken time series often muddy the picture, this dataset stands out and strengthens the case that the shift from sink to source isn’t a blip.

Models miss rainforest carbon losses

For years, many climate models have leaned on “CO₂ fertilization” – the idea that higher atmospheric carbon dioxide makes photosynthesis easier, helping forests grow faster, and store more carbon.

The new results suggest that bet is too optimistic when heat, drought, and storm damage surge at the same time.

The authors caution that current models likely overestimate how much carbon tropical rainforests can bank in a warmer world.

Updating those models to reflect observed mortality, cyclone impacts, and drought sensitivity is essential for credible national inventories and international targets.

Protect, connect, and restore forests

None of this means tropical forests have lost their climate value. It means their role is now more contingent on conditions we control.

Protecting intact stands, maintaining landscape connectivity, and reducing fuel loads at forest edges can dampen heat and wind damage.

Smart restoration – favoring drought-tolerant species and structurally diverse canopies – can speed recovery after storms. But the most effective fix sits upstream: cutting greenhouse gas emissions to ease the very stresses driving mortality.

Carbon goals hinge on forests

Because rainforests feature in everything from net-zero pledges to carbon offsets, clarity matters. Policymakers should treat sink capacity as conditional, not guaranteed, and avoid over-reliance on future forest uptake to balance ledgers.

The public deserves the straight story: forests remain vital allies, but they are not invulnerable. Their ability to help depends on the climate we create.

Australia’s wet tropics have long been a living laboratory for climate-forest dynamics. That they’ve now crossed from sink to source is a sobering marker of how far warming has pushed even rain-soaked, biodiversity-rich systems.

The study’s long, careful records make the message plain: the planet’s green safety net is fraying at the edges.

If we want tropical forests to stay on the right side of the carbon ledger, we need to cut emissions faster, update our models and targets, and invest in resilience where it counts – on the ground, tree by tree.

The study is published in the journal Nature.

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