Australia’s tropical forests have undergone a dramatic shift: the woody biomass of rainforests in north-eastern Queensland is now releasing more carbon dioxide into the atmosphere than it stores, according to a study published 16 October 2025 in Nature.

Analysing nearly fifty years of data from twenty permanent monitoring sites (established between 1971 and 1980) and around 11,000 trees across elevations from 15 metres to 1,200 metres, the research found that between 1971 and 2000 the forests absorbed on average around 0.62 megagrams of carbon per hectare per year via woody biomass growth. However, in the period 2010–2019 they emitted roughly 0.93 megagrams per hectare per year. The shift began approximately around the year 2000. The key driver appears to be accelerating tree mortality linked to climate-change stressors: rising temperatures, drought, atmospheric dryness, and stronger tropical cyclones.

The finding is the first recorded instance in a tropical forest of such a sink-to-source transition in woody biomass. Researchers emphasise that while roots, soils and non-woody vegetation were not part of this analysis, the results challenge longstanding assumptions that tropical rainforests will reliably continue to act as net carbon sinks. The limited “CO₂-fertilisation” effect — where higher atmospheric carbon dioxide might spur additional tree growth — appeared muted, possibly due to nutrient limitations (particularly phosphorus) in these forest systems.

This development carries major implications for global climate-change strategies, especially as the COP30 (United Nations Climate Change Conference) convenes in Belém, Brazil. Tropical forests have been counted on to absorb a significant portion of human CO₂ emissions. If large systems like Australia’s now emit rather than absorb carbon, the carbon-budget calculations and emissions-reduction pathways may need urgent revision.

The researchers caution that other tropical forests — such as those in the Amazon Basin, Congo Basin and Southeast Asia — are not yet documented to have made a full sink-to-source flip, but signs of decline in carbon-absorption capacity have emerged. In the Australian case, the authors describe the region as a “canary in the coal mine” for global forest ecosystems.

At a policy level, the study underscores the urgency of deep, rapid reductions in fossil-fuel emissions and of robust long-term monitoring of forest carbon dynamics. It suggests that relying on forests as stable carbon-sinks may be increasingly precarious. For Australia — already one of the highest per-capita emitters of carbon dioxide and a major fossil-fuel exporter — the results add fresh pressure on national climate-policy frameworks and highlight the need for intensified forest-resilience and adaptation measures.

In short, the era in which Australia’s tropical forests could be confidently counted as buffers against climate change appears to be entering its end. The future role of such forests in the global carbon cycle now appears more vulnerable than previously assumed, and the world may need to act accordingly as forest-carbon dynamics shift under climate stress.