Australia’s tropical rainforests are now emitting more carbon dioxide than they absorb, a reversal driven by climate change that could undermine global efforts to combat emissions, according to a new scientific study. This shift, documented for the first time in tropical forests worldwide, highlights the urgent need to reassess climate models and policies.
The study, published in the journal Nature, draws on 49 years of data from forests in Queensland, where researchers monitored over 11,000 trees from 1971 to 2019. It reveals that these ecosystems transitioned from carbon sinks to carbon sources around the year 2000, with the balance tipping due to increased tree mortality. Lead author Dr. Hannah Carle of Western Sydney University emphasized that this change challenges assumptions about forests’ role in offsetting fossil fuel emissions. The findings are based on the world’s longest and most comprehensive forest monitoring dataset, providing robust evidence of the trend.
Tropical rainforests typically function as carbon sinks by absorbing CO2 during tree growth and releasing it when trees die and decay. However, the research shows that dead trees’ woody biomass now contributes more carbon to the atmosphere than new growth can absorb. This reversal is primarily linked to climate change impacts, such as extreme temperatures and reduced water availability, which have accelerated tree death rates. Dr. Carle noted that forests, which should help curb climate change, are becoming less effective due to these stressors.
Key drivers include more frequent and intense cyclones, like Cyclone Larry in 2006, and severe droughts, such as the El Niño event of 1998. These extreme weather events, exacerbated by global warming, weaken trees, making them vulnerable to pests and pathogens. The cumulative effect has pushed mortality beyond natural replacement levels, with data showing a consistent decline in carbon storage capacity over decades. Unlike in regions like the Amazon, deforestation played a minimal role, underscoring the direct impact of climate change on intact ecosystems.
The implications for Australia’s climate targets are significant, as the country has committed to cutting emissions by 62-70% by 2035 from 2005 levels. Professor David Karoly from the University of Melbourne warned that the Paris Agreement’s reliance on natural carbon sinks may be flawed, potentially requiring stronger emissions reductions to limit global warming to 1.5°C. He described the findings as a wake-up call, suggesting that current models overestimate forests’ ability to absorb carbon, which could derail net-zero goals.
Experts stress that this shift serves as an early warning for other tropical forests globally, including those in South America and Southeast Asia. Dr. Carle likened the situation to a “canary in the coal mine,” indicating that similar changes could occur elsewhere if climate change persists. The study underscores that anthropogenic factors are degrading ecosystems that are vital for carbon sequestration, emphasizing the need for immediate action to reduce greenhouse gas emissions.
While carbon emissions from forests are smaller than those from fossil fuels, they add to the atmospheric load, accelerating global warming. The research calls for updated climate models to account for these dynamics and for policies that enhance forest resilience through conservation. Protecting these ecosystems is crucial not only for carbon storage but also for biodiversity and regional climate stability, as forests play a key role in regulating weather patterns.
Moving forward, researchers urge governments to strengthen emissions targets and invest in adaptive strategies to mitigate further ecosystem decline. Ensuring the health of tropical forests requires a dual approach: reducing fossil fuel dependence and supporting reforestation efforts. Without such measures, the loss of these carbon sinks could exacerbate climate crises, making it harder to achieve international climate goals and protect vulnerable communities.