🔗 Share this article Queensland Rainforest Trees Shift from CO2 Absorber to Emissions Source in World First Australian tropical rainforest trees have become the first worldwide by transitioning from serving as a CO2 absorber to becoming a source of emissions, due to rising heat extremes and drier conditions. Critical Change Discovered This crucial shift, which affects the trunks and branches of the trees but does not include the underground roots, started around 25 years ago, as per recent research. Forests typically absorb carbon as they develop and release it upon decay and death. Generally, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this absorption is assumed to increase with rising atmospheric concentrations. However, close to five decades of data collected from tropical forests across northern Australia has shown that this vital carbon sink may be at risk. Study Insights Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with increased tree mortality and insufficient new growth, according to the research. “It’s the first tropical forest of its kind to display this sign of change,” stated the principal researcher. “We know that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it could act as a future analog for what tropical forests will experience in global regions.” Global Implications One co-author mentioned that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are needed. But if so, the results could have major consequences for global climate models, carbon budgets, and climate policies. “This research is the first time that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not merely temporarily, but for 20 years,” stated an expert in climate change science. On a global scale, the share of carbon dioxide taken in by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was expected to persist under many climate models and strategies. But if similar shifts – from sink to source – were observed in other rainforests, climate projections may underestimate global warming in the coming years. “This is concerning,” it was noted. Continued Function Even though the equilibrium between gains and losses had changed, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels. Research Approach This study utilized a unique set of forest data starting from 1971, including records tracking roughly 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the changes in soil and roots. Another researcher emphasized the value of collecting and maintaining long term data. “We thought the forest would be able to store more carbon because [CO2] is increasing. But looking at these long term empirical datasets, we find that is not the case – it enables researchers to compare models with actual data and improve comprehension of how these ecosystems work.”