Researchers have recently discovered that tropical forests in Australia have become the first of their kind to emit more carbon than they absorb. The analysis found that Australian tropical forests are not taking in enough carbon dioxide to exceed or even match the carbon emissions that occur from their decaying trunks, signaling worrying implications for global ecosystems.
Tropical forests serve an important role as “carbon sinks” for cooling the planet, with their trees absorbing carbon dioxide from the air and storing it within their stems and branches. The fact that one Australian rainforest is doing the opposite has researchers concerned.
“These findings challenge our future reliance on forests as natural absorbers of extra atmospheric carbon,” the study’s authors wrote in The Conversation.
Published in the journal Nature on October 15, the research analyzed data from roughly 11,000 trees in wet tropical rainforests in Australia’s northeast region. Researchers examined woody biomass from the rainforest, which had been tracked for nearly 50 years. From the woody biomass, which typically contains large amounts of carbon, researchers found that the forest is emitting more carbon than it absorbs, and that this switch actually occurred 25 years ago.
During the process, when trees die and begin to rot, they release stored carbon back into the atmosphere. This shift indicates that the trees in this Australian rainforest are dying faster than they were a few decades ago, lead author Dr. Hannah Carle, a forest ecosystems researcher at Western Sydney University, tells the Australian Broadcasting Corporation’s Peter de Kruijff. It is thought that drier air, higher temperatures, and occurrences of drought might all have played a role in this change.
Each year from 1971 to 2000, the tracked plants absorbed about 553 pounds of carbon per acre. From 2010 to 2019, they emitted about 830 pounds of carbon per acre, on average, each year. This is the first rainforest that has been recorded as switching from a carbon sink to a carbon source. Carle says this phenomenon could reflect a declining ecosystem across all Australian wet rainforests. “That’s really significant,” Carle says. “It could be a sort of canary in the coal mine.”
No other rainforests have shown evidence of emitting more carbon than they absorb, for now. The Amazon rainforest has shown an overall decline in carbon capacity, partly due to some of the Amazon becoming a carbon source due to human-caused deforestation and fires. However, at the same time, its trees have responded to increased CO2 in the atmosphere by increasing their own growth.
Australia’s climate has become more extreme due to climate change, along with becoming more prone to severe cyclones and other natural disasters. Tree deaths resulting from cyclones also reduce the rainforest’s capacity to store carbon, the study found. While it has been known that CO2 in the atmosphere has been consistently increasing as humans burn fossil fuels, previous research suggested that tropical rainforests increased their capacity for carbon storage in response. This almost half-century of data challenges this understanding.
“Looking at these long-term empirical data sets, we find that is not the case,” Dr. Raphael Trouve, who researches forest dynamics at the University of Melbourne in Australia and was not involved in the study, tells The Guardian. Datasets like this one are essential in tracking changes to the climate and environment, Trouve noted.
