Scientists have documented a worrying trend in the rainforests of Australia: Tree lifespans have halved in the last 35 years, and it appears to be due to the effects of climate change on the ecosystems.
With these forests acting as significant carbon sinks, the consequences for the planet could be devastating, creating a feedback loop that’s both caused by global warming and which then contributes to it.
The signs of the increased death rate go back to the 1980s, suggesting that Earth’s natural systems have responded to shifts in temperature and atmosphere for longer than we might have realized.
“It was a shock to detect such a marked increase in tree mortality, let alone a trend consistent across the diversity of species and sites we studied,” says ecologist and lead author David Bauman from the University of Oxford in the UK.
“A sustained doubling of mortality risk would imply the carbon stored in trees returns twice as fast to the atmosphere.”
Researchers collected more than 70,000 data points from existing records to put together the study, with 24 different forest plots included. The earliest information goes back to 1971, enabling the team to track tree deaths over an extended period.
Atmospheric water stress driven by global warming is to blame for the increase in tropical tree mortality, the researchers think: The warmer air dries out trees more quickly.
The study authors compared the stress that rainforests have experienced to what’s been happening to the Great Barrier Reef, another delicately balanced ecosystem that is struggling with higher temperatures.
“The likely driving factor we identify, the increasing drying power of the atmosphere caused by global warming, suggests similar increases in tree death rates may be occurring across the world’s tropical forests,” says ecologist Yadvinder Malhi from the University of Oxford.
“If that is the case, tropical forests may soon become carbon sources, and the challenge of limiting global warming well below 2°C becomes both more urgent and more difficult.”
Other research suggests that a similar increased rate of tree death is happening in the Amazon rainforests, too, reducing the amount of carbon that the region is able to pull out of the atmosphere and store. The worry is that these forests will start contributing carbon to the atmosphere rather than taking it out.
The new study is particularly valuable because it uses a large pool of data gathered over many years – enabling scientists to cut through the noise of such busy and active ecosystems to spot these long-term trends.
As difficult as it is to put together research projects that last decades, more studies across a similar sort of timescale are urgently required to better understand the strain that the natural world is under.
“Long-term datasets like this one are very rare and very important for studying forest changes in response to climate change,” says ecologist Susan Laurance from James Cook University in Australia.
“This is because rainforest trees can have such long lives and also that tree death is not always immediate.”
The research has been published in Nature.