Forests close in on tipping point
Some of the world’s biggest forests might start losing their carbonsucking powers in just a few decades, says a study by New Zealand and US scientists.
We rely on plants to suck in a quarter or more of our fossil fuel emissions.
But unless we slow global heating, up to half the world’s forests and grasslands could pass their peak carbon dioxide uptake within the next two to three decades, according to a study in the journal Science Advances by researchers at Northern Arizona University (NAU) and the University of Waikato.
The lead author, Katharyn Duffy, a post-doctoral researcher at NAU, said the effect of global warming was as if Earth had a fever that was affecting plants’ ability to function normally.
If the globe kept heating at current rates, many of the plantfilled landscapes we rely on to reduce the impact of greenhouse emissions could flip from being carbon sinks to carbon sources, the study found.
Among the first areas to pass their peaks would be carbonloaded tropical rainforests in the Amazon and Southeast Asia, and snowy Taiga forests in Russia and Canada.
One senior plant ecologist who wasn’t involved with the study said the findings were ‘‘terrifying’’.
As record-breaking temperatures become routine, plants’ limits are being tested.
‘‘We wanted to ask, how much can plants withstand?’’ said Duffy.
The researchers used two decades’ of real-world records of carbon flux from measurement towers positioned above all major ecosystems in different parts of the globe, including one in Paeroa.
They found photosynthesis peaked at about 18 degrees Celsius in many of the world’s leafy areas, then went into decline.
However, respiration kept increasing, so plants were breathing out greenhouse gas faster, while taking progressively less in. With enough heat, they’d flip from sinks to sources.
Duffy said she noticed sharp declines in photosynthesis above a temperature threshold in nearly every kind of ecosystem across the globe, even after removing other effects such as water and sunlight.
Tipping points varied, but for the most crucial carbon-sucking ecosystems, the important threshold was about 18C, said University of Waikato Professor Louis Schipper, one of the study’s co-authors.
Vic Arcus, a biology professor at Waikato and another co-author of the study, said researchers still needed to study why carbon uptake peaks at 18C for so many ecosystems.
An Auckland University plant ecologist, Associate Professor Cate Macinnis-Ng, said the study was novel because it provided a more complete snap-shot of how temperature will influence carbon storage.
‘‘The results of this elegant study are equally fascinating and
terrifying,’’ she said.
Because plants’ respiration didn’t peak, it ‘‘continues to break down fixed carbon and return carbon to the atmosphere at faster and faster rates as temperatures rise,’’ she said.
‘‘Extreme events like drought and fire will also contribute to losses in global carbon sinks. This means the estimates of declining carbon storage [in the study] are likely to be conservative.
‘‘These potential runaway climate change mechanisms are what keep scientists awake at night.’’
Receiving the results back from Duffy had been personally upsetting, said Schipper. He and Arcus had subsequently moved to electric transport.