Forests are needed to absorb carbon, but the overheating planet might soon flip a critical switch
BANGKOK: New research shows that the fast warming of the planet could cause trees and plants to release more carbon than they absorb, a critical function reversal that would further accelerate climate change.
By as early as 2040, given the current projections of global temperature rise, land systems will only be able to take in half the amount of carbon compared to what they do today, according to the research, published in Science Advances this month.
The research analysed data from about 1,500 sites around the world to try and understand how temperature intersected with the absorption of carbon. What it uncovered was a “powerful tipping point”.
As temperatures continue to rise, the ability of trees and plants to perform photosynthesis - a function that utilises carbon dioxide in the air - is compromised. At the same time, in a warmer environment, the respiration of plants also increases due to heat stress, adding more carbon to the atmosphere.
If climate change continues unabated, no longer will the world’s rainforests, soil and grasslands be performing their important role as carbon sinks. Instead of slowing down climate change, they will be accelerating it.
“As a metaphor, you can think of the human body experiencing either hypothermia or heat stroke, it's the same for all living things: our function begins to shut down when we move outside of specific temperature ranges,” said co-author Katharyn Duffy, an earth-systems scientist at Northern Arizona University.
Land sinks currently help by absorbing about 30 per cent of anthropogenic emissions. For the globe’s forests alone, the amount of carbon being absorbed is significant - about 7.6 gigatonnes every year, according to another study published in Nature this month. That is the equivalent of China’s entire coal industry, or all of the world’s annual carbon emissions from gas.
Right now, only about 10 per cent of the biosphere experiences temperatures so hot that photosynthesis is degraded, according to the research published in Science Advances. But by 2100, about half of the biosphere could experience these conditions, including in parts of Southeast Asia.
Based on the examination of data from the largest continuous carbon monitoring network, FLUXNET, the researchers found that photosynthesis rates degraded significantly above 28 degrees Celsius in tropical forests. In temperate areas, it dropped at above 18 degrees Celsius.
“We realised how very close we currently are to the temperature maximum for global photosynthesis. Essentially we did not go looking for a tipping point, it emerged from the data,” Duffy said.
Scientists not involved in the research but observing the carbon tipping point phenomenon say it is clear that plants and trees are not able to adapt to the rising temperatures.
“I’m not surprised that, as the world is quickly warming up at an unprecedented rate, all physiological processes of plants and soil microbes are responding in ways that these organisms have not seen in their evolutionary history,” said Associate Professor Amos Tai from the Earth System Science Programme at The Chinese University of Hong Kong.
“I believe ultimately human activities are modifying climate at such an unprecedented pace that no organisms or any ecosystem as a whole can cope.”
Ancient rainforests across the island of Borneo were identified as areas that will be among the first to hit a heat threshold where the maximum temperature for photosynthesis will be exceeded for more than half of the year.
Already, these vast carbon-rich landscapes have grown smaller and are less able to perform their natural ecosystem functions, due to human activities. An island that once had about 75 per cent forest cover a century ago, has seen half of it disappear.
“Deforestation or removal of rainforests has certainly reduced the landscape capacity to absorb atmospheric carbon dioxide,” said Professor Daniel Murdiyarso, principal scientist at the Center for International Forestry Research (CIFOR), based in Bogor, Indonesia.
“Combined with the effect of climate change, it has caused the ecosystem to be more vulnerable to extensive droughts and wildfires. Rainforests are very fragile. Large-scale disturbance like deforestation or fires will not give them the chance to renew themselves” he said.
Across the globe in 2020, devastating wildfires raged across Australia, the Amazon, Siberia and the United States. On an annual basis, Indonesia also experiences heavily carbon-polluting peatland fires; last year was no exception as more than 1 million hectares burnt.
“Those wildfires have not only weakened forest carbon uptake, but also returned much of the stored carbon in the forests to the atmosphere in a short amount of time, essentially annihilating much of their precious carbon uptake,” Tai said, referring to the process of capturing and storing atmospheric carbon dioxide.
From a base in Malaysia’s Sabah state, scientists at the South East Asia Rainforest Research Partnership (SEARRP) have been observing forest changes in real time. Land clearing and the major expansion of palm oil plantations have had major impacts on raising local temperatures and influencing weather patterns.
“When you reduce forest cover through logging or shifts to agriculture, you reduce the capacity to sequester carbon. This also creates negative feedback with climate change, as reductions in forest cover in tropical forests are linked to reduced rainfall and increased temperatures,” said Dr Michael O’Brien, SEARRP’s Assistant Director of Science Coordination.
“We are seeing more days with no rain, more frequent long periods with no rain and more extreme rainfall events.
“In addition, severe drought years that are associated with El Niño Southern Oscillations are becoming more common. These years can cause increased tree mortality in many tropical forests,” he said.
The study’s findings also likely apply to all plants, meaning that there are potential implications for food production in many parts of the world. It found no evidence of plants adapting to their harsh new conditions.