Questionable trend: The green lungs of our planet are starting to weaken. The large tropical forests of Africa and South America now absorb significantly less carbon dioxide than 30 years ago. As a result, they have already lost a third of their buffering effect in the climate system, as researchers report in the journal “Nature”. As early as 2035, the Amazon rainforest could even fail completely as a CO2 sink.
The tropical rainforests are the “green lungs” of our planet – and important buffers in the climate system. Because the trees absorb CO2 from the air and store it as carbon in their tissues. This makes the tropical forests one of the largest sinks for greenhouse gases. However, there is evidence that these sinks less stable could be as thought. Researchers also fear that the tropical forests will continue to have a buffering effect due to progressive climate change could losen.
“Although additional CO2 promotes plant growth, this positive effect is increasingly being negated by the negative effects of rising temperatures and drought,” explains first author Wannes Hubau of the Royal Belgian Central Africa Museum.
CO2 absorption decreases
Hubau and his colleagues have examined in a long-term study what this means for the function of the tropical rainforests as sinks. Since 1968, they have been studying trees in 565 intact rainforest areas on the Amazon and in Africa. From their measurement and a model, the researchers then concluded that the carbon converted into biomass and thus the CO2 uptake by the trees.
The result: The tropical forests in South America and Africa have already lost part of their buffering effect. According to the measurement data, the forests still absorbed around 46 billion tons of CO2 from the air in 1990 – this corresponds to around 17 percent of the anthropogenic greenhouse gas emissions at that time. However, 20 years later, the CO2 uptake of these forests had dropped to just 25 billion tons, as the researchers report.
Affected by heat and droughts
Hubau and his colleagues see the main cause of this development as damage and loss of trees due to drought and heat. As a result, the CO2 binding in the individual forest areas has been reduced by 33 percent. They conclude that the negative effects of heat and drought outweigh the fertilizing effect of increasing CO2. In addition, the total area of intact forest areas decreased by 19 percent.
“The speed and scale of change in these forests suggests that the effects of climate change in the tropics are more serious than expected,” said co-author Bonaventure Sonké from Yaounde University in Cameroon. This confirms the sensitivity of these CO2 sinks to global warming.
Amazon particularly affected
The CO2 absorption in the Amazon region has decreased particularly rapidly and clearly: there the buffer effect of the trees has been decreasing since the 1990s. In the rainforests of tropical Africa, however, this effect only started around 15 years later, as the researchers report. They attribute this to the fact that the Amazon rainforest has greater heat and dryness exposed than the mostly higher lying forests of Africa.
“By combining data from Africa and the Amazon, we are beginning to understand why these forests are changing and that CO2 concentrations, temperature, drought and forest dynamics play a key role in this,” says Hubau. In addition to the pure Climate effects the Amazon rainforest has been intensified by in recent years Clearing and Forest fires affected. Both of these factors should have further reduced its buffering effect.
Loss of the sink function already from 2035?
But what does this mean for the future? Hubau and his colleagues have extrapolated this from the observed trends using a model. According to this, the CO2 absorption of the African rainforests will continue to decrease until 2040 and decrease by 14 percent compared to the values from 2010 to 2015.
The future development in the Amazon region could look even more drastic: “The Amazon valley will continue to weaken rapidly and could sink to zero in 2035,” the researchers report. From this point on, the Amazon rainforest would become a potential source of CO2 from a CO2 sink.
“It is imperative to stabilize the Earth’s climate in order to keep the carbon balance of intact tropical forests stable,” said co-author Simon Lewis from the University of Leeds. “If we do not take any measures to do this, it is only a matter of time before these forests no longer store carbon.” Quick and effective climate protection measures are urgently needed – also and especially for the green lungs of our planet. (Nature, 2020; doi: 10.1038 / s41586-020-2035-0)
Source: University of Leeds, Nature
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