The tropical rainforests are the “green lungs” of our planet – and important buffers in the climate system. But now a long-term study reveals that the tropical forests in Africa and the Amazon region are gradually losing their buffering effect. Accordingly, the uptake of carbon dioxide by intact tropical forest areas has decreased by around a third from 1990 to the 2010s. The main reason for this is the increased temperatures and tree losses due to droughts, as the researchers report. If this trend continues, the Amazon rainforest could change from a CO2 sink to a CO2 spinner in around 15 years.
Forests are one of the sinks in the Earth’s climate system – to build up their biomass, the trees absorb large amounts of CO2 from the air and bind the carbon in the plant material. The trees thus make a decisive contribution to regulating the concentrations of greenhouse gases in the atmosphere. It is estimated that around 250 billion tons of carbon are stored in the trees of the tropical rainforest – this corresponds to around 90 years of global anthropogenic greenhouse gas emissions and around half of the total carbon stored in biomass. Accordingly, they are of great importance for the stabilization of the global climate. But global warming does not leave the rainforests untouched. “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 has decreased since the 1990s
In a long-term study, Hubau and his colleagues from more than 100 research institutions worldwide have investigated what this means for the function of tropical rainforests as sinks. For this purpose, the researchers have been measuring and examining trees in 565 intact rainforest areas on the Amazon and in Africa since 1968. In the test areas, they recorded all trees with a trunk circumference of more than ten centimeters and used a model to determine the amount of carbon converted into biomass from their growth and size. They were able to determine how much CO2 these trees had absorbed since the last measurement.
The evaluations showed that in 1990 the tropical rainforests in South America and Africa absorbed around 46 billion tons of CO2 from the air – this corresponds to around 17 percent of the anthropogenic greenhouse gas emissions at that time. However, just 20 years later, the CO2 uptake of these forests had dropped to just 25 billion tons, as the researchers report. In 2010, the rainforests only compensated for six percent of anthropogenic CO2 emissions. The tropical forests have already lost part of their buffering effect in the climate system. The main cause of this development is primarily the damage and loss of trees due to drought and heat. As a result, the CO2 binding of the individual forest areas has been reduced by 33 percent. In addition, the area of ​​the intact forest areas decreased by a total of 19 percent, as Hubau and his colleagues report.
Heat and droughts as the main cause
“The pace and scale of change in these forests suggests that the effects of climate change in the tropics are more severe than expected,” said co-author Bonaventure Sonké of Yaounde University in Cameroon. The researchers found that the negative effects of heat and drought outweigh the fertilizing effect of increasing CO2. The connection with climate change was particularly clear when the scientists compared the development in the Amazon region and in Africa: While the CO2 uptake in the Amazon rainforest already began to decrease in the 1990s, this trend was only apparent in the tropical forests of Africa for 15 years later. Hubau and his colleagues attribute this to the fact that the African forests are higher on average and therefore less exposed to the heat. Droughts also occur less frequently in these forest areas than in the Amazon region.
“By combining the 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 order to find out how the green CO2 sinks could develop in the near future, he and his colleagues extrapolated 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 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. In fact, scientists have long feared that global warming could at some point cause the tropical rainforests to reach a tipping point at which they irreversibly lose their buffering effect in the climate system. Based on their results, Hubau and his colleagues once again underline the importance of fast and effective climate protection measures – also and especially for the green lungs of our planet.
Source: Wannes Hubau (Royal Museum for Central Africa, Tervuren) et al., Nature, doi: 10.1038 / s41586-020-2035-0