As Temperatures Increase, Forests are Having More Trouble Soaking up Carbon

On Earth, one of the most important factors regulating our climate is the carbon cycle. This refers to the processes by which carbon compounds are sequestered by biological (photosynthesis) and geological processes and released through volcanic activity and organic processes (decay and respiration). For billions of years, this cycle has kept temperatures relatively stable on Earth and allowed for life to flourish.

For the past few centuries, human activity has tipped the scales to the point that some refer to the current geological epoch as the Anthropocene. According to a new study by an international team of researchers, human activity is also leading to a situation where tropical rainforests (a major sequester of carbon dioxide) are not only losing their ability to soak up carbon but could actually be adding to the problem in the coming years.

The study that describes these findings, “Asynchronous carbon sink saturation in African and Amazonian tropical forests“, recently appeared in the journal Nature. This research effort was led by scientists from the Royal Museum for Central Africa in Tervuren, Belgium, and included research scientists from over 100 universities, forestry and conversation organizations from all around the world.

For the sake of their study, the international team consulted 30 years of data obtained from the study of over 300,000 trees from more than 500 tropical forest patches in South America and Central Africa. This included the UNESCO Heritage Salonga National Park located in the Democratic Republic of Congo, which is Africa’s largest tropical rainforest reserve.

Structurally intact tropical forests are well-known as a crucial global carbon sink that has been helping to slow the process of climate change by removing carbon from the atmosphere. Chief among them is the Amazon Rainforest and Congo Basin Rainforests, which previous climate models predicted would continue to act as carbon sinks for decades.

What’s more, satellite images taken over the past several decades have shown that tropical rainforests have been growing greener because of the growing presence of carbon dioxide in the atmosphere. However, this does not mean that the planet’s rainforests will continue to benefit from increased emissions or keep pace with all the added CO₂ in our atmosphere.

As Wannes Hubau, a researcher at the Royal Museum for Central Africa and the lead author on the study, explained in a Leeds University news release:

“By combining data from Africa and the Amazon we began to understand why these forests are changing, with carbon dioxide levels, temperature, drought, and forest dynamics being key. Extra carbon dioxide boosts tree growth, but every year this effect is being increasingly countered by the negative impacts of higher temperatures and droughts which slow growth and can kill trees.”

To examine the long-term trend, Habua and his colleagues looked at the three decades of tree growth, death, and carbon storage in the tropics. This consisted of measuring the diameter and height of individual trees in all 565 patches of forest and returning every few years to re-measure them. By tracking the carbon stored in trees that survived and those that died, the researchers were able to track changes in carbon sequestration over time.

The team then used a statistical model and records on carbon dioxide emissions, temperature, and rainfall to estimate how carbon storage will change until 2040. They then combined their data with information from two major research networks – the African Tropical Rainforest Observation Network (AfriTRON) and RAINFOR – which conduct observations of rainforests in Africa and Amazonia, respectively.

From all of this, the team concluded that the carbon absorption rates in the African and South American tropical rainforests peaked in the 1990s and early 2000s. During this period, these rainforests sequestered about 46 billion metric tons (51 US tons) of CO₂, which was roughly half of the global terrestrial carbon uptake and 17% of anthropogenic emissions.

During the 2010s, the amount of CO₂ they sequestered annually by the tropics dropped by one-third (on average), which was caused by a 19% drop in the area of intact rainforests and a 33% decrease in the amount of carbon the remaining forests could absorb. This occurred at a time when global carbon dioxide emissions soared by 46%.

By the end of the 2010s, an estimated 25 billion metric tons (27.5 US tons) had been removed, or just 6% of anthropogenic sources. In this decade, according to the team’s analysis, things will only get worse, with tropical rainforests sequestering only a third of what they absorbed during the 1990s – 15.33 metric tons (17 US tons).

Worst of all, by the mid-2030s, tropical forests will release more carbon than they absorb, thus depriving our planet of a key component in the carbon cycle. As Hubau said:

“We show that peak carbon uptake into intact tropical forests occurred in the 1990s… Our modelling of these factors shows a long-term future decline in the African sink and that the Amazonian sink will continue to rapidly weaken, which we predict to become a carbon source in the mid-2030s.”

In this respect, anthropogenic factors (i.e. industrialization, modern transportation, and fossil fuel consumption) are not only causing far more carbon to be produced but are also hurting the planet’s ability to sequester it. Ultimately, the combination of increased temperatures, drought, forest fires, pests, and unnatural deforestation (land clearance and logging) is causing the remaining trees to become overtaxed.

Simon Lewis, a professor of geography from the University of Leeds in the United Kingdom was another co-author of the study. As he explained, these findings make action on climate change all the more pressing:

“Intact tropical forests remain a vital carbon sink but this research reveals that unless policies are put in place to stabilize Earth’s climate it is only a matter of time until they are no longer able to sequester carbon. One big concern for the future of humanity is when carbon-cycle feedbacks really kick in, with nature switching from slowing climate change to accelerating it.

“After years of work deep in the Congo and Amazon rainforests we’ve found that one of the most worrying impacts of climate change has already begun. This is decades ahead of even the most pessimistic climate models. There is no time to lose in terms of tackling climate change.”

This research would not have been possible were it not for the tireless work of researchers from the many universities, forestry services, and conservation organizations in Cameroon, Liberia, Sierra Leone, the Democratic Republic of Congo, Gabon, the Central African Republic, and Indonesia that all contributed to the research.

In this respect, it also highlights the need for greater collaboration with scientists and researchers from Africa and South America and other parts of the world where tropical rainforests are to be found. On top of that, it highlights how these nations and locally-directed efforts are crucial to combatting climate change. As study author Professor Bonaventure Sonké from the University of Yaounde I in Cameroon said:

“The speed and magnitude of change in these forests suggests that climate impacts in the tropics may become more severe than predicted. African countries and the international community will need to seriously invest in preparation for ongoing climate change impacts in tropical regions.”

“For too long the skills and potential of African and Amazonian scientists have been undervalued. We need to change this by ensuring their work is properly supported,” added study co-author Prof. Oliver Phillips of Leeds University. “It will fall to the next generation of African and Amazonian scientists to monitor these remarkable forests to help manage and protect them.”

Climate change is affecting humanity collectively, with every corner of the world feeling the consequences. It therefore demands collective action to address and mitigate it. In the coming decades, significant changes are expected to occur and without drastic action, things are likely to get a lot worse before they can get better.

Further Reading: University of Leeds, EOS/AGU, Nature