Tropical forests are now carbon source, not carbon sinks

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The world’s tropical forests are no longer carbon sinks because of human activity, and these forests now emit more carbon than these are able to absorb from the atmosphere as a result of the dual effects of deforestation and land degradation, finds a new study.

The study tracking 300,000 trees over a period of 30 years finds: The ability of the world’s tropical forests to remove carbon from the atmosphere is decreasing.

The research report – “Asynchronous carbon sink saturation in African and Amazonian tropical forests” – published in research journal Nature on March 4, 2020, (Hubau, W., Lewis, S.L., Phillips, O.L. et al. Asynchronous carbon sink saturation in African and Amazonian tropical forests. Nature 579, 80–87, 2020, challenges the decades-long consensus that tropical forests are a moderate carbon sinks by storing more carbon than they emit due to natural processes and human activity.

The international scientific collaboration, led by the University of Leeds, reveals that a feared switch of the world’s undisturbed tropical forests from a carbon sink to a carbon source has begun.

Tropical forests are capable of storing large amounts of carbon. This is because trees absorb carbon from the atmosphere during photosynthesis and then use it to build new leaves, shoots and roots. But forests can also release carbon into the atmosphere. Some of this release is through natural processes such as plant respiration, droughts and wildfires. Emissions can increase further by human activities, such as deforestation and illegal logging.

The scientists assessed trends in the carbon sink using 244 structurally intact African tropical forests spanning 11 countries, compare them with 321 published plots from Amazonia and investigate the underlying drivers of the trends.

The study report said:

Structurally intact tropical forests sequestered about half of the global terrestrial carbon uptake over the 1990s and early 2000s, removing about 15 per cent of anthropogenic carbon dioxide emissions. Climate-driven vegetation models typically predict that this tropical forest “carbon sink” will continue for decades.

The carbon sink in live aboveground biomass in intact African tropical forests has been stable for the three decades to 2015, at 0.66 tonnes of carbon per hectare per year (95 per cent confidence interval 0.53–0.79), in contrast to the long-term decline in Amazonian forests.

Therefore, the carbon sink responses of Earth’s two largest expanses of tropical forest have diverged. The difference is largely driven by carbon losses from tree mortality, with no detectable multi-decadal trend in Africa and a long-term increase in Amazonia. Both continents show increasing tree growth, consistent with the expected net effect of rising atmospheric carbon dioxide and air temperature. Despite the past stability of the African carbon sink, our most intensively monitored plots suggest a post-2010 increase in carbon losses, delayed compared to Amazonia, indicating asynchronous carbon sink saturation on the two continents. A statistical model including carbon dioxide, temperature, drought and forest dynamics accounts for the observed trends and indicates a long-term future decline in the African sink, whereas the Amazonian sink continues to weaken rapidly. Overall, the uptake of carbon into Earth’s intact tropical forests peaked in the 1990s. Given that the global terrestrial carbon sink is increasing in size, independent observations indicating greater recent carbon uptake into the Northern Hemisphere landmass reinforce our conclusion that the intact tropical forest carbon sink has already peaked. This saturation and ongoing decline of the tropical forest carbon sink has consequences for policies intended to stabilize Earth’s climate.

Intact tropical forests are well-known as a crucial global carbon sink, slowing climate change by removing carbon from the atmosphere and storing it in trees, a process known as carbon sequestration. Climate models typically predict that this tropical forest carbon sink will continue for decades.

However, the new analysis of three decades of tree growth and death from 565 undisturbed tropical forests across Africa and the Amazon has found that the overall uptake of carbon into Earth’s intact tropical forests peaked in the 1990s.

By the 2010s, on average, the ability of a tropical forest to absorb carbon had dropped by one-third. The switch is largely driven by carbon losses from trees dying.

The study by almost 100 institutions provides the first large-scale evidence that carbon uptake by the world’s tropical forests has already started a worrying downward trend.

Study lead author Dr Wannes Hubau, a former post-doctoral researcher at the University of Leeds now based at the Royal Museum for Central Africa in Belgium, said: “We show that peak carbon uptake into intact tropical forests occurred in the 1990s.

“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.

“Our modeling 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 the 1990s, intact tropical forests removed roughly 46 billion tonnes of carbon dioxide from the atmosphere, declining to an estimated 25 billion tonnes in the 2010s.

The lost sink capacity in the 2010s compared to the 1990s is 21 billion tonnes carbon dioxide, equivalent to a decade of fossil fuel emissions from the UK, Germany, France and Canada combined.

Overall, intact tropical forests removed 17% of human-made carbon dioxide emissions in the 1990s, reduced to just 6% in the 2010s.

This decline is because these forests were less able to absorb carbon by 33% and the area of intact forest declined by 19%, while global carbon dioxide emissions soared by 46%.

In the 2000s, intact tropical forest sequestered 36 billion tonnes carbon dioxide, equivalent to 9% of human-made carbon dioxide emissions.

“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,” said Simon Lewis.

“There is no time to lose in terms of tackling climate change,” he said.

The findings represent the collaborative effort of roughly 100 institutions in which researchers tracked some 300,000 trees spanning 565 patches of undisturbed tropical forests across Africa and the Amazon over a 30-year period.

Researchers used measurements of tree growth and death, along with CO2 emissions, rainfall, and temperatures, to estimate carbon storage or “sequestration.”

“We show that peak carbon uptake into intact tropical forests occurred in the 1990s,” said another lead author Wannes Hubau of the Royal Museum for Central Africa in Belgium.

At that time, the forests were able to store 46 billion tonnes of CO2 from the atmosphere, representing about 17% of human-made carbon dioxide emissions.

Fast forward to the 2010s, and the researchers found the amount dropped to an estimated 25 billion tonnes, on par with roughly 6% of human-made carbon dioxide emissions.

Over the 30 years, the area of intact forest shrunk by 19% but global carbon dioxide emissions soared by 46%, the researchers noted.

The downward trend of carbon absorption didn’t happen in the zones at the same time, the study also found. The downward trend of sequestration hit the Amazon in the mid-1990s and the African forests about 15 years later.

The potential for the Amazon forests to switch from carbon sink to carbon source is not far off, with the study predicting it could happen as soon as the mid-2030s.

Hubau, in his statement, stressed need for ongoing monitoring “as our planet’s last great tropical forests are threatened as never before.”

For the moment, at least, humanity should still consider tropical forests carbon sponges. But, if urgent and bold measures aren’t taken soon, that could well change.

“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,” said Lewis, pointing to the possibility of a feedback loop being triggered.

“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,” Lewis said.

“By driving carbon dioxide emissions to net-zero even faster than currently envisaged, it would be possible to avoid intact tropical forests becoming a large source of carbon to the atmosphere. But that window of possibility is closing fast,” said Lewis.

Professor Douglas Sheil at the Norwegian University of Life Sciences, a contributing researcher to the study, put the findings in stark terms.

“Our results are alarming,” he said. “The word ‘alarming’ should not be used lightly,” continued Sheil, “but in this case it fits.”

An earlier study (Baccini et al., 2017, “Tropical forests are a net carbon source based on aboveground measurements of gain and loss”, Science, found that the world’s tropical forests gained 436m tonnes of carbon from 2003 to 2014. This was largely due to the recovery of previously disturbed forests, the scientists conducting the research said.

The tropical forests experienced a carbon loss of 861m tonnes of carbon over the same period.

This means that tropical forests experienced a net carbon loss of 425m tonnes of carbon over the study period – from 2003 to 2014. This figure is considerably higher than previous estimates of carbon loss from tropical forests.

The scientists collected carbon density measurements from forests across the tropics and used these to create a statistical model. The model then used to simulate fine-scale changes in carbon gain and loss in tropical forests over the course of 12 years.

The research findings suggest that curbing deforestation and protecting existing forests could be instrumental in removing greenhouse gases (GHG) from the atmosphere and fighting future climate change, he adds.

Calculating the balance between the uptake and release of carbon allows scientists to determine whether tropical forests are a “carbon sink”, meaning they take in more carbon than they release, or a “carbon source”, meaning their carbon emissions exceed their intake.

A recent rise in human activity in the world’s forested regions could have disrupted the balance between uptake and emissions, says Dr Alessandro Baccini, a researcher at the Woods Hole Research Centre in Massachusetts and lead author of the earlier study published in Science.

He said: “The main discovery is that forests in tropical regions are not a carbon sink, but a carbon source. That means that the amount of carbon emissions from tropical regions are actually bigger than the carbon removal that this region is able to achieve.”

The results find that the largest carbon losses took place in forests in Latin America, while the largest gains were made in Africa.

This pattern is largely driven by local levels of deforestation and land degradation, Baccini explained: “What we see is Latin America, especially Brazil, is a region that is giving off the largest amount of emissions. That is in part because the size of its forests but there is also a lot of disturbance and deforestation for cattle rearing going on. What we find in Africa there is a much smaller amount of disturbance and this is mainly driven by mining and deforestation for palm oil. In Asia, we see a lot of deforestation for palm oil.”

Calculating carbon loss

Traditionally, studies of carbon loss in tropical forests have relied on data taken from satellite images of tree cover.

This approach allows scientists to see the extent of deforestation in tropical regions, but it can overlook more subtle types of human activity, such as illegal logging, forest disturbance and land degradation.

Baccini said: “Degradation is a process where only a small portion of trees are removed from a forest. From a satellite image, the area will still look like an intact forest. But, when you lose even a small proportion trees, you lose a significant amount of carbon.”

Because of this caveat, the researchers opted instead to look for changes in “carbon density” from tropical forests spanning America, Africa and Asia.

Baccini said: “Carbon density is a measure of the weight of carbon that is held by forests. Even in the field we don’t really collect direct measurements of weight but we can collect direct measurements of the trees, such as their diameter and height, and then you use an equation to convert that into biomass.”

The figures found were considerably higher than that of previous estimates of carbon loss from tropical forests. This could be because previous research underestimated the impact of land degradation on carbon loss, said Baccini.

The research found that land degradation and disturbance accounts of 69% of total carbon losses from tropical forests.

Baccini explained: “We discovered that land degradation has a very significant effect on carbon loss. The carbon loss from land degradation is small but, because it happens a lot over a very large area, then it adds up to a lot of loss. We like to think that this is the first study where we can provide an estimate of the losses due to degradation over such a large area as the entire tropics.”

Although the world’s tropical forests are currently considered to be a carbon source, there are a number of steps that could be taken to turn them back into a carbon sink.

On this issue, Baccini said: “There is a possibility that by restoring the forest, by reducing, decreasing or stopping deforestation, by reforestation projects, we could actually make it into a sink. And it would be a pretty big sink.”

Restoring the ability of forests to remove carbon from the atmosphere could help us tackle climate change, he adds.

Baccini said: “Reforestation and afforestation could improve the quality of the planet through the conservation of biodiversity and an improvement in water quality and water resources, and all of this while we are reducing the concentration of CO2 in the atmosphere. It’s a win-win-win possibility.”




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