Tropical forests' capacity to trap CO2 and slow climate change is decreasing

Tropical forests will soon change from slowing climate change to potentially accelerating it, according to a high profile study from an international team of researchers including an academic from Manchester Metropolitan.

Study co-author Dr Martin Sullivan, Lecturer in Statistical Ecology, was part of the team that tracked 300,000 trees in more than 500 forest patches in Africa and the Amazon for over 30 years.

Intact tropical forests are well-known for being 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, a new study - published in and featured on the cover of Nature, the leading international weekly journal of science - highlights that the overall uptake of carbon into Earth’s intact tropical forests peaked in the 1990s.

From sink to source

Dr Sullivan said: “The carbon dioxide we emit into the atmosphere boosts tree growth, meaning that tropical forests act as a carbon sink.

“But our results indicate that the tropical forest carbon sink is reaching saturation point.

“Although climate models predict that carbon uptake by the world’s tropical forests will eventually decline, our results indicate that this is happening sooner than even the most pessimistic of model predictions.”

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.

While this represented 17% of human-made carbon dioxide emissions in the 1990s, it was 6% 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.

These long-term trends are matched by worrying predictions for the future.

Dr Sullivan said: “We used statistical models to predict how the size of the carbon sink will change in the future.

“These indicated that the sink is likely to stop in the mid-2030s, and tropical forests may even become a carbon source due to the negative effects in increasing temperatures and more frequent droughts.”

Differences between continents

By combining data from two large research networks of forest observations across Africa (AfriTRON) and Amazonia (RAINFOR), the authors show that the Amazon sink began weakening first, starting in the mid-1990s, followed by a waning of the African sink about 15 years later.

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: “The continental difference arises from a combination of Amazon forests being more dynamic than those in Africa, and Amazon forests facing stronger climate impacts.

“Typical Amazonian forests are exposed to higher temperatures, faster temperature increases and more regular and severe droughts than African forests.”

Forests are still important carbon stores

Despite the decline in the ability of tropical forests to take in more carbon, the authors stress that tropical forests are still huge reservoirs of carbon, storing 250 billion tonnes of carbon in their trees alone.

This storage is equivalent to 90 years of global fossil fuel emissions at today’s level.

Senior author Professor Simon Lewis, from the University of Leeds, said: “The immediate threats to tropical forests are deforestation, logging and fires. These require urgent action to protect forests and the carbon they store.

“In addition, stabilising Earth’s climate is necessary to stabilise the carbon balance of intact tropical forests.

“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.”

The study was a collaboration between a global network of scientists from 94 institutions, including 34 institutions in Africa.