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The rainforests in South America are close to becoming sources of carbon.


A dry tree stands out amid green trees in an Amazon forest.

2) out of the atmosphere every year.

The rainforests in South America are among the biggest carbon sinks globally, absorbing billions of metric tons of carbon dioxide (CO2) from the atmosphere annually.2

Every year, rainforests absorb a significant amount of carbon from the atmosphere. However, as the Earth’s temperature has increased, this ability has decreased. According to a recent study in Nature Climate Change, during the 2015-2016 El Niño, when certain areas of South America faced extreme heat and drought, the amount of carbon absorbed and emitted by these rainforests was almost equal.

During an El Niño event, there is a weakening of easterly winds and a rise in sea surface temperatures in the eastern Pacific Ocean. This results in a global increase in temperatures for several months, with disruptions to weather patterns in South America due to its proximity to the Pacific. This event provides scientists with the chance to evaluate the effects of heat and drought on forest ecosystems, both in the short and long term.

Over 100 researchers from various institutions conducted a study where they monitored thousands of trees in 123 forest plots across South America for a period of over 30 years. The plots, which were part of the Amazon Forest Inventory Network and the Brazilian Program for Biodiversity Research Information System, consisted of undisturbed areas of vegetation in the Amazon, the Atlantic Forest, and the Cerrado, a tropical savanna located in the Brazilian highlands.

The forests that have been helping us fight climate change may now become major sources of carbon, posing a serious threat to the global climate.

The scientists calculated the quantity of organic material and carbon stored in trees by monitoring changes in their trunk sizes over time. As trees mature, they absorb carbon from the atmosphere via photosynthesis and convert it into biomass, which comprises their trunks, roots, and foliage. However, trees also release carbon through respiration during their lifespan and through decomposition after they pass away.

The information revealed that the El Niño phenomenon resulted in elevated temperatures and dry conditions, which resulted in a greater number of trees dying and slower growth. As a result, the amount of carbon released equaled the amount of carbon absorbed. Prior to the 2015-2016 El Niño, each hectare of forest analyzed stored approximately one-third of a metric ton of carbon annually. However, during the event, this equilibrium was nearly non-existent.

2 emissions do not decrease, the Amazon rainforest could become a source of carbon rather than a sink.

Research has indicated that the Amazon rainforests, primarily in South America, have experienced a decrease in carbon absorption since the 1990s. Recent studies have also revealed that certain areas of the Amazon are now emitting more carbon than they take in. Scientists warn that if CO2 emissions continue to rise, the Amazon rainforest may switch from a carbon sink to a carbon source.2

If emissions continue at the present pace, the capacity to store carbon may come to an end prior to 2040.

Thaiane Rodrigues de Sousa, an ecologist at the Instituto Nacional de Pesquisas da Amazônia in Brazil, warns that the forests we have relied on to combat climate change may become major contributors to carbon emissions, resulting in a significant global impact on the climate.

Bigger trees are dying due to lack of water.

The latest research shed light on how varying rainforests react to increased temperatures. Certain experts theorized that hotter and dryer forests may be more capable of adapting to potential drought and extreme heat, as they have evolved in these conditions over many years. In reality, previous studies have demonstrated that African rainforests continue to act as effective carbon storage during El Niño occurrences.

However, recent studies have shown that the trend in South America has been the opposite. In areas with less rainfall, such as the transition zones in the Atlantic Forest and Cerrado, and certain parts of the southern Brazilian Amazon and northern Colombia, trees have been growing less and experiencing higher mortality rates. This risk of mortality is even higher for larger trees with lower wood density.

Sousa stated that larger trees are at a higher risk during extreme weather conditions as they require more energy to transport water from the soil to their leaves. When temperatures are elevated, these trees also experience a higher rate of evapotranspiration. This excessive transpiration without enough water absorption can result in dehydration for the plants.

Sousa commented that it’s as if they perish from lack of water.

Safeguarding the Remaining Woodlands

The recent research shed light on the varying reactions of rainforests to rising temperatures. Certain experts believed that forests in hotter and drier regions may be able to adjust more easily to upcoming drought and intense heat, as they have adapted to these conditions over many years. In fact, previous studies have demonstrated that African rainforests continued to absorb carbon during El Niño occurrences.

The researchers only observed undisturbed forest areas, thus they did not take into account the emissions of carbon dioxide caused by deforestation and wildfires, which are common occurrences in South American forests.

Deforested areas have reduced rainfall, causing the forests to experience higher temperatures, dryness, increased strain, and greater susceptibility to death.

According to climate scientist Luciana Gatti from the Instituto Nacional de Pesquisas Espaciais, who was not part of the research team, the findings serve as a reminder of the crucial need to preserve the few remaining untouched forests. Gatti has been conducting airborne measurements of carbon levels over the Amazon for over ten years. Her studies have demonstrated that intact forests absorb more carbon than those in areas affected by wildfires and near deforested areas. She believes this is due to the increased stress levels of trees in these impacted regions.

Gatti explained that deforested areas have reduced rainfall and as a result, the forests become warmer, drier, and more susceptible to stress and mortality.

Sousa concurred and advocated for increased funding in research and governmental measures to protect the forests. As a next measure, her research team will assess the impact of underwater reservoirs on a forest’s ability to withstand drought. This type of research can aid in identifying forests that are more resilient to climate change and potentially crucial for preservation. “Now more than ever, it is crucial that we prioritize the protection of these forests.”

I am Sofia Moutinho, a Science Writer from Brazil.

Reference: Moutinho, S. (2023). South American rainforests are close to becoming carbon-emitting areas. Eos, 104, https://doi.org/10.1029/2023EO230393. Published on October 17, 2023.

Text © 2023. The authors. CC BY-NC-ND 3.0

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