Forests losing ability to absorb carbon dioxide

Tropical forests such as the Amazon rainforest and the Western Ghats have traditionally been regarded as bulwarks against climate change, they might not be as effective now.

A view of the Western Ghats in Munnar. iStock

A forest fire in the Amazon  

Tropical forests such as the Amazon rainforest and the Western Ghats have traditionally been regarded as bulwarks against climate change because of their ability to soak up massive concentrations of carbon dioxide. Now, an international study shows that vegetation is absorbing less carbon than thought.

Plants and trees are generally able to pull large amounts of carbon dioxide (CO2) from the atmosphere during photosynthesis and store much of the carbon in wood. Deep forest areas are regarded as “carbon sinks” for their ability to absorb massive quantities of carbon. At the same time, plants can also contribute to greenhouse gas emissions through dead plant decay and forest fires.

A team of researchers, helmed by the NASA Jet Propulsion Laboratory (JPL), set out to discover whether a forest-like ecosystem is soaking up more carbon than they emit, amid ongoing challenges of carbon emissions and climate change.

Using NASA’s Geoscience Laser Altimeter System (GLAS) onboard ICESat and the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra and Aqua satellites, the researchers created maps of carbon sources and sinks from land-use changes spanning nearly two decades from 2000 to 2019. They discovered that the total amount of carbon emitted and absorbed in the tropics was four times larger than in temperate regions and boreal areas.

Causes

But then came the real bombshell-- the ability of tropical forests to absorb their expected levels of carbon has waned in recent years. The factors, according to the study which has been published in Science Advances, are myriad: large-scale deforestation, habitat degradation and the impacts of warming: frequent droughts and fires over the past two decades releasing carbon dioxide to the atmosphere.

Equally alarming is the discovery that up to 90% of all carbon being absorbed by forests is negated by the amount of carbon being released by major terrain and climate events such as deforestation and drought.

Sassan Saatchi, the principal investigator at JPL, said in a press note that while “the Amazon was considered a substantial carbon sink because of large tracts of pristine forest that soak up carbon dioxide... our results [now] show that overall, the Amazon Basin is becoming almost neutral in terms of carbon balance.”

In the Indian context, a 2019 paper by scientists at the Indian Institute of Science (IISc), also showed that the Western Ghats was waning in its ability to absorb CO2. The work, by Professor T V Ramachandra and Dr Setturu Bharath (a postdoctoral fellow), established that “anthropogenic activities such as burning fossil fuel, power generation, agriculture, industry, polluting water bodies and urban activities are responsible for increasing anthropogenic greenhouse gas footprint — out of which 72% comprises carbon dioxide.”

Change in land use

The IISc researchers traced the change using temporal remote sensing data which showed the decline of the evergreen forest by 5% with an increase in agriculture, plantations and built-up area. When the researchers simulated further likely changes in the region, they found that the Ghats would have only 10% evergreen cover, with 17% of the land taken up for agriculture, 40% for plantations and 5% consumed by built-up areas.

Quantification of carbon revealed that the Western Ghats forest ecosystem holds 1.23 MGg (million gigagrams) in vegetation and soil. But with the rate of carbon increasing annually at 37,507.3 gigagrams (Gg), a simulation of likely changes indicated the loss of 0.23 MGg in carbon sequestration potential “under business as usual scenario,” the researchers wrote.

“The results indicate that the future trends of deforestation and associated carbon stock loss would induce higher instances of flooding and drought due to changes in the climate,” the IISc researchers added.

The JPL findings help inform scientists which areas are serving as “sinks” and which are emitting. This also helps give them an idea about how forests and other vegetated regions respond to a changing climate.

However, one limitation of the study was the satellite-based carbon maps in this study covered about 100 square kilometres at a time. The study could not track changes happening on smaller scales.

The researchers expressed confidence that some of these information gaps could be remedied by higher-resolution carbon maps provided by newer satellites already in orbit, as well as upcoming missions like the NASA-Indian Space Research Organisation’s NISAR observing mission.