Global warming can affect the ability of forests to suck carbon from atmosphere, shows study

Forests are considered the most effective and abundantly available carbon sinks, capable of storing and sequestering millions of tonnes of carbon dioxide from the atmosphere. A new study from India joins emerging research that challenges this notion, showing that carbon uptake by forests is more volatile in a warming world than previously thought.

The goal to reach net-zero emissions by mid-century has popularised the idea of using forests as tools to aid climate mitigation, or carbon removal, in order to reduce global emissions. The Indian government bet on forests when it pledged to create a carbon sink capable of sequestering an additional 2.5-3 billion tonnes of carbon through forest and tree cover by 2030.

However, the study by researchers from the Indian Institute of Technology, Bombay , highlights the impacts of global warming on the forest ecosystem, suggesting that it may be affecting and reducing the carbon uptake potential of forests. Global warming could have reduced the carbon uptake potential in forests by around 6% over the last two decades, the research finds.

The findings send a “strong scientific message” that improvements in greening don’t necessarily result in improvements in carbon uptake. “This analysis also has significant implications on the scientific analyses for planning to achieve net zero by 2070, as committed by India,” says the paper, published in Nature in December 2023.

Scientists are still trying to fully understand the effects of global warming on the ability of forests to sequester carbon. “It’s still a very novel area of research, especially in India because we don’t have observational data that captures these changes,” says Subimal Ghosh, convener, Interdisciplinary Program in Climate Studies at the Indian Institute of Technology, Bombay, and one of the authors of the study.

The fate of forests and how they navigate a warming world is driven by multiple feedback loops with complex relationships between heat, moisture and carbon dioxide. The only way to meaningfully study how effective they are is to improve the robustness of India’s greenhouse gas monitoring network, several experts told Mongabay India.

The science of carbon intake

Trees are able to absorb carbon dioxide through photosynthesis, a crucial process for plant survival in which it uses carbon dioxide and water in the presence of sunlight to produce glucose and oxygen. Trees also release some carbon dioxide through respiration. Scientists measure this carbon uptake through gross primary productivity and call it net primary productivity when subtracting respiration from the rate of uptake.

Global warming can affect the way plants and forests function in a number of ways. For one, elevated levels of carbon dioxide in the atmosphere can drive photosynthesis, in a process called carbon fertilisation. But on the other hand, higher temperatures brought on by global warming can also reduce the rate of photosynthesis.

The study from IIT Bombay relied predominantly on Moderate Resolution Imaging Spectroradiometer satellite data and found that even though the Leaf Area Index – a measure of canopy cover – in India’s forests went up by 6.75% between 2001-2019, its carbon uptake reduced by 6.19%, possibly due to the effects of global warming. “At higher temperatures, it is possible that the rate of photosynthesis is reduced. The other explanation is that, if there are drought-like conditions, along with atmospheric aridity, plant productivity reduces, because there’s less water available,” said Ghosh.

The study found that the areas which experienced the greatest warming also saw the steepest declines in carbon uptake over the last two decades. Together, the Western Ghats, northeast India and the East Coast Peninsula are responsible for 32% of gross primary productivity and 34% of net primary productivity. Yet, these regions, which the paper describes as the “most biodiverse and pristine forest regions”, saw the most prominent declines in carbon uptake.

According to Ruth DeFries, a professor of ecology and sustainable development at Columbia University, the results do not bode well for net-zero plans that heavily rely on forests to act as carbon sinks. “Uptake of carbon by vegetation is a critical process in the earth system, but it is a fickle and unreliable partner in the fight against climate change,” she told Mongabay India over email, adding, “The promising ways forward are to reduce greenhouse gas emissions from fossil fuels and other sources which are the primary source of climate change and reduce deforestation and degradation so vegetation can continue to be a carbon sink as much as possible.”

India’s policies are aggressively geared towards afforestation and it is the second largest contributor to global greening since 2000 after China, although bulk of this greening comes from the cultivation of crops. Programmes such as the National Afforestation Programme and Green India Mission are both designed to enhance forest and tree cover, while afforestation is mandatory for project developers who are diverting forest land for their projects. One of the stated objectives is to scale up carbon sequestration.

Many unknowns

Climate change is generally said to limit the carbon uptake potential of forests globally. Governments across the world have thrown their weight behind forests in the fight against climate change in the absence of large-scale carbon removal technologies. A survey of the Long Term Low Emissions Development Strategies from 41 countries found that forest and soil sinks dominated the plans of most countries. Reliance on this “creates risks for both national and global net-zero”, ambitions because of instabilities in carbon sequestration and storage by forests, the survey said.

Emerging studies from Europe found that the droughts and heatwaves of 2022 negatively affected carbon uptake, and shrunk the capacity of forest carbon sinks. “Recurrent drought and heat challenges the net-zero goals of governments relying on forestry, and that forest management needs to be adapted to retain the forest carbon sink,” the paper says.

Forrest Fleischman, an associate professor of environmental policy at the University of Michigan, said the results of the Indian Institute of Technology, Bombay, study “are plausible”, but that in order to be more accurate, it needed more granular data as well as to take into consideration the impact of human activity on forests. “Absent data on how humans are modifying forests in India (data which doesn’t really exist) it is difficult to draw out these comparisons properly,” he said.

Another big limitation of the Indian Institute of Technology, Bombay, study is that it did not include the effects of carbon fertilisation on carbon uptake. In other words, it didn’t account for the possible effect of increased photosynthesis because of more abundantly available carbon dioxide in the atmosphere. “There are many uncertainties, and what it boils down to ultimately is that we need more and better observational data to support these projections,” said Ghosh.

The study also does not project what carbon uptake will look like in the future, when global warming rises and the effects of climate change grow more pronounced.

Scientists at the Indian Institute of Tropical Meteorology attempted to do just this, projecting carbon uptake potential in India’s forests till the year 2100, using multiple data sets and models. In the study, published in October last year, they find that even in the “most aggressive climate change projection,” gross primary productivity rose till the end of the century, with the Eastern Himalayan region absorbing the most.

According to Pramit Kumar Deb Burman, a scientist with Indian Institute of Tropical Meteorology and a co-author of the paper, improved carbon uptake in a climate altered future could be due to three factors – carbon fertilisation, early blooming due to warm temperatures and because shaded leaves at the bottom receive more scattered solar radiation, becoming more productive. “We are trying to do further studies to see which of these is the primary driver in India,” he said, adding, “The changes in the Indian monsoon season can also complicate the carbon uptake process, because the health of so many ecosystems depends on the monsoon.”

India lacks a robust network to measure fluxes in greenhouse gas emissions, which could support satellite data and modelled projections with actual, observed trends. Both Deb Burman and Ghosh said more concerted efforts were needed by both the government and the scientific community to improve observed data sets.

“Forests and our biodiversity is the most valuable system to absorb carbon, and we need more data not only to protect it, but so we know how vulnerable they are to climate change and the extent to which we can expect them to sequester and store carbon,” said Ghosh.