Global deforestation leads to more mercury pollution, study finds

About 10% of annual human-caused mercury emissions into the atmosphere are the result of global deforestation, according to a new MIT study.

The world’s vegetation, from the Amazon rainforest to the savannahs of sub-Saharan Africa, acts as a sink that removes toxic pollutants from the air. However, if the current rate of deforestation remains unchanged or accelerates, researchers estimate that net mercury emissions will continue to increase.

“We have overlooked an important source of mercury, especially in tropical regions,” says Ari Feinberg, former postdoctoral fellow at the Institute for Data, Systems and Society (IDSS) and lead author of the study.

The researchers’ model shows that the Amazon rainforest plays a particularly important role as a mercury sink, contributing about 30% of the global terrestrial sink. Curbing deforestation in the Amazon could thus have a substantial impact on reducing mercury pollution.

The team also estimates that global reforestation efforts could increase annual mercury uptake by about 5%. While this is important, the researchers emphasize that reforestation alone should not replace global pollution control efforts.

“Countries have made a lot of effort to reduce mercury emissions, particularly industrialized countries in the North, and for very good reasons. But 10% of the global anthropogenic source is substantial, and it is possible that this figure will be even greater in the future. (Cutting these deforestation-related emissions) needs to be part of the solution,” says lead author Noelle Selin, a professor at IDSS and MIT’s Department of Earth, Atmospheric, and Planetary Sciences .

Feinberg and Selin are joined in the paper by co-authors Martin Jiskra, former Swiss National Science Foundation Ambizione Fellow at the University of Basel; Pasquale Borrelli, professor at Roma Tre University in Italy; and Jagannath Biswakarma, postdoctoral fellow at the Swiss Federal Institute of Aquatic Science and Technology. The document appears in Environmental science and technology.

Mercury modeling

In recent decades, scientists have generally focused on studying deforestation as a source of global carbon dioxide emissions. Mercury, a trace element, has not received the same attention, in part because the role of the Earth’s biosphere in the global mercury cycle has only recently been better quantified.

Plant leaves absorb mercury from the atmosphere, the same way they absorb carbon dioxide. But unlike carbon dioxide, mercury does not play an essential biological function for plants. Most of the mercury remains in a leaf until it falls to the forest floor, where it is absorbed into the soil.

Mercury becomes a major concern for humans if it ends up in bodies of water, where it can be methylated by microorganisms. Methylmercury, a potent neurotoxin, can be absorbed by fish and bioaccumulated throughout the food chain. This can lead to dangerous levels of methylmercury in fish consumed by humans.

“In soils, mercury is much more tightly bound than it would be if it were deposited in the ocean. Forests provide a kind of ecosystem service, in that they sequester mercury for longer periods of time ” says Feinberg, who is now a postdoc at the Blas Cabrera Institute of Physical Chemistry in Spain.

Forests thus reduce the amount of toxic methylmercury in the oceans.

Many studies of mercury focus on industrial sources, such as the burning of fossil fuels, small-scale gold mining, and metal smelting. A global treaty, the 2013 Minamata Convention, calls on nations to reduce human-caused emissions. However, it does not directly take into account the impacts of deforestation.

The researchers launched their study to fill this missing piece.

In previous work, they built a model to study the role vegetation plays in mercury absorption. Using a series of land-use change scenarios, they adjusted the model to quantify the role of deforestation.

Emissions assessment

This chemical transport model tracks mercury from its emission sources to where it is chemically transformed in the atmosphere, and then finally to where it is deposited, primarily through precipitation or of its absorption in forest ecosystems.

They divided the Earth into eight regions and ran simulations to calculate deforestation emission factors for each, taking into account things like vegetation type and density, soil mercury content, and historical land use.

However, it has been difficult to obtain good data for some regions.

They lacked measurements from tropical Africa or Southeast Asia, two regions facing high deforestation. To fill this gap, they used simpler offline models to simulate hundreds of scenarios, which helped them improve their estimates of potential uncertainties.

They also developed a new formulation for mercury emissions from soil. This wording reflects the fact that deforestation reduces leaf surface area, which increases the amount of sunlight that hits the ground and accelerates the outgassing of mercury from soils.

The model divides the world into grid squares, each measuring a few hundred square kilometers. By changing land surface and vegetation parameters in certain squares to represent deforestation and reforestation scenarios, researchers can capture impacts on the mercury cycle.

Overall, they found that about 200 tons of mercury are released into the atmosphere as a result of deforestation, or about 10% of total human-caused emissions. But in tropical and subtropical countries, emissions from deforestation represent a higher percentage of total emissions. For example, in Brazil, emissions from deforestation represent 40% of total human-caused emissions.

Additionally, people often light fires to prepare tropical forest areas for agricultural activities, which causes more emissions by releasing mercury stored by vegetation.

“If deforestation were a country, it would be the second highest emitting country, after China, which emits about 500 tons of mercury per year,” adds Feinberg.

And since the Minamata Convention now addresses primary mercury emissions, scientists can expect deforestation to account for a larger share of human-caused emissions in the future.

“Policies aimed at protecting forests or cutting them down have unintended effects beyond their intended purpose. It is important to consider that these are systems and that they involve human activities, and we must understand them better in order to actually solve the problems that we know there are,” Selin says.

By providing this first estimate, the team hopes to inspire more research in this area.

In the future, they want to incorporate more dynamic Earth system models into their analysis, which would allow them to interactively track mercury uptake and better model the time scale of vegetation regrowth.

“This paper represents an important advance in our understanding of the global mercury cycle by quantifying a pathway that has long been suggested but not yet quantified. Much of our research to date has focused on primary anthropogenic emissions, those resulting directly from of human activity through the burning of coal or amalgams of mercury and gold burning in artisanal and small-scale gold mines,” says Jackie Gerson, assistant professor in the Department of Earth and Natural Sciences. environment at Michigan State University, which was not involved in this research.

“This research shows that deforestation can also lead to significant mercury emissions and needs to be considered both in terms of global mercury models and land management policies. It therefore has the potential to advance our field scientifically as well as promoting policies that reduce mercury emissions via deforestation.”

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news in MIT research, innovation and education.