March 1, 2024

Global deforestation leads to more mercury pollution, study finds

This article has been reviewed in accordance with Science X’s editorial process and policies. The editors have highlighted the following attributes, ensuring the credibility of the content:

checked

trusted source

review


Graphic summary. Credit: Environmental Science and Technology (2024). DOI: 10.1021/acs.est.3c07851

× to close


Graphic summary. Credit: Environmental Science and Technology (2024). DOI: 10.1021/acs.est.3c07851

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

The world’s vegetation, from the Amazon rainforest to the savannas 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 a significant source of mercury, especially in tropical regions,” says Ari Feinberg, former postdoc 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 land sink. Reducing deforestation in the Amazon could therefore have a substantial impact on reducing mercury pollution.

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

“Countries have made great efforts to reduce mercury emissions, especially northern industrialized countries, and for good reason. But 10% of the global anthropogenic source is substantial and there is potential for this to be even greater in the future. [Addressing these deforestation-related emissions] needs to be part of the solution,” says senior author Noelle Selin, professor at IDSS and MIT’s Department of Earth, Atmospheric, and Planetary Sciences.

Feinberg and Selin are joined on 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 paper 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 Earth’s biosphere in the global mercury cycle has only recently been better quantified.

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

Mercury becomes a serious 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 bioaccumulate through the food chain. This can lead to risky levels of methylmercury in the fish that humans eat.

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

In this way, forests reduce the amount of toxic methylmercury in the oceans.

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

The researchers launched their study to fill in the missing piece.

In previous work, they built a model to investigate 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.

Assessing emissions

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

They divided the Earth into eight regions and carried out simulations to calculate deforestation emission factors for each one, considering elements such as vegetation type and density, mercury content in soils and land use history.

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

They lacked measurements from tropical Africa or Southeast Asia – two areas experiencing heavy deforestation. To overcome this shortcoming, 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 formulation captures the fact that deforestation reduces leaf area, which increases the amount of sunlight reaching the soil and accelerates the release of mercury from soils.

The model divides the world into grid squares, each a few hundred square kilometers in size. 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 emitted into the atmosphere as a result of deforestation, or about 10% of total human-made 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 man-made 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 around 500 tons of mercury per year,” adds Feinberg.

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

“Policies to protect forests or cut them down have unintended effects that go beyond their target. It’s important to consider the fact that these are systems and involve human activities, and we need to understand them better to truly solve the problems we face . we know they’re out there,” says Selin.

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

In the future, they intend to incorporate more dynamic models of the Earth system into their analyses, which would allow them to interactively monitor mercury uptake and better model the time scale of vegetation regeneration.

“This paper represents an important advance in our understanding of the global mercury cycle, 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 human activity through the combustion of coal or the burning of mercury and gold amalgams in artisanal and small-scale gold mining,” says Jackie Gerson, assistant professor in the Department of Earth and Environmental Sciences at Michigan State University, who was not involved in this research.

“This research shows that deforestation can also result in substantial mercury emissions and needs to be considered both in terms of global mercury models and land management policies. Therefore, it has the potential to advance our field scientifically as well as promote policies that reduce mercury emissions through deforestation.”

More information:
Aryeh Feinberg et al, Deforestation as an anthropogenic driver of mercury pollution, Environmental Science and Technology (2024). DOI: 10.1021/acs.est.3c07851

Leave a Reply

Your email address will not be published. Required fields are marked *