A recent study from MIT suggests that roughly 10% of annual human-generated mercury emissions into the atmosphere stem from global deforestation.
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The planet's diverse vegetation, spanning from the Amazon rainforest to the savannahs of sub-Saharan Africa, serves as a crucial sink for absorbing this harmful pollutant from the air. Nonetheless, if the present pace of deforestation persists or intensifies, experts project a continued rise in net mercury emissions.
“We’ve been overlooking a significant source of mercury, especially in tropical regions,” states Ari Feinberg, a Former Postdoc in the Institute for Data, Systems, and Society (IDSS) and lead author of the study.
According to the researchers' model, the Amazon rainforest stands out as a crucial mercury sink, accounting for approximately 30% of the total global land sink. Therefore, halting deforestation in the Amazon could significantly mitigate mercury pollution.
Additionally, the team projects that global reforestation initiatives could enhance annual mercury absorption by around 5%. While this represents a notable contribution, the researchers stress that reforestation should complement, not replace, comprehensive pollution control measures worldwide.
Countries have put a lot of effort into reducing mercury emissions, especially northern industrialized countries, and for very good reason. But 10% of the global anthropogenic source is substantial, and there is a potential for that to be even greater in the future. [Addressing these deforestation-related emissions] needs to be part of the solution.
Noelle Selin, Study Senior Author and Professor, Institute for Data, Systems, and Society
Noelle Selin is also associated with MIT’s Department of Earth, Atmospheric and Planetary Sciences.
Feinberg and Selin collaborated with co-authors Martin Jiskra, formerly a Swiss National Science Foundation Ambizione Fellow at the University of Basel; Pasquale Borrelli, a Professor at Roma Tre University in Italy; and Jagannath Biswakarma, a Postdoctoral Researcher at the Swiss Federal Institute of Aquatic Science and Technology. The study was published in the journal Environmental Science and Technology.
Modeling Mercury
In recent decades, scientific research has predominantly centered on investigating deforestation as a significant contributor to global carbon dioxide emissions. However, the attention towards mercury, a trace element, has been comparatively limited.
This lack of focus is partly attributed to the fact that the terrestrial biosphere's involvement in the global mercury cycle has only been more accurately quantified in recent times.
Plant leaves have the capacity to absorb mercury from the atmosphere much like they absorb carbon dioxide. However, unlike carbon dioxide, mercury does not serve any essential biological function for plants. Once absorbed, mercury typically remains within the leaf until it eventually falls to the forest floor. Upon reaching the ground, the mercury is then absorbed by the soil.
Mercury poses a significant risk to humans when it enters water bodies, where microorganisms can methylate it. Methylmercury, a powerful neurotoxin, can be absorbed by fish and accumulate up the food chain, resulting in potentially hazardous 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. The forests are doing a sort of ecosystem service, in that they are sequestering mercury for longer timescales.
Ari Feinberg, Postdoc, Blas Cabrera Institute of Physical Chemistry
Forests play a crucial role in decreasing the levels of toxic methylmercury in oceans.
While numerous studies on mercury concentrate on industrial origins such as burning fossil fuels, small-scale gold mining, and metal smelting, the 2013 Minamata Convention—a global treaty—urges nations to decrease human-induced emissions but does not directly address the effects of deforestation.
Recognizing this gap, the researchers initiated their study to address this aspect.
In previous research, they developed a model to investigate the involvement of vegetation in mercury absorption. Through a range of land use change scenarios, they fine-tuned the model to assess the impact of deforestation and quantify its role in mercury uptake.
Evaluating Emissions
The chemical transport model traces the journey of mercury from its emission sources to its chemical transformation in the atmosphere and finally to its deposition, primarily through rainfall or absorption into forest ecosystems.
They partitioned the Earth into eight distinct regions and conducted simulations to compute deforestation emission factors for each region. Factors such as the type and density of vegetation, mercury content in soils, and historical land use were taken into account.
Nonetheless, obtaining reliable data for certain regions proved to be challenging.
Measurements from tropical Africa or Southeast Asia, regions characterized by significant deforestation, were notably absent. To address this limitation, they employed simpler, offline models to simulate numerous scenarios, aiding in refining their estimates of potential uncertainties.
Additionally, they crafted a novel formulation for mercury emissions originating from soil. This formulation accounts for the effect of deforestation on reducing leaf area, consequently intensifying sunlight exposure to the ground and hastening the release of mercury from soils.
The model divides the world into grid squares, each spanning a few hundred square kilometers. By adjusting land surface and vegetation parameters within specific squares to simulate deforestation and reforestation scenarios, the researchers can effectively depict the impacts on the mercury cycle.
Their analysis revealed that approximately 200 tons of mercury are emitted into the atmosphere due to deforestation, accounting for roughly 10% of total human-induced emissions. However, in tropical and sub-tropical nations, deforestation emissions constitute a larger proportion of the overall emissions. For instance, in Brazil, deforestation emissions contribute to 40% of total human-made emissions.
Furthermore, the practice of using fires to clear tropical forested areas for agricultural purposes exacerbates emissions by releasing stored mercury from vegetation.
“If deforestation was a country, it would be the second highest emitting country, after China, which emits around 500 tons of mercury a year,” Feinberg notes.
With the Minamata Convention now targeting primary mercury emissions, scientists anticipate that deforestation will become a more significant proportion of human-induced emissions in the future.
Policies to protect forests or cut them down have unintended effects beyond their target. It is important to consider the fact that these are systems, and they involve human activities, and we need to understand them better in order to actually solve the problems that we know are out there.
Noelle Selin, Study Senior Author and Professor, Institute for Data, Systems, and Society
By offering this initial estimate, the team aims to stimulate further research in this field.
Moving forward, they aspire to integrate more dynamic Earth system models into their analysis, allowing for interactive tracking of mercury uptake and enhanced modeling of the timescale associated with vegetation regrowth.
“This paper represents an important advance in our understanding of global mercury cycling 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 directly resulting from human activity via coal combustion or mercury-gold amalgam burning in artisanal and small-scale gold mining,” adds Jackie Gerson, an Assistant Professor in the Department of Earth and Environmental Sciences at Michigan State University, who was not involved with this research.
Jackie Gerson notes, “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. It therefore has the potential to advance our field scientifically as well as to promote policies that reduce mercury emissions via deforestation.”
The study was supported, in part, by the US National Science Foundation, the Swiss National Science Foundation, and the Swiss Federal Institute of Aquatic Science and Technology.
Journal Reference:
Feinberg, A., et al. (2024). Deforestation as an Anthropogenic Driver of Mercury Pollution. Environmental Science & Technology. doi.org/10.1021/acs.est.3c07851.