MIT Scientists Say That Mercury Pollution Has Been Declining

MIT researchers have uncovered some encouraging findings regarding global mercury emissions: “Mercury emissions from human activity have been declining over the past two decades, despite global emissions inventories that indicate otherwise.” Their study, recently published in the Proceedings of the National Academy of Sciences, reveals a 10% decrease in mercury concentrations in the atmosphere between 2005 and 2020. This reduction contradicts some prior expectations and brings new insights into the behavior of mercury in our environment.

Revealing Mercury’s Decline Through Data

The study involved data analysis from 51 monitoring stations across the Northern Hemisphere, revealing a marked decline in mercury levels. Researchers used two distinct modeling techniques to analyze the measurements and identify potential drivers of this trend, with both methods pointing to reduced human-driven emissions as the primary factor behind the decrease. Lead author Ari Feinberg from the MIT Institute for Data, Systems, and Society (IDSS) emphasized the importance of field data in refining emission models: “Our work shows that it is very important to learn from actual, on-the-ground data to try and improve our models and these emissions estimates. This is very relevant for policy because, if we are not able to accurately estimate past mercury emissions, how are we going to predict how mercury pollution will evolve in the future?”

Traditionally, global emissions inventories are created through estimates based on polluting activities, such as industrial production and the operation of coal-fired power plants. These models often rely on average emissions data and industry activity levels, which can miss nuanced trends captured by real-world measurements. Although inventories have often indicated a rise in mercury emissions, these projections have not aligned with the recent observed decline, highlighting a potential gap in how data is modeled and interpreted.

Understanding the Mismatch: Challenges in Mercury Emission Estimation

Part of the discrepancy between projected and observed mercury levels may stem from the unique characteristics of mercury itself. Mercury is the only metal that remains liquid at room temperature, which gives it distinctive properties. It can exist in multiple forms in the atmosphere and, once deposited into the ocean or onto land, can be re-released into the air over time. This cycle complicates efforts to pinpoint where mercury emissions originate and where they may be stored.

Another hurdle is that mercury is much more challenging to study in controlled settings than other pollutants, particularly given its toxicity. Mercury’s complex chemical behavior requires highly specialized study techniques, which limits scientists’ full understanding of the pollutant. In comparison to more common pollutants like methane or nitrous oxide, there are far fewer mercury monitoring stations, making it harder to capture comprehensive, long-term data trends.

Co-author and MIT professor Noelle Selin notes the value of continuing mercury monitoring efforts, particularly as many of the stations they analyzed are no longer in operation due to insufficient funding. “Mercury emissions are moving in the right direction, and could continue to do so, which is heartening to see. But this was as far as we could get with mercury. We need to keep measuring and advancing the science,” she explained.

To manage the data gaps and diverse data sources, researchers used advanced statistical methods. They aggregated data from 11 regions, which allowed them to evaluate broader regional trends despite limited data at each station. This aggregation, along with the application of two modeling techniques—biogeochemical box modeling and chemical transport modeling—helped them test a wide range of scenarios.

Biogeochemical box modeling enabled the team to run multiple simulations to evaluate hypothetical scenarios of mercury emissions, while chemical transport modeling allowed researchers to explore how regional meteorology and spatial differences affected mercury trends. This dual approach provided deeper insights and tested the feasibility of additional environmental “sinks” that could potentially absorb mercury from the atmosphere at rates higher than previously estimated. However, despite considering these hypotheses, the researchers ultimately concluded that reductions in human-generated mercury emissions are the most probable explanation for the decline in atmospheric mercury.

Policy Implications and the Role of the Minamata Convention

This research has significant policy implications, especially considering the Minamata Convention on Mercury, a global treaty targeting mercury emissions. Signed in 2013 and enforced in 2017, the convention was designed to curb mercury pollution from key sources, including coal-fired power plants and artisanal gold mining. Every five years, the 128 countries signed onto the convention, including major players like the U.S., China, and the EU, review global progress toward reducing mercury emissions. Despite these commitments, global inventory reports have sometimes suggested an uptick in emissions, creating concern and confusion for researchers and policymakers alike.

These inventories rely on emission factors—average estimates of mercury output from specific polluting activities—and assume uniformity across regions and industries. However, Feinberg and Selin’s findings suggest that real-world emissions have, in fact, declined, presenting an alternative narrative to global reports that indicated otherwise. “The big question we wanted to answer was: What is actually happening to mercury in the atmosphere and what does that say about emissions over time?” Selin said. The study emphasizes that actual data can help adjust these inventories to better reflect real-world conditions and improve future projections.

The researchers also identified potential gaps in current mercury data, with global inventories potentially lacking key regional details. For example, using a regional inventory from China helped the team resolve some inconsistencies, although gaps remained. Other challenges include determining how long it takes for mercury from products such as thermometers or laboratory equipment to be released into the atmosphere after disposal.

Looking Forward: Enhancing Global Mercury Monitoring

The next steps for the team and their international collaborators involve refining the models used to estimate mercury emissions. A coordinated approach across multiple countries and research institutions will be essential to building a more accurate global picture of mercury emissions. These advancements will not only help quantify current pollution levels more precisely but also inform strategies to mitigate mercury’s effects in the future.

Feinberg noted the importance of this research for long-term environmental management, as it will play a role in shaping mercury monitoring protocols and models that can adapt to evolving data. The improved models could help governments and organizations make more accurate predictions, ultimately supporting stronger and more effective environmental policies.

The project received support from several institutions, including the Swiss National Science Foundation, the U.S. National Science Foundation, and the U.S. Environmental Protection Agency. The backing of these organizations highlights the global commitment to advancing mercury research and working toward a cleaner and safer environment. The findings of Feinberg, Selin, and their colleagues underscore the value of continuous environmental monitoring and international cooperation in tackling persistent pollutants like mercury.