Global mercury impact synthesis: Processes in the Southern Hemisphere

Mercury is a global pollutant and the focus of an international multilateral environmental agreement (MEA), the Minamata Convention. Most of the mercury entering the environment comes from anthropogenic sources including artisanal small-scale gold mining activities, industrial plants, coal burning, incinerators, industrial mining, and associated processing. Anthropogenic activities have increased total atmospheric mercury concentrations by about 4.5 times natural levels (Lindberg et al. 2007; Driscoll et al. 2013; UNEP 2013; Hsu-Kim et al. 2018; Obrist et al. 2018; AMAP/UNEP 2019). Mobilized mercury moves readily into the biosphere and eventually makes its way into all water bodies. Mercury is ubiquitous in the environment and reaches levels of concern in fish not only in contaminated systems, but in remote and otherwise pristine ecosystems (Driscoll et al. 2007; Chen et al. 2012; Eagles-Smith et al. 2020). Fluxes of mercury in aquatic ecosystems have increased substantially with industrialization (Mason et al. 1994, 2012; Driscoll et al. 2013; Outridge et al. 2018), and mercury is now globally present at levels in aquatic and terrestrial ecosystems that can pose risks to humans and wildlife (Mergler et al. 2007; Karagas et al. 2012; Evers et al. 2016; Eagles-Smith et al. 2018).

Mercury travels predominantly through the atmosphere in its elemental form, which has an atmospheric lifetime of approximately six months to a year which means it has the ability to travel across oceans and continents (Driscoll et al. 2007, 2013; Giang and Selin 2016; Obrist et al. 2018). Mercury released in more reactive forms can enter ecosystems closer to sources (Selin et al. 2008; Selin 2009). A challenging property of mercury is that it can cycle between the atmosphere, land, and ocean, and as a result, past and present emissions can continue to affect the environment on timescales of decades to centuries (Amos et al. 2013). The mercury currently depositing to ecosystems represents largely a combination of (1) current anthropogenic emissions (e.g., from coal-fired power generation or use of mercury in artisanal and small-scale gold mining), (2) legacy contamination from historical anthropogenic mercury emissions; and (3) natural sources. This global biogeochemical cycling of mercury, and the different forms in which it is emitted and cycles in the environment are complex and becoming more complex with environmental changes due to climate (Sonke et al. 2023). Therefore, the process of detecting changes in mercury, and monitoring the effectiveness of mercury controls, is extremely complicated (Evers et al. 2016). A major challenge and need for scientific research and policymaking is to better understand and attribute the sources and environmental processing of mercury in ways that inform policy efforts to manage its risks (Selin 2014; Selin et al. 2018).

Methylmercury is the most toxic form of mercury, and its main exposure route for methylmercury to humans and wildlife is through consumption of shellfish and fish (Rice et al. 2000; Karagas et al. 2012; Oken et al. 2012; Eagles-Smith et al. 2018; Basu et al. 2018, 2023). Toxicological effects of methylmercury are of special public concern to high-risk populations including women and young children. Unborn fetuses are at greatest risk for neurological and developmental impairment since mercury can pass through the placenta to the fetal brain. People who consume elevated amounts of shellfish and fish contaminated with methylmercury are expected to have a higher body burden of the metal when compared to others (Karimi et al. 2012, 2014; Sunderland et al. 2018). The developing human nervous system is considered a sensitive target organ system for low-dose methylmercury exposure. Other organ systems possibly adversely affected include the cardiovascular system, the immune system, the kidneys, and the reproductive system (Karagas et al. 2012; Gribble et al. 2016; Eagles-Smith et al. 2018).

The global health risks to humans that result from exposure to this neurotoxin are significant, especially in populations that depend on fish for subsistence. Human exposure to mercury at levels with potential risks has been found in fish-eating populations all over the world and across all socioeconomic spectra (Trasande et al. 2016). These exposures and risks are particularly urgent in developing countries involved in the artisanal small-scale gold mining industry where both occupational and fish consumption exposures are prominent (Sherman et al. 2015; Basu et al. 2018; Lavoie et al. 2018; Basu et al. 2023). However, regulating dietary exposure by reducing fish consumption and related risk communication poses a particular challenge (Oken et al. 2003, 2012; Eagles-Smith et al. 2018; FAO 2020). Fish are highly nutritious with significant benefits to human health, they are a culturally important food source for many populations, and fish are also important to the global economy (Pirrone et al. 2005; Gribble et al. 2016; Basu et al. 2023). To date, the consumer guidance on fish consumption and mercury exposure is provided by multiple parties and is often contradictory and at best confusing (Groth 2010; Oken et al. 2012; Groth 2017). Improvements in risk communication by stakeholders are greatly needed, particularly in the consistency and clarity of their messages (Oken et al. 2012; Cleary et al. 2021).

Mercury science and management are the focus of attention worldwide. The Minamata Convention, the global MEA to protect human health and the environment from the adverse effects of mercury, entered into force in 2017 and since then, there have been four Conference of Parties (COP) meetings—2017, 2018, 2019, and 2022. To date, the Convention has been ratified by 140 parties. The Convention requires that countries around the world control both new and existing sources of mercury and monitor the effectiveness of those controls (Selin 2014; Evers et al. 2016; Selin et al. 2018). In many other countries, the use of mercury in artisanal small-scale gold mining (ASGM) is under investigation as the magnitude of associated mercury releases from this industry may have been underestimated (UNEP 2013, 2019; Keane et al. 2023). This is particularly important in developing countries in the Southern Hemisphere where much of the ASGM activity is conducted (Fisher et al. 2023; Keane et al. 2023). While initiatives to mitigate mercury impacts from emissions and releases have been implemented, the extent and rate of potential recovery are unclear because of uncertainties in our understanding of mercury cycling through the biosphere which are increasingly complex due to climate change (Selin et al. 2008; Selin 2009; Song et al. 2015; Zhang et al. 2016; Eagles-Smith et al. 2018; Hsu-Kim et al. 2018; Obrist et al. 2018; Sonke et al. 2023). Mercury transport, transformations, bioaccumulation, and exposure are affected by numerous interacting processes and phenomena (e.g., temperature, ocean circulation, nutrient loading, land use/cover, food web dynamics, human behavior, and decisions).

The existing scientific research on mercury fate in the environment and risk to human health is growing while international policy approaches are being developed and implemented. Currently, the Minamata Convention Secretariat, with guidance from the United Nations Environment Programme (UNEP), is developing guidance for monitoring programs in member countries to evaluate the effectiveness of the Minamata Convention. Scientific input is critical in the next years for guiding the policies related to effectiveness evaluation. In particular, the Southern Hemisphere is far less studied and there are major gaps in our understanding of the sources, fate, and effects in this region of the world. It is important to assess the current understanding of mercury cycling in this region to properly assess the sources and cycling globally. Moreover, the ability to evaluate the effectiveness of the Convention will require understanding of how environmental factors associated with climate change influence mercury cycling. Knowledge of the interactions between climate change and the fate of mercury in the environment is critical to understanding the impacts of international and domestic policy on mercury fate in the biosphere. The time is critical for providing syntheses of specific areas of mercury research to policymakers to guide decisions made under the Minamata Convention.

In July of 2022, the 15th International Conference on Mercury as a Global Pollutant (ICMGP) was held virtually for the first time in its history.1 The theme of this conference was "Reducing mercury emissions to achieve a greener world". The conference brought together 472 participants from 66 countries and included representatives of academia, government, industry, research institutions, and non-governmental organizations. The conference program provided a forum for communicating the advances being made in mercury science and management that included plenary presentations and special sessions on topics including artisanal small-scale gold mining, the implications of a changing climate on mercury fate, advances in research on mercury effects on human health, and mercury sources and cycling in the Southern Hemisphere. As with previous mercury conferences, a synthesis workshop and special section were organized for this ICMGP. This current synthesis of five papers that has emerged from the 2022 ICMGP follows on a series of syntheses that were produced from the 2006, 2011, and 2018 ICMGP's (The Madison Declaration on Mercury Pollution 2007; Driscoll et al. 2013; Chen and Driscoll 2018; Chen et al. 2018). The most recent 2017 mercury synthesis workshop entitled, “Integrating Mercury Research and Policy in a Changing World,” produced four synthesis papers published in the journal Ambio (Eagles-Smith et al. 2018; Hsu-Kim et al. 2018; Obrist et al. 2018; Selin et al. 2018) that were each summarized into 2-page factsheets provided to delegates and participants at COP1 and translated into the four other UN languages (Chen and Driscoll. 2018; Chen et al. 2018). As with each of the past syntheses, the goal of this synthesis has been to share and summarize data, enhancing communication among a diverse group of researchers, managers and policymakers, and synthesize, interpret and translate scientific findings that relate to policy questions on issues associated with mercury fate and transport and human exposure and health effects. The production of the current synthesis papers and future outreach products were timed to provide information to the 5th Conference of Parties to be held in October-November 2023.

In summer of 2021, lead authors of the 15th ICMGP 2022 synthesis were invited to develop and write papers in 2021–2022. These five papers later provided the basis for a virtual synthesis workshop that was held at the end of August 2022 to communicate the findings of the papers to policy makers, particularly those involved in the implementation of the Minamata Convention. This workshop and special section were sponsored by the US National Institute of Environmental Health Sciences. The workshop brought together participants of multiple disciplines to summarize and synthesize the current policy and research particularly as they relate to ASGM sources of mercury, impact of climate change on mercury cycling, exposures, and effects of mercury on human health, and natural and anthropogenic sources of mercury in the Southern Hemisphere. In addition to presentations of the synthesis papers by lead authors, members of the Minamata Secretariat and the Minamata Open-Ended Scientific Group (OESG) also provided mercury scientists with an update on the implementation of the Convention and the activities and needs for conducting the first effectiveness evaluation to be completed in 2023. Policy-makers working on other more mature environmental conventions also brought their experiences and suggestions to the table.

The four topics and five papers comprising the present special section were presented in the plenary sessions of the 15th ICMGP 2022 and in special sessions on their topic areas. The first paper provides a review of the use of mercury in ASGM and mercury-free alternatives. The second paper examines the effects of climate change on the biogeochemical cycling of mercury. The third paper reviews the evolution of the scientific understanding of the risks of mercury to human health. And lastly, the fourth and fifth papers focus on the natural and anthropogenic sources and cycling of mercury in the Southern Hemisphere. Each paper is described in greater detail below.

The first paper, entitled "Mercury and artisanal and small-scale gold mining: Review of global use estimates and considerations for promoting mercury-free alternatives" (Keane et al. 2023), discusses activities in over 80 countries, which employ about 15 million miners and serve as a source of livelihood for millions more. The artisanal and small-scale gold mining (ASGM) sector is estimated to be the largest emitter of mercury globally. The Minamata Convention on Mercury seeks to reduce and, where feasible, eliminate its use in ASGM. However, the total quantity of mercury used in ASGM globally is still highly uncertain, and the adoption of mercury-free technologies has been limited to date. This paper presents an overview of efforts to refine estimates of mercury use in ASGM, and then reviews and assesses technologies that can be used to phase out mercury use while increasing gold recovery rates. The paper concludes with a discussion of practical social and economic barriers to adoption of these technologies and recommends steps to overcome them.

The second paper, entitled "Global change effects on biogeochemical mercury cycling" (Sonke et al. 2023), considers the interactions of global change with the physical, biogeochemical, and ecological factors that control mercury cycling processes. The paper discusses atmospheric mercury transport and deposition processes that are sensitive to both meteorology and biome shifts citing modeling studies that predict future mercury deposition and re-emission patterns. These patterns are expected to shift at the regional level, not globally, due to biome shifts, deforestation, soil erosion, reservoir creation, and permafrost thaw, and legacy pools could be mobilized in extreme climate events such as floods and fires. In addition, mercury methylation determined by the presence of mercury methylating genes has been found to be present in oxic to anoxic systems all of which can be influenced by disruption of nutrient cycling associated with environmental change. Based on observations of two-fold decreases in Hg deposition and biotic concentrations in response to the two-fold reduction in mercury emission at the end of the twentieth century, the authors predict that an additional two-fold decrease in emissions under the Minamata Convention would be distinguishable from the effects of climate change at both regional and global scales but potentially not at local scales. The authors emphasize the need to continue mercury monitoring, conducting process studies and earth system mercury modeling in order to distinguish emission reduction effects from those due to climate change.

The third paper, entitled "Our evolved understanding of the human health risks of mercury" (Basu et al. 2023), discusses what the World Health Organization has identified as one of its top 10 contaminants of public health concern. Mercury concentrations in certain aquatic food items, consumer and industrial products, and occupational settings can be at levels deemed concerning to human health. In response, human biomonitoring studies have established that most people worldwide are exposed to mercury, and notably some populations are particularly vulnerable, including Indigenous Peoples (e.g., in Arctic and tropical biomes) and ASGM miners. In response, this review paper details how scientific understandings have evolved over time, from tragic poisoning events in the mid-twentieth century to important epidemiological studies in the late-twentieth century in the Seychelles and Faroe Islands, the Arctic and Amazon. Entering the twenty-first century, studies on diverse source-exposure scenarios (e.g., ASGM, amalgams, contaminated sites, cosmetics, electronic waste) from across global regions (e.g., tropics, small-island states, industrial areas) have expanded understandings and exemplified the need to consider socio-environmental variables and local contexts when conducting health studies. The authors conclude with perspectives on next steps for mercury health research in the post-Minamata Convention era.

The fourth paper, entitled "A synthesis of mercury research in the Southern Hemisphere, part 1: Natural processes" (Schneider et al. 2023), considers the contrast between what is known about mercury estimates and inventories in the Southern Hemisphere and what is assumed from knowledge based on the vastly greater number of studies in the Northern Hemisphere. The authors identify five key natural differences between the two hemispheres that must be considered when thinking about the mercury processes in the Southern Hemisphere: biome heterogeneity, vegetation type, ocean area, methylation hotspot zones, and occurrence of volcanic activity. The paper identifies major gaps in knowledge about mercury fate in the biomes specifically present in the Southern Hemisphere including properties, content, and storage of mercury in the soils present, pools of mercury moving from terrestrial environments to water bodies, and mercury concentrations in remote background sites particularly in aquatic ecosystems and tropical forests of Africa and Southeast Asia. There are also major data gaps for mercury in Patagonian dust and ice fields, Hg concentration in the southern oceans, and volcanic and geothermal sources of mercury in the many active volcanic regions of the Southern Hemisphere. These gaps in mercury science point to a need for local research on mercury sources and cycling in the global south.

The fifth paper, entitled "A synthesis of mercury research in the Southern Hemisphere, part 2: Anthropogenic perturbations" (Fisher et al. 2023), further considers the problems with extrapolating mercury data from the Northern Hemisphere to the Southern Hemisphere despite all the historical, political, and socioeconomic differences that impact mercury sources and sinks between the regions. The paper contrasts four anthropogenic perturbations that differ: lower historical production of mercury and other metals, more fire and deforestation driving re-mobilization of terrestrial mercury and destruction of an important mercury sink, prevalent use of mercury in ASGM, and significant mercury emission sources from coal-fired power stations and non-ferrous metal production. The authors point out the lack of long-term continuous measurements of mercury in the Southern Hemisphere that are needed for evaluating the effectiveness of the Minamata Convention.

The five papers contained in this special section provide a synthesis of what is currently known and not known about the estimates of mercury used in ASGM and current knowledge on mercury-free technologies, impacts and predictions of a changing climate on the fate of mercury, past and current understanding of its impacts on human health, and natural and anthropogenic sources of mercury in the Southern Hemisphere. These syntheses also provide recommendations for the areas of mercury research and monitoring that are needed to inform policies that will be effective in minimizing the environmental exposures and effects of mercury. It is the intention and hope that this synthesis will provide useful information to the next COP and to the implementation of the Minamata Convention going forward.