A synthesis of mercury research in the Southern Hemisphere, part 2: Anthropogenic perturbations

Abstract

Environmental mercury (Hg) contamination is a global concern requiring action at national scales. Scientific understanding and regulatory policies are underpinned by global extrapolation of Northern Hemisphere Hg data, despite historical, political, and socioeconomic differences between the hemispheres that impact Hg sources and sinks. In this paper, we explore the primary anthropogenic perturbations to Hg emission and mobilization processes that differ between hemispheres and synthesize current understanding of the implications for Hg cycling. In the Southern Hemisphere (SH), lower historical production of Hg and other metals implies lower present-day legacy emissions, but the extent of the difference remains uncertain. More use of fire and higher deforestation rates drive re-mobilization of terrestrial Hg, while also removing vegetation that would otherwise provide a sink for atmospheric Hg. Prevalent Hg use in artisanal and small-scale gold mining is a dominant source of Hg inputs to the environment in tropical regions. Meanwhile, coal-fired power stations continue to be a significant Hg emission source and industrial production of non-ferrous metals is a large and growing contributor. Major uncertainties remain, hindering scientific understanding and effective policy formulation, and we argue for an urgent need to prioritize research activities in under-sampled regions of the SH.

Keywords: ASGM; Deforestation; Legacy mercury; Mercury emissions; Methylmercury; Southern Hemisphere.

Fig. 1

Conceptual overview of the key hemispheric differences described in this paper, showing how anthropogenic factors differ in the SH and tropics relative to the NH (blue) and the associated impact on Hg biogeochemical cycling (orange). The bottom row indicates how the associated hemispheric-scale perturbation to Hg emission compares between hemispheres

Fig. 2

Estimates of Hg emissions from fires in the SH + T. Estimates are based on data published in Friedli et al. (2009), De Simone et al. (2015), Kumar et al. (2018), and Shi et al. (2019). De Simone 1, 2, and 3 refer to FINN, GFAS, and GFED, respectively, from De Simone et al. (2015). Regional data from Friedli et al. (2009) and De Simone et al. (2015) have been summed as follows: Central & South America = CEAM + NHSA + SHSA; Africa = NHAF + SHAF; Southeast & Equatorial Asia = SEAS + EQAS; Australia = AUST (with shorthand names defined in those papers). Kumar et al. (2018) used somewhat different regional definitions than those used here. For this plot, we used the results from De Simone et al. (2015) to calculate the fractional contribution of each sub-region (e.g., SEAS) to each of the Kumar et al. (2018) regions (e.g., EURAS) to re-calculate the estimated regional totals. The variation in the fractional contributions results in low- and high-end estimates shown in the dark and light colors, respectively, for the Kumar bars (pink). Note that Shi et al. (2019) did not provide an estimate for Australia

Fig. 3

Annual change in net forest area by region and decade (bars), along with conceptual diagram showing the expected implications for vegetation Hg uptake (green arrows) and atmospheric Hg concentrations (blue arrows). Net forest area change is calculated as the sum of all forest losses (deforestation) and all forest gains (forest expansion) in a given period and comes from the FAO Global Forest Resources Assessment 2020 (FAO 2020)

Fig. 4

Number of scientific articles on Hg use in ASGM as a function of projected mercury use in ASGM for countries with known ASGM activities. BOL Bolivia, COL Colombia, GHA Ghana, IDN Indonesia, PER Peru. Data sources: projected Hg use from the Global Mercury Assessment Report 2018 Technical Background Report (AMAP/UNEP 2019) Table A3.2.1; number of scientific articles on ASGM and mercury use in the country from Scopus as of 18 November 2022

Fig. 5

Anthropogenic Hg emissions by latitude, separated into ASGM (light green) and non-ASGM (dark green) emissions. Reprinted from Atmospheric Environment, Vol 211, Steenhuisen and Wilson, Copyright (2019), with permission from Elsevier

Fig. 6

Mercury emissions from non-ferrous metal production in South America, Africa, and Oceania, colored by metal produced. For clarity, only countries with estimated emissions of more than 1 Mg y⁻¹ are shown. Data source: GMA2018 Technical Background Report (AMAP/UNEP 2019)