Updated Global and Oceanic Mercury Budgets for the United Nations Global Mercury Assessment 2018

In support of international efforts to reduce mercury (Hg) exposure in humans and wildlife, this paper reviews the literature concerning global Hg emissions, cycling and fate, and presents revised global and oceanic Hg budgets for the 2018 United Nations Global Mercury Assessment. We assessed two competing scenarios about the impacts of 16th – late 19th century New World silver (Ag) mining, which may be the largest human source of atmospheric Hg in history. Consideration of Ag ore geochemistry, historical documents on Hg use, and comparison of the scenarios against atmospheric Hg patterns in environmental archives, strongly support a “low mining emission” scenario. Building upon this scenario and other published work, the revised global budget estimates human activities including recycled legacy emissions have increased current atmospheric Hg concentrations by about 450% above natural levels (prevailing before 1450 AD). Current anthropogenic emissions to air are 2.5 ± 0.5 kt/y. The increase in atmospheric Hg concentrations has driven a ∼ 300% average increase in deposition, and a 230% increase in surface marine waters. Deeper marine waters show increases of only 12–25%. The overall increase in Hg in surface organic soils (∼15%) is small due to the large mass of natural Hg already present from rock weathering, but this figure varies regionally. Specific research recommendations are made to reduce uncertainties, particularly through improved understanding of fundamental processes of the Hg cycle, and continued improvements in emissions inventories from large natural and anthropogenic sources.