As a result of EU policy1, European emissions of mercury have dropped considerably in recent decades, decreasing by about 60 per cent between 1990 and 2000. However, global emissions continue to rise. This report from the UN Environment Programme2, more recently published as a book, evaluated research on the distribution of mercury emissions and on models to predict future levels.
2503 tonnes of mercury emitted to the atmosphere each year comes directly from anthropogenic sources. This is about a third of all mercury emitted to the atmosphere. Fossil fuel power plant emissions contribute 1422 tonnes per year. China and India account for 62 per cent of mercury emissions from fossil fuel burning whilst Europe and USA account for 23 per cent.
Small-scale gold mining produces the second largest amount of direct anthropogenic emissions at 400 tonnes per year. Natural sources of mercury, such as oceans, rocks, volcanoes and forest fires, release about 5207 tonnes of mercury per year. Oceans account for over half of this (2682 tonnes per year) and the second largest source is burning biomass, such as forest fires (675 tonnes per year).
However, burning biomass is a mixture of natural processes and burning by humans. In fact, about two thirds of mercury emitted from natural processes has an anthropogenic origin – it is 'recycled' mercury that has previously been deposited from industrial sources.
Changes in global levels of mercury should be reflected in changes in background mercury concentration, but estimating global trends is difficult due to variations in area and time. The general consensus is that the southern hemisphere has a lower concentration at 1.1 to 1.3 nanograms (10-9 of a gram) per cubic metre than the northern hemisphere (1.5 to 1.7 nanograms per cubic metre). In the USA and Europe, trends vary according to region and for Asia there is a lack of data.
The report proposes that a global monitoring network is needed to assess progress on mandated reductions of anthropogenic mercury. It also recommends continued research on the processes of mercury movement and the development of models.
Little is known about the processes involved in the exchange of mercury between the air and the sea or the land and the air, or about the reactions of mercury with oxygen in the air. This information is needed to develop models which can accurately predict future mercury concentrations and the influence of human activity and climate change on mercury.