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Governments are on the verge of agreeing the first legally binding, global treaty to tackle mercury pollution. It aims to clean up the legacy of centuries of untrammelled emissions of the toxic metal, and to limit future contamination from sources as diverse as coal-fired power plants and gold mining.

Delegates from 128 countries are expected to meet next week in Geneva, Switzerland, for a fifth and final round of treaty negotiations. All agree that action is needed urgently to reduce mercury emissions, which pose risks to the environment and human health. But consensus on how to achieve that will not come easily. The current top emitters, in Asia, want to know why they should shoulder the burden of clean-up when much of the world’s mercury pollution is due to the past economic growth of developed nations — an argument that parallels one of the main stumbling blocks to an international greenhouse-gas agreement.

Yet with mercury able to drift freely through air, soil, rivers and oceans, it is crucial that the negotiations deliver “a global treaty that is going to be implementable in all the countries”, says Fernando Lugris, chairman of the Intergovernmental Negotiating Committee of the United Nations Environment Programme (UNEP), which is coordinating the treaty.

According to a draft version of UNEP’s 2013 Global Mercury Report, about 6,500 tonnes of mercury was emitted into the air in 2010. Roughly 30% came from human activities, and a further 15% from natural sources such as volcanoes and erosion. The remainder was from the re-emission from soils, water and vegetation of mercury released into the environment decades ago. “Once emitted into the air, it’s like a genie that has escaped the bottle,” says David Streets, an environmental scientist at Argonne National Laboratory in Illinois. “It takes decades or even centuries to get it fixed.”

The UNEP report points out that mercury concentrations in the upper 100 metres of the oceans have doubled in the past century; top marine predators in the Arctic Ocean contain up to 12 times more mercury than in pre-industrial times, and contaminated seafood is a significant source of human exposure. The World Health Organization (WHO) rates mercury as one of the top ten chemicals of major public health concern — it can damage the brain and is a particular hazard for fetuses.

Streets estimates that human activity has released a grand total of about 350,000 tonnes of mercury, with roughly 40% of those emissions occurring before 1850 (see ‘Quicksilver quantified’). Much of the rest came from silver and gold mining in the late nineteenth century, when mercury was used to extract precious metals. However, emissions have soared in recent decades, propelled by small-scale, or artisanal, gold mining in Africa and Latin America, and rapid industrialization in Asia (D. G. Streets et al. Environ. Sci. Technol. 45, 10485–10491; 2011). China is now the biggest emitter, contributing 30% of global anthropogenic emissions.

Source: D. G. Streets et al. Environ. Sci. Technol. 45, 10485–10491 (2011)/UNEP

Cleaning up industrial processes, for example by capturing mercury released from burning coal, involves costly technology. In the negotiations, “the biggest contention is who should pay”, says Ludovic Bernaudat, a mercury expert at the United Nations Industrial Development Organization in Vienna. “There are a lot of tensions right now.”

Before the latest round of negotiations, in June 2012, the discussions aimed at an agreement for all countries to cap mercury emissions. But developing nations such as China and India — second only to China as an emitter — were adamant that this would be unfair unless developed nations helped with the cost and technologies. Common measures for controlling air pollution have the potential to reduce mercury emissions from coal plants by about 36%, “but to go further you’d need specific mercury-control technologies that can remove 90% of emissions, which are only available in developed countries”, says Wang Shuxiao, an environment scientist at Tsinghua University in Beijing, who is part of the Chinese delegation.

Developed nations seem unlikely to commit to funding the transfer of such technologies. Negotiators may settle on an agreement that requires countries to set national targets that they can meet with the best mercury-control measures available to them, and to beef up monitoring programmes. UNEP predicts that such measures could reduce emissions in industrial regions by 25% by 2020, compared with an increase of up to 25% under a business-as-usual scenario.

The treaty also aims to limit emissions from artisanal gold mining, which is largely unregulated. Miners soak crushed ore in mercury to form an amalgam that leaves impurities behind; heating the amalgam frees the gold, but releases mercury into the air. “Most of them are unaware of the health hazards of mercury vapour and nobody wears a mask,” says Nicola Pirrone, director of the Institute of Atmospheric Pollution Research in Rome. The treaty is likely to recommend that countries register and monitor mining, and will encourage technologies that capture mercury vapour or use jets of water and air to separate gold from ores (see Nature 486, 306–307; 2012).

In late February, at the Global Ministerial Environment Forum in Nairobi, UNEP’s governing council will debate the draft treaty that will emerge from next week’s meeting. UNEP expects countries to ratify the treaty later this year. Even if the treaty does not set binding caps, “it should still build enough momentum for countries to commit to serious efforts to tackle the problem”, says John Munthe, an environment-policy researcher at the Swedish Environmental Research Institute in Stockholm. “There are plenty of low-hanging fruits that could make a big difference in reducing global mercury emissions.”