For our latest ‘On the horizon’ post, Colleen Seymour explains how climate change could release vast amounts of mercury into the environment as permafrost melts.

Hat making was a particularly hazardous occupation in the 17^th century.  Mercuric nitrate was used to soften pelts, and its use in poorly-ventilated rooms almost inevitably led to mercury (Hg) poisoning (‘mecurialism’), which manifested as a combination of psychoses, physical tremors, and other unsettling symptoms, often referred to as the ‘hatters’ shakes’ (as an aside, it is doubtful that the Mad Hatter in Alice and Wonderland exhibited mecurialism, because the main psychological symptoms are excessive timidity, shyness and diffidence – not traits the Mad Hatter had in abundance).

Mercury poses problems not just for hatters, however.  Mercury is transformed by microorganisms in the anaerobic sediments of water columns to produce methylmercury (MeHg), which has a variety of neurotoxic and physical effects that reduce animals’ ability to sense, move and reproduce, and can ultimately lead to their deaths.  It is found in soils, water and the atmosphere, with a large source occurring naturally in the Earth’s crust, including coal deposits. The amount of mercury on earth is constant, but the form in which it occurs, and where it ends up, influences its impacts on humans and animals alike.  In addition to being released naturally by volcanoes, mercury is also released during forest fires, biomass burning during vegetation clearing, and through coal burning. In fact, coal-burning power stations are amongst the planet’s greatest mercury emitters.

The combination of reduced reproductive success and mortality from mercury poisoning can severely reduce wildlife populations.  The impacts of MeHg are felt particularly at higher levels in food webs, as MeHg tends to bioaccumulate (that is, it becomes concentrated in animals’ bodies, and so is absorbed in ever-increasing quantities as animals prey on others that already contain mercury). Recent surveys have found that the concentrations of mercury in bird tissues and blood from numerous sites across North America already exceed toxicity benchmarks.  It’s probably safe to assume that this reflects the situation with regard to mercury concentrations in many other regions of the world.  Now a new possible source of mercury release is emerging: thawing permafrost.

Permafrost is ground (that is, soil, rock, ice and organic material) that has been at temperatures at or below 0 °C for two or more consecutive years. Permafrost covers a considerable area of the globe, about 24% of exposed Earth in the Northern Hemisphere (although there is much less permafrost in the southern Hemisphere, owing to less landmass in the high latitudes). Over thousands of years, various compounds, including mercury, have bound with organic material and become incorporated into the permafrost.  Now, with warming, this slowly accumulated material has the potential to be released.  We’ve been aware of the potential for mercury release from thawing permafrost for a few years now, but recent studies by Olson et al (2018)_and Schuster et al. (2018) show that the amount of mercury lurking in the northern hemisphere permafrost is far greater than previously assumed. In fact, the amount of mercury stored could be as much as twice the global average for all soils, the oceans and atmosphere combined. That amount is about 10 times that created by anthropogenic mercury emissions over the last 30 years, based on emissions estimates from 2016. Models predict a loss of between 6 and 29% of the permafrost per 1 °C of warming.  Given that much of the permafrost could thaw owing to climate change over the next century, most of its accumulated mercury will be transported from soil, via streams and rivers, to the oceans. The impacts will likely be acute, through chronic bioaccumulation and microbial conversion to methyl mercury. This could have potentially far-reaching impacts on terrestrial and aquatic organisms that would reverberate to affect an array of animals within ecosystems.

A Horizon Scan of Emerging Issues for Global Conservation in 2019 is available to read in Trends in Ecology and Evolution.

Read more from our ‘On the horizon’ series here:

Introduction  by Bill Sutherland and Nancy Ockendon

Climate change and the capacity of Antarctic benthos to store carbon by Nathalie Pettorelli

Deforestation expansion of plantations and infrastructure threaten Indo-Malay island species by Nafeesa Esmail and Alice Hughes

Options for cultivating rice as climate changes and salinity increases by Erica Fleishman

Plastic alternatives – the ecological impact is not always clear by Becky LeAnstey