Dr. Ashu Dastoor, a research scientist at Environment and Climate Change Canada, explains how computer models together with observations from fieldwork have helped to identify the sources of airborne mercury found in the Arctic.
Did you know Mercury can be transported through the air? It can evaporate, becoming invisible and odorless. As a vapor, mercury can travel for months by air, but it can also travel by water after it is deposited on land and oceans. Using both these travel modes, mercury can make its way to the Arctic to cycle between absorption by plants, soils, snow, ice and waters. Once in the Arctic, large amounts of mercury can be locked away for thousands of years in permafrost soils and glaciers. However, bacteria found in soils, sediments and waters convert mercury to its toxic form, methylmercury, which then accumulates in food chains, threatening the health of wildlife and human populations in the Arctic.
What is mercury?
Mercury is found naturally in the Earth's crust. It is better known in its liquid form, but we find it mostly as a vapor (or gas) in the air. It transforms into a gas when coal is heated or through small operation gold mining and other industrial processes. Sunlight can also break down some of the mercury deposits already present in land and oceans from past industrial emissions. It then evaporates and re-enters the atmosphere, eventually making its way to the Arctic.
Where does the mercury in the Arctic come from?
Most of the mercury found in the Arctic comes in through the air, ocean and rivers, but air transport is the primary route for its buildup in Arctic ecosystems. Mercury emissions to air from past and current industrial activity and some also from natural activities such as volcanic emissions, the breakdown of rock and minerals, or wildfires travel around the globe and deposit to the Earth's surfaces.
Scientists have been gathering data related to mercury emissions, levels and movement in global and Arctic environments for many years. They also work closely with northern Inuit communities to gather traditional knowledge of land and wildlife. Using this knowledge and data, computer models were developed to virtually reproduce real life situations of mercury transport pathways and deposits. We know that a large portion of airborne mercury in the Arctic comes from Asia, but it also makes its way up North from as far as South America. Scientists figured this out by using these computer models. These computer models can reproduce past and present situations and predict future scenarios.
Environment and Climate Change Canada senior research scientist, Dr. Ashu Dastoor, is the lead author of the third chapter of the 2021 AMAP Mercury Assessment. This chapter explains where mercury comes from, where it deposits and how it moves in the Arctic. According to Dr. Dastoor, computer models show that most of the mercury accumulating in the Arctic today comes from the recirculation of mercury emissions from previous global human activities, but that recent human activities are still adding one third of the mercury accumulation in the Arctic each year. "Reducing today's human-caused emissions is key to lowering mercury now as well as to preventing future build-up of recycled mercury affecting the Arctic," she says.
Where does the mercury go in the Arctic?
Computer models, along with data collected in the field help us not only determine where mercury comes from, but to calculate where mercury settles in the Arctic. We know that mercury travelling by air tends to be absorbed in large quantities by vegetation, eventually entering soils when the plants die, or to settle on the surface of the snow, ice and water. After long periods of darkness, the polar sunrise causes dramatic increases in chemical activity unique to the polar environment. It transfers mercury from the air to the extensive areas of coastal and marine snowpack. While some of this mercury is recycled back to air, some ends up in marine waters when the sea ice and snowpack melt. Spring floods also supply vast amounts of mercury stripped from surface soils and accumulated in the snowpack to the ocean waters through waterways from inland lakes and rivers and coastal erosion.
Figure 1: Plumes of mercury emissions transported by winds across the Pacific Ocean to the Arctic on May 19, 2015, shown as low (blue) to high (red) mercury levels in air around the globe. Locations of ECCC mercury observation sites, Alert (Nunavut) and LFL (Little Fox Lake, Yukon), are identified by pink stars. Model simulation by Dr. Andrei Ryjkov, ECCC scientist on mercury modelling team.
Once on land and waters, mercury is mostly attached to particles such as soils and organic matter, allowing it to settle in lakes, shallow coastal waters, called a shelf, and deep ocean sediments. "Piecing together data gathered on land, by ships and through computer modeling, we can create a complete picture of the presence of mercury, including how much there is of it and how it moves through the Arctic ecosystem," explains Dr. Dastoor. "We discovered that a lot more mercury than previously thought, arriving in the ocean from rivers and coastal erosion, ends up in the estuarine, a transition zone between rivers and oceans, and in shelf sediments." She points out that these sites are known to convert mercury to its toxic form, and estuaries are the primary hunting and fishing regions for northern Indigenous populations. "It is important that we understand the behaviour of mercury in sediments and its absorption in food chains in these regions," says Dr. Dastoor.
The need for continued research
Studying mercury in the Arctic is relatively new. There is a lack of data for how many regions of the Arctic tundra, boreal forests and ocean waters exchange mercury with air. Estimates are made using data from limited locations and seasons in the Arctic. "Models reveal that environmental changes such as warming temperatures and melting sea ice are profoundly affecting the atmospheric mercury cycling in the Arctic. This means wind patterns used to transport mercury and the way mercury is interacting with the Arctic ecosystems are shifting," explains Dr. Dastoor. On land, scientists are finding high concentrations of mercury in streams from permafrost thaw and glacial melt. These impacts of climate warming release mercury from soils that had accumulated and remained buried for over thousands of years. Permafrost thaw is happening everywhere in the Arctic, but its future impact is presently uncertain. We don't fully understand how mercury is mobilized from soils, and what is its fate in ocean waters, especially in the estuaries. "Improving geographical coverage of mercury monitoring and applying models that account for ecosystem variations under future climate change trajectories would help us anticipate the uneven effects of climate change on the impacts of mercury in the Arctic," says Dr. Dastoor.
Planning for the future
Knowing more on how mercury affects the Arctic is important, not only for the wildlife and humans who live in the Arctic region, but for all of us. Climate change may accelerate the probability of more mercury being released in the air and water. This could worsen the human impact in downstream Arctic ecosystems vulnerable to climate warming. Expected increases in wildfires, caused by a warmer and dryer climate may also contribute to an increase of mercury in the atmosphere, as will the changes in air circulation patterns due to warmer temperatures and lower sea ice concentrations caused by a warming climate.
The Minamata Convention on Mercury is one way that the world is coming together to protect the environment and human health from the adverse effects of mercury. This global treaty went into force in 2017 and includes several actions such as a ban on new mercury mines and control measures on emissions of mercury to air.
A deeper understanding of the impacts of mercury on the environment will allow us to better protect the Arctic from the impacts of mercury for future generations.
For more information on mercury in the Arctic, see the 2021 AMAP Mercury Assessment by the Arctic Monitoring and Assessment Program (AMAP), a working group of the Arctic Council. AMAP gathers together researchers, scientists and other experts to monitor and assess pollution and climate change in the Arctic, and proposes actions to reduce its threats for governments to consider.