During the industrial boom of the mid-twentieth century thousands of man-made chemicals were created to make chemical processes and products stronger and more durable.
The substances became useful in pest control and crop production, but it wasn't long before they also proved deadly, causing cancers, birth defects and other health problems.
Known as persistent organic pollutants (or POPs), this group of the world's most toxic compounds takes decades to degrade as they circulate through Earth's oceans and the atmosphere, gradually accumulating in the fatty tissues of humans and wildlife.
Once the connection between POPs and toxicity was scientifically proven, wealthy governments sprang into action to reduce the risks, eventually restricting or banning the use of 12 pollutants, including DDT and polychlorinated biphenyls (PCBs), at the 2001 Stockholm Convention on POPs.
Climatic forces were also helping to limit the chemicals' global reach.
In places like the Arctic, cold temperatures trapped POPs in snow, soil and oceans capped by sea ice, as the long-lived pollutants circled through the region. Between the POPs settling into the Arctic and other sinks — and the international campaign to regulate the chemicals — atmospheric levels of POPs steadily declined during the past decade.
New research, however, suggests that global warming is reversing this downward trend.
Climatologists at Environment Canada, the Canadian environmental agency, found that as climate change heats up oceans and melts sea ice and snow, the buried pollutants, known as legacy POPs, are being re-released back into the atmosphere.
This could undermine international treaties to restrict production and importation of the high-risk toxics and human exposure to them, the scientists say.
First to Make Warming Link
Over the years, various studies have indicated that a decrease in production of POPs due to regulation would result in lower levels of the toxics in the environment. This new paper was the first to consider whether climate impacts, like the melting Arctic, might be throwing a wrench into that assumption.
"An annoying feature of contaminant work in the Arctic in the early 2000s was that there was this notion of direct connectivity between the emissions controls [of POPs] and environmental exposure [to them]," said Robie Macdonald, a systems scientist at the Institute of Ocean Sciences, a division of Fisheries and Oceans Canada, who was not involved in the study.
"If you had a stable world, you could linearize everything," he explained. "But the problem is that the Arctic is changing rapidly. This paper is one of the first times scientists have actually put together a set of coherent data and looked at the trends responsibly in terms of how the [climatic] system is changing."
Macdonald said the research is key to developing more accurate ways to monitor the effectiveness of toxics regulation, such as the Stockholm Convention, especially since most governments and scientists only use linear atmospheric trends to measure whether chemicals policy is working.
The study, which was inspired by recent research, suggests that these trends have to be looked at in the context of changing ecosystems from global warming.
Teams of scientists from Canada and across the world had recently observed localized increases in atmospheric concentrations of POPs in the Arctic, even as the region was experiencing a general decline in levels following the Stockholm Convention. For example, a study earlier this year reported observed increases in the pesticide a-HCH over open water in the Hudson Bay and Beaufort Sea. A 2008 paper found an increase in PCBs along the edge of sea ice in the Atlantic Arctic.
The findings triggered earth systems scientist Jianmin Ma of Environment Canada and his colleagues to study whether there was a direct link between climate change and these localized rises in levels of POPs.
The researchers culled atmospheric data on POPs collected since the early 1990s at the Zeppelin Mountain Air Monitoring Station in Norway and the Alert Station in northern Canada. Plugging the data into a model, they first removed any factors affecting long-term trends of POPs not related to climate change, including the reductions that have been achieved from global agreements.
They found that since the early 2000s, higher levels of atmospheric POPs correlated with higher temperatures and lower sea ice extent, while lower POPs were linked to lower temperatures and greater sea ice.
"These chemicals are semi-volatile, meaning they don't stay in the air forever," Hayley H. N. Hung, a scientist at Environment Canada and co-author of the study, told SolveClimate News . "They drop down and get deposited into different materials. The organically rich, cold soil in the Arctic is a good trap for these chemicals, and POPs in the polar oceans are capped by sea ice, which literally puts a lid on the pollutants to keep them in place."
She continued: "When these deposits change — for example, when the sea ice melts — the POPs are 'uncapped' and are free to re-enter the atmosphere."
Predicting Future Trends
Ma and his colleagues then plugged the data of concentrations of POPs into models of three different Arctic-warming scenarios spanning the next five decades. The models showed significant increases in pollutant levels over the next 50 years, demonstrating that as snow pack and sea ice continue to melt in the coming decades, they will continue to discharge these stored POPs.
The re-release of the pollutants could have ripple effects worldwide. Once they are emitted back into the atmosphere, the POPs can catch a ride on the Earth's wind and ocean circulation systems, which can carry them as far south as Mexico and northern Africa.
This isn't just a problem for the Arctic, scientists say. POPs are also trapped in Antarctic snow and sea ice, although in less concentration since the majority of pollutants were emitted in Northern Hemisphere countries. As the Antarctic warms due to climate change, the Southern Hemisphere will likely get a fresh dose of newly unleashed toxics, the research suggests.
If these remobilized pollutants re-enter the food chain and start bioaccumulating once again as they move from one species to the next, the results could be disastrous for wildlife and humans.
The compounds have been shown to cause cancer, allergies, damage to the central and peripheral nervous systems, reproductive disorders and disruption to the immune system, according to the U.S. Environmental Protection Agency and the United Nations Environment Program.
The pesticide DDT, for instance, was present in such high levels in bald eagles in the mid-twentieth century that it thinned their eggshells, resulting in reduced fertility rates and a rapid decline in the species.
Because scientists don't know the actual concentration of POPs in the snow, oceans capped by sea ice and soil of the Arctic, the extent of this re-release remains unknown, said Hung.
"This paper is the beginning of a story," she said. "There were predictions that climate change would have an effect on POPs. Now we've shown that it does. Next, we need to figure out just how much of a change this will cause, both in the amounts being released and the impact it will have."