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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.