The warming of the Arctic is releasing toxic chemicals that had been trapped in the ice and cold water, scientists have discovered.

The researchers warn that the amount of the poisons in the polar region is unknown and their release could "undermine global efforts to reduce environmental and human exposure to them".

The chemicals seeping out as temperatures rise include the pesticides DDT, lindane and chlordane as well as the industrial chemicals PCBs and the fungicide hexachlorobenzene (HCB). All of these are know as persistent organic pollutants (Pops), and are banned under the 2004 Stockholm convention.

Pops can cause cancers and birth defects and take a long time to degrade. Over past decades, the low temperatures in the Arctic trapped volatile Pops in ice and cold water. But scientists in Canada and Norway have discovered that global warming is freeing the Pops again. They examined measurements of Pops in the air between 1993 and 2009 at the Zeppelin research station in Svalbaard and Alert weather station in northern Canada.

After allowing for the decline in global emissions of Pops, the team showed that the toxic chemicals are being remobilised by rising temperatures and the retreat of the sea ice, which exposes more water to the sun. The scientists' work is published in the journal Nature Climate Change.

Hayley Hung, at the air quality research division of Environment Canada and one of the team, said their work provided the first evidence of the remobilisation of Pops in the Arctic. "But this is the beginning of a story," she said. "The next step is to find out how much is in the Arctic, how much will leak out and how quickly."