Substantial quantities of the pesticide DDT are still being released from the world's oceans, despite widespread restrictions on its use during the 1970s, a computer simulation has revealed.
According to a report in Nature News, the scientists created a computer model to simulate the circulation of DDT between ocean and atmosphere between 1950 and 2002.
This revealed that since the 1970s, the re-emission of DDT from the ocean has become greater than from the three known modern releases of new DDT: its continued use in some countries for malaria control; degrading storage canisters; and other pesticides that contain DDT as a contaminant.
The calculations show that although remaining DDT use today tends to be in the southern hemisphere, its concentrations are actually growing in the northern hemisphere as it moves through the world's oceans and atmosphere.
An estimated 1.5 million tonnes of DDT were used worldwide between the 1940s and 1970s, both as an agricultural insecticide and to control disease-carrying insects such as mosquitoes - the chemical was a key weapon in the war against malaria, for example.
But, DDT is toxic to a wide range of aquatic life, and its eggshell-thinning effects also had a drastic impact on many bird species.
Concerns about its environmental toxicity led to a series of countries banning the agricultural use of DDT through the 1970s.
There has been a drastic decline in DDT's use since then, but its legacy is still very much with us, according to Irene Stemmler and Gerhard Lammel of the Max Planck Institute for Chemistry in Mainz, Germany.
DDT is continually re-entering the atmosphere from the ocean, before being dissolved again in a recurring cycle.
"DDT is simply sitting there, waiting for the next cycle," Lammel said.
A greater concern is that DDT is migrating towards northern latitudes because the pesticide evaporates more rapidly from warmer, southern waters, thus increasing concentrations in cooler seas over time.
This is of ecological concern because marine organisms concentrate DDT by factors of millions as it moves up the food chain, reaching levels where it can have toxic effects on fish, for example - or the animals that eat them.