Washington, Jan 8 (IANS) As if global warming was not alarming in itself, add one more sinister threat to the list -- mercury pollution.
It has already spurred public health officials to advise eating less fish, but it could become a more pressing concern in a warmer world.
Sue Natali, postdoctoral associate in botany at the University of Florida in a paper she co-authored compared mercury levels in soils under trees growing in air enriched with carbon dioxide to soil beneath trees in ambient air.
Carbon dioxide, the main greenhouse gas, has increased nearly 40 percent since the industrial revolution and is expected to continue climbing unless power plant and other emissions are drastically restricted or curtailed.
Natali's main finding: Soil samples from the carbon dioxide-enriched soil contained almost 30 percent more mercury -- apparently because the soil had greater capacity than soil in today's atmosphere to trap and hold on to mercury.
On the one hand, Natali said, increased capacity could slow the mercury's release into water -- its main conduit to aquatic wildlife and the fish that pose a hazard to people.
On the other, it means that even if policy makers manage to ban or severely restrict mercury emissions, the metal will remain a source of pollution for a long time.
'From the time you cut off mercury emission to the time it positively affects fish, you might have this lag, because the soils hold on to the mercury better,' Natali said.
Global mercury emissions today range from 4,400 to 7,500 tonnes per year, according to the Environmental Protection Agency. Natural sources such as volcanoes account for about half, with coal-fired power plants, smelters and incinerators contributing the remainder.
When mercury is belched into the air, it returns to Earth via rain, with bacteria and other natural processes converting it to methylmercury in lakes, rivers and oceans.
Methylmercury builds up through the food chain, with the flesh of the biggest, most sought-after predator fish - tuna, swordfish, king mackerel and so on - containing the highest concentrations, said a Florida release.
That's why the federal government has advised pregnant women, children and other groups considered vulnerable to limit their consumption.
Natali said scientists have long recognised mercury levels in soil spike under trees, averaging four times the concentration in open areas.
That's because trees effectively scavenge the poison from the atmosphere. Leaves and stems collect rainwater, and with it mercury; trees drop mercury-laden leaves on the ground, and trees take in the metal through their stomata, or breathing pores on leaves.
These findings appeared in a recent issue of Oecologia.
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