Ocean nitrogen only limited help for climate: study

By Alister Doyle, Environment Correspondent

OSLO (Reuters) - Rising amounts of nitrogen entering the oceans from human activities are less beneficial than previously thought as a fertiliser for tiny fertilizermarine plants that help slow global warming, scientists said on Thursday.

"As much as a third of the nitrogen entering the world's oceans from the atmosphere is man-made," according to a team of 30 scientists writing in the journal Science.

"It's not as good a thing as some people would like it to be," said Peter Liss, of the University of East Anglia in England which led the study with Texas A&M University.

Fossil fuels burnt in cars, factories or power plants release nitrogen that can be absorbed by the seas. And nitrogen-based fertilizers are often washed off farmland and end up in the sea.

In theory, extra nitrogen acts as a fertilizer to spur growth of microscopic plants that absorb carbon dioxide, the main greenhouse gas blamed for global warming.

But the study, by researchers in Germany, Italy, China, the Netherlands, Switzerland, Canada and Chile as well as the United States and Britain, said there were unwanted side-effects.

And rising levels would only have a muted benefit in slowing climate change that the U.N. Climate Panel says will spur more heat waves, storms and raise sea levels.

"Extra nitrogen draws down carbon dioxide which is good news from the climate perspective," Liss told Reuters. "The bad news is that two-thirds (of the carbon absorbed) is offset by the production of another greenhouse gas, nitrous oxide."