As much as a third of the nitrogen entering the world's oceans from the atmosphere is man-made, according to new findings by an international team of scientists. This has significant implications for global climate change because the nitrogen causes increased marine biological activity and CO2 uptake, which in turn produces the potent greenhouse gas nitrous oxide (N20).

Published in the journal Science on May 16, the research was led by Texas A and M University and the University of East Anglia (UEA).

It has long been known that man is enhancing the global nitrogen cycle through the use of fertilisers in agriculture and the burning of fossil fuels in power stations and cars. The effect of this on the land has been extensively studied. However, this is the first the time its impact on the open ocean has been properly quantified.

"Anyone concerned about climate change will be alarmed at the scale of man's impact on the world's oceans, as revealed by our new study," said Prof Peter Liss, an environmental scientist at the University of East Anglia.

"The natural nitrogen cycle has been very heavily influenced by human activity over the last century - perhaps even more so than the carbon cycle - and we expect the damaging effects to continue to grow. It is vital that policy makers take action now to arrest this.

"The solution lies in controlling the use of nitrogen fertiliser and tackling pollution from the rapidly increasing numbers of cars, particularly in the developing world."

'Impacts of atmospheric anthropogenic nitrogen on the open ocean' is published in Science on May 16. The paper is the culmination of a project involving 30 researchers from the UK, the US, Germany, Italy, China, the Netherlands, Switzerland, Canada and Chile.

The study found that increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for around one third of the ocean's external (non-recycled) nitrogen supply and up to three per cent of the annual new marine biological production.

While the increased biological activity has the beneficial effect of drawing down man-made CO2 from the atmosphere, the researchers found that around two-thirds of this is offset by the increase in harmful N20 emissions.

"This fertilization of the ocean by human activities has an important impact on the exchange of the greenhouse gases carbon dioxide and nitrous oxide and should be considered in future climate change scenarios," said Prof Robert Duce of Texas A and M University, lead author of the paper.