The world should widen a fight against global warming by curbing a string of pollutants other than carbon dioxide, the main greenhouse gas, the U.N. Environment Programme said on Friday.

Heat-trapping methane, nitrogen compounds, low-level ozone and soot are responsible for almost half of the man-made emissions stoking climate change in the 21st century, it said.

A wider assault on pollutants, twinned with cuts in carbon dioxide, would help toward a new U.N. climate pact due to be agreed in December and have other benefits such as improving human health, raising crop yields and protecting forests.

The science is showing us that global warming is happening faster and on a greater scale than anticipated, UNEP executive director Achim Steiner told Reuters on the sidelines of a World Climate Conference in Geneva.

There are other avenues by which we can move forward than cutting carbon dioxide, the main focus of a planned new U.N. climate deal to be agreed in Copenhagen in December. And there are multiple benefits.

Soot or 'black carbon', for instance, is among air pollutants blamed for killing between 1.6 and 1.8 million people a year, many from respiratory diseases caused by smoke from wood-burning stoves in developing nations.

FISH STOCKS

And ozone, a component of smog often linked to emissions of fossil fuels, has been blamed for loss of more than 6 billion euros ($8.56 billion) worth of crops in the European Union in 2000. U.S. studies suggest it cuts annual U.S. cereals output by 5 percent.

Nitrogen compounds, from sources such as sewage and inefficient use of fertilizers, stoke global warming and can cause dead zones in the oceans that cut fish stocks.

And methane, which comes from sources such as deforestation and livestock, contributes up to 20 percent of all greenhouse gas emissions.

We believe that those involved in the negotiations (on a new climate pact) should broaden their field of vision, Joseph Alcamo, UNEP chief scientist, told a news conference. It's not just a matter of carbon dioxide and energy.

Many of the non-carbon dioxide pollutants are not regulated by international treaties.

The U.N.'s existing Kyoto Protocol for combating global warming, for instance, sets limits only for developed nations on emissions until 2012 of six gases including carbon dioxide, methane and nitrous oxide.

(Editing by Ralph Boulton)

Global warming