The United Nations Environment Program (UNEP) said there was an urgent need to calculate risks of local pollutants such as soot, or "black carbon", in the Arctic. Soot darkens ice, making it soak up more of the sun's heat and quickening a melt.

Companies such as Shell, which this week gave up a push to find oil this year in the Chukchi Sea as the winter closed in, Exxon or Statoil say they are using the cleanest available technologies.

But the risks of even small amounts of pollution on the Arctic Ocean, emitted near ice with little dispersal by winds, have not been fully assessed.

"A lot of the concerns need urgent evaluation," said Nick Nuttall, spokesman of Naibori-based UNEP, referring to issues such as flaring of gas or fuels used by vessels in the Arctic.

"There is a grim irony here that as the ice melts...humanity is going for more of the natural resources fuelling this meltdown," he said. Large amounts of soot in the Arctic come from more distant sources such as forest fires or industry.

The extent of sea ice on the Arctic Ocean has shrunk this summer to the smallest since satellite records began in the 1970s, eclipsing a 2007 low. The melt is part of a long-term retreat blamed by a U.N. panel on man-made global warming, caused by use of fossil fuels.

"We're working to get a better documentation of the risks of black carbon in the Arctic," said Lars-Otto Reiersen, head of the Arctic Monitoring and Assessment Program (AMAP), part of the Arctic Council.

An AMAP report last year said that "regulation of black carbon production from all sources, especially those resulting locally from activities in the Arctic, is required at all scales."

400 FIELDS

More than 400 oil and gas fields within the Arctic region were developed by 2007, according to AMAP, mostly in West Siberia in Russia and in Alaska. Most of the undiscovered oil and gas is now estimated to be offshore.

Soot is an extra problem for planners, adding to risks such as of an oil blowout or a shipwreck. The U.N.'s International Maritime Organization is trying to work out a new "Polar Code" that might tighten everything from emissions to hull standards.

Still, for shipping, use of the Arctic route may be less damaging overall in terms of global warming, including soot, since it is a short-cut between some Atlantic and Pacific ports. That means ships burn much less fuel on the route.

"We are working on the net effect of the Arctic route compared to the Suez Canal," said Jan Fuglestvedt, of the Center for International Climate and Environmental Research in Oslo.

In 2009, the Bremen-based Beluga Group sailed from South Korea to Rotterdam across the Arctic, cutting 4,000 nautical miles off the route via Suez. This year, for instance, an icebreaker became the first Chinese vessel to cross the ocean.

One study indicated that increased use of the Arctic route might limit carbon dioxide emissions for global shipping by 2.9 million tons a year by 2050, or 0.1 percent, compared to use of the Suez Canal.

"If the Arctic route is really open by then it may reduce carbon emissions a bit on the global scale," said Leif Ingolf Eide, an author of the study at Norwegian-based risk management group DnV. The study did not assess soot, he said.

In a 2011 report, UNEP estimated that a global crackdown on soot, methane and ozone could slow global warming by 0.5 degree Celsius (0.9F). It would also protect human health and promote crop growth.

Almost 200 nations have agreed to limit climate change to below 2 degrees C (3.6F) above pre-industrial times, seeing it as a threshold to dangerous changes such as more droughts, floods or rising sea levels.

(Reporting By Alister Doyle; Editing by Rosalind Russell)

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