So far, shipping has avoided the same high-profile attention as the aviation sector, which accounts for about two percent of world emissions of climate warming carbon dioxide, but industry leaders say it is fast coming under the mainstream microscope.

"It is already facing that sort of pressure," said Bill Box, of the International Association of Independent Tanker Owners, one of the world's biggest tanker industry groups.

"A lot of transport industries have been working on air pollution and the shipping industry may be slightly behind on that but it's certainly catching up now."

No accurate figure of shipping's contribution to CO2 emissions exists but industry experts estimate it is slightly less than that caused by air travel.

The United Nations' International Maritime Organisation (IMO) in July launched a comprehensive study to assess its impact and hopes to reach a definitive conclusion by the end of the year.

"MORE CONSPICUOUS"

"As land based sources are targeted intensely in many areas of the world and shipping activities are growing together with the global economy, the contribution of ship emissions to air quality problems is growing percentage wise and is becoming more conspicuous," an IMO spokeswoman said.

"Shipping is now being targeted vigorously by local and regional regulators in parts of the world to help solve local air quality problems."

Moreover, the trillion-dollar industry, which carries around 90 percent of world trade by volume on about 50,000 merchant ships, also accounts for about 10 percent of global sulphur dioxide emissions, a cause of acid rain, as well as large amounts of toxic nitrous oxide and particulate emissions.

The International Council on Clean Transportation, made up of transport and air quality officials from around the world, estimates that by 2020 shipping will produce more sulphur and nitrogen oxides than all land-based sources in the EU combined.

In an indication of what might come, environmental campaign group Friends of the Earth said last week it was suing the US Environmental Protection Agency for failing to meet a deadline to regulate air pollution from large ships.

The group said emissions from global shipping were projected to double in North America in the next decade, exposing communities to fumes that contributed to respiratory illness, cancer, heart disease, and premature death.

"In Los Angeles alone, the ships in port spew more pollution than the metro area's six million cars combined," said Sarah Burt of Earthjustice.

Industry groups argue that shipping is at least two or three times cleaner than road or rail transport and around 20 times more environmentally efficient than air transport, and should be seen as part of the solution rather than part of the problem.

But there is an acceptance that the issue must be tackled.

"Addressing carbon emissions is indeed a major challenge, especially as maritime trade is expected to continue expanding," the International Chamber of Shipping said in its 2007 annual review.

BP's Gregory said it was essential that the shipping industry came up with its own regulations before prescriptive measures were imposed by regulators or other stakeholders.

"This is almost an exponential curve of attention, of regulation and of response," he said.

"What we need to do is calm that curve by responding more rapidly, and thereby alleviating some of the concerns that various stakeholders are raising with the shipping industry.

WHAT TO DO?

What action the industry should take is less clear.

"Everyone accepts that something needs to be done, it's exactly what needs to be done," Box said.

Some advocate switching from dirty high sulphur fuel to cleaner burning-distillate fuels. However, senior industry figures argue this would lead to an increase in CO2 emissions in the manufacturing process at oil refineries.

Another idea is to use exhaust scrubbing devices which reduce sulphur dioxide and other particulate emissions, but this could be expensive, costing the industry billions of dollars.

Another suggestion is for new markets to trade emissions of sulphur or greenhouse gases.

"I don't think there will be a silver bullet, a single answer that will solve the industry's problems. I think we will be operating off a menu of solutions," Gregory said.

The IMO also said it was essential to thrash out a long-term globally-accepted strategy.

"The shipping industry, constantly moving between different jurisdictions, cannot tolerate operating in a patchwork of differing regulations," the IMO spokeswoman said.

Gregory said having some certainty was key.