Mounting pressure on the shipping industry to reduce harmful exhaust emissions has forced a global shipowners' lobby group to call for a set of unified international standards.
The Baltic and International Maritime Council (BIMCO) said unified standards were needed to avoid the sort of controversy troubling airlines.
BIMCO also expressed concern as to whether there would be enough low sulphur fuel available when tougher regulations kicked in.
'There needs to be a global road map on pollution,' said Yudhishthir Khatau, BIMCO president. He said that with different emission standards mooted in Europe, North America and Asia, compliance would become 'very difficult'.
'Ships going into different regions can't change fuels,' Khatau said.
Ocean vessels are a big source of greenhouse gas emissions worldwide. Ship exhaust accounts for 18-30 per cent of nitrogen oxide pollution and 9 per cent of sulphur oxide pollution. The percentages will increase significantly by 2020 if no action is taken, according to the International Maritime Organisation (IMO).
Governments around the world are planning to introduce measures to control ship emissions.
Executives at BIMCO, which represents about 66 per cent of global shipowners, said different regions having different standards would lead to international disputes.
Arthur Bowring, managing director of the Hong Kong Shipowners Association, said: 'There is a row over the airline emissions-trading scheme, but there is also a simmering row about the regulation of shipping greenhouse gas emissions by Europe. The European Commission is under pressure from legislation to regulate shipping greenhouse gas emissions, since the IMO has not come up with an acceptable regulation. There are now calls by other governments and by [the] industry for the [commission] to put its effort into developing suitable regulation in the IMO rather than trying to do its own thing.'
Some areas set the limit of sulphur content in fuel to 0.5 per cent while others plan to introduce a much tougher cap of 0.1 per cent.
Torben Skaanild, BIMCO secretary general, said there was no assurance there would be adequate supplies of low-sulphur fuel.
An emission-control area in North America will start to be enforced from August when ships within 200 miles of the United States and the Canadian coast will have to use fuel with a sulphur content below 1 per cent. This will be cut to 0.1 per cent sulphur content by 2015.
Hong Kong will start offering rebates to ship firms whose vessels use fuel with sulphur content of 0.5 per cent and under while berthed in the city. The marine heavy fuel that most ships use now has a sulphur content of up to 4.5 per cent.
Apart from the three-year incentive scheme, the city has no tough controls on ship emissions.
At least 17 shipping lines including Orient Overseas Container Line, Cosco Container Lines, Nippon Yusen Kaisha and Hoegh Autoliners have opted to join the rebate scheme.
Shenzhen's environment commission said the city would announce its own ship emission policies by the end of this year. Ships using fuel with a sulphur content below 0.5 per cent are expected to get a reduction in port and related charges.
The Shenzhen authorities said Hong Kong and Guangzhou would also announce their respective plans by the end of 2013 on the mandatory use of low-sulphur fuel up to 24 nautical miles from the coast.
The IMO plans to introduce a set of global standards by 2020, pending an assessment in 2018 to see if there is sufficient supply of low-sulphur fuel.
BIMCO's Khatau said the planned use of low-sulphur fuel 'on the high seas from 2020 was creating a lot of uncertainty' over whether there would be sufficient fuel available.
Veronica Booth, senior project manager at Civic Exchange, said: 'For regions that may find the IMO process slow or perhaps cumbersome, the possibility of implementing a low emission zone in territorial waters that mimics an emission-control area may be appealing.
'Such regulation should be at 0.1 per cent sulphur content, as emission-control areas will tighten to 0.1 per cent by 2015.'
Michael Britton, general manager for Asia at container line Hamburg S?d, said more should be done to increase the supply of low-sulphur fuel in Hong Kong.
Approximate low-sulphur fuel threshold that the IMO plans to introduce under a set of global standards by 2020