Action urged on ships' carbon emissions
Greenhouse gas emissions from shipping should be included in the UK's climate change budgets, the Committee on Climate Change has recommended.
Under the Climate Change Act, the UK is committed to cutting all its climate-changing emissions by 80% - based on 1990 levels - by 2050.
But international aviation and shipping emissions are not currently included.
If the government agrees, it will mean tighter targets for other sectors such as motoring and electricity generation.
"Shipping could account for up to 10% of emissions allowed under the 2050 target, and that says this is a material issue," said Committee on Climate Change (CCC) chief executive David Kennedy.
The CCC's report says there are many ways for shipping to curb its carbon footprint - by improving fuel efficiency, deploying kites or sails, or allocating vessels more efficiently.
Some companies are already developing such techniques.
The CCC has recommended - and the government has adopted - a series of carbon budgets setting down the maximum scale of greenhouse gas emissions that the UK can emit over successive five-year periods.
They are designed as staging posts on the way to the 2050 target.
If the government does agree to include shipping and maybe aviation in the budgets, then constraints on other sectors must become tighter.
"If you include shipping in the 2050 target - especially if you throw in aviation as well - that implies full decarbonisation of electricity, heat and surface vehicles," said Mr Kennedy.
"And if the ambition is full decarbonisation [of those sectors], then we need to make good progress in the next two decades, otherwise we can't achieve the 2050 target."
The committee will put its formal recommendation on shipping and aviation to the government next year, and the government says that it will respond "in due course". Under law, it must decide by the end of 2012.
Ins and outs
The committee's analysts spent three months attempting to calculate UK shipping emissions "from the bottom up", scouring records of 150,000 shipping movements into and out of UK ports by vessels including cargo ships, tugs, fishing vessels, ferries and cruise liners.
It believes the UK should be responsible for half of all the emissions associated with ships entering or leaving national ports - the other half being borne by whichever countries lie at the other end of the journeys.
Having crunched the numbers, the committee concludes that the UK's share is 12-16 million tonnes of carbon dioxide (MtCO2) per year.
Globally, shipping emissions are growing by 3-4% per year, and could account for a quarter of all the world's greenhouse gas output by 2050.
The International Maritime Organisation agreed earlier this year on a programme to progressively increase vessels' fuel efficiency.
The CCC is basically saying the UK should lead an international effort to go further and faster down this track.
The UK Chamber of Shipping, which worked with the CCC on its analysis, welcomed the conclusion, but warned of potential impacts on competitiveness.
"This work is hugely important," said David Balston, the organisation's director for safety and environment.
"We do stress, however, that any solution must be global rather than regional to avoid distorting world trade and potentially damaging an industry that is vital to the future prosperity of the United Kingdom."
Mr Kennedy also suggested the priority was to end up with a global system of carrots and sticks for decarbonising shipping, and urged UK ministers to press for such a deal at and after the UN climate talks that begin in South Africa at the end of the month.
If international action proved impossible, the European Union would almost certainly introduce measure for traffic in and out of European ports, he said.
Environment group WWF, together with Oxfam, recently issued a report recommending that some kind of global shipping tax be used to raise some of the $100bn per year of climate-related cash that rich countries are committed to providing to the developing world by 2020.
"International shipping has, like aviation, been left out of efforts to reduce greenhouse gas emissions for too long, said Keith Allott, WWF-UK's head of climate change.
"An international deal to address shipping could be a win-win - addressing a rapidly growing source of emissions and at the same time providing a valuable source of funding for tackling climate change in the developing world."
Follow Richard on Twitter