Cleaner and better-managed seas and coasts would help boost economic growth and reduce poverty and pollution, a United Nations Environment Programme (UNEP) report said on Wednesday.
The report, produced with several other UN organisations, highlights the huge potential of a marine-based economy some five months before world governments meet to discuss pathways to more sustainable development at a UN conference in Rio de Janeiro, Brazil.
Around 40 per cent of the global population lives within 100 kilometres of a coast so the world's marine ecosystems provide essential food, shelter and jobs to millions of people.
But pollution from oil spills, fertilisers, waste, sewage and chemicals, as well as over-fishing, have damaged the health and productivity of the seas.
By using oceans to generate renewable energy and eco-tourism and shifting to more sustainable fisheries and transport, that trend could be reversed and islands in Asia and the Caribbean could reduce their vulnerability to climate change, UNEP said.
''Stepping up green investments in marine and coastal resources and enhancing international co-operation in managing these trans-boundary ecosystems are essential if a transition to low-carbon, resource efficient green economy is to be realised,'' said UNEP executive director Achim Steiner.
The report recommended key steps for ''greening'' the seas across areas such as tourism, fishing, transport, pollution, renewable energy and deep-sea mining.
Fertilisers such as nitrogen and phosphorous have helped to increase crop yields but their use has led to the degradation of marine ecosystems and groundwater.
The amount of nitrogen reaching the oceans has risen three-fold from pre-industrial levels and this could rise by almost another three times by 2050 if no action is taken.
Marine pollution costs US$100 billion ($123 billion) in the European Union alone, the report said, but this could be reduced by stricter regulation of fertiliser use, removal and management and through subsidies to encourage nutrient recycling.
The world economy could also gain up to US$50b a year by restoring fish stocks and reducing fishing capacity, according to the UN Food and Agriculture Organisation and the World Bank.
Investments in fuel-efficient fishing methods and environmentally-friendly feeds for aquaculture systems would help reduce the fishing sector's carbon footprint.
Cleaner fuels and more energy-efficient ship designs could help cut emissions from the maritime industry which currently accounts for almost 5 per cent of global C02 emissions.
This is expected to increase by 72 per cent by the end of the decade as demand for seaborne goods hits all-time highs.
Ships are also responsible for transporting harmful organisms in their ballast water which can harm marine ecosystems. The effects of the transfer in nuisance and on exotic aquatic species is estimated to cost US$100b a year.
Wind, wave and tidal power could create many jobs and help reduce greenhouse gas emissions, but in 2008 these technologies represented only 1 per cent of total renewable energy production.
Installed capacity is not likely to rise significantly until after 2020 as technology costs remain high and most projects are still in the demonstration phase.
Greater financial incentives are needed to encourage the private sector to develop more plants and bring costs down.
The report said deep-sea mining for minerals was a possible new revenue stream which could help boost the world's economy and relieve some of the burden on the terrestrial environment.
But the deep sea is a largely unknown environment and badly managed mining could put more pressure on already stressed marine ecosystems.