By Matt McGrath
Environment reporter, BBC News
A report prepared by Intertanko, which represents the majority of the world's tanker operators, says emissions have risen sharply in the past six years.
Previous International Maritime Organisation estimates suggested levels were comparable with those of planes.
Some 90,000 ships from tankers to small freighters ply the world's oceans.
Intertanko says its figures are the most realistic estimation of the current levels of CO2 from ships.
Its estimate suggests that the world's shipping uses between 350 and 410 million tonnes of fuel each year, which equates to up to 1.2 billion tonnes of carbon dioxide emissions.
Intertanko says that growth in global trade coupled with ships burning more fuel to deliver freight faster has contributed significantly to the increase.
Dragos Routa, the technical director of Intertanko, told the BBC the figures were a work in progress but the levels of emissions had risen sharply.
While there are few accurate measures and even fewer restrictions on the amounts of carbon dioxide that ships can emit at present, governments in many parts of the world are considering a clampdown as part of their efforts to tackle global warming.But Mr Routa argued that the much greater tonnage carried by each vessel, compared with aircraft, meant that shipping was still a much greener form of transporting freight around the globe.