Global emissions of carbon dioxide from shipping are twice that of aviation, a report by Intertanko, which represents the majority of the world's tanker operators, has said.
The report said emissions have risen sharply in the past six years.
Previous International Maritime Organisation estimates suggested emission levels comparable with those from aeroplanes.
Intertanko said its figures were the most realistic estimation of the current levels of CO2 emissions from ships.
It said the growth in global trade coupled with ships burning more fuel to deliver freight faster had contributed significantly to the increase.
Intertanko technical director, Dragos Routa said, the figures were a work in progress, but the emission levels had risen sharply, nevertheless.
According to a BBC report, some 90,000 ships from tankers to small freighters ply the world's oceans.