Shipping is slowing climate change by spewing out sunlight-dimming pollution but a clean-up needed to safeguard human health will stoke global warming, according to experts.
"So far shipping has caused a cooling effect that has slowed down global warming," said Jan Fuglestvedt, of the Centre for International Climate and Environmental Research Oslo.
"After some decades the net climate effect of shipping will shift from cooling to warming" because of cleaner fuels, he and colleagues in Germany, Britain and Norway wrote in this week's edition of the journal Environmental Science and Technology.
Toxic sulphur dioxide emitted by burning bunker fuel accounted for the deaths of an estimated 60,000 people worldwide in 2001 through cancer and heart and lung disease, according to a previous study. A clean-up would save thousands of lives.
But sulphur pollution from the fast-growing shipping industry also helps create clouds by providing tiny seeds around which droplets form. Clouds have a cooling effect since sunlight bounces off their white tops. The scientists argued against deliberate use of pollution from ships as part of possible schemes to shield the planet from sunlight, saying it was too risky and outweighed by the impact on human health.
"The available evidence suggests that 'climate cooling' by continued shipping emissions of sulphur dioxide would not be advisable," they wrote.
A clean-up of sulphur from ships will have a "double warming" effect - there will be more sunlight with less pollution and there will be ever more carbon dioxide, the non-toxic greenhouse gas emitted by burning fuel.
Shipping accounts for about 3.3 per cent of world carbon dioxide emissions from human sources, emissions the UN Climate Panel says will cause more droughts, floods, heat waves, rising sea levels and disease.
Some scientists, such as Nobel Prize winner Paul Crutzen, have suggested dumping sulphur in the upper atmosphere to slow global warming, one of several proposals for deliberate "geoengineering" to alter the climate system.