A new study by a team of German researchers from the University of Bremen has discovered that the sulphur exhaust from cargo ships produces a cooling effect on the atmosphere.
Shipping fuel is less refined than aeroplane fuel and contains more sulphur. As a result, tiny particles of sulphur dioxide contained in ship exhaust fumes create cloud tracks - similar to plane contrails - in the sky above a travelling vessel.
These tracks, however, tend to form where there is already some cloud cover - clear blue skies do not contain much moisture, so sulphur clouds form less easily.
When sulphur is pumped into a cloudy sky, the clouds get denser. These clouds then reflect radiation from the Sun back out into space, effectively cooling the area below.
Now, Mathias Schreier and his team have used satellite imagery to determine how these trails might be affecting global temperatures.
As part of the research, Schreier and his colleagues studied satellite pictures taken during 2004 to detect visible shipping contrails.
Then, using satellite measurements of the density of water droplets inside these tracks, they calculated how much they reduced the solar energy that reached the Earth.
The team estimates that the contrails reduced this energy by between 0.4 milliwatts and 0.6 milliwatts per square metre during the year.
Schreier said the results could alter predictions for future climate change, although some experts have noted that the effect is likely to be short lived, and so outweighed by the release of CO2 in the same plumes.
"There are also other wider issues such as the acidity of the air, or of the oceans. Increased sulphur dioxide in the atmosphere can result in acid rain," said Alice Bows of the Tyndall Centre for Climate Change Research in the UK.
Schreier further said his team was only able to quantify the effect of "visible" tracks. When a ship is hidden beneath a thick cloud, the effect cannot be seen from above.
As such, to quantify this hidden effect, the researchers made use of an analysis carried out by Axel Lauer of the German Institute for Atmospheric Research, Wessling.
Lauer estimated the total effect of sulphur emissions based on fuel sales, which he said was between 100 and 500 times greater than the effect observed by Schreier's team.
The study appears in the journal Geophysical Research Letters, reports New Scientist.