Sulphur emitted by ships may account for almost half the total going into the atmosphere in some parts of the oceans, according to US researchers.

The discovery suggest that this form of emission has a significant effect on climate change, by promoting cloud formation.

A team led by Spyros Pandis of Carnegie Mellon University in Pittsburgh, Pennsylvania, says sulphur from fossil fuel burnt by shipping in many areas almost equals the amount of sulphur flowing naturally from the oceans into the atmosphere.

The team, whose work is reported in the science journal Nature, says its discovery "forces a re-evaluation of our present understanding of sulphur cycling and radiative forcing over the ocean".

Quantifying shipping's role

It was known already that shipping made a significant contribution to sulphur emissions in some areas. What the team has done is to model how big that contribution actually is.

Shipping can account for nearly half the sulphur going from ocean to atmosphere

"Model calculations indicate that ship emissions account for greater than 60% of the July sulphur dioxide (SO2) prediction for large areas of the north Atlantic and north Pacific," the team reports.

"For most of the rest of the northern hemisphere oceans, ship emissions are responsible for greater than 30% of the predicted SO2.

"And in the southern hemisphere, ship contributions are generally less than 5%, except over large areas north and east of Australia where they contribute between 10 and 20%."

The researchers say that comparison of their modelling results with actual SO2 observations strengthens their conclusion.

Emissions from shipping can also have a marked effect on land, particularly in the form of acid rain. This is no surprise, as nearly 70% of ocean-going ship emissions occur within 400 km of land.

Aiding cloud formation

An important effect of ships' sulphur is the increase in the available nuclei upon which cloud drops form.

The researchers say the change in global atmospheric heat balance caused by the clouds which result from shipping emissions is appreciable.

The research will help to explain the part played by clouds

They put it at 14% of the estimated change caused by all sulphate from human activities in 1990.

Spyros Pandis said: "Sulphur emissions have a large role in the formation of aerosols, or tiny particles, on which water condenses to form clouds.

"The interactions of aerosols and clouds have been identified as one of the most important uncertainties in understanding the rate of climate change. This is because clouds reflect energy and thereby reduce the net warming effect of long-lived greenhouse gases."

Aerosols survive in the atmosphere for about a week, compared with decades and centuries for greenhouse gases.