Smokestacks From Ships at Sea Effect Global Climate

Scientists creating models of global climate change have concentrated on the effects of humans on land. Researchers knew that there was a great deal of sulfur in the air over the oceans, potentially affecting climate change, but assumed that natural forces had produced the sulfur.

"What has been miscalculated is how much has been contributed by humans," Spyros Pandis told ENS. Pandis is an engineering professor at Carnegie Mellon University in Pittsburgh and author of the study, published in the August 18 issue of the journal Nature.

Sulphate aerosols, tiny drops of sulfur compounds in the air, reflect sunlight away from the earth's surface. They also increase cloud formation, which further shields the earth from the sun. Together, aerosols and clouds force a reduction in the global temperature, in an effect called radiative forcing.

Radiative forcing is a measurement of the amount of energy contributed to the Earth by a substance like a greenhouse gas. Greenhouse gases like carbon dioxide cause positive radiative forcing, because they increase the energy on the Earth's surface. But sulphate aerosols decrease the energy reaching Earth, causing negative radiative forcing. Decreased energy ultimately means decreased temperature.

Previous studies of radiative forcing from sulphates have concentrated on the natural transfer of sulfur from the ocean to the atmosphere. When plankton dies, for example, it gives off sulfur to the air. But Pandis's study shows that ship sulfur emissions from the burning of fossil fuels nearly equal this natural sulfur source.

Calculations of how much global warming is caused by humans have discounted this important human effect, Pandis says. "The assumption was that we didn't cause any changes over the middle of the ocean," he says. "We have been kind of missing one piece of the puzzle."

Scientists were aware that ships could affect cloud formation, says Pandis -- they just didn't realize how much of an effect ship emissions could have. Astronauts observing clouds sometimes noticed long, straight bright lines in the clouds. Researchers were able to trace these lines to the paths of ships passing beneath the clouds.

The smokestack emissions from the ships would increase cloud formation directly above the ships, changing the clouds' composition. But while these "experiments of opportunity" showed that ships were having an effect, it did not provide information on the amount of sulfur contributed by ships.

"The potential of changing cloud properties was seen -- it was the magnitude that was not appreciated," says Pandis.

Pandis and his colleagues at Carnegie Mellon and Duke University in North Carolina used recent studies of ship emissions to estimate the contributions by ships to atmospheric sulfur concentrations over the oceans. Their model suggests that ship emissions account for more than 60 percent of the sulfur in the air over the north Atlantic and north Pacific.

For the rest of the north hemisphere oceans, ships account for about 30 percent of the sulfur. In the southern hemisphere, ships account for more than 40 percent of atmospheric sulfur in the oceans around Australia, New Zealand, South Africa and Argentina. Elsewhere, the percentage is lower because fewer ships travel these areas.

All this sulfur from ships means that human produced global cooling may offset a significant proportion of human produced global warming. Pandis says most scientists estimate that human produced carbon dioxide has, over the past 100 years, added about 1.5 watts of energy to every square meter of the Earth. Human produced sulfur, by Pandis's best estimate, has subtracted about .15 watts of energy per square meter -- or about 10 percent.

"In a sense, these particles have been masking the warming of the carbon dioxide, offsetting part of it," Pandis says.

To make good predictions about global warming, scientists need to use all available data on the effects that humans are having on the environment. Emissions from the international shipping industry should be included in studies of marine and coastal atmospheres, as well as climate change models.

"We need to understand what is going on," Pandis says. "We need to have all of these major players in our models to get a better idea of how we will affect the Earth."