Does sea spray keep Earth cool?

The cloud cover and haze caused by minute bubbles bursting on the surface of the sea may help to slow the pace of global warming, reports Dick Ahlstrom

Bursting bubbles on the sea could serve as a brake on climate change. Researchers in Ireland and Italy have identified a previously unknown source for cloud cover and haze that can reflect back sunlight and so reduce global warming.

The bubbles themselves don't make the difference; it is the organic matter they inject into the atmosphere when they burst that is important, says physics lecturer Prof Colin O'Dowd, of NUI Galway's Environmental Change Institute. The minute particles, known as aerosols, drift on the air and act like seeds for water droplets that accumulate to form clouds and haze.

O'Dowd and Dr Maria Cristina Facchini, of the Italian National Research Council's Institute of Atmosphperic Sciences and Climate, identified an unknown link between the growth of sea plankton and climate. They published their findings last week in the journal, Nature.

"What we found was that the largest source of aerosols in the marine environment is organic matter from the plankton, produced by bubble-bursting mechanisms at the surface," O'Dowd says.

They were studying the contribution made by aerosols to climate and the formation of clouds. They recorded the aerosols coming on to the coast at NUI Galway's Mace Head observatory, an important sampling point for European environmental scientists.

The long-held assumption was that iodine- and sulphur-based gases released by organisms such as plankton at the sea's surface made up most of the aerosols released into the atmosphere by the marine environment, he says. Their findings turned this assumption on its head.

The plankton's contribution is greatest in the spring and summer when warmer seas boost its growth. "This organic factor in the aerosol content has a large seasonal component," says O'Dowd. It never entirely goes away, however.

Organic matter at the surface is flung into the atmosphere to form aerosols when small bubbles burst at the surface in whitecaps tossed up by the wind. The aerosols, another name for small particles small and light enough to drift on the air, in turn become nucleation points for the water droplets that congregate to form clouds.

The finding is exceptionally important because the organic material represents a previously unknown source of aerosols. Its contribution is therefore left out of current models that attempt to predict future climate, says O'Dowd.

"Climate prediction models are huge for the development of policies and strategies against global warming," he says. "They are well-developed in terms of atmospheric gases but they are very poorly developed in terms of aerosols and associated cloud impacts. None of the current models take this into account."

The research showed that there was a tenfold increase in organic matter in the aerosols between winter and spring. Simulations run by the research team also showed that the extra organic derived aerosols could double the cloud droplet concentration and represented "an important component of the aerosol/cloud/climate feedback system".

This is why the discovery should have a major impact on climate models. Current thinking on global warming suggests that warmer sea waters will encourage plankton growth.

On the basis of O'Dowd's and Facchini's work, more plankton growth would mean more cloud cover and a greater potential for turning back solar radiation.

O'Dowd urges caution in relation to this feedback mechanism, however. There is no measure as yet of the strength of this feedback. Also, greenhouse gases live far longer in the atmosphere than aerosols, he warns.

"There is the potential for some degree of feedback," he says. "This should not be taken as a licence to emit pollutants as we wish."

The research does, however, clearly demonstrate the link between the marine biosphere and climate change, and the way in which living things can have a positive impact on our warming planet.