The ubiquitous one-celled ocean organisms, phytoplankton, play a significant and previously unknown role in warming the planet by capturing and absorbing the Sun's radiation, American researchers have found.

Researchers at the Scripps Institute of Oceanography in San Diego, California, used modelling and satellite imagery to show that solar radiation that would otherwise be reflected back into space was being absorbed by phytoplankton, contributing to global warming.

"Our paper shows that if we did not have phytoplankton in the ocean, we would have a cooler climate," said Dr Robert Frouin, a research meteorologist at Scripps, who worked with Dr Sam Iacobellis on the study. "Certainly the effect we have shown from phytoplankton is not negligible, so we need to look at it closely."

The Scripps study found that the radiation absorbed by phytoplankton raises global average temperatures by 0.1 to 0.6 degrees Fahrenheit (0.06 to 0.33 degrees Celsius) compared with computer models without phytoplankton, according to the paper in the current issue of the Journal of Geophysical Research.

Phytoplankton are microscopic plant-like organisms that free-float on the world's oceans, and are the major source of sustenance for animal life in the seas. They had not previously been associated with climate change.

The scientists argue that the 'phytoplankton effect' needs to be considered in future predictions of climate change. "Eventually, I hope that . . . will help policymakers make more far-sighted decisions," said Iacobellis, a member of the Climate Research Division at Scripps.

To further complicate the situation, the warming effect is not uniform. The natural variations in phytoplankton populations around the world will mean the climatological effect will also vary from region to region, they said.

In addition, some types of phytoplankton species absorb more radiation than others, and there are some types that reflect the Sun's radiation rather than absorb it.

The finding affects how humans interact with the ocean and climate, the paper argues. Changes in phytoplankton concentrations can affect the Earth's surface reflection of radiation from the Sun, which in turn influences how the planet's climate system interacts with human-generated greenhouse gases and aerosols.

The new findings will also need to be considered if 'iron fertilisation' of the oceans is ever used to reduce global warming, the researchers argue. Ocean-borne experiments suggest that if the oceans were seeded with vast quantities of iron sulphate (the lack of which limits phytoplankton growth), it would trigger a boom in phytoplankton rates and allow the organisms to draw more carbon dioxide from the atmosphere. But the new research indicates this could backfire.

Iron fertilisation would probably increase populations of phytoplankton. "Instead [of reducing global warming], you would contribute to warming the ocean by absorbing more radiation," said Frouin. The researchers argue that it would be important to calculate how much the initial decrease in temperature from fertilisation would be offset by the increase from radiation.

The calculations for the paper were averaged on a broad, global scale, and further detailed analyses need to be undertaken, they said. In particular, more research is needed on the complex biological feedback consequences that lead to more or less phytoplankton in certain areas.

"This just shows how intricate the climate system is," said Iacobellis. "It's like a ball of yarn all pushed together. It's difficult to unpiece the climate or put together what might happen in the future when all these things act together. One by itself may not be that important, but when thousands of these small things act together, what then?"

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