Iron fertilisation of the oceans would not substantially curb warming

5 October 2008

Large-scale manipulation of the environment to curb global warming - or geoengineering - will have a much smaller effect than previously estimated, at least with one much-touted solution.

Snapshot illustration of Southern Ocean salinity

A computer model showing the Southern Ocean surrounding Antarctica.

'Fertilising' parts of the Southern Ocean with an iron sulfate solution has been talked about as one method of slowing climate change. In theory, adding iron promotes growth of marine algae.

This has two effects. First, more algae means more carbon dioxide from the atmosphere is drawn down into the ocean.

It is not a big an impact as people are optimistically predicting.

But a second effect may be much larger. These tiny marine algae release a chemical called dimethyl sulfide, or DMS, to the atmosphere. DMS gives the ocean its distinct smell. Through further chemical reactions, DMS changes the optical properties of clouds, making them more reflective. So, in theory, more sunlight bounces back out to space, cooling the region.

Recent research (Wingenter 2007) has suggested that fertilising just two per cent of the Southern Ocean would lead to a significant regional cooling of 2°C, largely by the DMS influence.

On further reflection

But when Matthew Woodhouse and colleagues from the University of Leeds looked at how DMS might affect the reflectivity of clouds, their computer models estimated just a modest change in the number of small particles appearing in the atmosphere following fertilisation. It is these particles that influence cloud optical properties.

"Seeding the Southern Ocean with iron could lead to just 1·4 per cent more cloud condensation nuclei in the atmosphere, compared to a previous estimate of 10 per cent," says Woodhouse.

"This could still be important, but it is not a big an impact as people are optimistically predicting."

The results, published in the journal Atmospheric Environment, are yet to be tested in climate models, but Woodhouse plans to conduct further investigations with Met Office models next year.

Woodhouse adds that this is such a new area of investigation that all figures will undoubtedly be re-evaluated in the coming years.