Published online 26 September 2007 | Nature | doi:10.1038/news070924-8


Mixing the oceans proposed to reduce global warming

Could nutrients from the deep help remove carbon dioxide from the air?

Some coastal regions have natural upwelling, where nutrient-rich bottom waters surge to the top to feed local plant life.Some coastal regions have natural upwelling, where nutrient-rich bottom waters surge to the top to feed local plant life.JPL / NASA

Could mighty pumps be installed in the ocean to mix up the waters and cool the planet? At least some scientists and businessmen believe so — but the idea is controversial.

In a letter to the editor published in Nature this week1, James Lovelock and Chris Rapley suggest that this deus ex machina could be an "emergency treatment for the pathology of global warming". Large vertical pipes could, they say, be used to mix nutrient-rich waters from hundreds of metres down with the more barren waters at the surface. This could cause algal blooms at the surface, which would consume carbon dioxide (CO2) through photosynthesis. When the algae die, some of this carbon could sink into deep waters. The algae may also produce chemicals that spur cloud formation, further cooling the planet.

Lovelock is the well-known author of the Gaia theory, according to which the Earth is able to heal itself from environmental disturbances. Rapley, a former head of the British Antarctic Survey, is director of the London Science Museum.

Their 300-word letter to Nature is not peer-reviewed research or even a firm proposal, and they note it may fail or cause unwanted side effects. But the point, they say, is that the "stakes are so high" that they suggest using the Earth's own energy to help the planet heal itself.

The idea may seem far-fetched. But a wave-driven 'ocean upwelling system' to absorb CO2, very similar to what Lovelock and Rapley are proposing, is currently being developed by a company called Atmocean, based in Santa Fe, New Mexico.

Phil Kithil, chief executive officer of Atmocean, estimates that a pump-driven up-welling system, if deployed across 80% of the world's oceans, could help bring down to the ocean floor an additional 2 billion metric tonnes of carbon per year, potentially doubling the ocean's annual rate of CO2 sequestration. The company has developed floating tubes, 3 metres in diameter and 300 metres long, that it claims can do just this.

Putting it the test

The notion of fertilizing the ocean to increase biological productivity and decrease atmospheric CO2 is not new. And it is just one of several controversial geo-engineering techniques, including putting mirrors in space to reflect sunlight, that have been proposed as means to cool the climate (see 'Is this what it takes to save the world?').

Scientists and policy-makers are discussing various scientific and legal aspects of ocean fertilization at a conference this week at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts. "The timing of this proposal is great," says David Karl, a microbial biologist and oceanographer at the University of Hawaii at Manoa, who has led a lab study of the effect of nutrient availability on algae growth2.

Karl plans to conduct a large-scale field experiment using a pump supplied by Atmocean off the coast of Hawaii next year. He will investigate the consequences of mixing deep and surface ocean waters, and estimate the balance between carbon brought up and carbon sunk down. "This is the critical issue," he says.

Net gain or loss?

But some scientists fear that artificial mixing may ultimately release CO2, rather than removing the greenhouse gas from the atmosphere.


"The concept is flawed," says Scott Doney, a marine chemist at WHOI. He says it neglects the fact that deeper waters with high nutrients also generally contain a lot of dissolved inorganic carbon, including dissolved CO2. Bringing these waters to the lower pressures of the surface would result in the CO2 bubbling out into the air. So contrary to the desired effect, the scheme could result in a net 'outgassing' of CO2, he warns. "There is no technological fix for this problem," he says.

Others say such a project would have no net effect on CO2 in the atmosphere. "At every meeting I've been to, when they have talked about this idea for surface ocean CO2 removal, the point has been made that you would bring up nutrients and inorganic carbon in the same ratio as you remove as biomass," says Ken Buesseler, a marine chemist at WHOI. And there are potentially many harmful impacts on sea life, he says.

But according to Karl, the idea should not be dismissed without even giving it a try. "It's just too nice, conceptually — even though technically it may be difficult to tune," he says.

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  • References

    1. Lovelock, J. E. & Rapley, C. G. Nature 449, 403 (2007). LINK
    2. McAndrew, P. M. et al. Mar. Ecol. Prog. Ser. 332, 63-75 (2007).