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Introduction

Geoengineering is a name for various proposals to deliberately manipulate the Earth’s climate to counteract the effects of global warming. The National Academy of Sciences has defined geoengineering as “options that would involve large-scale engineering of our environment in order to combat or counteract the effects of changes in atmospheric chemistry”:

• Committee on Science, Engineering, and Public Policy (COSEPUP), National Academy of Sciences, Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base, Section 28: Geoengineering, 1992.

The 2007 IPCC report concluded that geoengineering options, such as ocean fertilization to remove CO2 from the atmosphere, remained largely unproven. It was judged that reliable cost estimates for geoengineering had not yet been published:

• IPCC Fourth Assessment Report: Climate Change 2007, Working Group III, Mitigation of Climate Change, C. Mitigation in the short and medium term (until 2030).

However, there is still considerable interest in geoengineering, because many people deem it difficult to reduce carbon emissions in time to prevent dangerous global warming.

Example proposals

• Enhanced weathering. Although relatively small-impact compared to some other schemes, still a huge projhect project that would involve global scale modification of the planet’s feedback mechanisms.

• Seeding the ocean with iron to increase phytoplankton populations. populations. The idea is that iron availability is the bottleneck preventing increasing phytoplankton growth, which would take more carbon dioxide from the atmosphere. This plan appears to depend upon the phytoplankton sinking to the ocean bottom, sequestering carbon dixoide. What happens otherwise is unclear.

• Increasing cloud cover to increase sunlight reflection via spaying machines at sea. The article notes one modelled side-effect of this would be increased preciptation precipitation over landmasses.

• Sunlight reflection from the upper atmosphere. Placing small particles of certain kinds in the upper atmosphere is apparently a relatively inexpensive way of reflecting a significant fraction of incoming sunlight back into space. Early proposals suggested sulphate particles, but these may react chemically with ozone in an adverse way. An alternative proposal is suspension of tiny, harmless particles (sized at one-third of a micron) at about 80,000 feet up in the stratosphere.

There is an analysis by Tim Lenton of the University of East Anglia which concludes that only geoengineering that reflects sunlight are able to have a large enough effect over a sufficiently short time interval. (He also projects that most of these are mechanisms that need continually replenishing, and in the event of discontinuing an even more dramatic rise in temperatures could occur.)

References

• Geoengineering, Wikipedia.

• Geoengineering faces ban is an excellent article in Nature News summarising some of the latest geoengineering developments.