The Oyster’s Garter

Geoengineering – the deliberate manipulation of the earth’s atmosphere in order to mitigate global warming – seems to be gaining more credibility worldwide. Just today, Wired reports that an iron fertilization experiment is being conducted in the Southern Ocean by Indian and German scientists.

Though iron fertilization is the best-known geoengineering proposal (thanks to Planktos’ shenanigans), there are several other serious methods under discussion. In a commentary published in Nature Geoscience in November, Philip Boyd summarized current geoengineering proposals and ranked them based on effectiveness, cost, risk, and time.

Here are the proposals Boyd ranked:

• Iron fertilization. Deliberately stimulating plant growth in the ocean with the aim that the excess material will be permanently sequestered in the deep sea. This would remove carbon from the atmosphere.
• Stratospheric aerosols. Injecting sulfer particles into the upper atmosphere to mimic a volcanic explosion and physically deflect sunlight. This would not remove carbon from the atmosphere, but would lower the overall amount of  heat reaching Earth.
• Cloud whitening. Spraying seawater droplets into marine clouds in order to increase their reflectance of sunlight. Like the stratospheric aerosols, this would not remove carbon from the atmosphere, but would lower the overall amount of  heat reaching Earth.
• Atmospheric carbon capture. Removing carbon from the atmosphere with a chemical absorbent (“scrubbers”) and then sequestering it.
• Geochemical carbon capture. Removing carbon from the atmosphere by using brine pools to transform it to a dissolved or solid state.

Boyd provides an handy, color-coded guide to the risks and benefits of each type of geoengineering. The more colored blocks a scheme has, the “better” it is.

Based on Boyd’s work, the highest-ranked proposal is cloud whitening. It has a reasonably solid rationale based on observations of ship tracks, it is reasonably affordable & safe, it would rapidly lower the Earth’s temperature if it worked (“mitigation rate”), and can be quickly stopped if there are unintended side effects (“emergency stop”).

The lowest-ranked proposal is (SHOCK!) iron fertilization. Though it has a relatively solid rationale based on experimental data, it has many unpredictable side effects and cannot easily be stopped in an emergency. Oh, and it probably won’t actually work.

Though Boyd’s ranking is certainly oversimplified – for example, cloud whitening, even if it worked, would do nothing to halt ocean acidification – that’s intentional. Boyd intends his system to be used as a logical starting point to figure out which geoengineering schemes deserve more research, and which should be trashed. He writes:

A transparent assessment should strive to increase public confidence in any selected tools, a prerequisite for tackling the difficult questions and complex issues raised by geopolitical, social and economic risks. Such an assessment of all of the
well-established proposals is urgently needed but so far entirely lacking.

Right now, I don’t feel confident in any of these proposals. I am dubious that we understand the ocean and atmosphere well enough to tinker with them. But since the nasty consequences of climate change are already coming fast and furious, I’m willing to listen to sober assessments of geoengineering risks and rewards. Just hold the pseudo-scientific hubris.

Thanks to Hao for the Wired article and Geoff for the geoengineering paper!

Philip W. Boyd (2008). Ranking geo-engineering schemes Nature Geoscience, 1 (11), 722-724 DOI: 10.1038/ngeo348