Can geoengineering reverse "irreversible" climate change?

So a fair degree of warming is inevitable, eh? That's the conclusion of a PNAS paper making the rounds this week. (I wrote about it yesterday.) But just how "irreversible" are the coming changes? As Arthur C. Clarke said, "When he states that something is impossible, he is very probably wrong."

The answer can be found in the same PNAS paper, in which the authors qualify their outlook for the next millennium by noting that "we do not consider geo-engineering measures that might be able to remove gases already in the atmosphere or to introduce active cooling to counteract warming."

In one of those curious coincidences, a pair of climate researchers have just come out with one of the first detailed examinations of geo-engineering schemes. The University of East Anglia's Tim Lenton, who has carved out a little niche as one of the leading explorers of climate "tipping points," and N. E. Vaughan of the Tyndall Centre for Climate Change Research compare a long list of ideas in Atmospheric Chemistry and Physics Discussions.

They break them into short-wave and long-wave geoengineering options, depending on whether the option tries to prevent short-wave radiation from reaching the Earth's surface, or it's an attempt at "interfering with a dynamic carbon cycle" that puts heat back into the atmosphere. They claim to have found several errors in previous efforts to calculate just how much radiative forcing can be offset by several schemes, and produce a handy ranking of the relative, theoretical potential of each.

The UEA press office produced this summary of the key findings:

• Enhancing carbon sinks could bring CO₂ back to its pre-industrial level, but not before 2100 - and only when combined with strong mitigation of CO₂ emissions
• Stratospheric aerosol injections and sunshades in space have by far the greatest potential to cool the climate by 2050 - but also carry the greatest risk
• Surprisingly, existing activities that add phosphorous to the ocean may have greater long-term carbon sequestration potential than deliberately adding iron or nitrogen
• On land, sequestering carbon in new forests and as 'bio-char' (charcoal added back to the soil) have greater short-term cooling potential than ocean fertilisation
• Increasing the reflectivity of urban areas could reduce urban heat islands but will have minimal global effect
• Other globally ineffective schemes include ocean pipes and stimulating biologically-driven increases in cloud reflectivity

And "the beneficial effects of some geo-engineering schemes have been exaggerated in the past."

This is not to say that none of the suggestions are worthy of further research:

"Will it be effective?" is certainly not the only criterion against which geoengineering proposals should be judged (Boyd, 2008), but it serves as a "knock out" criterion: Only measures that pass the basic test of potential effectiveness need be considered further.

It does seem to imply, however, that none are ready for deployment. For example, consider the notion of placing a large number of mirrors in space to shade the Earth:

If such a reduction in incoming solar radiation were achieved by placing a sunshade consisting of multiple "flyers" at the L1 point (Angel, 20 2006), it would require a total area of 4.1 million km² ... given that atmospheric CO2 is rising at 2 ppm yr^-1 and converting this to 0.0282Wm² yr^-1 using Eq. (9), a surface area of 31 000 km² would need to be added each year. This equates to 135 000 launches per year, each carrying 800 000 space flyers of area 5 0.2882m².

You can read the rest of "Radiative forcing potential of climate geoengineering" to see how other options stack up. But the basic trend is, the more feasible the idea, the smaller the contribution it can make. The pyrolysis (oxygen-free) burning of organic waste, which turns it into "bio-char" that can then be buried, for example, has been widely touted as one of the most promising proposals to sequester carbon. James Lovelock is talking it up as one of the only realistic options we might have to forestall catastrophic climate change. But Lenton and Vaughan conclude that, at best, it will only bring down CO₂ levels by 34 ppm.

That kind of drop might make the difference between catastrophic and manageable change. But unless we get on the decarbonizing bandwagon, it won't be nearly enough. As Lenton and Vaughan write, geoengineering really only makes sense as a part of a larger strategy that includes cutting back hard on greenhouse gas emissions.

Among the other options explored in the paper is another possibility that Lovelock floated a while back, although he seems to have dropped it since then: ocean pipes that move warm, CO2-rich water down to the depths. Of that, Lenton and Vaughan write it would have "trivial effects on any meaningful timescale" ;;;; drawing down only 0.1 ppm of CO₂

One can expect the advocates of some of ideas addressed in the paper to raise objections to the analyses that Lenton and Vaughan apply to their pet projects. I'm not qualified to weigh in at that level. But I do find one thing puzzling about their approach. They chose "pre-industrial" levels of CO₂ as the target against which to judge the various options. While that provides an objective point of comparison, it's a curious baseline that might overstate the technological challenges involved.

At 385 ppm, We're already 34% above pre-industrial levels, and not even James Hansen is suggesting we need to get back down that far. Hansen's preferred long-term target is 350 ppm, and others are still talking (hoping) that 450 will prove low enough to avoid the tipping points that the warming will inevitably bring.

Also worth keeping in mind when evaluating Lenton and Vaughan's evaluation is their observation that

Generating the energy and materials required for global scale geoengineering in turn costs money, and we have ignored economic constraints....

In truth, all the options would take some time to develop and deploy and may never reach their estimated potential.

--
T. M. Lenton and N. E. Vaughan (2009). The radiative forcing potential of different
climate geoengineering options Atmos. Chem. Phys. Discuss, 9 (1), 2559-2608