Why Levitt and Dubner like geo-engineering and why they are wrong

The geo-engineering option that is being talked about here is the addition of SO2 to the stratosphere where it oxidises to SO4 (sulphate) aerosols which, since they are reflective, reduce the amount of sunlight reaching the ground. The zeroth order demonstration of this possibility is shown by the response of the climate to the eruption of Mt. Pinatubo in 1991 which caused a maximum 0.5ÂșC cooling a year or so later. Under business-as-usual scenarios, the radiative forcing we can expect from increasing CO2 by the end of the century are on the order of 4 to 8 W/m2 – requiring the equivalent to one to two Pinatubo’s every year if this kind of geo-engineering was the only response. And of course, you couldn’t stop until CO2 levels came back down (hundreds, if not thousands of years later) without hugely disruptive and rapid temperature rises. As Deltoid neatly puts it: “What could possibly go wrong?”.

The answer is plenty. Alan Robock discussed some of the issues here the last time this came up (umm… weeks ago). The basic issues over and above the costs of delivering the SO2 to the stratosphere are that a) once started you can’t stop without much more serious consequences so you are setting up a multi-centennial commitment to continually increasing spending (of course, if you want to stop because of huge disruption that geo-engineering might be causing, then you are pretty much toast), b) there would be a huge need for increased monitoring from the ground and space, c) who would be responsible for any unanticipated or anticipated side effects and how much would that cost?, and d) who decides when, where and how much this is used. For point ‘d’, consider how difficult it is now to come up with an international agreement on reducing emissions and then ponder the additional issues involved if India or China are concerned that geo-engineering will cause a persistent failure of the monsoon? None of these issues are trivial or cheap to deal with, and yet few are being accounted for in most popular discussions of the issue (including the chapter we are discussing here).

Is geo-engineering a fix?

In a word, no. To be fair, if the planet was a single column with completely homogeneous properties from the surface to the top of the atmosphere and the only free variable was the surface temperature, it would be fine. Unfortunately, the real world (still) has an ozone layer, winds that depend on temperature gradients that cause European winters to warm after volcanic eruptions, rainfall that depends on the solar heating at the surface of the ocean and decreases dramatically after eruptions, clouds that depend on the presence of condensation nuclei, plants that have specific preferences for direct or diffuse light, and marine life that relies on the fact that the ocean doesn’t dissolve calcium carbonate near the surface.

The point is that a planet with increased CO2 and ever-increasing levels of sulphates in the stratosphere is not going to be the same as one without either. The problem is that we don’t know more than roughly what such a planet would be like. The issues I listed above are the ‘known unknowns’ – things we know that we don’t know (to quote a recent US defense secretary). These are issues that have been raised in existing (very preliminary) simulations. There would almost certainly be ‘unknown unknowns’ – things we don’t yet know that we don’t know. A great example of that was the creation of the Antarctic polar ozone hole as a function of the increased amount of CFCs which was not predicted by any model beforehand because the chemistry involved (heterogeneous reactions on the surface of polar stratospheric cloud particles) hadn’t been thought about. There will very likely be ‘unknown unknowns’ to come under a standard business as usual scenario as well – another reason to avoid that too.

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