Scientists and society in general are becoming increasingly concerned about the risks of climate change from the emission of greenhouse gases (IPCC 2007). Yet emissions continue to increase (Raupach et al 2007), and achieving reductions soon enough to avoid large and undesirable impacts requires a near-revolutionary global transformation of energy and transportation systems (Hoffert et al 1998). The size of the transformation and lack of an effective societal response have motivated some to explore other quite controversial strategies to mitigate some of the planetary consequences of these emissions.

These strategies have come to be known as geoengineering: 'the deliberate manipulation of the planetary environment to counteract anthropogenic climate change' (Keith 2000). Concern about society's inability to reduce emissions has driven a resurgence in interest in geoengineering, particularly following the call for more research in Crutzen (2006). Two classes of geoengineering solutions have developed: (1) methods to draw CO₂ out of the atmosphere and sequester it in a relatively benign form; and (2) methods that change the energy flux entering or leaving the planet without modifying CO₂ concentrations by, for example, changing the planetary albedo. Only the latter methods are considered here.

Summaries of many of the methods, scientific questions, and issues of testing and implementation are discussed in Launder and Thompson (2009) and Royal Society (2009). The increased attention indicates that geoengineering is not a panacea and all strategies considered will have risks and consequences (e.g. Robock 2008, Trenberth and Dai 2007).

Recent studies involving comprehensive Earth system models can provide insight into subtle interactions between components of the climate system. For example Rasch et al (2009) found that geoengineering by changing boundary clouds will not simultaneously 'correct' global averaged surface temperature, precipitation, and sea ice to present-day values. There is a tradeoff between cooling the planet and consequences to the hydrologic cycle and sea ice cover in the Arctic.

Ban-Weiss and Caldeira (2010) have taken another step in this exploration. They have treated geoengineering as an optimization problem and searched for an optimal solution by varying one aspect of a geoengineering methodology, imposing differences in the spatial location of the geoengineering—contrasting changes concentrated in polar regions with spatially uniform aerosol distributions (i.e. shielding the poles to protect the sea ice may have a different impact on the planet than shielding an equatorial region). They measured the impact by looking at the root mean square difference between the geoengineered world and present-day precipitation and temperature (as opposed to the global averaged changes in the Rasch et al study). They found that broad fixed location geoengineering is quite a crude mechanism for control of temperature and precipitation. Differences between uniform and optimal geoengineering distributions are quite modest, and the tradeoffs found in earlier studies are also found here. Solutions that minimize differences from present-day temperatures are not the best solutions in terms of differences in present-day precipitation.

The study is simple and idealized. The measures of desirability of climate to optimize for can be made more comprehensive, including other variables or measures, for example, of transient variability (seasonal, diurnal variability, or frequency of extreme events), and it is easy to identify ways to make the geoengineering strategy much more complex. The study is thought provoking, delivers clear and useful messages, outlines a methodology and helps to clarify ways to think about geoengineering consequences.

References

Ban-Weiss G A and Caldeira K 2010 Geoengineering as an optimization problem Environ. Res. Lett. 5 034009

Crutzen P J 2006 Albedo enhancement by stratospheric sulfur injections: a contribution to resolve a policy dilemma? Clim. Change 77 211–20

Hoffert M I, Caldeira K, Jain A K, Haites E F, Harvey L D, Potter S D, Schlesinger M E, Schneider S H, Watts R G, Wigley T M L and Wuebbles D J 1998 Energy implications of future stabilization of atmospheric CO₂ content Nature 395 881–4

IPCC 2007 Summary for policymakers Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change ed S Solomon, D Qin, M Manning, Z Chen, M Marquis, K B Averyt, M Tignor and H L Miller (Cambridge: Cambridge University Press) chapter 0, pp 1–18

Keith D W 2000 Geoengineering the climate: history and prospect Ann. Rev. Energy Environ. 25 245–84

Launder B and Thompson M 2009 A review of stratospheric sulfate aerosols for geoengineering Geo-Engineering Climate Change: Environmental Necessity or Pandoras Box? (Cambridge: Cambridge University Press) 332 pp

Rasch P J, Chen C-C and Latham J L 2009 Geo-engineering by cloud seeding: influence on sea-ice and the climate system Environ. Res. Lett. 4 045112

Raupach M R, Marland G, Ciais Ph, Le Quere C, Canadel J G, Klepper G and Field C B 2007 Global and regional drivers of accelerating CO₂ emissions Proc. Natl Acad. Sci. 104 10288–93

Robock A 2008 Twenty reasons why geoengineering might be a bad idea Bull. Atomic Scientists 64 14–8

Royal Society 2009 Geoengineering the Climate: Science, Governance, and Uncertainty (London: The Royal Society) ISBN: 978-0-85403-773-5

Trenberth K E and Dai A 2007 Effects of Mount Pinatubo volcanic eruption on the hydrological cycle as an analog of geoengineering Geophys. Res. Lett. 34 L15702