ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-5539-2009 The radiative forcing potential of different climate geoengineering options Lenton T. M. 1 2 Vaughan N. E. 1 2 School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Tyndall Centre for Climate Change Research, UK 06 08 2009 9 15 5539 5561 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/9/5539/2009/acp-9-5539-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/5539/2009/acp-9-5539-2009.pdf

Climate geoengineering proposals seek to rectify the Earth's current and potential future radiative imbalance, either by reducing the absorption of incoming solar (shortwave) radiation, or by removing CO<sub>2</sub> from the atmosphere and transferring it to long-lived reservoirs, thus increasing outgoing longwave radiation. A fundamental criterion for evaluating geoengineering options is their climate cooling effectiveness, which we quantify here in terms of radiative forcing potential. We use a simple analytical approach, based on energy balance considerations and pulse response functions for the decay of CO<sub>2</sub> perturbations. This aids transparency compared to calculations with complex numerical models, but is not intended to be definitive. It allows us to compare the relative effectiveness of a range of proposals. We consider geoengineering options as additional to large reductions in CO<sub>2</sub> emissions. By 2050, some land carbon cycle geoengineering options could be of comparable magnitude to mitigation "wedges", but only stratospheric aerosol injections, albedo enhancement of marine stratocumulus clouds, or sunshades in space have the potential to cool the climate back toward its pre-industrial state. Strong mitigation, combined with global-scale air capture and storage, afforestation, and bio-char production, i.e. enhanced CO<sub>2</sub> sinks, might be able to bring CO<sub>2</sub> back to its pre-industrial level by 2100, thus removing the need for other geoengineering. Alternatively, strong mitigation stabilising CO<sub>2</sub> at 500 ppm, combined with geoengineered increases in the albedo of marine stratiform clouds, grasslands, croplands and human settlements might achieve a patchy cancellation of radiative forcing. Ocean fertilisation options are only worthwhile if sustained on a millennial timescale and phosphorus addition may have greater long-term potential than iron or nitrogen fertilisation. Enhancing ocean upwelling or downwelling have trivial effects on any meaningful timescale. Our approach provides a common framework for the evaluation of climate geoengineering proposals, and our results should help inform the prioritisation of further research into them.

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