Today, the individually neutral words 'global' and 'warming' combine to provide an epithet whose consequences, already causing misery and premature death for millions, hold the prospect of unquantifiable change and potential disaster on a global scale for the decades to come. While the link between rising global temperatures and increasing atmospheric concentrations of CO2 has been known for more than a century (Arrhenius, 1896), there is increasingly the sense that governments are failing to come to grips with the urgency of setting in place measures that will assuredly lead to our planet reaching a safe equilibrium. Today, the developed world is struggling to meet its (arguably inadequate) carbon-reduction targets while emissions by China and India have soared. Meanwhile, signs suggest that the climate is even more sensitive to atmospheric CO2 levels than had hitherto been thought.

Alarmed by what are seen as inadequate responses by politicians, for a number of years some scientists and engineers have been proposing major 'last-minute' schemes that, if properly developed and assessed in advance, could be available for rapid deployment, should the present general concern about climate change be upgraded to a recognition of imminent, catastrophic and, possibly, irreversible increases in global temperatures with all their associated consequences. While such geoscale interventions may be risky, the time may well come when they are accepted as less risky than doing nothing.

The above sets out the main elements that led the Philosophical Transactions of the Royal Society A to publish a theme issue on such geo-engineering approaches and to subject them to critical appraisal by acknowledged experts from around the world. In inviting contributors to that issue (which the present editors also edited), we turned especially to speakers at a workshop on Macro-Engineering Options for Climate Change Management and Mitigation organised by Professor John Shepherd FRS and Professor Harry Elderfield FRS in January 2004 on behalf of the Tyndall Centre for Climate Change Research. Published under the theme title

Geoscale Engineering to Avert Dangerous Climate Change (Launder & Thompson, 2008), the issue has been and continues to be highly cited in the academic media and elsewhere.

Now, to extend and update this collection, and to bring it to a wider and more diverse audience, we are delighted to have worked with Cambridge University Press on this present book which contains edited versions of all the Phil. Trans. papers plus two entirely new articles prepared expressly for this volume.

The collected papers are here presented in three parts, the first of which, Scene setting, comprising five articles, provides a historical and philosophical overview combined with projections of future CO2 emission levels and the foreseen capacity (or limitations) of conventional sequestration. A further topic examined, in a chapter written specifically for this book, is the prediction of climate tipping points, an approach that may possibly be able to provide forewarning of impending abrupt climate change.

The section Carbon dioxide reduction (CDR) gives examples of three approaches to limiting CO2 growth or, possibly, in the longer term, even of reducing carbon dioxide concentrations. In a specially commissioned chapter for the present volume, Keith et al. assess the prospects for 'air capture', that is, the direct removal of CO2 from the atmosphere. This is a strategy first proposed (in the context of climate change) a decade ago (Lackner, 1999) which is now being intensely explored, with several pre-commercial prototypes undergoing testing in North America and Europe. Their actual final development and deployment will depend inter alia on the price paid for captured CO2, a subject on which there will surely be much discussion in the immediate future. Once captured, the carbon dioxide must be stored. While this could be by one of the routes considered with conventional carbon capture and storage (CCS) an alternative approach suggested in the chapter by Zeman and Keith is to recombine carbon dioxide with hydrogen to produce liquid hydrocarbon fuels. This section concludes with two chapters on stimulating the take-up of CO2 directly in the oceans. Over the years, such approaches have stimulated passionate debate for or against and the two chapters here included reflect this ambivalence.

In the final section of the book, Solar radiation management (SRM), two schemes for raising the proportion of incoming sunlight reflected back into space are presented. With both approaches the reflecting agents are in the atmosphere, in one case brightened, low-level marine stratus clouds and in the second stratospheric aerosols. While CDR schemes, because they attack the root cause of global warming, are commonly seen as the preferred geo-engineered route (albeit inferior to the unachieved goal of reducing the emissions of CO2 to a trickle), only SRM strategies can act swiftly following deployment should the risk of catastrophe appear imminent. While, with both SRM schemes, there are many legal, ethical, research and technological issues still to be resolved, there is the prospect that these should be soluble within the relatively short time-frame of 15-20 years. A further potentially important group of SRM strategies - space-based approaches -is not represented in this section largely because the development obstacles and the costs involved suggested to us a potential implementation date significantly further into the future. Mention of such alternative approaches is, however, included in the opening overview chapter by Stephen Schneider.

We wish to express our thanks to authors for adhering to (or nearly adhering to) the various deadlines set, and to the referees - at least two of whom reviewed each chapter in this volume - for their prompt and often very detailed and helpful responses. Finally we thank Suzanne Abbott of the Royal Society for facilitating the transfer of materials to Cambridge University Press.

Brian Launder J. Michael T. Thompson

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