Evidence that the climate is warming is overwhelming. Not only does this include land and sea surface temperature records, but also other indicators such as the coverage of Arctic sea ice. Few serious scientists doubt that anthropogenic activities, and specifically the emissions of greenhouse gases, are responsible for the major part of the observed warming. Projections of future global temperatures under a “business-as–usual” scenario vary between models but indicate that future global warming will probably well exceed the 2 °C global average commonly regarded as the upper tolerable limit, and serious consequences in terms of temperature extremes, storminess, droughts and floods can be expected unless action is taken. The broad consensus in the scientific community is that the much preferable form of action would be to effect a dramatic reduction in emissions of greenhouse gases, and particularly of carbon dioxide so as to stabilise, and if possible, reduce current atmospheric concentrations. To do this solely through a reduction in emissions is not only challenging but might prove politically unacceptable, and consequently other actions have been proposed. Such actions, which come under the general term “geoengineering”, include: interventions to stimulate natural processes of carbon dioxide removal (such as sequestration by terrestrial vegetation or within the oceans); chemical scrubbing from the atmosphere, changing planetary albedo; as well as more extreme measures such as using mirrors in space to intercept solar radiation before it reaches the earth; or creating a reflective aerosol layer in the stratosphere as a means of reducing solar input to the Earth's surface. This volume examines the scientific and engineering aspects of such options, as well as considering some of the associated governance issues.

The first chapter by John Thornes and Francis Pope describes the phenomenon of global warming and poses the question of why do we need solutions to global warming? It also sets the scene by describing the major options for geoengineering and considering the arguments for and against geoengineering research. This is followed by a chapter in which Stuart Haszeldine and Vivian Scott describe methods of carbon dioxide reduction which offer long timescales of storage. These are both on land and within the ocean, and the considerable complexities associated with seemingly straightforward measures are highlighted. The following chapter by Timothy Lenton goes further into the issue of carbon dioxide removal, looking at the overall global potential if all possible carbon sinks were optimised. The magnitude of the many possible removal options is assessed and a final conclusion reached that carbon dioxide removal has the physical potential to help stabilise atmospheric CO₂ in the middle of this century, if combined with reductions in carbon dioxide emissions. A complementary approach is to scrub carbon dioxide from the atmosphere by chemical means and Klaus Lackner explains the principles by which this might be achieved by means of “artificial trees” and estimates the likely financial cost of doing so. While current costs would be a huge increment on energy prices, the economics might become more favourable in the future.

Alternatives to carbon dioxide removal include those that aim to reflect more of the incoming solar radiation back to space, hence increasing the Earth's albedo, or reflectivity. One possibility is to adopt the growth of crops with a higher potential for reflectivity than those currently grown. Taraka Davies-Barnard sets out the basic principles and demonstrates what could be achieved by such a policy and concludes that it could make a useful though not large contribution to climate change mitigation. A complementary approach over the oceans involves creating artificial clouds by spraying large volumes of seawater into the atmosphere as fine droplets. Stephen Salter, Thomas Stevenson and Andreas Tsiamis describe the engineering technologies that would be needed to implement such measures, assess their possible effectiveness and also consider the likely economic costs. Moving to greater altitudes, Alan Robock describes how injections of sulfur gases into the stratosphere could be used to create a reflective sulfate layer which would lead to reduced surface temperatures. More detailed analysis shows that insolation reduction by this mechanism could keep the global average temperature constant, but global average precipitation would reduce particularly in summer monsoon regions, temperature changes would not be uniform and sea ice would continue to melt. Governance issues relating to such measures are also discussed. A perhaps more extreme option is to interpose large reflective objects or particle clouds in space between the sun and planet Earth. Colin McInnes, Russell Bewick and Joan Pau Sanchez describe the physical principles behind such technology and looks at the feasibility of its achievement.

Geoengineering raises huge ethical and governance issues which relate not only to the implementation of the technology but also to the research conducted into its feasibility. Alan Robock touches on these issues in his chapter, and in the final chapter of the book, Richard Owen gives an in-depth analysis of the issues and highlights some of his own experience in relation to research on geoengineering.

Geoengineering is a highly topical subject and hence very appropriate for coverage by the Issues in Environmental Technology series. We have been very fortunate to commission articles from some of the world's leading experts in this field and we believe that this volume provides an authoritative and highly informative overview not only of how geoengineering might be achieved but also of the likely financial costs, consequences and dis-benefits. This volume will prove of value to a very wide range of scientists and engineers dealing with global issues, as well as policy-makers, and students in a wide range of environmental science and engineering courses.

Ronald E. Hester

Roy M. Harrison