A casual look at the last few decades of debate about the CO2-climate problem might lead one to view geoengineering as a passing aberration; an idea that originated with a few speculative papers in the 1970s, that reach a peak of public exposure with the NAS92 assessment and the contemporaneous American Geophysical Union and American Association for the Advancement of Science colloquia of the early 1990s, an idea that is now fading from view as international commitment to substantive action on climate grows ever stronger. The absence of debate about geoengineering in the analysis and negotiations surrounding the FCCC supports this interpretation. However, I argue that this view is far too simplistic. First, consider that scientific understanding of climate has co-evolved with knowledge of anthropogenic climate impacts, with speculation about the means to manipulate climate, and with growing technological power that grants the ability to put speculation into practice. The history of this co-evolution runs through the century, from Eckholm's speculation about the benefits of accelerated fossil fuel use, to our growing knowledge about the importance of iron as a limiting factor in ocean ecosystem productivity.
This view of climate history is in accord with current understanding of the history of science that sees the drive to manipulate nature to suit human ends as integral to the process by which knowledge is accumulated. In this view, the drive to impose human rationality on the disorder of nature by technological means constitutes a central element of the modernist program. This link between understanding and manipulation is clearly evident in the work of Francis Bacon that is often cited as a signal of the rise of modernism in the seventeenth century.
Moreover, the disappearance of the term geoengineering from the mainstream of debate, as represented by the FCCC and IPCC processes, does not signal the disappearance of the issue. The converse is closer to the truth: use of the term has waned as some technologies that were formerly called geoengin-eering have gained acceptance.
To illustrate the point, consider the shifting meaning of carbon management. The recent Department of Energy "roadmap", an important agency-wide study of "Carbon Sequestration Research and Development"(Reichle, Houghton et al., 1999) serves as an example. The report uses a very broad definition of carbon management that includes (a) demand-side regulation through improved energy efficiency, (b) decarbonization via use of low-carbon and carbon-free fuels or nonfossil energy, and (c) carbon sequestration by any means, including not only carbon capture and sequestration prior to atmospheric emission, but all means by which carbon may be captured from the atmosphere. Although the report avoids a single use of the word geoengineering in the body of the text, one may argue from its broad definition of carbon management that the authors implicitly adopted a definition of geoengineering that is restricted to modifications to the climate system by any means other than manipulation of CO2 concentration.
In this review, in contrast, I have drawn the line between geoengineering and industrial carbon management at the emission of CO2 to the active biosphere. Three lines of argument support this definition. First, and most importantly, the capture of CO2 from the atmosphere is a countervailing measure, one of the three hallmarks of geoengineering identified in Section 10.2.1. It is an effort to counteract emissions, and thus to control CO2 concentrations, through enhancement of ecosystem productivity or through the creation of new industrial processes. These methods are unrelated to the use of fossil energy except in that they aim to counter its effects (Section 10.5.1). The second argument is from historical usage (Section 10.3.5); the capture of CO2 from the atmosphere has been treated explicitly as geoengineering (MacCracken, 1991; Keith and Dowlatabadi, 1992; Watson, Zinyowera et al., 1996; Flannery, Kheshgi et al., 1997; Michaelson, 1998) or has been classified separately from emissions abatement and grouped with methods that are now called geoengineering. Finally, the distinction between pre- and post-emission control of CO2 makes sense because it will play a central role in both the technical and political details of implementation.
As a purely semantic debate, these distinctions are of little relevance. Rather, their import is the recognition that there is a continuum of human responses to the climate problem that vary in resemblance to hard geoengineering schemes such as spaced-based mirrors. The de facto redefinition of geoengin-eering to exclude the response modes that currently seem worthy of serious consideration, and to include only the most objectionable proposals, suggests that we are moving down the continuum toward acceptance of actions that have the character of geoengineering (as defined here) though they no longer bear the name. The disappearance of geoengineering thus signals a lamentable absence of debate about the appropriate extent of human intervention in the management of planetary systems, rather than a rejection of such intervention.
Consider, for example, the perceived merits of industrial and biological sequestration. In the environmental community (as represented by environmental nongovernment agencies) biological sequestration is widely accepted as a response to the CO2-climate problem. It has been praised for its multiple benefits such as forest preservation and the possible enrichment of poor nations via the Clean Development Mechanism of the FCCC. Conversely, industrial sequestration has been viewed more skeptically as an end-of-pipe solution that avoids the root problems. Yet, I have argued here that biological sequestration - if adopted on a scale sufficient for it to play an important role - resembles geoengineering more than does industrial sequestration. Whereas industrial sequestration is an end-of-pipe solution, biological sequestration might reasonably be called a beyond-the-pipe solution. Such analysis cannot settle the question; it merely highlights the importance of explicit debate about the implications of countervailing measures.
Looking farther ahead, I speculate that views of the CO2-climate problem may shift from the current conception in which CO2 emission is seen as a pollutant to be eliminated, albeit a pollutant with millennial timescale and global impact, toward a conception in which CO2 concentration and climate are seen as elements of the Earth system to be actively managed. In concluding the introduction to the 1977 NAS assessment, the authors speculated on this question, asking "In the light of a rapidly expanding knowledge and interest in natural climatic change, perhaps the question that should be addressed soon is 'What should the atmospheric carbon dioxide content be over the next century or two to achieve an optimum global climate?' Sooner or later, we are likely to be confronted by that issue." (NAS77, p. ix).
Allenby argues that we ought to begin such active management (Allenby, 1999). Moreover, he argues that failure to engage in explicit "Earth system engineering and management" will impair the effectiveness of our environmental problem solving. If we take this step, then the upshot will be that predicted in NAS83: "Interest in CO2 may generate or reinforce a lasting interest in national or international means of climate and weather modification; once generated, that interest may flourish independent of whatever is done about CO2".
Although the need for improved environmental problem solving is undeniable, I judge that great caution is warranted. Humanity may inevitably grow into active planetary management, yet we would be wise to begin with a renewed commitment to reduce our interference in natural systems rather than to act by balancing one interference with another.
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