I am sympathetic to those who expressed the concern about a moral hazard - that the very knowledge of the potential of geo-engineering to offset some inadvertent global change disturbances could provide ammunition to those who wished to 'solve' the side effects of indefinite expansionary consumption and population trends with a dubious technological fix rather than a fundamental change in the political acceptability of the fossil-fuel-based growth paradigm. Nevertheless, I, too, share the concern of others that, should the middle to high end of the current range of estimates of possible inadvertent climatic alterations actually begin to occur, we would indeed face a very serious need for drastic, unpopular action to change consumption patterns in a short time - a politically dubious prospect. Thus I, somewhat reluctantly, voted with the majority of the NAS panellists in 1991 who agreed to allow a carefully worded chapter on the geo-engineering options to remain in the report - provided it had enough explicit caveats that the committee could not possibly be interpreted as advocating near-term use of such schemes, only a study of their climatic, ecological, social and economic implications. For example, in the report of the mitigation panel of the NAS committee, it was noted that:
it is important to recognize that we are at present involved in a large project of inadvertent 'geoengineering' by altering atmospheric chemistry, and it does not seem inappropriate to inquire if there are countermeasures that might be implemented to address the adverse impacts... Our current inadvertent project in 'geoengineering' involves great uncertainty and great risk. Engineered countermeasures need to be evaluated but should not be implemented without broad understanding of the direct effects and the potential side effects, the ethical issues, and the risks.
It seems that this volume a decade and a half later than the NAS report has, in virtually every chapter, echoed the warnings that have been implicit in all responsible essays on geo-engineering since Budyko in the mid-1970s.
In the context of one geo-engineering proposal, the deliberate use of augmented stratospheric aerosols as a seemingly inexpensive advertent attempt to offset a few watts per square metre of inadvertent global-scale heating from anthropogenic greenhouse gases, it is likely that stratospheric dust cooling could not possibly be a perfect regional balance to greenhouse warming owing to the very patchy nature of the greenhouse forcing itself (see Schneider 1996). That is, aerosols injected in the stratosphere would, owing to high winds and a lifetime measured in years, become relatively uniformly distributed in zonal bands over the hemisphere. This means that they would reflect sunlight back to space relatively uniformly around latitude zones. Their reflection of sunlight would also vary with latitude because the amount of incoming sunlight and its relative angle to the Earth changes with latitude and season. This fairly uniform, zonally averaged rejection of sunlight would be balancing a relatively non-uniform trapping of heat associated with greenhouse gases owing to the patchy nature of cloudiness (Schneider 1994) in the lower atmosphere (to say nothing of the very patchy nature of soot and sulphate aerosols in the lower atmosphere).
Thus, even if we somehow could manage to engineer our stratospheric aerosol injections to exactly balance on a hemispheric (or global) basis the amount of hemi-spherically (or globally) averaged heat trapped by human-contributed greenhouse gases such as CO2 and methane, we would still be left with some regions heated to excess and others to deficit. This would inevitably give rise to regional temperature anomalies and induce other regional climatic anomalies that could very well lead to greater than zero net global climate change even if the hemispheric average of radiative forcing for all human activities were somehow reduced to zero. This also suggests the need to create a hierarchy of anthropogenic climate forcing indicators, of which global average radiative forcing is but the simplest aggregate modulus.
Govindasamy & Caldeira (2000) tested this concern with a climate model and argued that although perfect offsets are not possible at regional scales, zonal injections of aerosols make a CO2-doubled world that has aerosol geo-engineering compensations look much more like an undisturbed greenhouse effect world. However, let me remind the reader that it is well established that climate models do not represent regional climatic patterns as well as they simulate continental to hemispheric scale projections (IPCC 2007a).
I will not argue that regional climatic anomalies arising from some aerosol geo-engineering offset scheme would necessarily be worse than an unabated 3-6 °C warming before 2100 - a range that implies a very high likelihood for dangerous anthropogenic interference with the climate system (IPCC 2007b). Rather, I simply wish to reiterate why the strong caveats, which suggest that it is premature to contemplate implementing any geo-engineering schemes in the near future, are stated by all responsible people who have addressed the geo-engineering question. Such caveats must be repeated at the front, middle and conclusion sections of all discussions of this topic, as they indeed are in this volume as well.
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