Although we have tried in the preceding discussion to provide some rough quantification of the potential magnitude of the contribution that may be achieved through various management options, as well as the timescale on which it may be possible to achieve these contributions, it is important to recognize that there is great uncertainty about both the cost and the efficacy of many options. We do not view these uncertainties as a basis for delay or inaction (Caldeira et al. 2003). The evidence of a growing problem is sufficiently compelling that action is clearly needed today. Economic, business, and ecological theory suggests that when faced with large uncertainty, the best option is to invest in a broad portfolio that creates a diversity of future options.
Our ability to project population, per capita GDP, political and social revolutions, and so on, is quite limited (Nakicenovic and Swart 2000). For example, the widely used IS92a scenario of the Intergovernmental Panel on Climate Change (IPCC) was tech nologically overoptimistic but failed to anticipate reduced CO2 emissions associated with the end of the Soviet Union; thus the IS92a scenario overestimated year 2000 CO2 releases. Experts meeting a century ago could not have anticipated developments like world wars, jet travel, nuclear power (and its rejection in many places), the computer, and the Internet. We assume we are in an equally disadvantageous position regarding the prediction of technical, social, and political innovation likely to occur this century.
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