Alternative earth-based and space-based techniques for mitigating global climate change: What can we learn by examining them?

When climate change mitigation strategies are discussed in reports such as the Intergovernmental Panel on Climate Change published in 2001 (Metz et al, 2001), the authors inevitably center on reduction of emissions of greenhouse gases and, to a lesser extent, reduction in aerosol emissions such as black carbon aerosols. Reductions of carbon emissions to the atmosphere includes strategies to not only capture and store carbon emissions from, for example, coal fired power plants, but also includes strategies such as increasing energy efficiency, greater use of renewable energy sources, and greater use of fuels that do not contain carbon (such as nuclear) or are carbon- neutral. Over the next several centuries most or all of the earth\´s fossil fuel reserves (about 4000 Gt-C) may be consumed, with the hope that carbon capture and storage (CCS) will minimize future greenhouse gas emissions to levels that are not dangerous to the planet. However, as discussed in this paper, it may not be possible for repositories to reliably store the massive amounts of carbon generated without imposing environmental risks. Further, while control of global warming is of immediate concern, possibilities of other types of climate change exist. Abrupt changes could be triggered that, for example, might reduce the deep North Atlantic circulation and produce a cooling over much of the earth. Second, the earth has been in an interglacial warm period for over 10,000 years, and if the climate cycles observed in the previous 500,000 years continue, it is possible that the earth will return to a colder period. Ultimately, control of global cooling may be a more challenging problem than control of global warming. If reduction of greenhouse gas emissions turns out not to be the complete solution to climate change mitigation requirements (which may involve both warming and cooling periods), what is? In this paper, earth-based and space- based techniques are explored that are designed to supplement gre- - enhouse gas reduction approaches. While the strategies presented here may be futuristic and expensive, they do provide us with an outlook on controlling global climate change that may spawn ideas for other mitigation techniques. General design considerations are presented based on a global energy balance model to predict global temperature changes not only in response to changing greenhouse gas concentrations, but also to changing conditions in the earth system, such as the net incoming solar radiation or the planetary albedo. The approaches are intended to perturb the earth\´s energy balance in a controllable manner. Today, for example, data show that the earth is not in an equilibrium energy state. About 1.0 W/m² more energy is coming into the earth than is leaving, indicating that the earth is "committed" to this amount of global warming and its attendant consequences, even if greenhouse gas concentrations in the atmospheredo not continue to increase. This energy imbalance is equivalent to a further warming of 0.5degC to 1degC, comparable to the observed global warming that has occurred over the past century. One benefit of the supplemental mitigation techniques would be to help "uncommit" earth from such energy imbalances, as well as to provide a basis for addressing future climate change.