Regardless of what we might consider to be prudent or imprudent with respect to CO2 emissions into the atmosphere, these emissions continue to increase and as a result atmospheric CO2 concentrations also continue to increase. In the absence of countervailing measures, continuing increases in atmospheric CO2 concentrations can be expected to lead to warmer near-surface temperatures. No one can be certain of the consequences of these changes for human or natural systems. If we were faced with an imminent climate catastrophe where further warming would push us over some critical 'tipping point', and we chose to address this situation via CO2 emissions reductions, a near-complete cessation of CO2 emissions would be required to prevent further warming (Matthews & Caldeira 2008), one whose abruptness might make the likelihood of its attainability appear remote.
However, the idealized model results shown here indicate that there may be considerable opportunity to diminish some adverse consequences of CO2 emissions to the atmosphere through intentional climate modification. Nobody claims that such climate engineering would be perfect or is devoid of risks. Furthermore, it is clear that such climate engineering will not reverse all adverse effects of carbon dioxide emission; for example, climate engineering will not reverse the acidifying effect of carbon dioxide on the oceans (Caldeira & Wickett 2003). Furthermore, plant growth may be augmented in a high CO2 world with relatively low temperatures, due to CO2 fertilization of photosynthesis without corresponding increases in transpiration and respiration rates (Bala et al. 2006). This may affect natural ecosystems, but is likely to be advantageous for crop yields.
Let us consider the counterfactual situation in which we already had zero-CO2-emissions energy economy. For example, assume that we were already using some combination of nuclear fission, renewables, carbon with capture and storage, electric vehicles, etc. Nobody knows what such total emission abatement would really cost, but let us assume for the present discussion that, with reasonably optimized research, development and deployment, a CO2-neutral energy system would cost all of us an additional 2 per cent of our income, worldwide and for ever. Now, let us assume that somebody proposed that we could increase our income by 2 per cent, but that we would thereby heat up the planet's surface, acidify the ocean, risk melting major ice sheets, shift precipitation patterns and so on. Would we trade that added environmental damage and risk of damage for 2 per cent more income globally and for ever? If we would not want to 'sell out' such a carbonneutral energy system if we already had one, then the basic issue is the difficulty of creating the system, not the additional cost of 'operating' it per se.
There are several major challenges in obtaining deep reductions in CO2 emissions. For the world to cut CO2 emissions deeply, nearly all actors would need to nearly totally eliminate their CO2 emissions. However, the burning of fossil fuel produces immediate benefits to the user of the energy thereby attained. By contrast, the climate costs of that fossil fuel burning will be borne broadly throughout the world and primarily by future generations. The fundamental political challenge of CO2 emissions reduction is to create institutions that would make it in the self-interest of the vast majority of actors to sharply curtail their CO2 emissions (Barrett 2003), starting from the condition wherein benefit accrues rapidly to the emitter and the climate costs are borne primarily by others distant in space and time.
Of course, it would be strongly preferable to obtain international consensus and cooperation before deployment and operation of any climate engineering system. However, unlike CO2 emissions reduction, the success of climate engineering does not depend fundamentally on such consensus and cooperation. Putting aside the question of whether or not such a course of action would be wise, a climate engineering scheme could be deployed and operated unilaterally by a single actor, perhaps at remarkably low economic expense. As already observed, the cost of climate engineering may be several orders of magnitude smaller than the '2 per cent for ever' cost of emissions reduction just hypothesized. Just as with substantial CO2 emission reductions, climate engineering is likely to result in relative winners and losers; all such circumstances are pregnant with political tensions.
Modelling of climate engineering is in its infancy. However, continued growth in CO2 emissions and atmospheric CO2 concentrations, combined with preliminary numerical simulations such as those presented here, constitute a prima facie case for exploring climate engineering options - and associated costs, risks and benefits - in greater detail.
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