3.3.8 Reduction of Solar Radiation
Several proposals have been made to reduce incoming sunlight by placing mirrors in outer space, or reflective balloons or aerosol particles in the troposphere or stratosphere (17, 52-54, 92, 95). Each of these is based on the idea that reducing the amount of solar radiation reaching the surface of the earth will reduce the amount of re-radiated infrared radiation that can be absorbed by the GHGs. In addition, by intercepting this radiation before it reaches the lower atmosphere, initial absorption of shortwave radiation by water vapor, about 20% of incoming solar, will also be prevented.
Conceptually, each of these could work. In fact, long-term climate modeling done recently shows that a 1.8% reduction in incoming solar radiation will completely offset the forcing from a doubling of carbon dioxide from the pre-industrial level (280 to 550 ppmv) or from recent levels (355 to 710 ppmv) without causing any significant temperature rise or climate change (96, 97).
To achieve this reduction, however, would require either placing mirrors with diameters in excess of 1000 Km in outer space or billions of reflective balloons in the atmosphere. Neither of these is practical today and by the time such technology could be available, we probably will also have solved the problems regarding fuel cells and carbon management.
Atmospheric injection of sulfate aerosol particles that reflect sunlight would produce effects similar to those caused by volcanic emissions. This could possibly be done using naval rocket shells, but knowledge of atmospheric chemistry is still too imperfect to guarantee success. The aerosols might have a negative impact on stratospheric ozone and the rainout of these particles could exacerbate acid rain. At best, this idea should be considered as a last resort, although waiting until a climate crisis is underway to attempt an untested procedure is also a gamble we cannot afford to take.
3.3.9 Conclusions
We have presented here a fairly pessimistic analysis of the options for reducing or offsetting the effects of man-made GHG emissions and their likelihood for success. We don’t want to leave the impression that the future is necessarily bleak. Instead, our point is that it may take 50 years or more for those countermeasures that are found to be cost-effective to be implemented. In another 50 years, the additional GHG loading of the atmosphere may result in significant damage to the environment and change the climate in ways that cannot be easily reversed.
For these reasons, it is imperative that other options be considered for implementation as soon as possible to minimize the effects of global warming for as long as possible. Thus, a bridging technology is needed to give these new technologies the time to be developed and implemented. Such a technology would be available today or within a few years and would be capable of slowing the rise in the temperature of the biosphere over the next 100 years.
To this end, we are proposing the use of land surface albedo enhancement as a stopgap treatment. This will reduce the amount of re-radiated solar radiation that can be absorbed by the GHGs and in doing so, reduce the warming of the atmosphere, land and oceans. Because it can be applied incrementally over a 60-year period, there will be ample time to study its effectiveness and refine its application.
