initially proposed, would actually have relatively little effect on the amount of energy reaching the surface-troposphere system (and thus would not offset the warming influence of the increasing concentrations of greenhouse gases). In addition, reducing the amount of UV radiation reaching the troposphere might well adversely affect tropospheric chemistry and reduce the chemical cleansing capacity of the atmosphere; on the other hand, reducing the down-coming UV radiation would increase the time it takes to become sunburned and so perhaps reduce incidence of skin cancer.

As another alternative in its analysis, NAS (1992) considered an approach involving lofting of millions to trillions of larger, reflective, high-altitude balloons. Maintaining enough aloft would likely make this approach costly due to the need to provide lift and to limit deterioration due to high-energy UV radiation. Such balloons could be configured like corner reflectors or especially coated on their upper side to increase reflectivity. Whether so many balloons might interfere with communications would need to be considered.

The most recent proposal (Keith, 2008b) is to inject small particles into the mesosphere. By clever design and choice of materials, the injected particles could be self-levitating, taking advantage of the electrostatic and magnetic fields present at high altitudes. Except to the extent that the particles would age and be damaged in the harsh high-altitude stream of solar radiation, these particles could have a lifetime of several years or more, thereby balancing the greater cost for their production and injection. Theoretically, at least, the particles could also be designed to primarily reflect wavelengths that are not in the visible part of the spectrum (so, for example, being in the near-infrared range), thus reducing side effects (except perhaps for species making use of a wider spectral band than humans). While the approach appears viable, developing an optimal design and checking for as yet unrecognized side effects remain to be done.

**_Troposphere:_** The present global tropospheric sulfate loading, which results primarily from SO2 emissions from elevated stacks of coal-fired power plants,<sup>10</sup> creates the whitish haze that extends over and often far downwind from major industrialized areas. Because there are both direct and indirect (i.e., cloud-brightening) components of the global cooling influence, the strength of the cooling influence is only roughly known. IPCC’s best estimate is that aerosol cooling offsets ~40% of the overall warming influence of the human-induced increase in the concentrations of greenhouse gases, an amount equivalent to the warming influences of the increases in concentration of all of the non-CO2 greenhouse gases (Forster and Ramaswamy, 2007).<sup>11</sup> Because the SO2 emissions are primarily in the Northern Hemisphere, the cooling influence is also, and it was likely the increase in SO2 emissions from elevated sources in the mid-20<sup>th</sup> century that led to a sharp increase in the Northern Hemisphere’s sulfate loading and inhibited

> 10 Surface-level emissions of SO2 have an atmospheric lifetime of only a day or two before being removed by dry or wet deposition, so only a small fraction of the emitted SO2 is typically converted to sulfate. Emissions from tall stacks, by contrast, have lifetimes of a week or more, allowing a significant fraction of the SO2 to be converted to sulfate aerosols.

> 11 An inadvertent consequence of this near-perfect offsetting has been to largely limit many of the discussions of policy actions to limiting emissions of CO2, thereby sometimes hiding the important and cost-effective opportunities that exist for control of the non-CO2 greenhouse gases. That the offset is so close is purely fortuitous and is very unlikely to persist as emissions and control measures change.

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