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Ashwin Kumar

Mar 4, 2013


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Thanks for the proposal! 1) As you mention in places in your proposal, the suggestion is to send mirrors in orbit that aren't just opaque, they reflect sunlight back to outer space-- is that correct? Because an opaque object could absorb sunlight, heat up, and radiate longwave energy downward towards the Earth. 2) One thing to consider is that to work effectively, the reflecting surface of these mirrors would have to be turned towards the sun while in different locations in orbit around the Earth. Or, at least when these mirrors are over much of the summer pole. How would this be achieved?

Michael Maccracken

May 2, 2013


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A panel convened by the National Academy of Sciences considered the option of mirrors in low Earth orbit in their 1992 publication Policy Implications of Greenhouse Warming (see page 448, for example; the report is downloadable from Given the notion of reducing solar absorption in high latitudes, the mirrors would be mainly active during on the few months of spring/summer when substantial sunlight is reaching the polar regions, so a lot of reflection would have to be done over a short time, necessitating a lot of mirrors that would basically be inactive 2/3 of the time or more--so it seems a bit inefficient unless one is also thinking one would use them to reflect energy reaching the high latitudes of the Southern Hemisphere, and generally also reflecting energy during the rest of its orbit as well. The proposals that have been made for mirrors in low Earth orbit generally suggest that the mirrors would orbit around low latitudes to catch the most sunlight. Taking all the energy out of low latitudes, however, might well disrupt the weather because of how it disrupted the latitudinal input of energy from the Sun. One unique idea here is thus to consider polar orbits instead of equatorial orbits. Were all mirrors oriented optimally at any given time (so perpendicular to incoming solar radiation, that the mirrors would become concentrated in high latitudes would likely mean a much greater fraction of solar energy reflected there than at low latitudes, thus over-compensating for the case of equatorial orbiting mirrors. Presumably some optimum could be found. I think the main problem, then, would be the shear number of mirrors that would be needed. Reflecting enough energy to counterbalance about half of the additional trapping resulting from a CO2 doubling would require covering about 1% of the Earth with such mirrors, which would mean, as the NAS calculated, about 50,000 mirrors in orbit, each about 10 km by 10 km. Aside from issues of the costs to loft all of these mirrors and steer them, the navigational logistics of this as they all cross the pole would be virtually impossible, and the sky in high latitudes would experience a flickering due to frequent shading of the Sun. Going to more very small mirrors (or even particles) would reduce the lifetime of the mirror against the solar wind. Going bigger would make solar flickering tend to be worse and the importance of avoiding collisions of the mirrors even more important. The cost analysis the NAS panel did suggested that this approach was just not the most cost effective, among other problems.

2013geoengineeringjudges 2013geoengineeringjudges

Jul 6, 2013


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Thank you for sharing your ideas and for the work invested to create this proposal. We have considered this proposal carefully, but the issues raised in previous comments by the contest Advisor have not been addressed sufficiently for the proposal to move forward.