**_Preserving the ocean’s meridional overturning circulation:_** Johnson (1997) argues that, to reduce the likelihood of a reduction in the Meridional Overturning Circulation and consequent cooling of the North Atlantic, damming the Strait of Gibraltar might be useful. Taking this action would tend to keep Europe and the North Atlantic region warm by cutting off the cooling influence of the high salinity waters that form in the Mediterranean. Other, less developed, ideas have also been proposed. The very high-resolution ocean models that are becoming available provide a useful tool for evaluating such ideas.

**_Preserving Arctic sea ice:_** Although there was interest during the anomalously cold 1960s in the possibility of taking actions to melt Arctic sea ice (e.g., see Weart, 2004), recent attention has focused on ways to preserve it. Several authors have suggested that damming the Bering Strait or Siberian rivers flowing into the Arctic, which could be done to alter the salinity, stabilization, and or heat transport, might be useful in controlling what happens in the Arctic Ocean. Ocean circulation models are finally becoming adequate to undertake such investigations.

Rather than altering ocean currents, sharply reducing incoming solar radiation coming in over the Arctic<sup>12</sup> seems likely to offer a number of the benefits of stratospheric aerosol injection while avoiding the global consequences of sharply modifying the ratio of diffuse and direct solar radiation. Reductions in the radiation could, theoretically, be accomplished by lofting of sulfate aerosols or other reflectors (e.g., corner reflecting balloons or particles) in large amounts over the high latitudes where natural ecosystems are used to dealing with low light levels. The injection altitude could be chosen to ensure that the aerosols would stay aloft only during the sunlit months, thereby limiting their contribution to intensification of springtime ozone depletion. The diminution in solar radiation would be made sufficient to promote earlier and more extensive formation of Arctic sea ice. Augmenting the sea ice cover, with its high albedo, would in turn reduce absorption of solar radiation and induce further cooling. The longer duration of a cold sea-ice surface would extend the time during which the overlying air that later spreads to midlatitudes would be cooled, thus tending to restore traditional weather patterns.

Calculations by Caldeira and Wood (2008) indicate that sufficiently reducing solar radiation in the Arctic could indeed offset greenhouse-gas-induced warming in that region and that limitations would extend southward through the northern mid-latitudes. While a substantial aerosol loading or lofting of balloons would be needed to fully counteract a CO2 doubling (e.g., enough to reduce radiation north of about 60oN by about 10% or north of 70oN by 25%, or about half this amount for 50% of this effect), it might be possible that, rather than darkening the skies every year, a heavier injection every few years might promote sufficient Arctic cooling that the additional sea ice with its higher albedo would persist for several years.

An interesting result learned from the model simulations is that, while the region’s temperature and sea ice cover could theoretically be returned to preindustrial conditions, the increase in precipitation caused by global warming would persist. This persistence would occur because most global evaporation occurs over the mid and lower latitude oceans. As a result, cooling the Arctic would not substantially reduce the warming, and so the evaporation, in those regions that feeds moisture to the Arctic. Because the existing and additional precipitation would occur

> 12 Proposed initially by the author of this article at the Department of Energy's 2001 workshop (E. Khan, unpublished manuscript).

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