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BACKGROUND

The fundamental physics that control the global mean surface temperature are well understood: about one-third of the incoming solar radiation is reflected back to space by Earth’s albedo and the remaining two-thirds is absorbed at the surface, then emitted as longwave energy. In this way the incoming and outgoing energy at the top of the atmosphere largely balances the energy leaving, after partially trapping some of the energy by the greenhouse effect of atmospheric water vapor and clouds as well as greenhouse gases (IPCC 2007). These interactions constitute the Earth’s radiative energy balance, as illustrated in Figure 1. Higher surface mean temperatures (Tsurf) are due to the greenhouse effect, caused by the man-made release of CO2 and other greenhouse gases. Increasing albedo (α) can offset CO2-enhanced greenhouse warming by increasing the shortwave reflection of clouds. And clouds can be brightened to increase their reflectance by adding aerosol particles, which increase the number and decrease the mean size of cloud droplets. An example of such brightening is provided by the “ship tracks” created by the emissions of cargo ships crossing the Pacific Ocean, as shown in Figure 2.

Keeping in mind that maintaining global mean surface temperature does not imply that regional temperatures or precipitation patterns are kept constant, engineering the global mean surface temperature to reduce changes from present-day conditions could be sufficient to alleviate some of the most severe effects of global warming. Adding aerosol is straightforward, since particle production is a side effect of most combustion processes as well as a result of vaporization of liquids in condensable conditions. The real challenge in engineering aerosol particles to offset climate change by brightening clouds is predicting how the earth system, and in particular its clouds, will affect the albedo response to increased particles.

RECENT MODEL SIMULATIONS OF CLOUD BRIGHTENING

Model simulations have established the climate impacts of distributing enough particles to modify enough clouds to offset sufficient global warming to delay or lessen some of the effects expected in Earth’s changing climate (Latham 1990, 2002; Latham et al. 2008). Some schemes focus on a perceived need for engineering and development of new technology, such as Flettner rotors and high-efficiency seawater atomization (Salter et al. 2008). Other studies use detailed global modeling investigations to show what fraction of clouds are brightened, with more aggressive increases in brightening resulting in exacerbation of climate in some regions even as others are improved (Rasch et al. 2009). Global simulations have also shown that where clouds are targeted is important because some choices result in exacerbation of drought conditions in some regions (Korhonen et al. 2010; Rasch et al. 2009). In addition, recent studies have investigated the com-



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