spraying a fine seawater spray in the air (Latham, 2002). This approach may be able to offset some or most of the radiative forcing associated with a doubling of atmospheric CO2 (Bower et al., 2006; Latham, 2002). Process understanding relevant to this approach (e.g., cloud physics) can be tested at relatively small scales (Salter et al., 2008), although such tests would not permit direct inference of climate consequences of large-scale deployment. Another proposed cloud-based approach involves the seeding of high cirrus clouds with heterogeneous ice nuclei to reduce their coverage, potentially using commercial airplanes (Mitchell and Finnegan, 2009). While this method is not technically an example of SRM, it could potentially increase the amount of longwave (infrared) radiation emitted to space, which would cool the Earth.
It has been proposed that global warming could be slowed by whitening roofs to reflect more sunlight back to space (Akbari et al., 2009). Under certain circumstances, whiter roofs could both reduce heating costs and help keep the Earth cool by reflecting sunlight back to space. Others have proposed growing more reflective crops (Ridgwell et al., 2009). Both approaches, if applied on a global scale, could potentially yield a modest cooling effect (The Royal Society, 2009), and white roofs also have the potential for co-benefits such as reducing urban heat islands (see Chapter 12). To date, studies indicate limited potential for such approaches, and the efficacy and environmental consequences of these approaches have yet to be carefully studied.
The overall climatic and environmental responses to SRM approaches are not well characterized. All proposed approaches have the potential for unintended negative consequences for both environmental and human systems. While the magnitude of the consequences is generally proportional to the scale on which the approach is deployed (painting an individual home white would yield fewer impacts—and be more easily reversible—than injecting millions of tons of sulfur into the stratosphere), several issues associated with large-scale deployment merit discussion.
First, none of the SRM approaches would stem ocean acidification (see Chapter 9) associated with enhanced atmospheric CO2 levels. This is a key difference between SRM approaches and the CDR approaches discussed in Chapters 9 and 14 and in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c).