The Ethics of Geoengineering:
Investigating the Moral Challenges of Solar Radiation Management
Solar Radiation Management (SRM)*
Solar Radiation strategies do not modify CO2 levels in the atmosphere. Instead, they reflect incoming radiation to reduce the atmosphere's solar energy content and restore its natural energy balance. Proposed reductions of solar radiation absorption are usually 1-2% ; around 30% is already reflected naturally by the earth's surface and atmosphere. The methods are space, land, or ocean-based and involve either introducing new reflective objects within or outside of the atmosphere, or an increase in the reflectivity or albedo  of existing structures and landforms. SRM could reduce increases in temperature, but it may not address the non-temperature aspects of greenhouse-induced climate changes. SRM strategies would generally take effect more quickly than CDR strategies. However, once started, some would likely require constant maintenance and/or replenishment to avoid sudden and drastic increases in temperature.
Some SRM proposals include:
Stratospheric Sulfate Injections – A spray of sulfates into the second layer of earth’s atmosphere  could reflect incoming solar radiation to reduce absorption. This process occurs naturally after a volcanic eruption, in which large quantities of sulfur dioxide are released into
the stratosphere. 
White roofs and surfaces – Painting the roofs of urban structures and pavements of urban environments white would increase their albedo by 0.15-0.25 (15-25%). This strategy was suggested by DOE Secretary Steven Chu in May of 2009 at the St. James Palace Nobel Laureate Symposium.
Cloud brightening / Tropospheric Cloud Seeding – A fine spray of salt water or sulfuric acid is injected into the lowest level of our atmosphere to encourage greater cloud formation over the oceans, which would increase the local albedo.
Land use changes – Portions of the earth’s natural land cover could be modified for more reflective growth patterns, such as light colored grasses. Also, existing agricultural crops could be genetically modified to reflect more sunlight.
Desert reflectors – Metallic or other reflective materials could be used to cover largely underused desert areas, which account for 2% of the earth’s surface.
Space-based reflective surfaces – One large satellite or an array of several small satellites with mirrors or sunshades could be placed in orbit to reflect a portion of sun radiation before it reaches the earth’s atmosphere. Reflectors could also be placed at the sun-earth Lagrange (L1) point, where the gravitational pulls from each body act with equal force and therefore allow objects to “hover” in place.
*As defined by the US House of Representatives Committee on Science and Technology Hearing Charter, "Geoenegineering: Assessing the Implications of Large-Scale Climate Intervention" <http://democrats.science.house.gov/Media/File/Commdocs/hearings/2009/Full/5nov/Hearing_Charter.pdf>
10 The Royal Society report suggests a reduction of 1.8% (RS 23) <http://royalsociety.org/Geoengineering-the-climate/>
11 Novim 8. This inherent reflectivity of the earth is often referred to as “planetary albedo.”
12 Albedo is usually presented as a number between 0 and 1, 0 representing a material in which all radiation is absorbed and 1 a material which reflects all radiation.
13 Roughly 6 to 30 miles above the earth’s surface
14 The naturally-occurring sulfur emissions from the 1991 eruption of a volcano in the Philippines, Mt. Pinatubo, are thought to have decreased the average global temperature by ~0.5° C for a 1-2 year period by increasing global albedo. Another example of such short term atmospheric cooling is often attributed to the eruption of El Chicón in March 1982.