2.4 Surface Albedo Enhancement
2.4.1 Principle
Increasing the reflectivity of the surface of the earth will reduce the amount of re-radiated solar radiation that can be absorbed and re-emitted back to the surface by the GHGs and in doing so reduce the warming of the earth. The earth’s reflectivity for solar radiation or albedo at the surface (with 1 being 100% reflective and 0 being 100% absorptive) ranges from 0.02 for ocean water to 0.9 for the polar ice caps.
Surface albedo plays a major role in determining local, regional and global climate. Increases since 1750 in the land albedo of the earth due to deforestation and other development may be responsible for a negative forcing of –0.2 Wm-2 since that time and may also explain why N. America is cooler than climate models predict.
Studies of the urban heat island effect, in which the dark asphalt and concrete surfaces of cities absorb and emit more infrared radiation than surrounding rural areas, have found that because of this, some cities create their own weather in the form of increased numbers of thunderstorms during the summer. It is estimated that raising the albedo of all of the pavement and roofs in the Los Angeles area by 7.5% along with planting 10 million trees, would reduce temperatures there by 5?F as well as ground level ozone whose formation is temperature dependent, completely offsetting the urban heat island effect created by urbanization. Finding ways to reduce the albedo of cities would, therefore, be very helpful.
On a smaller scale, white plastic mulch is used around vegetable plants to help them grow in hot weather. The mulch cools the soil and air around the plants. White roofs are used to reduce the need for air conditioning during the summer.
2.4.2 Suitable Locations
Given the potential advantages from land surface albedo enhancement, suitable locations have been identified that could have their surface reflectivity increased on a scale large enough to offset some or all of the additional radiative forcing due to GHGs expected during much of the 21st century.
The ideal candidate locations are certain desert lands without significant use or potential for use for human habitation, crop production, grazing, mining or development of other natural resources. These lands are mostly devoid of vegetation, biologically unproductive and thus do not remove much carbon dioxide from the air. They have relatively low year-round albedos in the range of 0.2-0.4, so that there is potential for increasing them. They also receive a high flux of solar radiation with few cloudy days, thereby maximizing the effectiveness of any surface covering. Consisting of relatively flat and stable gravel plains and dry lakebeds, without many sand dunes or mountainous areas, some 4.5 million square miles of the Sahara, Arabian and Gobi deserts, among others, qualify as candidates for increasing their surface albedo by applying a reflective cover.
2.4.3 Coverage Areas Required to Achieve Desired Level of Forcing Offsets
Assuming little is done to reduce emissions, the projected increase in radiative forcing from 2010-2070 may be around 2.8 Wm-2. To offset this, i.e., keep the forcing where it is in 2010 and thus prevent additional global warming, will require covering 67,000 sq. mi. per year for 60 years, a total of 4 million sq. miles, with a cover that increases the reflectivity from 0.36 to 0.8. Offsetting the forcing needed to meet the U.S. Kyoto target in 2012 requires covering a total of 290,000 sq. miles, while 390,000 sq. miles would offset all forcing due to U.S. electric power generation from 1750-2070.
