The conversion of a natural landscape to an urban environment often induces a local warming trend due to the introduction of impervious and light-absorbing surfaces, such as dark-colored rooftops and pavements. This is known as the "urban heat island" effect. Warmer temperatures in urban environments have a number of negative impacts, such as higher energy demand for air conditioning and related emissions (pollutants and greenhouse gases), thermal stress to local ecosystems, and an increased risk of high temperature episodes . Strategies for reducing urban surface temperatures in cities include the use of "cool" building materials, e.g., light-colored pavement, to increase the surface albedo.
Strategies already in place to mitigate urban heat islands will likely also serve to slow global climate change to some extent by lowering fossil fuel use and increasing surface reflectivity. However, geoengineering projects to substantially alter global climate by modifying Earth's surface reflectivity pose a significant engineering challenge. A major physical limitation is location - only half of the incident solar radiation actually penetrates to the surface of the planet. A second major limitation is the availability of suitable regions to modify. Many areas of the Earth have naturally high albedo, such as snow- or ice-covered regions and deserts. Covering these areas of earth with solar-reflective material would have little effect on the planetary albedo.
Mitigation Option: Cover low albedo portions of Earth with reflective coverage. Proposals include covering large areas of the ocean with highly reflective material, such as manufactured material, films, foams, biological organisms, or sea ice. Land-based proposals include use of reflective building materials, genetically-modified high albedo vegetation, or spraying vegetation with bright material [34, 39, 40].
Feasibility: Flannery et al.  calculated that covering 10% (or approximately 36 million km2) of Earth's ocean with highly reflective material would compensate for a doubling of CO2. This basic theoretical calculation alone illustrates the extensive surface coverage that would need to be constructed and maintained in order to mitigate warming. Other proposals to mitigate warming by boosting land albedo do not reach the target of offsetting a doubling of CO2, but in combination could cause substantial cooling effect . An important factor to consider in all cases is the potential diminishing reflectivity of the surface coverage under aging, which may require continued maintenance and/or larger area coverage to sustain the desired albedo. While the use of reflective surface materials in urban areas is a well-established method for combating urban heat islands, large-scale modification of Earth's albedo is currently restricted to the point of a theoretical proposal. The feasibility of implementing such large-scale modification and expected costs are unknown at this time.
Co-benefits and undesirable consequences: Large-scale modification of either land or ocean surface properties is bound to cause substantial disturbances to ecosystems and in regional and global hydrological cycles. In particular, blocking solar radiation to the ocean or land surface will likely reduce the amount of PAR available to photosynthetic organisms beneath the modified surface. Covering the ocean surface to the extent suggested by Flannery et al.  can also be expected to reduce the evaporation of water into the atmosphere, altering precipitation patterns to an unknown degree. While not an intentional change, reduced solar flux, due to the presence of a dense plume of air pollution particles over the Indian Ocean, appears to have weakened the Asian monsoon, leading to drought in Northern China and southern flooding . To the authors' knowledge, co-benefits of large-scale surface modification beyond improving urban building materials are nonexistent.
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