Reflective surfaces (climate engineering)

Reflective surfaces, or ground-based albedo modification (GBAM), is a solar radiation management method of enhancing Earth's albedo (the ability to reflect the visible, infrared, and ultraviolet (UV) wavelengths of the Sun, reducing heat transfer to the surface). The IPCC described GBAM as "whitening roofs, changes in land use management (e.g., no-till farming), change of albedo at a larger scale (covering glaciers or deserts with reflective sheeting and changes in ocean albedo)."

The most well-known type of reflective surface is a type of roof called the "cool roof". While cool roofs are primarily associated with white roofs, they come in a variety of colors and materials and are available for both commercial and residential buildings. Painting roof materials in white or pale colors to reflect solar radiation is encouraged by legislation in some areas (notably California).

This technique is limited in its ultimate effectiveness by the constrained surface area available for treatment. This technique can give between 0.01 and 0.19 W/m² of globally averaged negative forcing, depending on whether cities or all settlements are so treated. This is small relative to the 3.7 W/m² of positive forcing from a doubling of atmospheric carbon dioxide. Moreover, while in small cases, it can be achieved at little or no cost by simply selecting different materials, it can be costly if implemented on a larger scale.

A 2009 Royal Society report states that "the overall cost of a 'white roof method' covering an area of 1% of the land surface (about 10¹² m²) would be about $300 billion/yr, making this one of the least effective and most expensive methods considered." However, it can reduce the need for air conditioning, which emits carbon dioxide and contributes to global warming.

As a method to address global warming, the IPCC 2018 report indicated that the potential for global temperature reduction was "small," yet was in high agreement over the recognition of temperature changes of 1-3 °C on a regional scale. Limited application of reflective surfaces can mitigate urban heat island effect.

Reflective surfaces can be used to change the albedo of agricultural and urban areas, noting that a 0.04–0.1 albedo change in urban and agricultural areas could potentially reduce global temperatures by overshooting 1.0 °C.

The reflective surfaces approach is similar to passive daytime radiative cooling (PDRC) in that they are both ground-based. Yet, PDRC focuses on "increasing the radiative heat emission from the Earth rather than merely decreasing its solar absorption."

Cool roofs in hot climates can offer both immediate and long-term benefits, including: