4.0 Surface Albedo Enhancement
Reflecting incoming solar radiation from outside or just inside the atmosphere will reduce the amount of that radiation reaching the surface of the earth and the amount of re-radiated infrared radiation that can be absorbed by the GHGs. However, since this isn’t practical, increasing the reflection of sunlight reaching the surface should be considered as an alternative. The process will not be as efficient, because on average, only half the incoming radiation (168/342 Wm-2) reaches the surface. Thus, any benefits to be gained in reduction of solar radiation absorbed must come from the 168 Wm-2 that reaches the surface.
This must be accomplished by increasing the surface reflectivity or albedo, a measure of how much incoming solar radiation is reflected back into space. Albedo (A) is measured on a scale of 0 to 1, with 0 being 100% absorption of all incoming radiation and 1, 100% reflection. As examples, a black asphalt surface has an albedo of around 0.1 (10%), while fresh snow is close to 0.9 (90%).
It is important to realize that although a material may have a high reflectivity in the visible range, i.e., the whiter it appears, the higher its albedo, for solar radiation, visible light is only about half the total. Thus, the solar reflectance (R) of a surface over the total range of wavelengths may be somewhat lower due to absorption of these other invisible wavelengths (98).
Solar radiation or shortwave radiation reaching the surface consists of ultraviolet (<0.3 um), visible (0.4-0.7 um) and near-infrared light (0.7-2.5 um) (12). The visible light makes up 46%, near-infrared 49% and ultraviolet 5%. The near-infrared or NIR is not the same as the infrared (IR) or thermal radiation which covers the wavelength range 4-40 um.
Although the earth’s albedo is often cited as close to 0.3, this actually refers to the overall albedo from above the atmosphere and includes the reflection of sunlight from cloud tops. The reflectivity of the surface (land and oceans) is much lower, around 0.09 (20 Wm-2), somewhat misleading in that it is based on the 235 Wm-2 that pass through the atmosphere and not on the incoming radiation at the top of the atmosphere. This figure is also somewhat misleading since most of the earth’s surface is covered by dark blue water that has an albedo that ranges from 0.02-0.1. Albedos of land surfaces vary greatly from the tops of green forests that are around 0.15 to deserts that can range from 0.2 to 0.5 to the polar ice caps that are generally around 0.8-0.9. (99, 100). Thus, one has to be careful in taking these planetary averages and applying them to smaller areas as well as know the downwelling shortwave radiation in Wm-2 on which the albedo number is based.
By increasing the solar reflectivity of a surface, the surface itself is made cooler by virtue of reduced heating from the infrared radiation generated by direct contact with sunlight. The atmosphere is also made cooler due to the reduction of upwelling IR and the accompanying lessened absorption by the GHGs. The surface is also made cooler because the downwelling IR flux is also reduced.
These facts have not been lost on building designers, farmers, the researchers who study the “urban heat island effect” and anyone who spends time outside in the sun. Everyone knows that white clothing is cooler than dark clothing in the summer. Farmers have learned to use both black plastic to warm the soil around plants in cold weather and white plastic to keep it cooler in hot weather. Building designers have used this knowledge to make roofs whiter to reduce electricity demand in summer from air conditioning (94).