Other changes do alter the surface albedo, however. Felling a broad-leaved, deciduous forest to provide farmland increases albedo, and converting a coniferous forest to meadow has an even larger effect. As albedo increases, more solar radiation is reflected and the surface temperature decreases. Changing forest to farmland has a cooling effect. Planting forests, on the other hand, may have a warming effect, because it reduces albedo, although the warming this caused would not be sufficient to offset the benefits of increasing the absorption of carbon dioxide by expanding the forest area.
The biggest change results from building roads and expanding urban areas. Grassland and farm crops have an albedo of 0.10-0.25. Concrete has an albedo of 0.17-0.27 and a black road of 0.05-0.10.
We can and do alter the albedo over substantial areas of the Earth's surface. Whether this is climatically significant is another matter, because the albedo also changes naturally. The albedo of the ocean changes during the course of a day from 0.99 around dawn to 0.02 at midday to 0.99 around dusk. In other words, more than 70 percent of the Earth's surface changes from white to black in the course of every day. If there is a storm at sea, the surface may be hidden by cloud, but around the center of the storm there may be cloud-free areas where the winds are strong enough to whip the sea into white-capped waves. Again, the albedo changes. If the storm happens in the middle of the day it changes from black to white. A large forest fire releases huge clouds of smoke. The smoke is pale in color because it contains particles of gray wood ash and a large amount of water droplets. This increases the albedo, possibly from about 0.20—a forest in full leaf—to 0.70—a gray cloud.
Albedo changes constantly, but the natural changes tend to cancel. The albedo of the ocean surface changes in a regular way through the day, so its average remains constant. Sea storms rise up and die down, and they do so at an average rate over the world as a whole. Even forest fires have only a temporary effect. Eventually they are extinguished.
Changes we make ourselves, by converting the land surface from one use to another, have a more permanent effect. The albedo does not revert in a few hours, days, or weeks. Consequently, we should bear in mind that when we plant forests or clear-fell them, allow overgrazing to reduce pasture (albedo 0.15) to desert (albedo 0.27), or build roads and cities on what was formerly natural countryside, we alter the color of the surface. When we alter the surface color, we alter the albedo, and on a large enough scale this might affect the climate.
Some clouds are more reflective than others
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