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Albedo


The albedo (from the Latin for ‘white’) of the planet is an important feedback mechanism as extra ice can increase cooling or reduced ice can lead to warming. The amount of incoming radiation which is reflected from the earth (albedo) has an important bearing on the earth’s response to climate change. The albedo of different surfaces varies with vegetation and cover. The average albedo of the earth is normally taken to be 0.31. Typical ranges of values are:


Surface

Range of Albedo

Fresh snow

0.80 to 0.90.

Old/melting snow

0.40 to 0.80

Desert sand

0.40

Grassland

0.25

Deciduous trees

0.15 to 0.18

Coniferous forest

0.08 to 0.15

Tundra

0.20

Ocean

0.07 to 0.10


Albedo by latitude

Figure 1 shows the average albedo of the earth in different bands of latitude. The shape of the graph reflects the fact that the oceans and arid tropical regions have low albedo but that the polar regions, and bordering latitudes, have snow cover for much of the year.


Variation in albedo over time

The albedo of the earth is not constant but varies with the season of the year. As can be seen in Figure 2 during the course of a year the earth’s albedo has two peaks; the first, lower one, corresponds to the time when the Antarctic sea ice is at its maximum and the second, higher peak, corresponds to the time when at higher latitudes much of the land mass is covered with snow.


Seasonal variation in albedo

The largest difference in albedo is between snow-covered surfaces and surfaces without snow. Figures 1 and 2 show the variation of albedo with latitude and over time. Figure 3 shows the difference in albedo at different latitude at different times of the year. Although labelled “seasonal” difference it is in fact the maximum difference in the 4-year satellite albedo record. It is not possible to extend the graph further as for part of the year the polar regions are not visible to the satellite. Figure 3 shows quite clearly that the maximum difference in albedo of 0.3 occurs at around 60 ºN. This is the latitude of the boreal (northern) forests, mainly pine. It is also the latitude in which July insolation as predicted by Milankovitch cycles is thought to determine the sequence of ice ages and warm periods.


Importance of albedo

The importance of albedo can be gauged from looking at the effect of whether or not the northern forests are covered with snow. Such forests cover about 12% of the total land area which in turn covers 29% of the earth’s surface. The difference in albedo between snow-covered and bare forests is at least 0.5. The product of these values indicates a difference in average albedo for the whole earth of 0.017. If the average albedo increases from 0.31 to 0.327 then, other things being equal, the earth’s temperature would increase by 3.8 ºC.


Data sources

The data came from: Earth Radiation Budget Experiment (ERBE) S4G Global Clear-Sky Top Atmosphere Monthly Albedo at 2.5 and 1 Degree Resolutions (1986-1990). They were provided by the NASA Langley Research Center (LaRC) Distributed Active Archive Center (DAAC). Dr. Bruce Barkstrom was the Principal Investigator of the Earth Radiation Budget Experiment (ERBE). The original data sets can be accessed at the following web site: http://eosweb.larc.nasa.gov/PRODOCS/erbe/table_erbe.html

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