Basics of Environmental Science

Earth Sciences / 43

About 70 per cent of the Earth’s surface is covered by

water. Its albedo varies according to the angle at which

sunlight reflects from it. When the Sun is low in the

sky much more light is reflected and the water looks

brighter than when the Sun is high; in the latter case

most of its light penetrates the water and is absorbed,

and the water looks dark. Early and late in the day,

when the water is calm, the occupants of open boats

can develop sunburn quite quickly, even in cool

weather. Table 2.2 compares the incident angle of

radiation with the resultant albedo for water.

Reflected radiation does not warm the surface and so

albedo has an important climatic effect, and one that can

be modified by human intervention, although the

relationship is more complex than it may seem. The

clearance of tropical rain forest to grow field crops, for

example, involves little change, but if that change is from

forest to pasture for feeding livestock, the albedo could

double. In this case, the ground would absorb less heat,

so there would be less evaporation of water and less cloud

would form. This would reduce the average cloud albedo,

however, so increasing the amount of radiation reaching

the surface and warming it again, evaporating more water

and increasing cloudiness once more, but not necessarily

to its original value.

This rather intricate relationship illustrates an important

point. Climate is strongly subject to feedback effects.

In most cases, as in our example, these tend to stabilize

conditions, as negative feedback, but positive feedback

also occurs. It exaggerates effects and so has a

destabilizing effect which can be felt rapidly, as in the

onset of glaciation and glacial melting. Eventually,

positive feedback is overridden by negative feedback

and a destabilized system finds a new level of stability.

Clean air reflects no light, but air is seldom clean. It

contains very small particles, called ‘aerosols’. In the

upper troposphere and at lower levels over the open sea,

concentrations range from about 100 to 600 per cm^3 , but

at low level over continents they are much higher and in

industrial regions can reach millions. They vary in size

from 10-3 to 10^2 μm and gravity has little effect on them, because they are so small. They tend to reflect

short-wave radiation, thus increasing planetary albedo, and in saturated air water vapour will condense on

aerosols smaller than about 0.5 μm, encouraging cloud formation and also increasing albedo (and removing

the aerosols in precipitation). Increasing the albedo has a cooling effect, but aerosols absorb radiation at

infra-red wavelengths, so those in the lower atmosphere also have a warming influence; for those reaching

the stratosphere, on the other hand, the cooling influence of increased albedo is dominant. To complicate

matters further, aerosols settling on to clean snow ‘dirty’ it, reducing its albedo.

Table 2.1 Albedos of various surfaces

Table 2.2 Effect of the incident angle

of radiation on water’s

albedo