44 / Basics of Environmental Science
Aerosols are released into the air by volcanoes, as salt crystals formed when droplets of sea spray
evaporate, from forest fires, and as tiny soil particles raised by the wind. They are also produced by
a range of human activities, especially the burning of fuels. It is difficult to separate natural sources
from those linked directly to human activities, and both vary from season to season and year to year
but, on average, agriculture and industry account for about one-third of the particulate matter in the
air. Forest clearance and the overgrazing of marginal land in semi-arid regions, for example, leads to
large injections of small particles as wind-blown soil.
From time to time people suggest altering albedos to trigger climatic change. Particles injected into
the upper troposphere might increase the formation of cirriform cloud, for example, and particles
injected into the stratosphere might increase planetary albedo for several years. In both cases the
quantities of particles required would be huge. It might also be possible to reduce albedo in deserts,
by colouring large areas black (perhaps by covering them with plastic sheeting). Such ‘thermal
mountains’ would stimulate convection, hopefully leading to the formation of cumuliform clouds
that would release rain. Most climatologists are wary of such schemes, suspecting that in the unlikely
event that they worked the unanticipated consequences might be unpleasant. Happily, perhaps, their
high cost makes them unattractive to governments.
Varying albedo means some surfaces absorb more solar energy than others, but there are also wide
differences in the way heat is absorbed. On a really hot summer day the surface temperature of sand
on a beach may be high enough to make it painful to walk across it in bare feet, whereas the water is
cool, yet both sand and water are exposed to the same amount of insolation. Dig your feet into the
sand, however, and you soon reach a cooler level. The differences were measured over a Saharan
sand dune at 1600 hours, when the temperature reached its maximum. The air temperature was a
little over 40°C, that of the sand surface 65°C, but 30 cm below the surface the temperature was
about 38°C and 75 cm below the surface it was 25°C (BARRY AND CHORLEY, 1982, p. 288).
Soon after sunset, of course, the sand surface would feel cool.
Different materials vary in their response to radiant energy because they have different heat capacities.
Heat capacity is calculated as the ratio of the amount of energy applied, to the resulting rise in
temperature. The heat capacity of water is much greater than that of rock. This means that much
more energy is needed to raise the temperature of water than of rock, or any substance made from
rock. It also means that water loses heat much more slowly than rock (HIDORE AND OLIVER,
1993, p. 58). Consequently, water responds to insolation by warming and cooling slowly and land by
warming and cooling quickly. This explains the difference in temperature between the sand on a
beach and the water beside it, but it also has profound climatic implications.
The rate at which temperature decreases below the surface depends on the conductivity of the material
and its mobility, which affects the transfer of heat by convection. Sand grains conduct heat poorly,
which is why a layer of cool sand lies at quite shallow depth. Although water is not a very good
conductor of heat, heat moves through it readily by convection, and turbulence due to the wind mixes
warmed surface water with cooler water immediately beneath it.
13. The greenhouse effect
Since the solar constant is known it is possible to calculate what the ‘black-body’ temperature of the
Earth should be: it is 250 K (-23°C) (HARVEY, 1976, pp. 43–44). This is what the average temperature
at the surface would be were it not for the absorption of long-wave radiation by the atmosphere,
which delays the loss of heat and thus warms the planet. The absorption of radiation modifies the