But Deserts Make Themselves

In addition to all of the more traditional climatology, there is another factor whose importance is only now becoming understood. Deserts partly owe their existence to the fact that they themselves exist. The desert makes the desert, internally modifying its own climate so that less rain falls!

So the link from climate to vegetation, in Chapter 1, has been turned on its head. A fundamental fact of the earth system, that climate scientists are only now becoming fully aware of, is that vegetation can make the climate too. The mechanisms by which deserts reinforce their own dry climate was apparently first explored in the mid-1970s by Joseph Otterman, who represented his ideas in a landmark paper which inspired a whole new way of thinking. He took apart the basic physics of the local atmosphere and surface—the "mesoscale climate" that is built up from the microclimatic factors explained in Chapter 4—and he thought about what might happen if you changed the vegetation cover. One thing that he knew was important was the brightness of the surface, the albedo (roughly meaning "whiteness" in Latin).

Seen from above, green leaves look a lot darker than a bare soil surface. For example, if you look out of the window of a plane flying high above dry country, areas of dense tree and shrub cover look almost black, in contrast to the blinding brightness of patches of bare soil. The brightness of that bare soil is solar energy— sunlight—reflected straight back up into space. This is energy that might have gone into heating the surface if it had been absorbed, but instead it has been wasted. Thus, the lighter the surface, the less energy is absorbed and more is thrown right out into space. Table 5.1 shows a range of typical albedo values found for different surface types in the world.

Above bare soil with high albedo, the atmosphere is deprived of some of the warm air that would otherwise be rising up from the ground, so (other things being equal) it is cooler than it would be if it was darker. There is less convection to carry heat aloft, and the atmosphere is relatively calm and stable. The relatively shallow convection above high-albedo land tends to let air, coming in from above, descend and form a "cap" on the top, which supresses rain cloud formation. Normally, rain clouds tend to form where warm air keeps rising up from the surface carrying some water vapor and then begins cooling, causing water droplets to condense out to give clouds and then rain. Over a bare, bright surface, air tends to do the opposite thing— descending and heating as it does so. These are the sort of conditions which prevent any rain from falling (Figure 5.1). So, if we imagine two land surfaces that start off equally moist as one another, the darker low-albedo one is going to generate more rain for itself than the bright high-albedo one.

To anyone who has traveled a lot around the world, it is intuitively hard to believe that lack of vegetation cover would make the surface cooler. Arid areas of the world tend to get very hot, whereas the rainforest zones of the tropics have fairly mild temperatures most of the time. But, in fact, areas with plenty of dark vegetation are absorbing a lot more energy overall than deserts. The air temperature at ground level in these densely vegetated zones would be even higher than in a bare arid desert, except that the leaves of the forest are usually evaporating water which sucks away

Table 5.1. Typical values for various land surface types. Ice and snow, deserts, bare soil and grassland yellowed in the dry season all have substantially higher albedo than evergreen rainforest or conifer forests, deciduous forests in leaf, and water surfaces. From: A range of sources.

Surface

Albedo

Natural

Fresh snow

0.80-0.95

Old snow

0.45-0.70

Desert

0.20-0.45

Glacier (dusty)

0.20-0.40

Soil (dry)

0.23-0.40

Cropland

0.18-0.25

Grassland (dry season)

0.25

Deciduous forest (in leaf)

0.10-0.15

Coniferous forest

0.05-0.15

Tropical rainforest

0.12

Water

0.03-0.10

Urban

Road

0.05-0.20

Roof

0.08-0.35

Paint

White

0.50-0.90

Red, brown, green

0.20-0.35

Black

0.02-0.15

heat as "latent heat of evaporation". If you need to be convinced of the difference that albedo makes to the amount of sunlight converted into heat at the surface, try walking across from a light concrete surface to dark recently-laid asphalt on a hot sunny day. The air hovering above the dark asphalt will be much hotter than that over the light concrete—often almost unbearably hot. If they were not continually evaporating water from their leaves, forests would also be even hotter than bare land.

In a desert, then, because the land surface is bare of vegetation, this tends to result in weaker upwards movement of warm air rising from the ground, which allows descending air to come in from above, and does not give rain. And, of course, without rain there can be no vegetation. So the chain of causes goes in two different directions depending on the starting position;

Bare land ^ weakly rising air ^ no rain ^ bare land

Vegetated land ^ strongly rising air ^ rain ^ vegetated land

In effect, once there is a lack of vegetation cover in an already fairly arid environment, it stabilizes its own aridity in a vicious cycle. Indeed, it may well exaggerate its own

' ^ rk Rising air condenses out clouds and then gives rain

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