Geoscience Reference
In-Depth Information
of cold ocean water immediately offshore, either because of cold upwelling ocean
water or because of a cold ocean current flowing parallel to the coast. The fourth
factor is location in the lee of a high mountain range running parallel to the coast, an
effect termed the rain-shadow effect, because most of the moist air flowing in from
the coast loses much of its moisture as it rises and cools on the windward side of the
ranges.
At intervals in the past, changes in global atmospheric circulation have occurred at
time scales ranging from decades to millennia, causing the deserts and their margins
to become more or less arid. Evidence of past desert expansion is seen in the vast
areas of now fixed and vegetated desert dunes that lie hundreds of kilometres beyond
the margins of great deserts like the Sahara or in the now stable and soil-covered
expanses of desert dust in central China. During episodes in the past when the deserts
were less arid than they are today, integrated river systems flowed across them, lakes
were abundant and fresh, and prehistoric human populations were able to graze their
herds in areas now devoid of both water and pasture. In short, in some presently humid
areas there is evidence of former aridity, just as some previously wetter areas are now
arid.
