Geology Reference
In-Depth Information
trees and evaporates from both forest and marshland. With less evapotranspiration and
less cloud cover, the treeless land surface has warmed by about 0.7
0
C—a catastrophic-
ally high increase.
The northern Sahara provides another example of the tight coupling between plants
and climate. It is now a desert, but 6,000 years ago much of it was grassy bush country
with scattered trees and shrubs, populated by a stunning assembly of large mammals,
birds, insects and other wildlife. What could have brought about the shift from savannah
to desert? Almost certainly around 6,000 years ago the earth's orbit brought us closer to
the sun, triggering more rainfall in the region. The reasons for this are difficult to fully
unravel, but it seems that changes to complex relationships amongst air, ocean, vegeta-
tion and sea ice in widely scattered parts of the globe were involved. Grasses and shrubs
expanded their ranges in the slightly wetter climate, setting in train a positive feedback
that encouraged even more rain to fall over the entire region. The ability of the soil to
hold on to rainfall increased as plant roots permeated the once barren sand. As the plants
grew, they sucked up water through their roots, releasing it into the air as water vapour
that condensed into rain-bearing clouds with the help of cloud-seeding chemicals emit-
ted by the plants themselves. The dark green surfaces of the plants encouraged the en-
hancement of evaporation. The feedbacks look something like this
(Figure 30)
:
Figure 30: The drying of north Africa.
