Geoscience Reference
In-Depth Information
and these are now termed 'Heinrich events'. Bond et al. (
1997
;
2001
) found that
Holocene episodes of ice-rafted debris (IRD) into the North Atlantic ('Bond cycles')
occurred at intervals of approximately 1,500 years, possibly linked to changes in solar
activity.
3.5 Conclusion
Several immediate conclusions may be drawn from this brief survey. First, tectonic
factors involving lateral migration of the continents, opening up of the oceans and
Cenozoic uplift of major mountain ranges likes the Himalayas, Rockies and the Andes
were the primary causes of Cenozoic cooling and desiccation. The timing of these
tectonic events varied from region to region, which means that the inception of aridity
and the birth of the major deserts were not everywhere synchronous, suggesting that
local influences need to be considered in any account of climate change in deserts.
During the Quaternary period, which began 2.6 Ma ago, astronomically controlled
changes in the amount of solar radiation received by the earth determined the duration
and severity of the successive glacial-interglacial cycles. Over the past 0.7 Ma, the
100 ka cycle has been dominant, although the 20 ka precessional cycles have also had
a major influence on climate. During times of maximum high latitude glaciation, sea
surface temperatures were lower, intertropical rainfall diminished, rainforests shrank
and the great tropical deserts in Africa, Arabia, India and Australia expanded. The
reverse was true during interglacial and interstadial episodes, with re-integration of
former drainage systems, desert lakes becoming abundant, and formerly active desert
dunes and loess plains along the desert margins becoming vegetated and stable. It
was during these occasions that prehistoric human groups were able to penetrate
the deserts and travel across the presently forbidding deserts of the Sahara, southern
Negev and Arabia. Finally, the onset of aridity in Africa, Asia, Australia and the
Americas predates any human presence in those regions, and so humans cannot be
considered the cause of these deserts, as some have erroneously claimed.
