Geoscience Reference
In-Depth Information
Rivers of central India cut down
30 m into their former floodplains at or before
8ka
Ice-rafted debris (IRD) events in North Atlantic at 11, 10.3, 9.4, 8.1 and 5.9 ka
Donggwe Cave speleothem record (southern China) shows strong Asia Monsoon (AM) at
9-7 ka and weak AM events at 8.3, 7.2, 6.3 and 5.5 ka
Oman speleothems show reduced rainfall at 9.5, 9, 8.3, 7.4 and 6.3 ka
IV. 5-0 ka
Climates in the intertropical zone become less humid and more seasonal
Summer monsoon still strong but less vigorous than in the previous phase and its spatial
domain probably somewhat reduced
Lakes dry out in arid and semi-arid areas, including the Sahara, which becomes abandoned
by Neolithic pastoralists who move south into West Africa or east into the Nile Valley
Weaker summer monsoon; ITCZ retreats
500 km to the south in North Africa during the
NH summer
Sporadic glacier advances culminating in the most recent Little Ice Age
Increase in the frequency of El Ni no-Southern Oscillation (ENSO) events, leading to
more frequent extreme floods and droughts in the Americas, Africa, Australia and Asia;
interaction between ENSO and summer monsoon leads to more variable rainfall regime in
both hemispheres in regions influenced by them and to highly seasonal or very variable
river flow regimes
Impact of human activities on global climate (and on river basins) may have begun with
the advent of agriculture and has increased ever since, not always to the advantage of
either
IRD events in North Atlantic at 4.2, 2.8 and 1.4 ka
Donggwe Cave speleothem record (southern China) shows weak Asia Monsoon events at
4.5, 2.7, 1.6 and 0.5 ka
to the overgrazing of rangelands in every continent - to the detriment of both the land
and the humans dependent on it. Accelerated clearing of the native vegetation has
denuded the land, making it more and more susceptible to soil loss by wind and water.
In certain regions, such as semi-arid southern Australia (Powell,
1976
; Powell,
1988
),
such clearing has caused the regional groundwater-tables to rise, bringing dissolved
salts to the surface and leading to what has been called dryland salinization. Attempts
to grow crops in desert regions using irrigation water from rivers originating well
outside the deserts, such as the Indus River in Pakistan, the Nile in Egypt and Sudan,
and the Amur Darya and Syr Darya in Uzbekistan, have often led to waterlogging and
salt accumulation in soils, as well as problems downstream, including the silting up
of rivers and drying out of desert lakes and inland seas, such as the Aral Sea in central
Asia and a number of once large lakes in Iran.
Given the magnitude of these problems of land degradation, groundwater depletion,
and loss of plant cover and biodiversity in the desert world, it is important that we
identify a template or a set of guidelines for both present and future sustainable use of
the deserts and their extensive semi-arid and dry subhumid margins. Two principles of
