Geography Reference
In-Depth Information
Although population size in the dry zone comprised only 15 % of the Northern
Hemisphere total, 37.5 % (33) of the population collapses happened there. The
population growth rate in the dry zone fluctuated along with temperature change.
Every warming/cooling resulted in population decline before 1800 (Fig. 14.4 b,
orange line). Twenty-nine population collapses happened in the dry zone in the
Medieval Warm Period, C1 and C3. Population grew slowly in mild phases. From
AD 1 to 1800, total population increased by only 76.9 %, while population in
warmer humid and cooler humid zones increased by 493.1 % and 890.6 %,
respectively.
The dry zone was the only zone that suffered from population collapses in the
Medieval Warm Period. After the population drop of the Medieval Warm Period,
the population sizes of some countries there (Egypt, Libya, Tunis, and Iraq) did not
reach their previous high until AD 1800 (McEvedy and Jones 1978 ). This indicates
that the agro-ecosystem in arid and semi-arid regions is highly sensitive to changes
in temperature and precipitation (Zhang et al. 2007a ). Furthermore, most of the
countries and regions in the dry zone were long-civilized and underwent long-
term intensive cultivation. The resultant land degradation might have magnified
and prolonged the ecological impact of climate change (i.e. shrinkage of land
carrying capacity). This explains why the average population collapse duration in
the dry zone (102.8 years) was much longer than that in other zones (50.3 years)
(Fig. 14.4 a). Although the Muslim agricultural revolution spread to all arid areas in
northern Africa and western Asia beginning in the eighth century, it could not help
dissipate the demographic impact of climate change. All population collapses in the
Medieval Warm Period located in the dry zone's major agricultural regions (Algeria,
Arabia, Egypt, Libya, Morocco, Tunisia, and Turkey) were primarily triggered by
nomadic tribal invasions (Issar 1995 ). As the zone became hotter and drier in the
Medieval Warm Period (von Rad et al. 1999 ; Enzel et al. 2003 ), nomadic tribes in
pastoral areas suffered most, forcing them to invade adjacent agricultural regions for
subsistence. Subject to further warming, farmlands in those regions were degraded
to pastoral land (McEvedy and Jones 1978 ; Issar 1995).
Warming and cooling were detrimental to the dry zone, but in warmer humid
and cooler humid zones, in middle and high latitudes, only cooling was detrimental.
Forty-five out of the total of 51 population collapses in those zones occurred in
cold phases, while warming boosted population growth rates by increasing the local
land carrying capacity (Galloway 1986 ; Lee et al. 2008 ; Lee and Zhang 2010 ;
Lee et al. 2009 ). Population size in warmer humid and cooler humid zones made
up nearly 70 % of the Northern Hemisphere total, which largely determined the
overall trend of Northern Hemisphere population growth (Fig. 14.4 b). A disparity
of agro-ecological sensitivity between the two zones may be revealed by their
respective population growth rates. The cooler humid zone had a gentle fluctuation
warmest month above 10 ı C; (3) cooler humid - rainy climate with severe winter, coldest month
below 0 ı C, warmest month above 10 ı C; and 4) dry - dry climate; limits are defined by formulae
based on rainfall effectiveness.
Search WWH ::




Custom Search