Quantitative Analysis of Climate Change and Human Crises in History - Space-Time Integration in Geography and GIScience - page 245

of population growth (Fig. 14.4 b, turquoise line), while the rate in the warmer
humid zone oscillated more (Fig. 14.4 b, red line). As agricultural production in the
southern part of the warmer humid zone (i.e. southern Europe and China) involves
multiple cropping that is only sustainable in a warm climate, the land carrying
capacity in the warmer humid zone was more vulnerable to cooling relative to other
zones, resulting in greater fluctuations in population growth.
The demographic impact of deteriorating climate was not great in the tropical
humid zone. The zone had steady population growth and only four population
collapses, owing to the rich biodiversity and large land carrying capacity there.
There were two slight drops in population growth (Fig. 14.4 b, green line): one in
the Medieval Warm Period because of the hot-drought climate in India (Walford
1970 ) and one from AD 1550 to 1750, caused by the colonization-induced disasters
in the New World (McEvedy and Jones 1978 ).
Given the significant association between temperature change and population
collapses, regressions were run to estimate the relative sensitivity of population
growth in the Northern Hemisphere and various climatic zones to climate change 21
(Table 14.5 )(Zhangetal. 2011b ). For the Northern Hemisphere, temperature was
positive and highly significant in the regressions in which a 10 % increase in
temperature produced on average a 3.1 % increase in population growth rate. At the
regional level, the warmer humid zone was shown to be the region most sensitive to
temperature change, in which a 10 % increase in temperature produced on average a
2.9 % increase in population growth rate. This is consistent with the above findings
regarding temperature change and population collapses in the warmer humid zone.
In tropical humid, cooler humid, and dry zones, even though temperature positively
correlated with population growth, their association (elasticity) was relatively weak
compared with that of the warmer humid zone. This might be attributable either to
their relatively low population density (McEvedy and Jones 1978 )ortofavorable
geographic context, which provides some spare land carrying capacity to buffer
climate-induced subsistence shortage and its demographic impact (Zhang et al.
2007a ). Dry zone is the only region in which population collapses occurred in
both warm and cold phases. Such a non-stationary relationship may dampen the
regression results.

14.2.3 Socio-political Chaos

In China over the last millennium, the six cold phases could be further divided into
long ones (each lasting 105-133 years) and short ones (each lasting 25-43 years). 22

21 See footnote no. 13 for the specifications of the regression models.
22 See footnote no. 3 for the dates of the cold phases in China.