Quantitative Analysis of Climate Change and Human Crises in History - Space-Time Integration in Geography and GIScience - page 258

14.5 Conclusions

Previous research about the climate-crisis relationship was constrained by the
availability of paleo-climate reconstructions (cf. Introduction). In recent years, the
situation has been completely changed. There has been a blossoming of paleo-
climate reconstructions, which are not only in high temporal resolution, but also in
fine spatial resolution. Subject to the increasing availability and precision of paleo-
climate data, climate-crisis research in pre-industrial societies was further facili-
tated. On the other hand, the linkages and feedback loops for the climate-population
relationship may be non-stationary. Two specific methods are recommended to
tackle the possible non-stationary components of the relationship: cross-correlation
function and wavelet analysis. Cross-correlation function measures overall associ-
ations for the whole frequency-time domain. The overall cross-correlation function
associations are often determined by predominant associations of some frequencies,
which often mask weak associations of other frequencies or time periods (Bloom
et al. 2007 ). Wavelet analysis is used to examine non-stationary time series,
particularly the causal links of multiple non-stationary time series (Cazelles et al.
2008 ; Zhang et al. 2009 ). The continuous wavelet transform decomposes the time
series into both time and frequency components, the calculation of the wavelet
power spectrum quantifies in the time-frequency domain the distribution of the
variance of the time series (Cazelles et al. 2008 ; Grinsted et al. 2004 ). Those who
are interested in the details and application of the above methods can refer to Zhang
et al.'s ( 2010 ) study. Hopefully, with the application of more sophisticated methods,
the role played by climate change in human history can be fully unveiled in the near
future.

Acknowledgements This research was supported by the Hui Oi-Chow Trust Fund (201302172003
and 201205172003), HKU Seed Funding Programme for Basic Research (201109159014), and
Research Grants Council of The Government of the Hong Kong Special Administrative Region of
the People's Republic of China (HKU745113H, HKU758712H, HKU7055/08H, and HKU705-
HSS-12).

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