Debate over the relationship between climate and human crisis has gone on for more than a century, and recent studies have shown significant temporal correlations between the two – but it was only with the publication in December of research led by the Faculty of Geography’s Professor David Zhang that the specific causal mechanisms underlying this relationship were properly analysed.

Advances in paleoclimatology enabled the extensive research team – who spread across the globe and covered multiple disciplines – to use temperature data and climate-driven economic variables to simulate the climate in Europe and the Northern Hemisphere between 1500 and 1800. They looked into every major conflict and crisis and correlated them to 14 economic, social, agricultural and demographic variables.

“Our findings indicate that climate change was the ultimate cause and climate-driven economic downturn was the direct cause of large-scale human crises in pre-industrial Europe and the Northern Hemisphere,” says Professor Zhang.

Cognitive research

He explains that “The theoretical contribution of this paper is greater than its discovery value.” Many other researches have covered this area, but the difference this time was “not the idea but the methodology. We undertook cognitive research not qualitative – new methodology using economics, sophisticated statistics including the Granger method of ecometrics. There were 14 sets of data from different disciplines – it all took about four years.”

Their aim was to use better records to confirm the causal effects of climate change. “We used five criteria and those five criteria were satisfied, thereby proving causal linkage. It was the verification we had been waiting for,” says Professor Zhang.

Clearly, it was very complicated and involved different disciplines which was why he invited so many people to join him in carrying out the research. It also meant the students involved were not all geographers – “which broadened my horizons as well as theirs,” he says.

The study was carried out on European countries for the simple reason that they have more sophisticated records from those times than Asian countries. “It’s partly down to religion,” he says. “In Europe there are Church records of human heights, harvest yields etc. There are no such detailed records in China from that time.”

In addition, the period between 1500 and 1800 included plenty of variables in terms of temperature and of crises, taking in not only the Little Ice Age, but also including the General Crisis of the 16^th century.

The researchers found that every change in average temperatures was related to a change in agricultural output and food supply and that led to crises. The effect was not immediate, so that at the start of a cold period the population would continue to rise, but then after several bad harvests the effects would be felt, with less grain produced but more mouths to feed. That in turn led to higher food prices and more hungry people who would then tend to revolt, migrate or starve.

The time lag between the drop in temperature and the crisis was usually about 15 years. For example, a drop in temperatures between 1264 and 1359 led to the Great Famine of the late Middle Ages. Climate change even led to loss in stature – during a lengthy cold spell from 1559 to 1652 average heights in Europe shrank by nearly an inch.

“Also, well-organised societies fared better and those with economic power suffered less,” says Professor Zhang. “That’s evidently true today – Hong Kong produces next to nothing in the way of harvests, but can buy everything it needs because it is financially strong.“

Economic failure

Overall, the implication from the study is: if you cut off the climate factor, then what effects human most directly would be economics. “Economic failure leads directly to human suffering – take Greece today,” he says. “But one step behind economic failure is climate change that leads to the poor harvest which in turn leads to economic failure.”

Of course what goes down also goes up and the findings also indicates that climate also influenced the better times – for example the Renaissance may have been at least partly sparked by a return of warmer temperatures.

Professor Zhang has been working in similar areas for many years, and started publishing his first papers on global climate change, war and population decline in 2005. Currently, he is working on how climate has effected the geopolitical rhythms of Imperial China over the centuries.