October 8, 2011 — andyextance

People’s struggle to feed themselves has led to economic chaos and war through history, and the most vicious struggles can be traced directly back to climate change. That’s the conclusion of an ambitious attempt to work out exactly how climate and human social crises are linked by David Zhang from the University of Hong Kong and his colleagues. “Climate change has been scientifically proven to be the ultimate cause of significant human crises in pre-industrial Europe and the Northern Hemisphere,” Zhang stated boldly on Tuesday.

Many studies have shown that civilizations have collapsed in times of climate change, typically in periods when the world cooled. Zhang has been especially active in this area. “Over the last 7 years I have published over 20 papers on climate change and social responses,” he told Simple Climate. But, he noted, other researchers’ work had been criticised for a lack of evidence that definitively showed that climate causes upheaval in human societies. Zhang’s response, in a paper published in Proceedings of the National Academy of Sciences of the USA on Monday, calls on great volumes of data to show that there are, in fact, strong statistical links.

The researchers from Hong Kong and China brought together measurements on 16 different climate, agricultural, and social factors between 1500 and 1800 AD. “The great challenge is collecting and analysing this huge amount of data,” Zhang said. “The datasets come from different disciplines, and we have read over a thousand pieces of literature. For the same reason, we invited other economists, geographers and anthropologists to join us in the research to make sure there were no mistakes.”

Cold and hungry

During the measured period, the Northern Hemisphere’s most dramatic climate change was a cold period from 1560 to 1660 where temperatures fell approximately 0.2°C. Plant-growing seasons shortened and farmed land area shrunk, triggering more famine. That lack of food reduced human height by 0.8 inches in the late 16th century, with Europeans only getting taller again slowly with warming after 1650. But the data the scientists brought together showed that throughout the 300 year period, agricultural and food supply changes followed cooling periods closely. They then tended to be followed 5-30 years later by social disturbance, wars, migrations, and disease epidemics.

Though Zhang’s team thought they knew how links between temperature and farming productivity, for example, would build up into a larger network, the point of gathering so much information was so that they could test the links using statistical tools. The tools they used included a method devised by Nobel-prize winning economist Clive Granger, considered able to show whether changes in one measurement cause changes in others. Their analysis revealed that factors further down the chain of consequences from temperature were less closely linked to climate change, indicating the effects were offset by factors such as human adaptation. But these later factors, like famine and migration, were strongly linked to food supply per person, or food supply per capita. This therefore became the most important link in the chain determining climate change’s effects on society.

“Climate influences human society by way of food supply per capita,” Zhang emphasised. “That means that people either need enough money or enough land to survive the bad climate.” He now feels that the strength of the relationships shown in this analysis supports previous studies linking climate and upheavals in society. “The achievement of this research is solid verification of causal relationships between climate change and human crisis,” Zhang said. “Climate changes not only influence war, population and grain price – which I had already found in the past few years – but also all major aspects of human society.”

Debate continues

Despite Zhang’s best efforts, that claim has again been met with similar criticisms to his previous work. For example historian William Atwell from Hobart and William Smith Colleges in Geneva, New York, pointed out to Science Now that the data overlooks the impact of factors like trade and religion. Meanwhile, in the same article, paleoclimatologist Sebastian Wagner of Helmholtz Zentrum Geesthacht in Germany noted that the study doesn’t look at the effect of rainfall. However, Zhang says that while the social causes that Atwell mentions can be responsible for local crises, this study looks at upheaval on a larger scale. He also defends using temperature alone, though he does plan to look at precipitation in future.

“The temperature distribution is the key element of climate and it controls many other elements at the large scale,” he said. “It controls surface pressure distribution, and in turn air circulation and rainfall distribution. Therefore, precipitation is influenced by temperature. During the last few years in my research experience, I’ve found temperature played a very important role in climate-human relationship, especially in Europe and China. Of course, precipitation also played an important role in history, and I am currently investigating the role of precipitation on human civilization and hopefully the results will be submitted for publication very soon.”

Other criticisms include that it’s not clear whether these findings still apply – though Zhang warned Simple Climate that they do. “Most of the population in developing countries still rely on agricultural products as their main resource,” he said. “Climate change, of course, would affect these countries. Thanks to globalisation, any disturbance of agricultural production would affect the world market and consequently, global stability.”