Case study: rise and fall of civilizations

After spending much of the time talking about the mechanisms of economic analysis in mitigating climate change (or abrupt climate change), I would like to give some examples of how environmental change has affected past human development. In particular, I would like to highlight the paper I recently read by Yancheva et. al 2007. 

Yancheva et. al 2007 presents 'the role of climate and environmental change in the success or failure of societies'. They propose that major changes in Chinese dynasties occurred when the East Asian winter monsoon was strong and that 'dynastic transitions tended to occur when the summer monsoon was weak and rainfall was reduced'. A reduction of rainfall led to phases of crop failure and famine which leads to uprisings and riots. They also link the phases in dynasties with the phases of collapse of the classic mayan civilisation (Fig. 1). 

Fig. 1: Paleoclimate records during the past 4,500 years in the context of major events in the cultural history of China (Yancheva et. al 2007).

The mechanism that they propose for abrupt climate change is the shifts in the inter-tropical convergence zone (ITCZ) which represents the band of rainfall in the tropics. When the ITCZ is displaced to the North, summer monsoons will strengthen and vice versa. On the topic of the ITCZ, Timmermann et. al. (2007) and Zhang and Delworth (2005) proposes that there is an intimate link between the strength of the thermohaline circulation in the Atlantic Ocean and the meridional position of the ITCZ. With a weakening of the thermohaline circulation, there will be less heat transported to the northern high latitudes and consequently, a southward flow of heat. This causes the ITCZ to migrate southwards.

A weakening of the thermohaline circulation might be due to several reasons but one widely cited cause of a slow down in the circulation is a massive and abrupt injection of freshwater into the Northern Atlantic. This mechanism has been widely accepted to be the cause of abrupt climate change events such as the Younger Dryas event and the 8.2 kyr cooling event. 

The Chinese and the Mayan were not the only civilizations affected by climate change. Other studies have shown that the Norse abruptly abandoned their settlements in Greenland during the little ice ages in the 14th and 19th century as well as the Akkadian empire in Mesopotamia 4,200 years ago. Putting this in the perspective of climate change today, it is highly likely that abrupt climate changes might occur as a weakening of the thermohaline circulation seems like a possibility in the future (e.g. due to the melting of ice sheets in Greenland). This might then have an impact on countries situated in the tropics and especially in countries where the people are heavily reliant on the patterns of precipitation for agricultural or farming purposes. For example, China is heavily dependent on the agricultural sector to support her growing population and economy and hence, will be subjected to the impacts of a migrating ITCZ. 

References:

Timmermann, A., Okumura, Y., An, S, I., Clement, A., Dong, B., Guilyardi, E., Hu, A., Jungclaus, J. H., Renold, M., Stocker, T. F., Stouffer, R J., Sutton, R., Xie, S. P. and Yin, J. (2007) ‘The influence of a weakening of the Atlantic meridional overturning circulation on ENSO’, Journal of Climate, 20, 4899-4919.

Yancheva, G., Nowaczyk, N. R., Mingram, J., Dulski, P., Schettler, G., Negendank, J. F. W., Liu, J., Sigman, D. M., Peterson, L.C. and Haug, G. H. (2007) ‘Influence of the intertropical convergence zone on the East Asian monsoon’, Nature, 445, 74-77.

Zhang, R. and Delworth, T. L. (2005) ‘Simulated Tropical Response to a substantial weakening of the Atlantic thermohaline Circulation’, Journal of Climate, 18, 1853-1860.