Tuesday, June 27, 2006

Climate Change and History

A study of ice cores has shown that the earth is coming out of a cool spell that began about 5,200 years ago. The cores show a sudden change, likened to a "cold snap," suggesting that future change due to warming could also come suddenly. The same ice core study corrobates that a sudden warming has occurred in the past half-century, and the change is out of line with normal warming and cooling patterns over the past 2,000 years.

More interesting to me is the other end of the study's time frame.

But the finding likely to cause the most debate is Thompson's conclusion that a swift and sudden cooling of the climate five millennia ago occurred simultaneously with key changes in civilizations.

"It represents a time where, for many parts of the world, people ceased to be hunters and gatherers and formed cities," he said....

Thompson said he does not know what caused the abrupt change -- one possibility is a "mega La NiƱa" shift in upper air currents. But he said the evidence from such diverse sources as Mount Kilimanjaro; African lakes; Greenland and Antarctic ice cores; the Andes and the Alps point to a sudden arrival of cool and often wet conditions, all about the same time.

That time saw cities form in the Nile Valley and Mesopotamia, his paper says, and the end of a humid period in Africa that "seems to have begun and ended abruptly, within decades to a century."

There is no doubt that environmental factors have played an important role in shaping human history. At the most basic level, cities are planned and constructed based on the availability of natural resources - water for drinking and transportation, fertile land for raising food and livestock, mineral deposits, elevated or difficult terrain for defense from attack, and so on. Epidemic disease and repeated crop failures can weaken a civilization - especially one confined to a small area - to the point that resistance to outside pressure becomes impossible.

Relatively stable features of the landscape and sudden disasters are easy to measure. The former mostly stay the same or at least leave evidence behind. The latter will also leave evidence - sometimes physically (like a volcanic eruption) or as part of the written record (like the Black Death). Long-term influences like climate change are harder to connect to specific historical events, as their "fingerprints" are less obvious. That makes them no less real. If the rise of complex civilizations can be correlated to changes in climate, it would go some way to understanding why it happened at that particular time, and in those particular places.