11 October 2006

Ancient Human Induced Climate Change

So, here's a bogus post. But it makes for a fun thought exercise. Could the desertification of the Sahara some 3,000 to 8,000 years ago have been caused by human induced climate change?

Draw a line of latitude at about 30 degrees North through the Sahara and further east. You'll go through Iraq and Iran, northern India, and southern China. Picture this entire expanse 10,000 years ago as a grassy plain with stands of wood in some areas. In the wide Tigris-Euphrates river valleys, herds of cattle grazed and people were starting to experiment with agriculture. Throughout this area, agriculture was being developed, population densities were increasing, and demand for wood (for construction and for fires) was increasing.

It is not unlikely that over the relatively short timespan of 1000 years, much of the woodland cover was stripped bare to create fields for planting, firewood, and construction materials. This would have dramatically changed the local climate. Less water would be held in the soil. Less water would have been transferred to the air by trees. This entire area would have become much more arid over a relatively short time span.

(To the inexperienced, it may not be clear how much more water forests can hold in their soil than bare land can hold. I live in California where our gardens grow in clay, the sun is hot, and the air is dry all summer long. When watering roses growing in bear clay with a hose, the water immediately runs off the top of the soil and very little water soaks into the soil. Apply a mulch of redwood bark around the roses, and now when you water, the bark soaks up and holds the water. The water content of soil rich in organic matter is easily 10 times higher than the water content of bare land.)

As this entire latitude became drier, the marginal grasslands dried up and became desert.

Over subsequent millenia, forest cover and other problems caused by humans living in an area slowly built up. As the quality of the land in the Tigris-Euphrates declined, the center of western civilization moved west to Egypt, Palestine, Greece, and Rome. Each new civilization gradually depleted the land and the outskirts gradually became stronger.

As I said, it's a fairly bogus scenario. But fun to think about.