by Eugene Linden
Akkad was the capital of the Akkadian Empire. Situated in what is now northeastern Syria, it controlled much of Mesopotamia and was perhaps the first of the great Mesopotamian empires. However, the Empire collapsed sometime around 2100 BCE. For years, historians have assumed that this collapse occurred in the “conventional” manner: barbarians galloped in, killing and destroying. There was no evidence to support this assumption. Indeed, there was no evidence at all. Akkad simply disappeared from the world scene.
Archaeologists have located the ruins of the city: they are buried in a mound now called Tell Leilan. The site has been partly excavated, yielding a remarkable discovery: the level of the city of Akkad is covered in 3 feet of windblown soil that was so dry that it was never occupied by earthworms -- that’s very dry! Moreover, the remains of the city show no signs of burning or deliberate destruction. Particularly revealing was a puzzling arrangement of construction stones around a wall. They seemed too neatly organized to have simply fallen there. The wall was never completed. The Akkadians simply abandoned the city before finishing this wall.
There is now a hypothesis that Akkad was not destroyed by human activity; it was starved to death by a long drought. Evidence of this drought shows up all over the Middle East. As yet, this hypothesis has not conquered the archaeological community, but it has strong support. If it be correct, then Akkad represents the first civilization to be destroyed by changes in climate -- but not the last. History is peppered with cases of entire civilizations collapsing for no apparent reason. The Mayan civilization fell apart late in the first millennium, and archaeologists have yet to come up with a compelling explanation for the Mayan collapse -- but a long drought is one of the more popular hypotheses. The Anasazi people of northwestern New Mexico appear to have succumbed to a long drought somewhat later. A similar drought also destroyed the Mississippian culture further east.
Droughts are part of the regular rough-and-tumble of weather, but these were not regular droughts -- they spanned centuries. They reflect major shifts in climate. These were regional changes, not global changes; at the same time the Mayans were starving to death, the Anasazi culture was enjoying lush rainfall and experiencing a population explosion.
Eugene Linden explains these processes in this book. After examining the historical examples of civilizations destroyed by long-term drought, he discusses El Nino, a periodic phenomenon that appears to have effects all over the globe. Heat tends to accumulate in the eastern Pacific, and after some years -- we still don’t know quite why -- the warm water shifts eastward across the Pacific towards Peru. In the process, it upsets weather patterns all over the globe, although the effects are most strongly felt along the west coasts of the Americas.
The most disturbing portion of the book is its discussion of ocean currents. Most of the heat transfer on this planet is accomplished through these currents. Most people know about the Gulf Stream, but they may not know that is only one leg of a planet-wide system that sends warm water from the tropics to the poles, and cold water from the poles to the tropics. It courses through the Atlantic, Pacific, and Indian Oceans. And this system controls all our weather. Warm water warms the air over it; cool water cools the air over it. The atmosphere moves in response to the temperature gradients, and the ocean currents control those temperature gradients.
Only now are scientists beginning to realize just how sensitive these ocean currents are. Any number of small alterations in the planetary arrangement could dramatically alter the paths taken by these currents, which in turn would yield big changes in weather over large portions of the earth’s surface. Two factors that have particular significance are temperature gradients and salt concentrations.
Temperature gradients are merely the differences between warm regions and cold regions. Big temperature gradients drive large amounts of heat transfer, strengthening, speeding up, and possibly altering the directions of ocean currents. Small temperature gradients yield weaker, slower ocean currents that might deviate from their earlier courses. Salt concentrations are important because salt water is denser than fresh water; it sinks to the ocean floor more quickly, thus feeding the return courses of the deep ocean currents carrying cold water towards the tropics.
Human kind is altering both of these factors. Anthropogenic climate change is dramatically heating the polar regions, reducing the magnitude of the temperature gradient that drives ocean currents -- and ultimately weather. Moreover, the massive amounts of water melting in Greenland are mixing with the salty ocean water and diminishing its salinity, reducing the sinking of cold water and weakening the Gulf Stream.
How will these changes affect ocean currents? We don’t know. Theory suggests that the currents should weaken, but they might also change direction, carrying their vital heat to different climes. It is entirely possible that the current arrangement of ocean currents could completely change, which would in turn lead to dramatic changes in weather all over the planet.
Here’s the biggest kicker: it could happen suddenly. The historical information suggests that some of these killing droughts descended quickly, without any ramping up. Scientists are finding more and more evidence of this phenomenon, called abrupt climate change, in the historical record for the last 11,000 years. We once thought that climate shifted slowly, on a time scale of centuries, and that remains true on a planetary scale. But on a regional scale, climate can change dramatically in less than a decade. Ten years from today, the damp redwood forests of California could be a desiccated and ghostly stand of dead giants. The cornfields of Iowa could be a dustbowl even as Washington D.C becomes a rain-swollen swamp. I used to think that such dramatic changes were the stuff of overwrought imaginations. After reading this book, I’m not so confident. Climate change might not be a problem for our grandchildren; we ourselves may be called upon to pay for our sins against the planet.