Saturday, July 4, 2009

Summer Reading, Part 1

Each Fall Semester I teach a course, SOC260 Population, Resources and Change, that examines the interrelationships between human societies and the environment, focusing on modern industrial societies. Consequently each summer, I try to read a couple of new (to me) books on the general topic of the environment and society. This summer I thought I would post reviews of books as I finish them -- with the thought that this might prompt me to finish more! The first book I will discuss is The Great Warming: Climate Change and the Rise and Fall of Civilizations by Brian Fagan, Bloomsbury Press, 2008.

Let me begin by saying that The Great Warming has lots of fascinating information about the interplay between climate and society, drawing upon research on dozens of societies on eight continents across thousands of years of human history. It is well researched, entertaining and lively and worth reading. Each of the stories shows the importance of climate in both the making and the breaking of humans societies. However, the book does not live up to its title, nor does it deliver on the basic premise set forth in its preface.

Fagan's thesis, as set out in the preface, is that the "Medieval Warm Period" was a global warming event affecting the entire planet, and that the primary lesson to be drawn from this global event was that global warming (even when it is on a lesser scale than the anthropogenic warming of the present day), creates devastating drought across much of the world.

The term "Medieval Warm Period" refers to the higher than average temperatures, documented by several forms of temperature proxy research, in Europe between approximately 800 AD to 1300 AD. Proxy methods for establishing past temperature regimes include: ice cores, deep sea an lake sediment cores, coral records, and tree rings. Through out the book, Fagan refers to the period between 800-1300 AD as either the "Medieval Warm Period" or more generally as the "warm centuries;" but when he gets down to the specifics the regional temperature proxy information he presents often indicates prolonged centuries of colder climate for regions such as Eurasia, the Sahara/Sahel in Africa, the Andes of South America, and the middle and south Pacific.

We know, in the present day, that an overall warming of the earth, is consistent with the occasion pattern of cooling in specific regions. Not every location on earth, experiences a constant, upward warming pattern. Present day climate change research emphasizes statistical averages and the global pattern while recognizing local variation. Fagan does not produce sufficient evidence to support a claim that the overall earth's temperature rose during the period 800-1300 AD, only that some widespread regions experienced warming and that equally wide spread regions experienced cooling. Perhaps that evidence exists, but it was not presented in this book.

Moreover, although Fagan's primary aim is to show the connection between warmer climate and drought, many of the examples of drought come from regions where temperatures were cooler, or where there are no proxy measures of temperature available, only measures of rainfall. For example, drought in the Sahara/Sahel during the 800-1300 AD period is primarily related to cooler temperatures. Cooler temperatures over the Pacific during these centuries is also associated with drought on the west coast of California, and in the South American Andes.

Other examples of drought come from regions such as India where both warming and cooling occurred in different regions, and even shifted from time period to time period as the oscillation between El Nino/La Nina shifted the timing and location of the monsoons. With China, Fagan's evidence of warming comes from eastern China, while the evidence of drought comes from Huguangyan in south China where lake cores indicated cooler climate (during the early part of the target period) and the northern Tibetan highlands (during the latter part of the target period).

If one ignores Fagan's attempt to build a grand argument about global warming, much can be learned in this book about the importance of climate, and especially the impact of flood and drought, in the course of human history from the specific evidence about particular societies.

1 comment:

Chris Crawford said...

Yes, the relationship between temperature and rainfall can be confusing, but the basic controlling principle is simple: warm ocean water means that more water vapor is absorbed by the atmosphere. Cold water doesn't release much water vapor into cold air. Antarctica is one of the driest places on earth.

So warming should, in general, lead to lots more precipitation -- but only if you're in the right places for precipitation, not too far from a seacoast on the side of the continent favored by the prevailing winds. The American Southwest is dry for several reasons, one of which is that the Humboldt Current carries very cold water down the West Coast.

In any case, cooling ALWAYS leads to less precipitation. Cold ocean water doesn't release much water into the atmosphere, and that's the starting point for all precipitation.