THEGREENGROK    Planetary Watch

Climate Change in the Rearview Mirror

by Bill Chameides | October 11th, 2011
posted by Erica Rowell (Editor)

Permalink | 2 comments


You’ve heard about going to a particularly unpleasant place in a handbasket? Try this climate-change trip on for size.

Climate scientists often talk about climate change in terms of global temperature swings of a few degrees or even less. This can often leave the non-scientist wondering what the big deal is. Temperatures can change by tens of degrees over the course of a day, and daily highs and lows also change by tens of degrees from one season to another. Why should we worry about such miniscule changes in global average temperature?

The answer of course is that global average temperatures are a metric that tracks the entire energy balance of the atmosphere and a small change in this metric can lead to significant changes in other important elements of the climate — like rainfall rates — that can affect us in profound ways. Such explanations are all well and good but don’t quite get at the human significance of the changes that can occur.

Another tack, I’ll call it the historic one, can be far more compelling. Probably most famously adopted by Jared Diamond in his bestseller Collapse: How Societies Choose to Fail or Succeed, the approach looks at past societal calamities and argues that climate change and a failure to confront those changes in a timely manner contributed to the demise of the civilizations of both the Anasazi of the southwestern United States and the Maya of Central America, among others.

A Look at Past Swings in Climate and the Social Upheavals That Accompanied Them

Now, David Zhang of the University of Hong Kong and co-authors in a paper published last week in the Proceedings of the National Academy of Sciences have taken this historical approach one step further by using sophisticated statistical methods in conjunction with historic data on climate, agriculture, the economy, and demography to try to establish a quantitative and causal relationship between climate change and major social upheavals.

Much of their analysis focuses on the period from 1500 to 1800, which spanned the so-called Little Ice Age when the Northern Hemisphere experienced relatively cool temperatures. (It has been hypothesized that this cool period was caused by low solar activity and/or high rates of volcanism, although this remains a subject of some scientific debate.)

The authors’ analysis suggests that the seemingly modest plunge in average temperatures in the Northern Hemisphere (of about one degree Fahrenheit) beginning in about 1560 led to precipitous drops in European grain yields and increases in grain prices. This in turn was followed by major changes in metrics we would normally associate with human well-being, such as famines, decreases in human height, increases in plagues and wars, and ultimately a drop in human population. I don’t know about you, but sounds like a society firmly ensconced in that handbasket headed in the wrong direction.

Fortunately, the handbasket ride was not permanent. When temperatures began to recover in about 1650, these other variables trended back to their pre-cooling levels. The authors also found that although abbreviated cooling in 1700 and 1750 “caused minor fluctuations in grain yield, real wages, grain price, famine, war, social disturbance, and migration, its impact was not strong enough to cause general crisis and population collapse.”

Given what’s been going on with the climate of late, this is scary stuff. Of course it is to be noted that the Zhang et al study focused on cooling episodes, and our climate is warming. Perhaps we only ride the handbasket when the climate cools? We can hope, but then again Zhang et al point out that there were widespread famines during the Medieval Warm Period in the 11th and 12th centuries. All aboard?

filed under: climate change, faculty, global warming, Planetary Watch, rainfall, temperatures
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  1. John Bell
    Oct 20, 2011

    Bill, There is a perfect example, history shows us, that slightly higher temps are good and lower temps caused trouble, but you want to believe that both are bad, which is illogical. Firstly, our recent global temps have been quite steady, well within normal variation. Also I notice that you are still using copious carbon just like other Americans. I am reading your blog, so go on over to and read my blog titled “Global warming – my take” thanks John

  2. John Mashey
    Oct 16, 2011

    See CO2 over last millennium, specifically the unique sharp drop into 1600AD: While it remains to be seen, a case is starting to be made that at least some of that was massive reforestration in the Americas caused by the biggest die-off in human history. See the 2 papers referenced in Columbus blamed for LIA: It is an interesting attribution problem to model the effects of this plus natural causes, although note this preceded the Maunder Minimum, so it wasn’t that.

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