It's sort of understood these days that global warming carries with it cascading impacts—higher sea levels, for example, or shifts in arable land—which in turn promise a bit of social unrest.

Climate shifts might well make kings out of former paupers (witness the boom in potato yields in icy Greenland, which now has a longer growing season), but the expected increase in competition for resources (water, anyone?) has long inspired experts to warn of new and looming bouts of local, regional and international envies—some of which could lead to conflicts.

Of course, that sort of predictability—climate change leads to conflict—has been tough to quantify, though a study out of the University of Hong Kong, published late last year in the Proceedings of the National Academy of Sciences, makes a convincing stab at establishing a historical correlation between cooling periods and warfare.

That's right: cooling temperatures.

Eric Wagner of Conservation Magazine (source of the chart above), interpreted the study from David Zhang and his colleagues, which looked at historical temperature fluctuations and conflicts—as well as other variables like food prices and population—between 1400 and 1900:

Global cooling, their hypothesis goes, led to decreases in rainfall, which made the land less fertile. The result: food shortages, skyrocketing food costs, and sometimes famine. Communities fell into disarray, fomenting broad social unrest. Armed conflicts surged. The number of wars in years with the coolest average temperatures—1450, 1650, and 1820—was nearly twice that of the mild eighteenth century. When temperatures rose again, relative calm was restored.

The findings were in keeping with an earlier analysis by Zhang et al that focused exclusively on China. In that study, the researchers note, "the number of war outbreaks and population collapses in China is significantly correlated with Northern Hemisphere temperature variations and that all of the periods of nationwide unrest, population collapse, and dynastic change occurred in the cold phases of this period."

Before anyone assumes that concerns over rising temperatures nowadays are undermined by the study, the researchers suggest that the correlation is likely to have more to do with the rate and duration of climate fluctuations in either direction than specifically with cooling periods. That is to say, dramatic shits of the mercury either upward or downward would be expected to yield similarly rowdy times—unless...

Yes, there's one more caveat. The period studied may well have been more susceptible to Darwinian fits because many of the mechanisms that might mitigate social unrest were either non-existent or suppressed by political or geographical circumstance.

Animals, for example, can respond to resource stresses in really only two ways: decreasing population size (i.e., starvation), or migration to areas where resources are more plentiful. Humans have more tricks up their sleeves: they can go to war, as we've seen, or migrate, but there's also, according to Zhang et al, "economic change, innovation, trade, and peaceful resource redistribution."

Political boundaries may have prevented mass migrations, the authors note, and when they did occur, war often ensued. And fundamental economic adaptations, developments in new technologies, etc., were painfully slow.

Today, the authors imply, human beings have developed "international and national institutions" that might well be strong enough to buffer the tensions caused by food resource scarcity.

We'll see.

Chart: Conservation Magazine

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