The Old World Exchange had devastating consequences. Cities shrank, trade declined, tax revenues fell, and fields were abandoned. And as if all this were not enough, every source of evidence—peat bogs, lake sediments, ice cores, tree rings, strontium-to-calcium ratios in coral reefs, even the chemistry of algae—suggests that the weather, too, turned against humanity, ending the Roman Warm Period. Average temperatures fell about 2°F between 200 and 500 CE, and since the cooler summers of what climatologists call the Dark Age Cold Period reduced evaporation from the oceans and weakened the
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