Abrupt Climate Transitions and Humans

Rainfall patterns at low latitudes shifted abruptly northward 14,700 years ago, and again 11,700 years ago. From studies of ice cores and tree rings, it is clear that these transitions were extremely fast, on the order of a decade, with up to half of the change occurring in one year. Remarkable archaeological evidence from western Asia, including precise radiocarbon dating, suggests that major changes 1) away from nomadic lifestyles and 2) initiation of agriculture began within error at precisely these dates, respectively. The ice core, cave deposit, and marine sediment record show that these abrupt events recurred hundreds of times over the past million years, with a roughly millennial periodicity, raising the question of whether they influenced human development. Radical environmental change ought to select for individuals capable of innovation and learning, rather than pre-programmed behavior. Finally, the study of these events has shown conclusively that they are caused by shifts of the Earth’s thermal equator in a north-south direction, wrought by ocean and atmosphere circulation changes. This raises a warning flag for our future: if we humans decide to manipulate the climate, we must be careful not to cool one hemisphere without cooling the other, because the tropical rain belts will move toward the warmer hemisphere. For example, if sulfate aerosol is sprayed into the northern hemisphere, but (to save money) it is not done in the southern hemisphere, we would likely trigger a southward shift of rain belts away from African and Asian monsoon regions where billions of people depend on rain for their livelihood.