The main question is the extent to which humans have caused this warming. The Earth, the conventional wisdom goes, is rebounding from an ice age, but in the past 150 or 200 years, we have accelerated the pace as a byproduct of our use of fossil fuels. Prior to that, we were puny creatures incapable of affecting the global environment, and it is only modern technology that changed this.

But is that true? Not the modern technology part--few Analog readers would disagree that we have the ability to geoengineer the Earth on a large scale, and that the future will give us ever greater power. "Our leverage [over climate] keeps growing as our science gets better," David Keith, of the University of Calgary, put it at the Fall 2005 meeting of the American Geophysical Union.

But until recently, climate had a much more obvious effect on us than we had on it. Harvey Weiss, an archaeologist from Yale University, goes so far as to argue that civilization was created in reaction to a climate fluctuation that occurred about 8,200 years ago. He bases this argument on the fact that humans have been on the planet a long time, but it was only then, in ancient Mesopotamia (today's Iraq), that they began banding together into anything more complex than scattered tribes and villages.

"The biggest question in Mesopotamian archaeology is why there even is a Mesopotamian archeology," he said at a 2003 geophysics meeting. That's because, at first glance, Mesopotamia isn't the most inviting place. It's a desolate area "that looks like what you see on CNN every night. Bleak, dismal, and parched, only watered where the Euphrates has its course." It can be farmed, but only at the cost of a lot of work, building and maintaining irrigation canals. Cooperating to do this was obviously a boost to civilization--but why bother?

Weiss argues that the answer lay in the aforementioned climate change: an abrupt cold shift and drought that lasted 200-300 years. This forced people to migrate to the water, where they had to work together to learn irrigation. By the time the climate moderated, civilization was established.

Sara Parcak, of the University of Cambridge, believes that another abrupt shift, about 4,200 years ago, produced droughts that contributed to the collapse of Egypt's once-powerful Old Kingdom. Similarly, many archaeologists believe that drying climate in the American Southwest may have forced the Anasazi to abandon the cliff dwellings that delight today's tourists. More recently, a series of wet decades in the early twentieth century lured farmers to places as unlikely as California's Mojave Desert and the sagebrush steppes of eastern Oregon, where ghost towns still dot the land. And in early 2006, South African scientists calculated that global warming would eliminate a sizeable percentage of the continent's arid-region creeks by the end of this century--a potential catastrophe for some of the world's poorest countries.