Listen to part of a lecture in an earth science class.

Professor: Although we've been keeping accurate climate records for maybe a few centuries, the field of paleoclimatology looks at the geological record to find out about the earth's climate in the distant past. We recently analyzed a stalagmite from a cave in western China that sheds some light on the relationship between global climate change and human society.

Professor: Now think of a stalagmite and think about how it forms, who can tell us how stalagmites are formed. Jenny?

Student: Sure. Stalagmites form when cave…when water drips down from the roof of a cave, there's like some sort of mineral in the water that gets deposited on the cave floor or something. Over thousands of years, the deposits build up.

Professor: That's right. Ground water carries minerals, mostly calcium and deposits them when it drips through a cave ceiling, resulting in a kind of layering of the deposits, that's the stalagmite. Now, much of China's weather in the southeast part of the country is affected by monsoon rains, the going through one rainy and one dry season each year. In the northwest, there's a more temperate, drier climate. And this particular cave we're talking about is located in a region between a monsoon and temperate climate. So, it's sensitive to drought years when less rain falls in the rainy season. So, stalagmites may grow faster or slower, depending on the amount of rainfall. More water means more growth. Max, question?

Student: Don't all the stalagmites grow that way? What makes this one so important?

Professor: Its location. If it were squarely in a tropical monsoon climate, it probably wouldn't be affected so much by drought. Also, the ground in this area, the ground water isn't just seeping through calcium, but also through an unusually high concentration of uranium. Uranium is a radioactive element. So it decays at a predictable, regular rate. And the uranium is the reason that we can accurately date the stalagmite. So, this case has produced an extremely accurate climate record dating back 1,800 years.

Another thing researchers did was they checked the stalagmite against Chinese historical records. What they found was that there is a strong correlation between the climate, the rainfall that the stalagmite records and what was going on in society at the time. For example, when the stalagmite shows years of abundant rainfall, Chinese society was at peace and culturally productive. Times of drought seemed to have sped up, exacerbated social problems and probably led to the downfall of several Chinese dynasties. Jenny?

Student: Well, that makes sense, right? If there's abundant rainfall, there's lots of crops and everybody's happy. A drought would mean less food and increasing social strife. True?

Professor: This pattern wasn't just limited to ancient Chinese dynasties. There is evidence that when the Tang dynasty was in decline about 1,000 years ago, partially due to a prolonged drought, something similar was happening to classic Mayan civilization in what is now southern Mexico and Central America. Here we see...

Student: Wait, how could a Chinese drought affect Mayan civilization? Professor: It couldn't, not really, we think that something was affecting the climate in both China and the territory of the Mayans. We're trying to find out what could do that on such a global scale. And the stalagmite offers some clues. We found cyclical, wet, then dry periods throughout the stalagmite’s history, a cycle of about 11 years. Now, the neat thing about this seems to correspond with an 11-year cycle in the sun's activity. See, the sun goes through cycles of increasing and decreasing energy output. The peak years of solar activity tend to correspond to wet periods. And years of weak solar activity correspond to drier periods.

Student: So, the sun ruined ancient civilizations? Professor: Well, not exactly. You see, the prevailing theory is that in times of decreased solar activity, less ocean water evaporated at the equator, which means less moisture in the atmosphere that could fall as rain. So, there was a dry period. In this case, in China during the downfall of the Tang dynasty. But keep in mind what we're talking about the link between potential global climate change and ancient civilizations decline. It's just too complex for a simple cause-effect relationship. We are discussing a single piece of evidence from one cave in a very large continent. Climate probably played a role, but we don't know how much of a role.