Dealing with Drought
Short Version: Drought or other natural disasters can cause even really smart people to fail badly if they don't get enough help. However, with plenty of fossil-fueled tools and trade, the dangers of natural disasters have been reduced greatly. Here, we consider two cases of people responding to severe droughts — one before the age of fossil-fuel energy, the other during the age of fossil-fuel energy.
Friendlier but Longer Version: We could tell many stories about the benefits of fossil fuels. Here is one. The details of this story are not especially important, but the basic idea is greatly important—our ability to use fossil fuels to power our tools makes us much better off.
A few years ago, a great group of Penn State students, faculty, and film professionals toured many of the national parks of the US southwest. We hiked to the bottom of the Grand Canyon, rafted the Colorado below the Glen Canyon dam, slept on the slick rock at Canyonlands, and otherwise had a truly wonderful trip.
Many of us were especially fascinated by Mesa Verde. Ancestral Puebloan (often called Anasazi) people lived at that site for roughly 700 years—much longer than the history of the Americas since Columbus—first on top of the mesa, but then moving to build intricate dwellings in caves down the mesa sides, commuting up ladders and steps carved in the rock to work the fields on top. But, after most of a millennium, the people left.
Archaeological sites are almost always open to interpretation and argument. We know what was left behind, and we can learn much of what was going on around the area, but the record is necessarily incomplete and viewed through the lens of who we are.
Still, much of the Mesa Verde story is rather clear. The national park rangers showed us the little holes that the people painstakingly carved in the rock in the dwelling caves to capture a trickle of water. We marveled at the carefully constructed check dams, stones set to stop the erosion of the mesa top and catch a little soil and water to grow a little more corn. Food-storage structures were built in places that were very difficult to reach. And, toward the end, windows between different parts of the cliff dwellings were blocked with rocks, dividing people.
Video: Mesa Verde Story (8:29)
Some of the evidence we saw at Mesa Verde of people dealing with hard times caused by a drought.
The evidence is very clear that the people were conserving water and soil, working to maintain and improve their ability to grow food. The hard-to-reach food storage might be a truly serious version of someone hiding something on the top shelf so they don’t eat it before they should, and the window-blocking is at least suggestive of increasing social stresses.
To learn more of this story, scientists went to Long House Valley in Arizona, a simpler place nearby that was occupied by the same people. Recall that the age of a tree can be learned by counting its yearly rings. These rings are easy to see in places where there are pronounced seasons because trees grow rapidly during the spring and early summer, putting on a lot of new wood that appears lighter in color, and then during the fall and winter, the growth slows way down and very little wood is added; this late-season wood is denser and darker. So, one thick light band and a thin darker band make up one year. This is sometimes not the case for trees that grow in the tropics, where there may be little difference between summer and winter, however, if tropical settings with defined wet and dry seasons, trees do develop annual rings. The important thing is that there needs to be a seasonality for trees to develop annual rings. In the dry climate of a place like Long House, trees grow better when it is wetter in the growing season, so a tree will thicker annual rings — the ring thickness is directly correlated to the amount of rainfall. In colder climates, the ring width can be correlated to temperatures during the growing season — warmer temperatures lead to thicker rings.
Thus, tree rings preserve a record of the climate history — rainfall in drier regions and temperature in colder regions. And, living trees overlap in age with trees that were used in construction, or trees that died but haven’t rotted yet. Using the pattern of thick and thin years to match the modern and older wood (a technique called cross-dating), the history of rainfall can be extended beyond the life of a single tree. Cross-dating has enabled us to produce continuous tree ring records that go back about 12,000 years even though the oldest living tree is just a bit over 5,000 years.