Part 4: Megadroughts and the People of Pueblo Bonito
In the 1930s, the Great Plains region of the United States was devastated by drought. The 150,000-square-mile area, encompassing the Oklahoma and Texas panhandles and neighboring sections of Kansas, Colorado, and New Mexico, had little rainfall, for several years. The combination of poor land-use practices (destructive plowing), light soil, and high winds formed a destructive combination. When drought struck in the mid 1930s, the soil lacked the stronger root system of grass as an anchor, so the winds easily picked up the loose topsoil and swirled it into dense dust clouds, called "black blizzards." The region became know as the Dust Bowl. Recurrent dust storms wreaked havoc, choking cattle and pasture lands, and driving 60 percent of the population from the region.
Although the drought of the 1930s was a significant one, paleoclimatologists a science discipline that focuses on climatic conditions and their causes and effects in the geologic past using evidence found in glacial deposits, fossils, tree-rings and sediments have identified periods in our history when droughts in a region have spread over a decade or more instead of a few years. These extreme weather events are termed megadroughtsa prolonged drought lasting two decades or longer. In this part of the lab, you will investigate a long-term data set produced by tree-ring records as a proxy to identify the existence of drought and megadroughts that may have affected the people of Pueblo Bonito and the surrounding region.
Instructions
1. Learn about megadroughts in the past and future. Download and read the one-page article Water Resources about mega droughts by Dr. Ed Cook. Cook's article is found on the right side of the document.
Stop and Think
4.1 How do megadroughts of the past compare to those that have occurred in the past 100 years?
4.2 What is one possible cause of megadroughts in the past?
4.3 What factors are increasing the possibility of catastrophic megadroughts in the future?
4.4 Based on the graphical data given in the article (figure 1, bottom of the page on the right side), did your state, or your community, experience a severe drought in any of the three periods shown?
2.Investigate the long-term tree ring data
The time-series graph below shows a very long tree-ring chronology developed by Dr. Henri Grissino-Mayer and others, from an amazing site called El Malpais, located in New Mexico. At this site, trees are growing on an ancient lava flow. Because the lava holds so little water, the trees grow very slowly and their annual rings are sensitive to changes in the amount of summer precipitation. In this harsh environment, dead trees can resist decay for a long time, so if a tree dies and falls over, it may not rot for hundreds or even a thousand years.
Drop into El Malpais and take a look around at the ancient lava beds and look for dead trees as well as living ones. El Malpais
3. Scientists can collect samples from both living and dead trees at sites like El Malpais, and if there is enough overlap between the two tree-ring records to confirm the dating of the dead trees (about 100 years), then they can combine the data to produce a continuous record of changes in precipitation, marked by varying tree-ring growth over the past 2000 years. The data from the El Malpais site was also used to confirm the dates of many of the ancestral pueblos in New Mexico.
The vertical axis (Y axis) of the time-series graph below is the precipitation in a given year in millimeters.
4. Use the sliders at the bottom of the interactive to explore periods of time with scant rainfall. Are there any megadroughts that last 10 to 20 years or more?
Stop and Think
4.5 After studying the 360 image of the El Malpais site, what two factors have made this an exemplary site for tree-ring research?
4.6 List any periods of megadrought you can locate over the past 1000 years.
4.7 How would drought impact you? Think about and list how a 1 or 2 year drought might impact your life. Now compare that to how a 10-year drought might impact your life. How would you respond to these types of droughts and how might they affect you and your family?
4.8 Use the graph below to answer this question: How often do extreme 1- to 2-year droughts occur? How often do megadroughts occur?
4.9 Scientists estimate that the ancestral pueblo people left Pueblo Bonito in the early 1200s. Based on the data that you measured, and the El Malpais data plot, could a megadrought be a primary cause of their abandonment of their village?
4.10 Take a look at the most recent decades on the El Malpais plot. Based on the data, is this region of New Mexico in a period of drought or have the past few decades been average?
4.11 Given what you have learned about drought through time based on tree-rings, would it change your mind about living in that region? What about farming in that region? If you were a water manager for New Mexico, where Chaco Canyon, Pueblo Bonito, and the El Malpais data are located, how would this data affect your thinking about managing water?
