Droughts of the Past
Part B: Recent Droughts
Drought is all over the map: compare these two maps showing the Palmer Drought Severity Index (PDSI) for the weeks ending June 25, 2005 and June 26, 2008. Though each map depicts conditions at the end of the month of June, you'll note several differences between the two images that are three years apart.
- Which areas went from moist in 2005 to dry in 2008? Which areas went from dry to moist?
- Were there areas that showed drought during both periods?
- Go to the Climate Prediction Center's Drought Monitoring site. This site offers access to past PDSI maps for every week of the year from 1998 through the present.
- Choose the same week for 2011. What pattern or trend has emerged?
- Next, choose a week of interest to you, for instance, the week of your favorite holiday or your birthday. Compare the PDSI maps of that week for three or more years to get a sense of the variability of drought patterns.
- Get a longer view of regional drought patterns by examining this set of graphs and images. Each graph shows the PDSI from 1895 through 1995 at the location indicated. Yellow bars in the graphs highlight years of exceptional drought at that location. Maps next to each graph show conditions across the contiguous U.S. for the years indicated. Note that the color scale for interpreting the images is in the lower right of the large image.
- In 1937, in addition to drought in the "dust bowl" area of the southern plains, what other region was experiencing drought?
- While the Pacific Northwest endured almost a decade of drought beginning in 1984, what was it like in the "sunbelt" states of Arizona, New Mexico, and west Texas?
Historical patternsInstrumental records of drought for the United States only extend back to 1895. To understand how major droughts of the 20th century compare to those of previous millenia, paleoclimatologists seek environmental records that document the occurrence of drought. Relatively thin widths of climate-sensitive tree rings and pollen counts from layered lake sediments are two pieces of environmental evidence used to reconstruct past conditions.
- Go to the North American Drought Variability site. Read the introductory paragraph to learn how drought conditions across the contiguous U.S. were deduced from a network of sampling sites.
- Click the color map to launch another window that shows drought maps for North America from 1730 through 1995.
- Use the animation controls to move though the years, watching for widespread or persistent drought events. You'll see that from 1895 to 1995, maps produced from instrumental recordsactual measurements taken with weather toolsare shown side by side with maps from reconstructed recordsconditions that were deduced from tree ring measurements.
- Learn more about environmental records and how scientists reconstruct climate history from them by reading the page at Paleoclimatology and Drought. Then answer the questions, below.
NOTE: The interactive animation window works better in some browsers than others. If you're having problems, try using another browser. Note also that the maps use an expanded PDSI that extends from -6 (dry) to +6 (wet).
- List at least 5 years from the animation that you would identify as major drought years for some portion of the continental U.S. 1730, 1820, 1861, 1937, 1953, 1961, 1988
Stop and Think1. After comparing the notable drought years with other years in the animation, describe what you would consider as the criteria for designating a year as a drought year or not.
2. Consider how measurements of tree-ring widths taken from trees at each of PDSI reconstruction gridpoints can result in the reconstructed maps. Describe a method that could be used to produce the maps.
3. Based on your comparison of reconstructed and instrumentally derived drought maps, describe how confident you are in the accuracy of the reconstructed maps. Give examples to support your answer.