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 months of May in 2013 and 2016. Though each map depicts conditions at the end of the month of May, you'll note several differences between the two images that are three years apart.

1. Go to the Climate Prediction Center's Historical Palmer Drought Indices. This site offers access to past PDSI maps for every month of the year from 1900 to now.
2. Choose the start and end as May 2018 and compare to the 2013 and 2016 maps of May above. What pattern or trend has emerged?
3. Next, choose a month of interest to you, for instance, the month of your favorite holiday or your birthday. Compare the PDSI maps of that month for three or more years to get a sense of the variability of drought patterns. Compare the PDSI maps for one of those months with three or more months in a row to view short-term changes. Select Play in the bar above the map to create a slideshow of your selected dates.
4. Get a longer view of regional drought patterns by examining this set of graphs and images (Acrobat (PDF) 903kB Oct23 18). 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 below the sets of maps in the image.

Historical Patterns

Instrumental 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 millennia, 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.

1. Read an article on Tree-Ring Data and Historic Drought to learn how drought conditions across the contiguous U.S. were deduced from a network of sampling sites.
2. Visit the North American Drought Atlas site, which creates its drought reconstruction maps using the tree-ring data in the article. Click North American Drought Atlas Animation AD 0000-2005 on the webpage to launch the animated maps that show drought across North America from year 0 through 2005.
• NOTE: The animation will display in a popup window and may take a moment to load. Buttons at the top allow control over the speed and movement through the animation. You can also drag the slider to speed up getting to certain time periods. Note that the animation's maps use an expanded PDSI that extends from -6 (dry) to +6 (wet).
3. Click the color map to launch another window that shows drought maps for North America from 1730 through 1995.
4. Use the animation controls to move though the years, watching for widespread or persistent drought events.
5. Look at and compare these sets of maps (Acrobat (PDF) 2.7MB Oct31 18), taken from the Drought Atlas data, of 1911, 1934, and 1984. The sets include instrumental record maps - produced from actual measurements taken with weather tools, side by side with reconstructed record maps - produced by deducing conditions from tree ring measurements.
6. Learn more about tree-ring records and how scientists reconstruct climate history from them by watching the video on Tree Ring Analysis. Then answer the questions, below.
   
   If the video does not play, go to the Tree Ring Analysis video on YouTube.

Checking In

• List at least 8 years, between 1730 and 1995, from the animation that you would identify as major drought years for some portion of the continental U.S.
  Show me some notable drought years
  Hide
  1735-36, 1756-58, 1772-73, 1776, 1782-83, 1789, 1800, 1808-09, 1817-18, 1829, 1848, 1851, 1855, 1863-65, 1880, 1895, 1902, 1911, 1924-25, 1931, 1934, 1936, 1940, 1951, 1953-54, 1956, 1961, 1988, 2002,

Stop and Think

1. 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 PDSI reconstruction gridpoints can result in the reconstructed maps. Describe a method that could be used to produce the maps.
3. Why do you think there is data from only certain parts of the continent for certain time periods?
4. 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.

Optional Extension

Using years of your own interest, compare maps created from reconstructed climate data from tree-rings to those created from instrument data from the North American Drought Atlas site.

1. In the toolbar at the top of the site, select Single Year from the Maps dropdown menu.
2. Select Reconstructed Dataset and then Next.
3. Input a year to Display between 1895 and 2005. Then, in the map under Options, resize the black box to cover the Continental U.S. For the color bar, select the center option that transitions red to violet. Then select Create Map in bottom left.
4. Keeping the reconstructed map, open a second window of the North American Drought Atlas site and repeat Step 1.
5. Select Instrumental Dataset and then Next.
6. Input the same year to Display that you did in Step 3. Then select Range of Months with a Start Month of June and the End Month of August. Then, in the map under Options, resize the black box to cover the Continental U.S. For the color bar, select the center option that transitions red to violet. Then select Create Map in bottom left.
7. Put the two windows with the different maps side by side to compare. What differences do you notice between the maps?