According to recent scientific articles, the U.S. drought of 2012 was probably not made more likely by climate change. For the details, see my previous post. But what about Missouri? Was 2012 a historic drought?

It is complex to try to estimate drought conditions for a place like Missouri, because different kinds of drought affect our lives differently. Intense, hot droughts during the summer affect crops, even if they are only a month or two long. But they don’t affect river and groundwater levels. Those are affected by droughts that may not be as intense, and which may not be hot, but which last a long time – a year or more.

The the Palmer Drought Index (PDI, also known as the Palmer Drought Severity Index, or PDSI) is a midterm measure of drought that is useful for estimating agricultural impacts. When summarized into an annual number and applied over the entire state, it becomes a longer term measure that is useful for exploring general trends over time. The first chart at right shows the annual PDI for Missouri from 1895-2012. The blue columns show wet years, the green columns show midrange years, the orange columns show moderate drought, and the red columns show severe drought or worse. The black line shows the 20-year moving average.

The chart shows that in Missouri, the drought of 2012 was not a severe or extreme mid- or long-term drought. That is not to say that there were no areas in the state that were severely impacted, or that there were no months of severe short-term drought. But the spring of 2012 was unusually wet, and the chart takes the whole year into consideration.

The chart seems to show that the opening of the 20th Century was wetter than average. There were dry years during the 1930s, the Dust Bowl years, but the drought was not as severe here as it was in some other states. The really dry years in Missouri were during the 1950s. Since 1965, Missouri has been mostly wetter than average.

The National Climate Data Center (NCDC) divides each state into regions. In Missouri, Region 1 is the northwestern part of the state. The second chart on the right shows the Palmer Z-Index for Region 1 for the months of June-August each year. The Palmer Z-Index is a short-term measure, meant to capture drought over as little as a month. This chart confirms that Missouri has recently been wetter than average, but in 2012 the northwestern portion of the state experienced its 3rd worst short-term drought in history. Other regions of the state were dry, but not as dry.

There is nothing in this chart to suggest that Region 1’s summers are trending dryer over time.

What about the nation as a whole – did it experience a record drought in 2012? One way to measure this is to count the percentage of regions in the country that are “very dry.” A region is “very dry” if the precipitation is in the lowest 10% of all periods going back to 1895. The third chart at right shows the percentage of regions in the country that were very dry for every month from 1895-2012. The red line represents the monthly percentage; the black line represents a 24-month moving average.

The record belongs to October, 1952, when the very dry percentage reached 77.51%. The peak in the 24-month moving average occurred a year-and-a-half later, in May, 1954, at 16.05%. The largest dry percentage of 2012 was 33.90%, 28th highest in history, occurring in November. The 24-month moving average increased throughout 2012, peaking at 14.42% in December, a mid-range value. (At any given time, some parts of the country are very dry. The average very dry percentage is 9.94%, the average very wet percentage is 10.22%.)

Thus, while the drought of 2012 was a severe short-term drought in some places, including some in Missouri, it was not a severe long-term drought in Missouri or in the nation as a whole. Those were during the 1950s and the 1930s.

Sources:

Peterson, Thomas, Hoerling, Martin, Stott, Peter, and Herring, Stephanie (eds.). (2013). Explaining Extreme Events of 2012 from a Climate Perspective. Special Supplement to the Bulletin of the American Meteorological Society, 94,(9). http://www.ametsoc.org/2012extremeeventsclimate.pdf.

PDI data for Missouri was created using a data portal at the National Climatic Data Center: NCDC > Climate Monitoring > Climate At A Glance. http://www.ncdc.noaa.gov/cag/time-series. The following selections were made:

Parameter > Palmer Drought Severity Index
Time Scale: Annual
Start Year: 1895
End Year: 2013
State/Region: Missouri
Climate Division: Statewide

Wet vs. Dry percentages for the United States were created using a data portal at the National Climatic Data Center: Home > Climate Monitoring > Temp, Precip, and Drought > U.S. Percentage Areas (Very Warm/Cold, Very Wet/Dry). http://www.ncdc.noaa.gov/temp-and-precip/uspa/index.php?area=wet-dry&month=0.