Early 21st century U.S. drought ranks as one of the most severe in 120 years
At the beginning of the 21st century, several regional droughts were plaguing parts of the contiguous United States. By 2012, these droughts had combined into a national-scale event the likes of which hadn’t been seen in decades. With two-thirds of the Lower 48 states in drought by the end of September, many began to speculate that it was as severe as the episodes of the 1930s and 1950s. But how did this early 21st-century drought really compare to those two previous droughts? NCEI scientist and drought expert, Richard Heim, set out to answer that very question.
In his paper, A Comparison of the Early Twenty-First Century Drought in the United States to the 1930s and 1950s Drought Episodes published in the Bulletin of the American Meteorological Society, Heim found that the 1998–2014 drought shared several characteristics with the droughts of the 1930s and 1950s. Using the Palmer Drought Index, Standardized Precipitation Index, and U.S. Climate Extremes Index, Heim was able to provide a historical perspective on the early 21st-century drought’s intensity, extent, and duration.
What defines drought?
The United States is divided into 344 “climate divisions” or locations. Using the Palmer Drought Index, a location is in drought when the Index goes negative, that is, demand for water exceeds the supply. A “stretch” of drought is the length of time the Index is in the negative for that location. The stretch of drought at that location ends when the Index goes to zero or “wet.”