Drought is a very familiar foe in parts of the US, significant regions of Africa, and much of Australia. Drought is often called a creeping disaster as it decimates a region slowly. We will talk a lot more about drought in Modules 9 and 10. Here we briefly consider the recent record of drought. As we saw previously with research on drought and the Mayans, oxygen isotopes of stalactites can be interpreted in terms of precipitation. Another indicator of drought is the width of tree rings. One of the most comprehensive tree-ring data sets is shown in a compilation of precipitation in New Mexico from 137 BC to 1992 (The New York Times, The Longest Measure of Drought: 21 Centuries of Rainfall in New Mexico). The data set shows that with the exception of a wet phase in the last decade, much of the 20th century was dry in New Mexico, and, in general, the data suggest that western North America has become drier over the last 1000 years.
As it turns out, there is solid evidence that the severity of droughts has increased over the long term in many parts of the world. And, as it turns out, climate models predict that drought will become a part of everyday life in many regions in the coming century. In the US, these areas include large parts of the west and south central (Kansas, Oklahoma, Texas). So even though the amount of moisture in the atmosphere is increasing, its distribution is very uneven.
So how is the severity of drought quantified? The most widely used method to measure drought today and in the past is the Palmer Drought Severity Index (PDSI). This index is an accounting of the balance of supply of moisture via precipitation and demand for moisture via potential evapotranspiration (PE). The PE is the amount of water that could be evaporated given an unlimited supply of water. The National Oceanic and Atmospheric Association (NOAA) publishes a map of the PDSI over the US every month (see figure below).
The PDSI has increased (become drier) for the driest parts of the globe since 1950. For most of the US, the PDSI has decreased for this time period. However, the last century has seen severe droughts in certain regions. For example, the major drought of the 1930s Dust Bowl in the Central US caused severe hardship for farmers and others in this region. The 1950s and 1980s also saw severe droughts in the Great Plains of the US. We will find out in Modules 3 and 4 how models project that the severity of droughts will increase in many parts of the globe with future climate change. Moreover, we will learn more about the impact of drought on water supply in Module 8 and on food in Module 9.