Rainfall records used to evaluate drought extend back just over 100 years, and are too short to answer these questions. Data from the natural archive are used to answer these questions and to evaluate 20th century North American droughts in the context of hundreds to thousands of years in the field of paleoclimatology.
Paleoclimatology is the study of past climate.
Paleoclimate is climate that existed before humans began collecting instrumental measurements of weather such as temperature from a thermometer, precipitation from a rain gauge, sea level pressure from a barometer, and wind speed and direction from an anemometer. Instead of instrumental measurements of weather and climate, paleoclimatologists use natural environmental records to infer past climate conditions. Paleoclimatology includes the collection of evidence of past climate conditions, and the investigation of the climate processes underlying these conditions.