How Can Paleoclimatology Tell Us About Drought?
Drought is a unique climate event, which often begins with subtle effects but can end up being incredibly costly and devastating. Paleoclimatology, or the study of past climate prior to instrumental records, allows scientists not only to collect evidence of past climate conditions, such as drought, but also provides them with a means to investigate the climate processes underlying these conditions. Climate records from tree rings, lake and dune sediments, historical records, and archaeological remains—also called proxy data—have all provided information about past droughts in the United States. Each record provides a piece of the puzzle, and together, they provide a more complete history than any one proxy would.
Historical records, such as diaries and newspaper accounts, can provide detailed information about droughts for the last 200 or 300 years. Tree ring records can extend back 300 years in most areas, and thousands of years in some regions. In trees that are sensitive to drought conditions, tree rings provide a record of drought for each year of the tree's growth. For records longer than those provided by trees and historical accounts and for regions where they may not find trees or historical accounts, scientists turn to sediments from dunes and lakes.
Lake sediments can provide information about climate variations occurring at frequencies ranging from millennia to less than a decade in length. Lake level fluctuations can be recorded as geologic bathtub rings when beach material sediments are deposited either higher (under wetter conditions) or lower (under drier conditions) within a basin as the water depth changes in response to drought. Droughts can also increase the salinity of lakes, changing the species of small lake organisms that dwell within them.
Additionally, pollen grains get washed or blown into lakes and accumulate in sediments. Different types of pollen in lake sediments reflect the vegetation around the lake and the climate conditions that are favorable for that vegetation. So, a change in the type of pollen found in sediments from, for example, an abundance of grass pollen to an abundance of sage pollen, can indicate a change from wet to dry conditions.
Scientists can find records of more extreme environmental changes by investigating the layers within sand dunes. The sand layers are interspersed among layers of soil material produced under wetter conditions, between the times when the sand dune was active. For a soil layer to develop, the climate needs to be wet for an extended period, so these layers reflect slower and longer-lasting changes.
To reconstruct drought or drought-related variables from environmental recorders, scientists calibrate proxy data with the instrumental record to determine how well the natural record estimates the climate record. Scientists then define the mathematical relationship between the proxy data and the climate record and use that relationship to produce a statistical model. They then use the model to reconstruct drought from the proxy for the length of the proxy record.
Taken together, these different proxies record variations in drought conditions on the order of single seasons to decadal and century-scale changes, providing scientists with the information about both rapid and slow changes as well as short and long periods of drought. These records are needed to put individual droughts in perspective as well as to characterize droughts of the 20th century.
For more information on paleoclimatology and drought, see North American Drought: A Paleo Perspective.