DROUGHT: Understanding Drought from a Paleoclimate Perspective

Image of a scientist extracting a tree-ring core with image of tree rings

Scientists typically harvest full tree-ring slices from only dead trees. For examining live trees, they use a tool called an increment borer to extract a tree-ring core, which is a small pencil shaped piece of wood. After the sample is collected, sap fills in the hole and the tree easily heals itself.

Looking back in time can help scientists better understand how droughts unfold. Unfortunately, only about 150 years of instrument records exist across the U.S. To supplement that instrument record, scientists use paleoclimate data, which allow them to examine droughts that happened centuries ago. Paleoclimate records are developed from natural environmental records, such as the thickness of tree rings or the chemical makeup of sediments at the bottoms of oceans and lakes.

To reconstruct drought or drought-related variables from these environmental data, scientists calibrate their paleoclimate records with the instrumental record to determine how well the natural record estimates the climate record. They then define the mathematical relationship between the paleoclimate data and the climate record and use that information to produce a model. This model allows scientists to reconstruct climate conditions as far back as they have paleoclimate data, which can often be hundreds or thousands of years.

Tree-ring records can extend the record 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. A dry year produces a narrow growth ring, and a wet year produces a wider one. Temperature can also influence ring width, and scientists often examine a calibration interval (when drought records are also available) to determine which trees are uniquely sensitive to temperature.  For records longer than those provided by trees or for areas in which these data are not available, scientists turn to ocean and lake sediments.

Lake sediments can provide climate information in the form of geologic bathtub rings. Beach material sediments are deposited either higher or lower within a basin as the water depth and thus the lake level changes, often in response to drought. Sediments are deposited higher up and further from the center of the lake under wetter conditions, and they are deposited closer to the center of the lake under drier conditions. Droughts can also increase the salinity of lakes, changing the species of small, lake dwelling organisms that occur within them.

Pollen grains can also help scientists learn about past droughts. Pollen grains get washed or blown into lakes and accumulate in the 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.

Records from tree rings, lake sediments, and pollen grains as well as from other sources like historical records, sand dunes, and archaeological remains 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 picture of the historical climate. Taken together, these paleoclimate records can indicate drought conditions and changes over decades and centuries, providing scientists with information about both short and long periods of drought. These records also allow scientists to put individual droughts in perspective as well as to characterize droughts of the 20th century.

This is last in a series of articles about monitoring and assessing drought conditions across the United States. Check out the products used for drought monitoring.