Paleoclimatology and Drought

Paleoclimatology is the study of past climate. The word is derived from the Greek root paleo-, which means ancient, and the term "climate" meaning the weather conditions over an interval of time, usually several decades. Paleoclimate is climate that existed before humans began collecting instrumental measurements of weather (e.g., temperature from a thermometer, precipitation from a rain gauge, sea level pressure from a barometer, wind speed and direction from an anemometer). Instead of instrumental measurements of weather and climate, paleoclimatologists use

For more information regarding Paleoclimatology, please visit NOAA's Paleoclimatology Program's - Education and Outreach pages.

How do we reconstruct drought from paleoclimatic data?
Records of rainfall (or other variables that reflect drought, such as changes in lake salinity, vegetation, or evidence of blowing sand) are preserved in tree-rings, buried in the sediments of sand dunes and lakes, contained within historical documents, and preserved in archaeological remains. These recorders of climate are called proxy climate data - that is they substitute for rain gauges and other instrumental recorders of drought. By analyzing records taken from these proxy sources of paleodrought data, scientists can extend our records of drought far beyond the 100-year record provided by instruments.

To reconstruct drought or drought-related variables from environmental proxy data, the proxy data are calibrated with the instrumental record to determine how well the natural record estimates the climate record. The mathematical relationship between the proxy data and the climate record is defined, then used to produce a model. The model is then used to reconstruct the instrumental record from the proxy record for the length of the proxy.

How does paleoclimatic data help us understand drought?
Proxy records from tree rings, lake and dune sediments, historical records, 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 history than any one proxy would.

Historical records, such as diaries and newspaper accounts, can provide detailed information about droughts for the last two hundred (mid-western and western U.S.) or three hundred (eastern U.S.) 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 we may not find trees and/or historical accounts), scientists turn to sediments from dunes and lakes.

Lake sediments, if the cores of the sediments are sampled at very frequent intervals, can provide information about variations occurring at frequencies less than a decade in length. Lake level fluctuations can be recorded as geologic bath tub rings as beach material sediments are deposited either high (further from the center under wetter conditions) or lower (closer to the center under drier conditions) within a basin as the water depth and thus lake level changes in response to drought. Droughts can increase the salinity of lakes, changing the species of small, lake-dwelling organisms that occur within a lake.

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.

Records of more extreme environmental changes can be found 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 of time, so these layers reflect slower, longer-lasting changes.

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, and 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.

On to... The Data.