Studying climates from the past is an important part of climate science because it gives researchers information about how climate conditions in different parts of the world have changed over time. Such studies have shown that past climates can change abruptly over short periods of time—crucial information for human societies.

The study of climate conditions before the availability of instrumental records of temperature and precipitation is called paleoclimatology. Because instrumental records only go back a few hundred years, researchers must make hypotheses about past climates from incomplete evidence. They base their hypotheses on a kind of data known as indirect or proxy. These data are defined as data that cannot be obtained by direct measurement but which can be constructed, or inferred, from other data. Inference is a kind of scientific reasoning that uses a true statement or judgment to evaluate the truth of a statement that follows from it.

Indirect or proxy data are clues to past climates that are hidden in the natural environment—in sediments on ocean floors, inside coral reefs, and locked in patterns within glacial ice or tree rings. These data can be called natural archives because they are data from the past preserved in the natural world. Other types of proxy climate data are from documentary evidence created by humans. This includes written data such as historical records, newspaper articles, books, and even the illustrations found in books.

Scientists use proxy data from natural archives in very flexible and imaginative ways. For example, they gather tree ring data not only from living trees, but also from dead trees, lumber from old buildings, tree stumps, and even tree trunks preserved in bogs, lake sediments, or river bottoms.

Coral records, like tree rings, often show annual growth bands reaching back many thousands of years. Pollen data can come from many sources, including lake sediments, ice cores, peat bogs, and land surfaces.

Other kinds of proxy data include the microscopic algae that live in lakes, small animals from lakes such as insects, and preserved samples of moss. Most of these data can be analyzed by comparing the same stable chemical markers or isotopes, often of oxygen, present in samples in order to infer conditions about atmospheric moisture and air temperature.

Print Background Essay