Proxy - (n.)

In paleoclimatic reconstruction, a proxy is a measure of climate conditions of the past. A proxy (or proxy variable) yields clues as to temperature, precipitation, productivity, or other environmental conditions. Examples of proxies are: presence and absence of fossils; the abundance ratios of fossils; and the chemical composition of fossils, growth rings of trees and corals. Proxies can be classified according to the type of properties of a sediment they describe (physical, chemical, isotopic, or biological remains) or according to the target which they are supposed to represent. The first approach equates proxies (correctly) with sediment properties (e.g. oxygen isotope ratio in fossil shells). The second approach emphasizes climate-relevant parameters, such as temperature. Thus, in the case where proxies are classified by the targets they are thought to represent, we speak of temperature proxies, sea-level proxies, and rainfall proxies.