Clues to past climate can be contained in many indicators that clima-tologists refer to as climate proxies. (Proxy means "substitute.") The term is used because climatologists cannot obtain direct temperature or other climatic data from proxies but can only infer past climatic conditions based on information obtained from the proxies themselves. As an illustration, it is possible to infer the climate of an area just by looking at a photograph. If the photograph, such as that shown on the facing page, is of a beach dotted with palm trees and tropical flowers along an ocean front with blue skies, it can be inferred that the location is a tropical setting somewhere near the equator, because we know those conditions exist today. If another photo portrays a snow-swept plain, snow-capped mountains in the distance, and a team of huskies being driven across the plain by a fur-clad musher, it can be inferred that the location shown is a high-latitude, cold climate.
There are three types of climate proxies commonly used: (1) geo-morphic landform proxies, (2) geological-geochemical proxies, and (3) biotic proxies. Geomorphic landform proxies involve identifying features on the Earth's surface indicative of specific climatic events, such as glacial landforms, landforms caused by desert environments, land-forms caused by running water, and those that result from wind. Geo-logical-geochemical proxies involve movements of the Earth's materials through the climate system either as physical particles (such as sediments) or dissolved chemicals. Examples include sediments deposited by water, ice, and wind in soil profiles; rocks; varves; glacial ice; loess deposits; paleolimnology; and cave formations. Chemical weathering processes involve dissolution (the dissolving of carbonate and evaporite rocks) and hydrolysis (the addition of water to rocks during weathering). Biotic proxies include evidence obtained from the fossil data of plants and animals, including pollen, plankton, cones, seeds, leaves; corals; insects; paleofire; and tree rings. These proxies will be described in greater detail in the following three chapters.
Another type of proxy data is historical data—a written record. These documents can contain a wealth of information about climates of the past. Over time, people have kept records in all kinds of settings:
farm records, ship logs, explorer's accounts, travelers' diaries, newspaper stories, civic records, and personal accounts. Many sources of verified historical data have augmented the instrumental records of the past 150 years, further extending direct knowledge of the climate backward in time.
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