Which methods allow the reconstruction of past climates ?

General principle

The climate of an area is defined on the basis of mean values (usually computed over 30 years) of different parameters, especially temperatures and precipitation.

Nowadays these parameters are measured by standardized methods in many points on the earth. But these measurements began only recently, and there is no archive of direct measurements older than 150 years. Indirect methods are thus necessary; they are based on climate indicators called proxies.

Proxies are physical, chemical or biological variables related to the variations in temperature, humidity, atmospheric composition, or other climate elements.

First, the relationship between a climate indicator and measurements of climatic parameters must be determined. Once established, this relationship may be applied to indicators recorded in old sequences, which allows the reconstruction of palaeoclimate parameters. All these methods involve some uncertainty.

Some methods :

Climate indicator = proxy	applied to	relationship with climate	goes back	resolution
tree growth rings	fossil or living woods	relationship between ring width and local conditions of temperature and moisture	11000 years	1 year
fossil assemblages (plants, animals) in sediments	pollens and spores, diatoms	relationship vegetation - climate	hundreds of thousands years	± 50 years
	foraminifers,...	relationship with ocean temperature	hundreds of thousands years	variable
isotopic composition (ex : 16O/18O ratio)	marine shells, diatoms	relationship with ocean temperature	hundreds of million years	variable
	ice	relationship with global temperature	hundreds of million years	1 year or > 1 year
concentration in greenhouse trace-gases (CO2 et CH4 ) in trapped air bubbles	ice	relationship with global temperature	hundreds of thousands years	1 year or > 1 year
corals	coral skeleton	relationship with sea level	tens of thousands years	variable
geomorphology	traces of glacial erosion	relationship with advance/retreat of glaciers and ice sheets	a few billion years	variable