Paleoclimate studies analyze the variation of the climate in past geologic times, prior to instrumental measurements. Paleoclimate is expressed by its parameters—paleotemperature, precipitation in the past, circulation, sea surface temperature (SST) and sea level.
The general state of Earth's climate is dependent upon the amount of energy the Earth receives from the solar radiation, and the amount of energy the Earth releases back to space in the form of infrared heat energy. Causes of climate change involve any process that can alter the global energy balance (climate forcing). Climate forcing processes can be divided into internal and external types. External processes include variations in Earth's orbit around the Sun. These variations change the amount of energy received from the Sun, and also cause variations of the distribution of sunlight reaching Earth's surface. Long-periods of solar luminosity variations cause variations of the global climate, although lower intensity variations of luminosity may not produce detectable changes in local climate if circulation patterns modulate them. Internal processes operate within Earth's climate system, and include changes in ocean circulation and changes in the composition of the atmosphere. Other climate forcing processes include the impacts of large volcanic eruptions, and collisions with comets or meteorites.
Over much of Earth's geologic history, the global climate has been warmer and wetter than at present. Global temperatures early in Earth's history were 8–15°C (46–50°F) warmer than today. Polar regions were free of ice until periods of glaciations occurred 2,300 million years ago. For about the past last one billion years, Earth's dominant climate pattern has been one of tropical regions, cool poles, and periodic ice ages. Most recent glaciers reached their maximum thickness and extent about 18,000 years ago, and then glaciation ended abruptly about 10,000 years ago. Since the last glacial period, sea levels changes from -120 m (-132.2 yd) during glaciation to +10 m (+10.9 yd) during interglacial maximum, due to ice sheet melting.
The Medieval Climatic Optimum occurred around 1000 to 1250 A.D.. The Northern Hemisphere experienced a warm and dry climate. Most of Greenland was ice free, therefore, was named Greenland. The Little Ice Age was a period of rapid cooling, which began after the end of the Medieval Warm period and lasted nearly until the end of the eighteenth century, reaching its peak from 1460 to 1705. During this period, the average global temperature dropped between 1 and 2°C (33.8–35.6°F). Solar activity may have lead to climatic changes like the Little Ice Age and Medieval Climatic Optimum. The Little Ice Age coincides with a period of absence of aurora form 1460–1550 called the Spoerer Solar Minimum, and an absence of sunspots from 1645–1715 called the Maunder Minimum. The number of sunspots has been related to solar output and the emission of the radiant heat from the sun.
Reconstructions of Paleoclimate are made by use of records of different proxies (models) of different climatic parameters. These models can be divided to quantitative, qualitative, and indirect from the point of view of the precision of the reconstruction of past climates they provide. Quantitative models are able to reconstruct the exact values of the temperature, annual precipitation or sea level, and to estimate its error. Qualitative models are able to reconstruct only their principal variations expressed through the variation of the model. Indirect models do not express variations of the climate, but of something dependent on climate through a complicated mechanism such as distribution of a certain plant type.
Speleothems (stalagmites, stalactites, and flow-stones) are producing a tremendous range of reconstructions of different types of paleoclimatic parameters, including many quantitative records. Calcite speleothems display luminescence, which is produced by calcium salts of humic and fulvic acids derived from soils above the cave. The luminescence of speleothems depends exponentially on the solar insolation (if soil surface is heated directly by the Sun) or on the air temperature (if the cave is covered by forest or bush). Therefore, luminescence records represent solar insolation or temperature in the past. Luminescence of many speleothems is exhibited by annual bands much like tree rings. Distance between them is a quantitative proxy of annual precipitation in the past.
Changes in the thickness of the tree rings records temperature changes if derived from temperature-sensitive tree-rings, or records precipitation if derived from precipitation-sensitive tree rings. These records are modulated to some degree by the other climatic parameters.
The stable isotope records of past glaciations are preserved in glacier ice and in sea cores. These records are primarily a measure of changing volume of glacier ice. The ratio of stable isotopes in water is temperature dependent, and is altered whenever water undergoes a phase change. Now, the volume of land ice is relatively small, but during glacial periods, much isotopically light water was removed from oceans and stored in glaciers on land. This caused slight enrichment of seawater, while glacier ice had lower values of the ratio. Sea cores do not allow for a better resolution than 1000 years, and cannot be dated precisely. Corals and speleothems often allow measurements with minor time increments. Plants and animals adapt to the climatic changes, so may be used as indirect paleoclimatic indicators. Fossil evidence provides a good record of the advancing and retreating of ice sheets, while various pollen types indicate advances and retreats of northern forests.
See also Atmosphere observation; Geochemical analysis; Geochemistry; Geographic and magnetic poles; Geologic map; Geologic time; Geology; Geometry; Geomicrobiology; Meteorology; Paleobotany; Paleoecology; Paleomagnetism; Precession of the equinoxes; Seasons.
Noller, J.S., Janet M. Sowers, and William R. Lettis. Quaternary Geochronology: Methods and Applications. Vol. 4, AGU Reference Shelf Series, 2000.
Totman Parrish, Judith T. Interpreting Pre-Quaternary Climate from the Geologic Record. New York: Columbia University Press, 1998.
Turcotte, Donald L., and Gerald Schubert. Geodynamics. 2nd ed. Cambridge, UK: Cambridge University Press, 2001.
Alverson K., F. Oldfield, and R. Bradley, eds. "Past Global Changes and Their Significance to the Future." Quaternary Science Reviews 19, no. 1–5 (2000): 1–479.