Understanding climatic fluctuations since the last ice age provides a context for evaluating the extent to which the present global warming trend is an anthropogenic phenomenon. Correlation of archaeological and historical records allows projections of the impact of global warming on human society.

Climatic changes since the end of the last ice age form the backdrop for much of human prehistory and are viewed by some as a driving force in the rise and fall of civilizations. The retreat of continental glaciers began in earnest 13 million years ago, with a gradual warming trend that reached its peak around 6,000 years ago during a period known as the Hypsithermal or Holocene climatic optimum.

Proxy records, supplemented by historical data in more recent times, suggest six periods of abrupt cooling in the Holocene, 9,000-8,000, 6,000-5,000, 4,200-3,800, 3,500-2,500, 1,200-1,000, and 650-150 years before the present. Within warm periods and cold periods, there is considerable fluctuation on scales ranging from decades to centuries. Temperature variations as measured by a variety of proxies are more dramatic near the poles, while variations in rainfall associated with temperature-induced fluctuations in oceanic currents predominate in the tropics. Overall, climatic variability in the Holocene is considerably less than it was in the Pleistocene, and what fluctuations have occurred in the Holocene have decreased over the course of the period.

Climatologists are continually modifying the prevailing picture of climate change in the Holocene as more high-resolution studies become available from areas other than Europe and eastern North America. Climatic shifts typically appear earlier in the Southern Hemisphere than in the Northern Hemisphere. Signs of the 8.2ka event, a period of dramatic European cooling due to disruption of currents in the Atlantic Ocean, are much less evident in western North America and are absent in New Zealand.

Systematic instrumental records of weather in parts of Europe and North America exist for the last 150 years (Alverson, 2003). Agricultural records, historic narratives, and even legendary sources chronicle catastrophic events throughout human history.

Until recently, much of the available information about climate in prehistoric times came from archaeological investigations. The study of artifacts and settlement patterns reveals a great deal about the climate in which ancient people lived. For example, prolonged drought in the American Southwest, corresponding to the culturally benign Medieval Warm Period in Europe, is evident in shifting patterns of cultivation, declining population, skeletal deformities due to malnutrition, compressed tree rings in construction timbers, and eventual abandonment of cliff dwellings.

Vegetation is a good climatic indicator. Leaves, woody material, and particularly pollen occur in abundance in bogs, lake sediments, and areas of human settlement. Pollen analysis is a powerful tool, because pollen is extraordinarily decay-resistant. Many pollen grains can be identified to genus, and relative abundance provides a fairly complete picture of a region's flora. Wind-pollinated plants with narrow ecological niches are particularly useful (Fagan, 2004).

In Europe, the arctic-alpine herb Dryas octopetala indicates arctic-alpine conditions, spruce (Picea) indicates a cold, humid climate, and oak (Quercus) provides evidence of a drier, warm climate. Pollen of Plantago, a weed in grain fields, suggests cultivation. In marine sediments, relative abundance of planktonic types serves as a proxy for water temperature. Various geological formations permit high-resolution analysis of local climate. Moraines and scouring document the advance and retreat of glaciers. Varves, which are layers of sediment in lakebeds, provide a record of stream flow into lakes. When precipitation is high, increased runoff and sediment load create thick varves and rapid deposition of alluvial fans at the mouths of rivers. Terraces along lake and ocean shores document rises and falls in water level. In some areas, the land may also be rising or subsiding relative to sea level.

Ice cores taken from glaciers in Greenland and Antarctica provide evidence of climate over the last 400,000 years, including rates of precipitation, amounts of atmospheric dust, and concentrations of carbon dioxide (CO2) in trapped air (Alverson, 2003). Analysis of the ratios of carbon and oxygen isotopes in carbonates and of oxygen isotopes in ice also provides clues to climate, since both biotic and abiotic processes use isotopes selectively. Isotope ratios can also be used as proxies for sunspot activity, a suspected factor in warming and cooling trends.

References

1. Alverson, Keith D., Raymond S. Bradley, and Thomas Pedersen, eds. Paleoclimate, Global Change, and the Future. Berlin: Springer Verlag, 2003.

2. Fagan, Brian. The Long Summer: How Climate Changed Civilization. New York: Basic Books, 2004.

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