Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
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The Holocene
Climate Change and Variability
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Climate Change and Variability
A series of climatic changes has occurred throughout the Holocene. In fact, scientists have recorded up to 18 climatic cycles during this time. Since the Bronze Age (starting about 3500 BC), it has become increasingly challenging to separate “natural” climatic trends from human-induced effects on the environment. Geologists and climatologists separate the climatic history of the Holocene into three temperate substages that relate mainly to the Northern Hemisphere (North America and Eurasia). These include an early cool stage (about 10,000 BC) a middle “climatic optimum” (also termed the altithermal, 9000-4000 BC), and a late stage cooling that contains the “little ice ages” (starting about AD 1300). A xerothermic or hypsithermal substage also exists between these last two stages.

Three other factors must be added to this: complex solar cycles, Earth's orbital variations (also called Milankovitch cycles, which occur over intervals of tens to hundreds of thousands of years), and different rates of change and climatic conditions depending on location. The result is a complicated picture of the transition from glacial to postglacial conditions. For example, the retreat of the continental glaciers of the Wisconsinan stage began in central North America around 10,000 years ago, but it did not occur in northern Canada and Alaska until nearly 6000 years ago. The retreat has yet to occur in Greenland. Sea level rise from the melting of the glaciers affected coastal areas globally, so much so that in the late 1800s some scientists believed that this sea level rise should be the defining characteristic of the Holocene.

The retreating glaciers in Europe and North America generated a transition in vegetation types that is visible in the pollen record. This provides reliable information for monitoring climate change and identifying climate variability in the recent past. The sediment found in deep-ocean cores also gives valuable information on past climates. For example, the appearance of the foraminiferan Globigerina menardi menardi indicates a major environmental shift toward warmer conditions. Oxygen isotope analyses can be used to measure changes in surface water temperature in the Tropics. Such changes can track shifts in ocean conditions, which can then be linked to changes on land over the last 10,000 years. This type of analysis has shown that the great continental ice retreat and worldwide warming in the late Wisconsinan stage was accompanied by increased precipitation in the tropical regions, as seen in the record of Nile River floods, for example.

In Africa, during the climatic optimum interval approximately 4500 years ago, the Sahara became lush and green, with lakes and a rich vertebrate fauna where now there is only dry sand. Changes in the faunas and floras of North America during the latest Pleistocene and Holocene mainly involved shifts in geographic distributions, which are documented in extensive databases.


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