Earth's Climatic History
"Climatologists have used various techniques and evidence to reconstruct a history of the Earth's past climate. From this data, they have found that during most of the Earth's history global temperatures were probably 8 to 15 degrees Celsius warmer than today. In the last billion years of climatic history, warmer conditions were broken by glacial periods starting at 925, 800, 680, 450, 330, and 2 million years before present.
The period from 2,000,000 - 14,000 B.P. (before present) is known as the Pleistocene or Ice Age. During this period, large glacial ice sheets covered much of North America, Europe, and Asia for extended periods of time. The extent of the glacier ice during the Pleistocene was not static. The Pleistocene had periods when the glacier retreated (interglacial) because of warmer temperatures and advanced because of colder temperatures (glacial). During the coldest periods of the Ice Age, average global temperatures were probably 4 - 5 degrees Celsius colder than they are today.
The most recent glacial retreat is still going on. We call the temporal period of this retreat the Holocene epoch. This warming of the Earth and subsequent glacial retreat began about 14,000 years ago (12,000 BC). The warming was shortly interrupted by a sudden cooling, known as the Younger-Dryas, at about 10,000 - 8500 BC. Scientists speculate that this cooling may have been caused by the release of fresh water trapped behind ice on North America into the North Atlantic Ocean. The release altered vertical currents in the ocean which exchange heat energy with the atmosphere. The warming resumed by 8500 BC. By 5000 to 3000 BC average global temperatures reached their maximum level during the Holocene and were 1 to 2 degrees Celsius warmer than they are today. Climatologists call this period the Climatic Optimum. During the Climatic Optimum, many of the Earth's great ancient civilizations began and flourished. In Africa, the Nile River had three times its present volume, indicating a much larger tropical region.
From 3000 to 2000 BC a cooling trend occurred. This cooling caused large drops in sea level and the emergence of many islands (Bahamas) and coastal areas that are still above sea level today. A short warming trend took place from 2000 to 1500 BC, followed once again by colder conditions. Colder temperatures from 1500 - 750 BC caused renewed ice growth in continental glaciers and alpine glaciers, and a sea level drop of between 2 to 3 meters below present day levels.
The period from 750 BC - 800 AD saw warming up to 150 BC. Temperatures, however, did not get as warm as the Climatic Optimum. During the time of Roman Empire (150 BC - 300 AD) a cooling began that lasted until about 900 AD. At its height, the cooling caused the Nile River (829 AD) and the Black Sea (800-801 AD) to freeze.
The period 900 - 1200 AD has been called the Little Climatic Optimum. It represents the warmest climate since the Climatic Optimum. During this period, the Vikings established settlements on Greenland and Iceland. The snow line in the Rocky Mountains was about 370 meters above current levels. A period of cool and more extreme weather followed the Little Climatic Optimum. A great drought in the American southwest occurred between 1276 and 1299. There are records of floods, great droughts and extreme seasonal climate fluctuations up to the 1400s.
From 1550 to 1850 AD global temperatures were at their coldest since the beginning of the Holocene. Scientists call this period the Little Ice Age. During the Little Ice Age, the average annual temperature of the Northern Hemisphere was about 1.0 degree Celsius lower than today. During the period 1580 to 1600, the western United States experienced one of its longest and most severe droughts in the last 500 years. Cold weather in Iceland from 1753 and 1759 caused 25% of the population to die from crop failure and famine. Newspapers in New England were calling 1816 the year without a summer.
"The period 1850 to present is one of general warming. Figure 7x-1 describes the global temperature trends from 1880 to 2006. This graph shows the yearly temperature anomalies that have occurred from an average global temperature calculated for the period 1951-1980. The graph indicates that the anomolies for the first 60 years of the record were consistently negative. However, beginning in 1935 positive anomolies became more common, and from 1980 to 2006 most of the anomolies were between 0.20 to 0.63 degrees Celsius higher than the normal period (1951-1980) average."
In the 1930s and 1950s, the central United States experience two periods of extreme drought. In the seventeen year period from 1990 to 2006, ten of the warmest years in the last 100 years and possibly since the Little Climatic Optimum have occurred. Proxy and instrumental data indicate that 2005 was the warmest year globally in 1200 years of Earth history. Many scientists believe the warmer temperatures of the 20th and 21st centuries are being caused by the human enhancement of the Earth's greenhouse effect. "
PhysicalGeography.net
