The holocenerecent climates years ago to present

The Holocene brings geologic time up to the present day. The major ice sheets of the last ice age reached their maximum extent about 18,000 years ago and then began retreating around 14,000 years ago. However, this time span was not one of a continual upward trend in temperatures. The climate varied from significant cooling to thermal episodes. There were four notable periods of climatic cooling or warming during this period: (1) the Younger Dryas Cooling, (2) the Mid-Holocene Thermal Maximum, (3) the Late Holocene Neoglacial Fluctuations, and (4) the Little Ice Age.

Younger Dryas cooling

One of the more notable periods of cooling is known as the Younger Dryas Cooling. This period occurred 10,000 to 11,000 years ago, which coincides with the geologic boundary of the Pleistocene and Holocene epochs. Glacial ice advanced from the north and moved southward to about 45°N latitude (roughly the same latitude as Minneapolis, Minnesota, and Portland, Oregon). This was a significant advance, because ice levels proceeded within about 10° of where they had reached during the last glacial maximum.

Mid-Holocene Thermal Maximum

The Mid-Holocene Thermal Maximum occurred about 6,000 years ago. Paleoclimatologists have determined that this period was somewhat warmer than it is today. Scientists have uncovered a significant amount of evidence relating to this period and have made great strides in understanding the global issues concerning the cause and extent of temperature change. Changes in the Earth's orbit eventually combined to alter the amount of solar radiation that was reaching various areas of the Earth, according to NOAA. They have concluded that temperatures above summer temperatures today in the Northern Hemisphere did occur. Other evidence that supports the occurrence of this event includes vegetation zones that migrated to different latitudes and changes in precipitation patterns. Experts believe the mid-Holocene warmed the Earth 1.7-3.3°F (1-2°C) warmer than it is today. Additional fieldwork has provided evidence that regions were much wetter during this period than they were during earlier arid conditions.

Late Holocene Neoglacial fluctuations

This period was witness to a number of climate fluctuations spread over 4,500 years. Overall, experts say that temperatures were lower during this time than during the Mid-Holocene Thermal Maximum. From 2,500 to 4,500 years ago, a cooling period called the Iron Age neoglacia-tion occurred. The climate then heated around the time of the birth of the Roman Empire but cooled again during the Dark Ages (476-1000 c.E.). After this period, the climate warmed during what was referred to as the Medieval Optimum, or Medieval Warm Period. At this time, climate was sufficiently favorable to allow large-scale exploration and expansion of civilizations.

The Little Ice Age

The Earth's climate remained relatively warm until approximately 1450 c.E., then it took another turn toward cold. The subsequent cold period lasted from 1450 to 1890 and has become known as the Little Ice Age. The Little Ice Age was a time of renewed glacial advance and affected the North Atlantic, Europe, Asia, and North America. It alternated between colder and less cold periods. The two coldest time segments occurred in the 1600s and 1800s. The 1500s and 1700s were less cold. Because the Little Ice Age occurred in such recent times scientists were able to make scientific observations, take measurements, and keep historical records, enabling this event to be well documented.

According to the Intergovernmental Panel on Climate Change (IPCC), this episode was a modest cooling that mainly affected the Northern Hemisphere. Temperatures cooled about 1.7°F (1°C). The majority of the evidence of the period occurs in the Northern Hemisphere, although there are a few indications in the Southern Hemisphere that it, too, felt some of the effects.

There are several scientific opinions of when it actually began. Some claim it was the mid-1200s because that was when the Atlantic pack ice was first documented as beginning to advance south; others say the early 1300s because that marked the period when warm summer months were no longer dependable in Europe and farmers began to face extreme hardships growing food and supporting the populations that depended on them for their food supply. To support this, the period from 1315-17 is known as the Great Famine.

There were many effects from the Little Ice Age that were recorded and can be found in historical accounts today. Many written records discuss the extremely hard winters, especially those experienced in Europe and North America. As glaciers advanced in Switzerland, Germany, and Austria, farmers lost their farms, and entire villages were buried and destroyed by ice and snow. Holland, known for its internal waterway passages, became frozen over and unnavigable, as did others, including major waterways such as the River Thames in southern England. There are many accounts of people skating on the rivers and holding what they called frost fairs for recreation and entertainment.

The Little Ice Age also had serious ramifications for military, civil, and cultural issues. In 1794-95, the French army, under the leadership of General Jean Pichegru, overran Holland and captured the Dutch fleet with cavalry after their vessels became locked in the ice. Had the river not been frozen, history may have been different.

During the harsh winter of 1780, New York harbor froze over. Historical accounts say that people were able to walk across the ice from Staten Island to Manhattan. Business and commerce was devastated in Iceland during this same period because sea ice closed all the island harbors to ships, and they were not able to enter or leave ports. The hardships were so extreme that many people died. The same fate met the Vikings who had settled colonies in Greenland during the prosperous warm period prior to the Little Ice Age. Conditions became so harsh and cold in Greenland that the Norse could not farm the land, which caused them to die off. Winters in North America were also harsh, and records of numerous struggles were kept by the American Indians and colonists. They documented that winters were extremely cold and wet.

Throughout Europe and North America, traditional farming practices had to be drastically changed to accommodate the short, unpredictable growing seasons that had suddenly become the norm. Some of the extremely cold temperatures recorded on the eastern coast of North America are thought to have been the result of a change in the strength of the North Atlantic thermohaline circulation, a major ocean current responsible for bringing warmth from the equatorial regions to the polar regions.

Effects were also felt in other places worldwide. Ethiopia, for example, had permanent snow on mountain peaks where it no longer exists today; the Niger River in western Africa flooded (it has never flooded since); ocean sediment cores taken from Antarctica reflect relative ice age conditions during this period; it snowed in 1836 in Sydney, Australia, which is the only time since European occupation that has occurred; and tropical Pacific corals show indications of intense El Niño activity in the mid-1600s.

Experts, such as Gavin Schmidt, Drew Shindell, and David Rind of NASA GISS, have proposed a couple of major external forcings as the reasons for the cause of the Little Ice Age. They believe it may have been caused by a decrease in solar activity and an increase in volcanic activity. They have also proposed that a drastic decrease in population in Europe, Asia, and the Middle East because of famine and death coupled with a decrease in agriculture as well as the absorption of CO2 through reforestation may have prolonged the Little Ice Age. Therefore, it probably was not due to one factor but to a combination of several global and regional internal and external factors.

The decrease in solar activity theory is supported by the known Maunder Minimum from 1645-1715. This was a well-documented period of low solar activity and happens to coincide with the coldest phase of the Little Ice Age. In addition, an increase in volcanic activity was documented during the same period, and repeated eruptions of sulfur and ash could have played a significant role in blocking incoming solar radiation. One well-documented example is the eruption of Tambora in Indonesia in 1815. The following year has become known in historical records as the Year Without a Summer, because frost and snow existed throughout July in the United States and northern Europe.

The Little Ice Age ended abruptly around the mid-1800s. Although some skeptics of present-day global warming suggest the Earth is still warming today in response to its recovery from the Little Ice Age, most

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