Climate Through Human History

The climate of the Holocene

The last glacial maximum occurred 18 Ka (thousand years ago). The most recent glacial retreat is still going on. We call the temporal period of this retreat the Holocene epoch.

Temperatures of the Last 18,000 years

Warming of Earth and glacial retreat began about 14,000 years ago (12,000 BC). The warming was shortly interrupted by a sudden cooling at about 10,000 - 8500 BC known as the Younger-Dryas. 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. 6,000 years ago the Sahara was far more fertile than today and supported large herds of animals, as evidenced by the Tassili N'Ajjer frescoes of Algeria (right).

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.

Hvalsey church, in southwest Greenland, is the best-preserved artifact of Norse Greenlanders, who mysteriously disappeared in the 15th century.

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.

During this period occurred the abandonment of settlements in the Southwest United States, including those in Chaco Canyon and Mesa Verde. Tree ring analysis has identified a period of "no" rain between 1276 and 1299 in these areas.

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 degree Celsius lower than today.

The cold winters of the little Ice Age were recorded in Dutch and Flemish paintings such as Hunters in the Snow by Pieter Bruegel (c. 1525-69)

Extreme weather during this period might have played an important role in the genesis of the Black Death (bubonic plague):

• Horrendous floods devastated China in 1332 (reported to have killed several million people)
• movement of wildlife including rats (disease is spread by the animal feces)
• mixing of different populations of rats is a triggering factor in an disease outbreak
• once new virulent strain emerged spread was controlled by events largely unrelated to weather and climate.

The Little Ice Age was not continuously cold: 13th-14th centuries cold; then interval of more favorable conditions; return of more severe weather mid 16th-mid 19th centuries.

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.

Suggestion of the initiation of post-Holocene cooling towards the next glaciation interrupted by human intervention or natural variability.

Summary of distinct climatic periods during the Holocene epoch

Period	Name	Climate conditions
14,000 years ago	Holocene warming	Slow warming from the last ice age; large ice melt
10,000 - 8500 BC	Younger-Dryas	Rapid cooling period
5000 - 3000 BC	Climatic optimum	Warm conditions; temperatures were perhaps 1 to 2 degrees Celsius warmer than they are today. Great ancient civilizations began and flourished.
3000 - 2000 BC		Cooling trend; drops in sea level and the emergence of many islands.
2000 - 1500 BC		Short warming trend
1500 - 750 BC		Colder temperatures and renewed ice growth, sea level drop of between 2 to 3 meters below present day levels.
750 BC - 150 BC		Slight warming not as warm as the Climatic Optimum.
150 BC - 900 AD		Cooling trend; Nile River (829 AD) and Black Sea (800-801 AD) froze
900 - 1200 AD	Little Climatic Optimum or Medieval Optimum	Warm; warmest climate since the Climatic Optimum, Vikings established settlements on Greenland and Iceland.
1200 - 1550 AD		Cool and more extreme weather; abandonment of settlements in the Southwest United States,
1550 to 1850 AD	Little Ice Age	Coldest temperatures since the beginning of the Holocene. Populations die from crop failure and famine in Europe.
1850 AD - present	Contemporary climate	Steady warming trend