Historical impacts of climate change
Climate has affected human life and civilization from the emergence of hominins to the present day. These historical impacts of climate change can improve human life and cause societies to flourish, or can be instrumental in civilization's societal collapse.
Role in human evolution
Changes in East African climate have been associated with the evolution of hominini. Researchers have proposed that the regional environment transitioned from humid jungle to more arid grasslands due to tectonic uplift and changes in broader patterns of ocean and atmospheric circulation. This environmental change is believed to have forced hominins to evolve for life in a savannah-type environment. Some data suggest that this environmental change caused the development of modern homimin features; however there exist other data that show that morphological changes in the earliest hominins occurred while the region was still forested. Rapid tectonic uplift likely occurred in the early Pleistocene, changing the local elevation and broadly reorganizing the regional patterns of atmospheric circulation. This can be correlated with the rapid hominin evolution of the Quaternary period. Changes in climate at 2.8, 1.7, and 1.0 million years ago correlate well with observed transitions between recognized hominin species. It is difficult to differentiate correlation from causality in these paleopanthropological and paleoclimatological reconstructions, so these results must be interpreted with caution and related to the appropriate time-scales and uncertainties.
Role in human migration and agriculture
Climate change has been linked to human migration from as early as the end of the Pleistocene to the early twenty-first century. The effect of climate on available resources and living conditions such as food, water, and temperature drove the movement of populations and determined the ability for groups to begin a system of agriculture or continue a foraging lifestyle.
Groups such as the inhabitants of northern Peru and central Chile, the Saqqaq in Greenland, nomadic Eurasian tribes in Historical China, and the Natufian culture in the Levant all display migration reactions due to climactic change.
Further descriptions of specific cases
In northern Peru and central Chile climate change is cited as the driving force in a series of migration patterns from about 15,000 B.C. to approximately 4,500 B.C. Between 11,800 B.C. and 10,500 B.C. evidence suggests seasonal migration from high to low elevation by the natives while conditions permitted a humid environment to persist in both areas. Around 9,000 B.C. the lakes that periodically served as a home to the natives dried up and were abandoned until 4,500 B.C. This period of abandonment is a blank segment of the archeological record known in Spanish as the silencio arqueológico. During this break, there exists no evidence of activity by the natives in the lakes area. The correlation between climate and migratory patterns leads historians to believe the Central Chilean natives favored humid, low-elevation areas especially during periods of increased aridity.
The different inhabitants of Greenland, specifically in the west, migrated primarily in response to temperature change. The Saqqaq people arrived in Greenland around 4,500 B.P. and experienced moderate temperature variation for the first 1,100 years of occupation; near 3,400 B.P. a cooling period began that pushed the Saqqaq toward the west. A similar temperature fluctuation occurred around 2,800 B.P. that led to the abandonment of the inhabited Saqqaq region; this temperature shift was a decrease in temperature of about 4 °C over 200 years. Following the Saqqaq dominance, other groups such as the Dorset people inhabited west Greenland; the Dorset were sea-ice hunters that had tools adapted to the colder environment. The Dorset appeared to leave the region around 2,200 B.P. without clear connection to the changing environment. Following the Dorset occupation, the Norse began to appear around 1,100 B.P. in west Greenland during a significant warming period. However, a sharp decrease in temperature beginning in 850 B.P. of about 4 °C in 80 years is thought to contribute to the demise of initial Norse occupation in western Greenland.
In Historical China over the past 2,000 years, migration patterns have centered around precipitation change and temperature fluctuation. Pastoralists moved in order to feed the livestock that they cared for and to forage for themselves in more plentiful areas. During dry periods or cooling periods the nomadic lifestyle became more prevalent because pastoralists were seeking more fertile ground. The precipitation was a more defining factor than temperature in terms of its effects on migration. The trend of the migrating Chinese showed that the northern pastoralists were more affected by the fluctuation in precipitation than the southern nomads. In a majority of cases, pastoralists migrated further southward during changes in precipitation. These movements were not classified by one large event or a specific era of movement; rather, the relationship between climate and nomadic migration is relevant from "a long term perspective and on a large spatial scale."
The Natufian population in the Levant was subject to two major climactic changes that influenced the development and separation of their culture. As a consequence of increased temperature, the expansion of the Mediterranean woodlands occurred approximately 13,000 years ago; with that expansion came a shift to sedentary foraging adopted by the surrounding population. Thus, a migration toward the higher-elevation woodlands took place and remained constant for nearly 2,000 years. This era ended when the climate became more arid and the Mediterranean forest shrank 11,000 years ago. Upon this change, some of the Natufian populations nearest sustainable land transitioned into an agricultural way of life; sustainable land was primarily near water sources. Those groups that did not reside near a stable resource returned to the nomadic foraging that was prevalent prior to sedentary life.
Historical and prehistoric societies
The rise and fall of societies have often been linked to environmental factors.
Societal growth and urbanization
Approximately one millennium after the 7 ka slowing of sea-level rise, many coastal urban centers rose to prominence around the world. It has been hypothesized that this is correlated with the development of stable coastal environments and ecosystems and an increase in marine productivity (also related to an increase in temperatures), which would provide a food source for hierarchical urban societies. Cultures on Great Britain progressed differently than cultures on the European mainland due to flooding of the Dover-Calais land bridge around 428,000 B.C.E.
Climate change has been associated with the historical collapse of civilizations, cities and dynasties. Notable examples of this include the Anasazi, Classic Maya, the Harappa, the Hittites, and Ancient Egypt. Other, smaller communities such as the Viking settlement of Greenland have also suffered collapse with climate change being a suggested contributory factor.
There are two proposed methods of Classic Maya collapse: environmental and non-environmental. The environmental approach uses paleoclimatic evidence to show that movements in the intertropical convergence zone likely caused severe, extended droughts during a few time periods at the end of the archaeological record for the classic Maya. The non-environmental approach suggests that the collapse could be due to increasing class tensions associated with the building of monumental architecture and the corresponding decline of agriculture, increased disease, and increased internal warfare.
The Harappa and Indus civilizations were affected by drought 4,500–3,500 years ago. A decline in rainfall in the Middle East and Northern India 3,800–2,500 is likely to have affected the Hittites and Ancient Egypt.
Notable periods of climate change in recorded history include the Medieval warm period and the little ice age. In the case of the Norse, the Medieval warm period was associated with the Norse age of exploration and Arctic colonization, and the later colder periods led to the decline of those colonies.
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