Historical climatology is the study of historical changes in climate and their effect on civilization from the emergence of homininis to the present day. It is concerned with the reconstruction of weather and climate and their effect on historical societies, including a culturally influenced history of science and perception. This is done in the broader context of environmental history. This differs from paleoclimatology which encompasses climate change over the entire history of Earth. These historical impacts of climate change can improve human life and cause societies to flourish, or can be instrumental in civilization's societal collapse. The study seeks to define periods in human history where temperature or precipitation varied from what is observed in the present day.
The primary sources include written records such as sagas, chronicles, maps and local history literature as well as pictorial representations such as paintings, drawings and even rock art. The archaeological record is equally important in establishing evidence of settlement, water and land usage.
Methods In literate societies, historians may find written evidence of climatic variations over hundreds or thousands of years, such as phenological records of natural processes, for example viticultural records of grape harvest dates. In preliterate or non-literate societies, researchers must rely on other techniques to find evidence of historical climate differences.
Past population levels and habitable ranges of humans or plants and animals may be used to find evidence of past differences in climate for the region. Palynology, the study of pollens, can show not only the range of plants and to reconstruct possible ecology, but to estimate the amount of precipitation in a given time period, based on the abundance of pollen in that layer of sediment or ice. The distribution of diatoms in sediments can also be used to examine changes in salinity and climate over geologic eras.
The Palgrave Handbook of Climate History provides a comprehensive overview of the methods and results of historical climatology in a global context. According to this, information in historical documents should not be taken at face value without verification. The most credible are reports by contemporaries that were recorded shortly after the event and dated documents from institutions. Transcripts of sources are prone to error. Furthermore, attention must be paid to correct dating. Globally, there have been two major systems of calendars: solar calendars based (approximately) on the revolution of the Earth around the sun, and lunar calendars based on the orbit of the moon. The former have historically been used in Europe (and its colonies), India, and Iran, while lunar calendars were historically used in the Islamic world and imperial China. It is also important to distinguish between Julian (“old style”) (Julian Calendar) and Gregorian (“new style”) dates (Gregorian Calendar). When "Saints'" days are used for dating, it must be determined whether the dates refer to the new or to the old calendar. Any corrections must be made accordingly. The presented almanac for the year 1600 displays the two dating styles including the Saints days side by side. The “old” Julian calendar to the right celebrates “Chris" [i.e. Christmas] 10 days later than the 'new” Gregorian calendar to the left, namely on 4 January. If Saints Days are used for dating attention must be paid to the calendar used, as the reproduction of the almanac illustrated shows
Historical climatologists have created an interface between narrative and proxy data in the form of an ordinal index that allows converting disparate documentary evidence into continuous quantitative data that can be presented as time series A differentiated representation of thermal and wetness conditions was developed for Europe in terms of a temperature and precipitation index for nearly every month for which evidence of sufficient data density and quality is available. The so-called Pfister index comprises seven ordinal classes on a scale from -3 to +3. referring to deviations from a reference period in the 20th century that must be declared. Chroniclers included proxy data on plant development and the frequency of snow and ice to compare the meteorological conditions of a given year with those of other years. The interpretation draws on the complementarity between proxy data and the description of the associated weather pattern which needs to be consistent with the proxy.. Proxy information must be calibrated, meaning they must be statistically compared. against instrumental data. Chroniclers also describe effects of weather and climate on the human and the natural environment. This information is case-specific and relates to the historical context. If no relevant proxy data is available, narrative data should therefore be just assigned to index classes 1 ‘warm’ or -1 ‘cold’, regardless of the wording to prevent overestimation
Until the early 2020s, documentary climate data was mainly limited to regional and local scales. In contrast to natural‑science proxy communities—such as the dendrochronologiclal community, which is embedded in globally coordinated research networks—documentary sources had rarely been integrated into large‑scale or global frameworks. Cultural and linguistic barriers long impeded the development of a global perspective. To unlock the potential of quantitative documentary evidence, researchers at the Oeschger Centre for Climate Change Research led by Stefan Brönnimann, compiled a global inventory of climate‑relevant information contained in historical documents. As part of a meteorological Reanalysis, documentary data spanning the period from 1421 to 2008 was evaluated in conjunction with natural proxy records, particularly tree‑ring data A dedicated software enables the generation of flexible spatio-temporal reconstructions, including maps and time series
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 hominin 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.
Ice ages The eruption of the Toba supervolcano, 70,000 to 75,000 years ago reduced the average global temperature by 5 degrees Celsius for several years and may have triggered an ice age. It has been postulated that this created a bottleneck in human evolution. A much smaller but similar effect occurred after the eruption of Krakatoa in 1883, when global temperatures fell for about 5 years in a row.
Before the retreat of glaciers at the start of the Holocene (~9600 BC), ice sheets covered much of the northern latitudes and sea levels were much lower than they are today. The start of our present interglacial period appears to have helped spur the development of human civilization.
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 climatic 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."