The reconstructed depth of the Little Ice Age varies between different studies (anomalies shown are from the 1950–80 reference period).

The Little Ice Age (LIA) was a period of cooling that occurred after the Medieval Warm Period (Medieval Climate Optimum).^[1] While not a true ice age, the term was introduced into the scientific literature by François E. Matthes in 1939.^[2] It may be conventionally defined as a period extending from the 16th to the 19th centuries,^[3][4][5] or about 1350 to about 1850 ^[6] though climatologists and historians working with local records no longer expect to agree on either the start or end dates of this period, which varied according to local conditions. NASA defines the term as a cold period between 1550 AD and 1850 AD and notes three particularly cold intervals: one beginning about 1650, another about 1770, and the last in 1850, each separated by intervals of slight warming.^[7] The Intergovernmental Panel on Climate Change (IPCC) describes areas affected by the LIA:

Evidence from mountain glaciers does suggest increased glaciation in a number of widely spread regions outside Europe prior to the 20th century, including Alaska, New Zealand and Patagonia. However, the timing of maximum glacial advances in these regions differs considerably, suggesting that they may represent largely independent regional climate changes, not a globally-synchronous increased glaciation. Thus current evidence does not support globally synchronous periods of anomalous cold or warmth over this time frame, and the conventional terms of "Little Ice Age" and "Medieval Warm Period" appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries... [Viewed] hemispherically, the "Little Ice Age" can only be considered as a modest cooling of the Northern Hemisphere during this period of less than 1°C relative to late 20th century levels.^[8]

Several causes have been proposed: cyclical lows in solar radiation, heightened volcanic activity, changes in the ocean circulation, an inherent variability in global climate, or decreases in the human population.

Dating[link]

There is no agreed beginning year of the Little Ice Age^[9][10], although there is a frequently referenced series of events preceding the known climatic minima. Starting in the 13th century, pack ice began advancing southwards in the North Atlantic, as did glaciers in Greenland. There is anecdotal evidence of expanding glaciers almost worldwide. Based on radiocarbon-dating of roughly 150 samples of dead plant material with roots intact collected from beneath ice caps on Baffin Island and Iceland, Miller et al. (2012)^[11] state that summer cold and ice growth began abruptly between 1275 and 1300 AD, followed by "a substantial intensification" from 1430 to 1455 AD.^[12] The three years of torrential rains beginning in 1315 ushered in an era of unpredictable weather in Northern Europe, which did not lift until the 19th century^{[citation needed]}.

In contrast, a climate reconstruction based on glacial length^[13][14] shows no great variation from 1600 to 1850, though it shows strong retreat thereafter.

For this reason, any of several dates ranging over 400 years may indicate the beginning of the Little Ice Age:

• 1250 for when Atlantic pack ice began to grow
• 1275 to 1300 based on radiocarbon dating of plants killed by glaciation
• 1300 for when warm summers stopped being dependable in Northern Europe
• 1315 for the rains and Great Famine of 1315–1317
• 1550 for theorized beginning of worldwide glacial expansion
• 1650 for the first climatic minimum.

The Little Ice Age ended in the latter half of the 19th century or early in the 20th century.^[15][16][17]

Northern Hemisphere[link]

The Frozen Thames, 1677

Winter Landscape with a Bird Trap, 1565, Pieter Bruegel the Elder

The Hunters in the Snow by Pieter Brueghel the Elder, 1565

Europe and North America[link]

The Little Ice Age brought colder winters to parts of Europe and North America. Farms and villages in the Swiss Alps were destroyed by encroaching glaciers during the mid-17th century.^[18] Canals and rivers in Great Britain and the Netherlands were frequently frozen deeply enough to support ice skating and winter festivals.^[18] The first River Thames frost fair was in 1607; the last in 1814, although changes to the bridges and the addition of an embankment affected the river flow and depth, hence diminishing the possibility of freezes. Freezing of the Golden Horn and the southern section of the Bosphorus took place in 1622. In 1658, a Swedish army marched across the Great Belt to Denmark to invade Copenhagen. The winter of 1794-1795 was particularly harsh, when the French invasion army under Pichegru could march on the frozen rivers of the Netherlands, while the Dutch fleet was fixed in the ice in Den Helder harbour. In the winter of 1780, New York Harbor froze, allowing people to walk from Manhattan to Staten Island. Sea ice surrounding Iceland extended for miles in every direction, closing harbors to shipping.

The last written records of the Norse Greenlanders are from a 1408 marriage in the church of Hvalsey — today the best-preserved of the Norse ruins.

The population of Iceland fell by half, but this was perhaps caused by fluorosis after the eruption of the volcano Laki in 1783.^[19] Iceland also suffered failures of cereal crops, and people moved away from a grain-based diet.^[20] The Norse colonies in Greenland starved and vanished (by the 15th century), as crops failed and livestock could not be maintained through increasingly harsh winters, though Jared Diamond noted that they had exceeded the agricultural carrying capacity before then. In North America, American Indians formed leagues in response to food shortages.^[21] In Portugal, snowstorms were much more frequent than today. There are reports of heavy snowfalls in the winters of 1665, 1744 and 1886.^[22]

Winter skating on the main canal of Pompenburg, Rotterdam in 1825, shortly before the minimum, by Bartholomeus Johannes van Hove

In 1995, Hubert Lamb said that in many years, "snowfall was much heavier than recorded before or since, and the snow lay on the ground for many months longer than it does today."^[23] Many springs and summers were cold and wet, although there was great variability between years and groups of years. Crop practices throughout Europe had to be altered to adapt to the shortened, less reliable growing season, and there were many years of dearth and famine (such as the Great Famine of 1315–1317, although this may have been before the LIA proper).^[24] According to Elizabeth Ewan and Janay Nugent, "Famines in France 1693–94, Norway 1695–96 and Sweden 1696–97 claimed roughly 10% of the population of each country. In Estonia and Finland in 1696–97, losses have been estimated at a fifth and a third of the national populations, respectively."^[25] Viticulture disappeared from some northern regions. Violent storms caused massive flooding and loss of life. Some of these resulted in permanent loss of large areas of land from the Danish, German and Dutch coasts.^[23]

The extent of mountain glaciers had been mapped by the late 19th century. In both the north and the south temperate zones, snowlines (the boundaries separating zones of net accumulation from those of net ablation) were about 100 m lower than they were in 1975.^[26] In Glacier National Park, the last episode of glacier advance came in the late 18th and early 19th centuries.^[27] In Chesapeake Bay, Maryland, large temperature excursions were possibly related to changes in the strength of North Atlantic thermohaline circulation.^[28]

In Ethiopia and Mauritania, permanent snow was reported on mountain peaks at levels where it does not occur today.^[29] Timbuktu, an important city on the trans-Saharan caravan route, was flooded at least 13 times by the Niger River; there are no records of similar flooding before or since.^[29] In China, warm weather crops such as oranges were abandoned in Jiangxi Province, where they had been grown for centuries.^[29] Also, two periods of most frequent typhoon strikes in Guangdong coincide with two of the coldest and driest periods in northern and central China (AD 1660-1680, 1850–1880).^[30] In North America, the early European settlers reported exceptionally severe winters. For example, in 1607-1608, ice persisted on Lake Superior until June.^[23] The journal of Pierre de Troyes, Chevalier de Troyes, who led an expedition to James Bay in 1686, recorded that James Bay was still littered with so much floating ice that he could hide behind it in his canoe on July 1.^[31]

Antonio Stradivari, the famous violin maker, produced his instruments during the Little Ice Age. It has been proposed that the colder climate caused the wood used in his violins to be denser than in warmer periods, contributing to the tone of Stradivari's instruments.^[32]

The Little Ice Age by anthropology professor Brian Fagan of the University of California at Santa Barbara tells of the plight of European peasants during the 1300 to 1850 chill: famines, hypothermia, bread riots, and the rise of despotic leaders brutalizing an increasingly dispirited peasantry. In the late 17th century, writes Fagan, agriculture had dropped off so dramatically that "Alpine villagers lived on bread made from ground nutshells mixed with barley and oat flour." ^[33] Historian Wolfgang Behringer has linked intensive witch-hunting episodes in Europe to agricultural failures during the Little Ice Age.^[34]

The Reverend Robert Walker Skating on Duddingston Loch, attributed to Henry Raeburn, 1790s

Depictions of winter in European painting[link]

Burroughs analyzes the depiction of winter in paintings, as does Hans Neuberger.^[35] Burroughs asserts that this occurred almost entirely from 1565 to 1665 and was associated with the climatic decline from 1550 onwards. Before this, there were almost no depictions of winter in art, and he hypothesizes that the unusually harsh winter of 1565 inspired great artists to depict highly original images and that the decline in such paintings was a combination of the "theme" having been fully explored and mild winters interrupting the flow of painting.

The famous winter paintings by Pieter Brueghel the Elder (e.g. Hunters in the Snow) all appear to have been painted in 1565. Snow also dominates many village-scapes by Pieter Brueghel the Younger, who lived from 1564 to 1638. Burroughs states that Pieter Brueghel the Younger "slavishly copied his father's designs. The derivative nature of so much of this work makes it difficult to draw any definite conclusions about the influence of the winters between 1570 and 1600..."

Winter landscape with iceskaters, c. 1608, Hendrick Avercamp

Dutch painting of the theme appears to begin with Hendrick Avercamp after the winter of 1608. There is then an interruption of the theme between 1627 and 1640, with a sudden return thereafter; this hints at a milder interlude in the 1630s. The 1640s to the 1660s cover the major period of Dutch winter painting, which fits with the known proportion of cold winters then. The final decline in winter painting, around 1660, does not coincide with an amelioration of the climate; Burroughs therefore cautions against trying to read too much into artistic output, since fashion plays a part. He notes that winter painting recurs around the 1780s and 1810s, which again marked a colder period.

Neuberger analyzed 12,000 paintings, held in American and European museums and dated between 1400 and 1967, for cloudiness and darkness.^[35] His 1970 publication shows an increase in such depictions that corresponds to the LIA,^[35] peaking between 1600 and 1649.^[36]

Paintings and contemporary records in Scotland demonstrate that curling and ice skating were popular outdoor winter sports, with curling dating back to the 16th century and becoming widely popular in the mid-19th century.^[37] As an example, an outdoor curling pond constructed in Gourock in the 1860s remained in use for almost a century, but increasing use of indoor facilities, problems of vandalism, and milder winters led to the pond being abandoned in 1963.^[38]

Central America[link]

An analysis of several proxies undertaken in Mexico's Yucatan Peninsula, linked by its authors to Mayan and Aztec chronicles relating periods of cold and drought, supports the existence of the Little Ice Age in the region.^[39]

Southern Hemisphere[link]

Since the discovery of the Little Ice Age, there have been doubts about whether it was a global phenomenon or a cold spell restricted to the Northern Hemisphere. In recent years, several scientific works have pointed out the existence of cold spells and climate changes in areas of the Southern Hemisphere and their correlation to the Little Ice Age.

Africa[link]

In Southern Africa, sediment cores retrieved from Lake Malawi show colder conditions between 1570 and 1820, suggesting the Lake Malawi records "further support, and extend, the global expanse of the Little Ice Age."^[40]

A novel 3,000-year temperature reconstruction method based on the rate of stalagmite growth in a cold cave in South Africa suggests a cold period from 1500 to 1800 "characterizing the South African Little Ice age."^[41]

Antarctica[link]

Kreutz et al. (1997) compared results from studies of West Antarctic ice cores with the Greenland Ice Sheet Project Two (GISP2) and suggested a synchronous global Little Ice Age.^[42] An ocean sediment core from the eastern Bransfield Basin in the Antarctic Peninsula shows centennial events that the authors link to the Little Ice Age and Medieval Warm Period.^[43] The authors note "other unexplained climatic events comparable in duration and amplitude to the LIA and MWP events also appear."

CO₂ mixing ratios at Law Dome

The Siple Dome (SD) has a climate event with an onset time that is coincident with that of the LIA in the North Atlantic based on a correlation with the GISP2 record. This event is the most dramatic climate event seen in the SD Holocene glaciochemical record.^[44] The Siple Dome ice core also contained its highest rate of melt layers (up to 8%) between 1550 and 1700, most likely because of warm summers during the LIA.^[45]

Law Dome ice cores show lower levels of CO₂ mixing ratios during 1550–1800 AD, leading investigators Etheridge and Steele to conjecture "probably as a result of colder global climate."^[46]

Sediment cores in Bransfield Basin, Antarctic Peninsula, have neoglacial indicators by diatom and sea-ice taxa variations during the period of the LIA.^[47]

Australia, New Zealand and Pacific Islands[link]

There is limited evidence about conditions in Australia, though lake records in Victoria suggest that conditions, at least in the south of the state, were wet and/or unusually cool. In the north of the continent, the limited evidence suggests fairly dry conditions, while coral cores from the Great Barrier Reef show similar rainfall as today but with less variability. A study that analyzed isotopes in Great Barrier Reef corals suggested that increased water vapor transport from southern tropical oceans to the poles contributed to the LIA.^[48]

Sea-level data for the Pacific Islands suggest that sea level in the region fell, possibly in two stages, between AD 1270 and 1475. This was associated with a 1.5°C fall in temperature (determined from oxygen-isotope analysis) and an observed increase in El Niño frequency.^[49]

Borehole reconstructions from Australia suggest that, over the last 500 years, the 17th century was the coldest in that continent, although the borehole temperature reconstruction method does not show good agreement between the Northern and Southern Hemispheres.^[50]

On the west coast of the Southern Alps of New Zealand, the Franz Josef glacier advanced rapidly during the Little Ice Age, reaching its maximum extent in the early 18th century, in one of the few places where a glacier thrust into a rain forest.^[33] Based on dating of a yellow-green lichen of the Rhizocarpon subgenus, the Mueller Glacier, on the eastern flank of the Southern Alps within Aoraki/Mount Cook National Park, is considered to have been at its maximum extent between 1725 and 1730.^[51]

South America[link]

Tree-ring data from Patagonia show cold episodes between 1270 and 1380 and from 1520 to 1670, periods contemporary with LIA events in the Northern Hemisphere.^[52][53] Eight sediment cores taken from Puyehue Lake have been interpreted as showing a humid period from 1470 to 1700, which the authors describe as a regional marker of LIA onset.^[54] A 2009 paper details cooler and wetter conditions in southeastern South America between 1550 and 1800 AD, citing evidence obtained via several proxies and models.^[55]

Although it only provides anecdotal evidence, in 1675, the Spanish explorer Antonio de Vea entered San Rafael Lagoon through Río Témpanos (Spanish for "Ice Floe River") without mentioning any ice floe and stated that the San Rafael Glacier did not reach far into the lagoon. In 1766, another expedition noticed that the glacier did reach the lagoon and calved into large icebergs. Hans Steffen visited the area in 1898, noticing that the glacier penetrated far into the lagoon. Such historical records indicate a general cooling in the area between 1675 and 1898, and "The recognition of the LIA in northern Patagonia, through the use of documentary sources, provides important, independent evidence for the occurrence of this phenomenon in the region."^[56] As of 2001, the border of the glacier has significantly retreated compared to the borders of 1675.^[56]

Climate patterns[link]

In the North Atlantic, sediments accumulated since the end of the last ice age, nearly 12,000 years ago, show regular increases in the amount of coarse sediment grains deposited from icebergs melting in the now open ocean, indicating a series of 1-2°C (2-4°F) cooling events recurring every 1,500 years or so.^[57] The most recent of these cooling events was the Little Ice Age. These same cooling events are detected in sediments accumulating off Africa, but the cooling events appear to be larger, ranging between 3-8°C (6-14°F).^[58]

Causes[link]

Scientists have tentatively identified these possible causes of the Little Ice Age: orbital cycles, decreased solar activity, increased volcanic activity, altered ocean current flows, the inherent variability of global climate, and reforestation following decreases in the human population. The most recent study found that an especially massive tropical volcanic eruption in 1258, followed by three smaller ones in 1268, 1275, and 1284 that did not allow the climate to recover, may have caused the initial cooling, and that the 1452–53 eruption of Kuwae in Vanuatu triggered a second pulse of cooling.^[12][11] The cold summers can be maintained by sea-ice/ocean feedbacks long after volcanic aerosols are removed.

Orbital cycles[link]

Orbital forcing due to cycles in the earth's orbit around the sun has, for the past 2,000 years, caused a long-term northern hemisphere cooling trend that continued through the Medieval period and the Little Ice Age. The rate of Arctic cooling is roughly 0.02 degrees Celsius per century.^[59] This trend could be extrapolated to continue into the future, possibly leading to a full ice age, but the 20th-century instrumental temperature record shows a sudden reversal of this trend, with a rise in global temperatures attributed to greenhouse gas emissions.^[59]

Solar activity[link]

Solar activity events recorded in radiocarbon

There is still a very poor understanding of the correlation between low sunspot activity and cooling temperatures.^[60][61] During the period 1645–1715, in the middle of the Little Ice Age, there was a period of low solar activity known as the Maunder Minimum. The Spörer Minimum has also been identified with a significant cooling period between 1460 and 1550.^[62] Other indicators of low solar activity during this period are levels of the isotopes carbon-14 and beryllium-10.^[63]

On the other hand, in a 2012 paper, Miller et al. link the Little Ice Age to an "unusual 50-year-long episode with four large sulfur-rich explosive eruptions, each with global sulfate loading >60 Tg," and notes that "large changes in solar irradiance are not required."^[11]

Volcanic activity[link]

Throughout the Little Ice Age, the world experienced heightened volcanic activity.^[64] When a volcano erupts, its ash reaches high into the atmosphere and can spread to cover the whole earth. This ash cloud blocks out some of the incoming solar radiation, leading to worldwide cooling that can last up to two years after an eruption. Also emitted by eruptions is sulfur in the form of sulfur dioxide gas. When this gas reaches the stratosphere, it turns into sulfuric acid particles, which reflect the sun's rays, further reducing the amount of radiation reaching Earth's surface. The 1815 eruption of Tambora in Indonesia blanketed the atmosphere with ash; the following year, 1816, came to be known as the Year Without a Summer, when frost and snow were reported in June and July in both New England and Northern Europe. Other volcanoes that erupted during the era and may have contributed to the cooling include Billy Mitchell (ca. 1580), Mount Parker (1641), Long Island (Papua New Guinea) (ca. 1660), and Huaynaputina (1600).^[18]

Ocean Conveyor slowdown[link]

Thermohaline circulation or Oceanic Conveyor Belt illustrated

Another possibility is that there was a slowing of thermohaline circulation.^[26][65][66] The circulation could have been interrupted by the introduction of a large amount of fresh water into the North Atlantic, possibly caused by a period of warming before the Little Ice Age known as the Medieval Warm Period.^[33]>^[67][68] There is some concern that a shutdown of thermohaline circulation could happen again as a result of the present warming period.^[69][70]

Decreased human populations[link]

Some researchers have proposed that human influences on climate began earlier than is normally supposed and that major population declines in Eurasia and the Americas reduced this impact, leading to a cooling trend. William Ruddiman has proposed that somewhat reduced populations of Europe, East Asia, and the Middle East during and after the Black Death caused a decrease in agricultural activity. He suggests reforestation took place, allowing more carbon dioxide uptake from the atmosphere, which may have been a factor in the cooling noted during the Little Ice Age. Ruddiman further hypothesizes that a reduced population in the Americas after European contact in the early 16th century could have had a similar effect.^[71][72] Faust, Gnecco, Mannstein and Stamm (2005)^[73] and Nevle (2011)^[74] supported depopulation in the Americas as a factor, asserting that humans had cleared considerable amounts of forests to support agriculture in the Americas before the arrival of Europeans brought on a population collapse. A 2008 study of sediment cores and soil samples further suggests that carbon dioxide uptake via reforestation in the Americas could have contributed to the Little Ice Age.^[75] The depopulation is linked to a drop in carbon dioxide levels observed at Law Dome, Antarctica.^[73]

Inherent variability of climate[link]

Spontaneous fluctuations in global climate might explain past variability. However, it is very difficult to know what the true level of variability from only internal causes might be, since other forcings, as noted above, exist; and their magnitude may not be known either. One approach to evaluating internal variability is to use long integrations of coupled ocean-atmosphere global climate models. These have the advantage that the external forcing is known to be zero; but the disadvantage is that they may not fully reflect reality. These variations may result from chaos-driven changes in the oceans, the atmosphere, or interactions between the two.^[76] Two studies have concluded that the demonstrated inherent variability is not great enough to account for the Little Ice Age.^[76][77]

See also[link]

References[link]

Further reading[link]

• Fagan, Brian M. (2001). The Little Ice Age: How Climate Made History, 1300–1850. Basic Books. ISBN 0-465-02272-3. http://books.google.com/books?id=LwvkmXt5fQUC. 

External links[link]

Wikimedia Commons has media related to: Little Ice Age

• Abrupt Climate Change Information from the Ocean & Climate Change Institute, links to Woods Hole Oceanographic Institution articles
• "The Next Ice Age". Discover. September 2002. http://discovermagazine.com/2002/sep/cover/.  (discussion of Woods Hole research)
• "Huascaran (Peru) Ice Core Data". NOAA/NGDC Paleoclimatology Program. 1995. http://www.ngdc.noaa.gov/paleo/pubs/thompson1995/huascaran.html. 
• Dansgaard cycles and the Little Ice Age (LIA) (It is not easy to see a LIA in the graphs.)
• Tyson, P.D.; Karlen, W.; Holmgren, K.; Heiss, G.A. (2000). "The Little Ice Age and Medieval Warming in South Africa" (PDF). South African Journal of Science 96 (3): 121–6. http://ruby.fgcu.edu/courses/twimberley/EnviroPhilo/Tyson.pdf. 
• Was El Niño unaffected by the Little Ice Age? (2002)
• Evidence for the Little Ice Age in Spain, circa 2003
• The Little Ice Age in Europe, updated 2009
• "The Little Ice Age, Ca. 1300–1870". Timeline of European Environmental History. http://www.eh-resources.org/timeline/timeline_lia.html.  undated review article
• What's wrong with the sun? (Nothing) (2008)
• HistoricalClimatology.com, further links and resources, updated 2012

An artist's impression of ice age Earth at glacial maximum. Based on: "Ice age terrestrial carbon changes revisited" by Thomas J. Crowley (Global Biogeochemical Cycles, Vol. 9, 1995, pp. 377–389)

The Antarctic ice sheet. Ice sheets expand during an ice age.

Variations in temperature, CO₂, and dust from the Vostok ice core over the last 400,000 years

An ice age, or more precisely, a glacial age, is a period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers. Within a long-term ice age, individual pulses of cold climate are termed "glacial periods" (or alternatively "glacials" or "glaciations" or colloquially as "ice age"), and intermittent warm periods are called "interglacials". Glaciologically, ice age implies the presence of extensive ice sheets in the northern and southern hemispheres.^[1] By this definition, we are still in the ice age that began at the start of the Pleistocene epoch, because the Greenland and Antarctic ice sheets still exist.^[2]

More colloquially, "the Ice Age" refers to the most recent glacial period that peaked approximately 20,000 years ago, in which extensive ice sheets lay over large parts of the North American and Eurasian continents. This article will use the term ice age in the former, glaciological, sense: glacials for colder periods during ice ages and interglacials for the warmer periods.

Origin of ice age theory[link]

In 1742 Pierre Martel (1706–1767), an engineer and geographer living in Geneva, visited the valley of Chamonix in the Alps of Savoy.^[3][4] Two years later he published an account of his journey. He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the fact that the glaciers had once extended much farther.^[5][6] Later similar explanations were reported from other regions of the Alps. In 1815 the carpenter and chamois hunter Jean-Pierre Perraudin (1767–1858) explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers previously extending further.^[7] An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss-German geologist Jean de Charpentier (1786–1855) in 1834.^[8] Comparable explanations are also known from the Val de Ferret in the Valais and the Seeland in western Switzerland^[9] and in Goethe's Scientific Work.^[10] Such explanations could also be found in other parts of the world. When the Bavarian naturalist Ernst von Bibra (1806–1878) visited the Chilean Andes in 1849–1850 the natives attributed fossil moraines to the former action of glaciers.^[11]

Meanwhile, European scholars had begun to wonder what had caused the dispersal of erratic material. From the middle of the 18th century some discussed ice as a means of transport. The Swedish mining expert Daniel Tilas (1712–1772) was, in 1742, the first person to suggest drifting sea ice in order to explain the presence of erratic boulders in the Scandinavian and Baltic regions.^[12] In 1795, the Scottish philosopher and gentleman naturalist, James Hutton (1726–1797), explained erratic boulders in the Alps with the action of glaciers.^[13] Two decades later, in 1818, the Swedish botanist Göran Wahlenberg (1780–1851) published his theory of a glaciation of the Scandinavian peninsula. He regarded glaciation as a regional phenomenon.^[14] Only a few years later, the Danish-Norwegian Geologist Jens Esmark (1763–1839) argued a sequence of worldwide ice ages. In a paper published in 1824, Esmark proposed changes in climate as the cause of those glaciations. He attempted to show that they originated from changes in the Earth's orbit.^[15] During the following years, Esmark’s ideas were discussed and taken over in parts by Swedish, Scottish and German scientists. At the University of Edinburgh Robert Jameson (1774–1854) seemed to be relatively open towards Esmark's ideas. Jameson's remarks about ancient glaciers in Scotland were most probably prompted by Esmark.^[16] In Germany, Albrecht Reinhard Bernhardi (1797–1849), professor of forestry at Dreissigacker, adopted Esmark's theory. In a paper published in 1832, Bernhardi speculated about former polar ice caps reaching as far as the temperate zones of the globe.^[17]

Independently of these debates, the Swiss civil engineer Ignaz Venetz (1788–1859) in 1829, explained the dispersal of erratic boulders in the Alps, the nearby Jura Mountains and the North German Plain as being due to huge glaciers. When he read his paper before the Schweizerische Naturforschende Gesellschaft, most scientists remained sceptical.^[18] Finally, Venetz managed to convince his friend Jean de Charpentier. De Charpentier transformed Venetz's idea into a theory with a glaciation limited to the Alps. His thoughts resembled Wahlenberg's theory. In fact, both men shared the same volcanistic, or in de Charpentier’s case rather plutonistic assumptions, about earth history. In 1834, de Charpentier presented his paper before the Schweizerische Naturforschende Gesellschaft.^[19] In the meantime, the German botanist Karl Friedrich Schimper (1803–1867) was studying mosses which were growing on erratic boulders in the alpine upland of Bavaria. He began to wonder where such masses of stone had come from. During the summer of 1835 he made some excursions to the Bavarian Alps. Schimper came to the conclusion that ice must have been the means of transport for the boulders in the alpine upland. In the winter of 1835 to 1836 he held some lectures in Munich. Schimper then assumed that there must have been global times of obliteration ("Verödungszeiten") with a cold climate and frozen water.^[20] Schimper spent the summer months of 1836 at Devens, near Bex, in the Swiss Alps with his former university friend Louis Agassiz (1801–1873) and Jean de Charpentier. Schimper, de Charpentier and possibly Venetz convinced Agassiz that there had been a time of glaciation. During Winter 1836/7 Agassiz and Schimper developed the theory of a sequence of glaciations. They mainly drew upon the preceding works of Goethe,^[21] of Venetz, of de Charpentier and on their own fieldwork. There are indications that Agassiz was already familiar with Bernhardi's paper at that time.^[22] At the beginning of 1837 Schimper coined the term ice age ("Eiszeit").^[23] In July 1837 Agassiz presented their synthesis before the annual meeting of the Schweizerische Naturforschende Gesellschaft at Neuchâtel. The audience was very critical or even opposed the new theory because it contradicted the established opinions on climatic history. Most contemporary scientists thought that the earth had been gradually cooling down since its birth as a molten globe.^[24]

In order to overcome this rejection, Agassiz embarked on geological fieldwork. He published his book Study on glaciers ("Études sur les glaciers") in 1840.^[25] De Charpentier was put out by this as he had also been preparing a book about the glaciation of the Alps. De Charpentier felt that Agassiz should have given him precedence as it was he who had introduced Agassiz to in depth glacial research.^[26] Besides that, Agassiz had, as a result of personal quarrels, omitted any mention of Schimper in his book.^[27]

Altogether, it took several decades until the ice age theory was fully accepted. This happened on an international scale in the second half of the 1870s following the work of James Croll including the publication of Climate and Time, in Their Geological Relations in 1875 which provided a credible explanation for the causes of ice ages.^[28]

Evidence for ice ages[link]

There are three main types of evidence for ice ages: geological, chemical, and paleontological.

Geological evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratics. Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret. Furthermore, this evidence was difficult to date exactly; early theories assumed that the glacials were short compared to the long interglacials. The advent of sediment and ice cores revealed the true situation: glacials are long, interglacials short. It took some time for the current theory to be worked out.

The chemical evidence mainly consists of variations in the ratios of isotopes in fossils present in sediments and sedimentary rocks and ocean sediment cores. For the most recent glacial periods ice cores provide climate proxies from their ice, and atmospheric samples from included bubbles of air. Because water containing heavier isotopes has a higher heat of evaporation, its proportion decreases with colder conditions.^[29] This allows a temperature record to be constructed. However, this evidence can be confounded by other factors recorded by isotope ratios.

The paleontological evidence consists of changes in the geographical distribution of fossils. During a glacial period cold-adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or are squeezed into lower latitudes. This evidence is also difficult to interpret because it requires (1) sequences of sediments covering a long period of time, over a wide range of latitudes and which are easily correlated; (2) ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed; and (3) the finding of the relevant fossils.

Despite the difficulties, analysis of ice core and ocean sediment cores^{[citation needed]} has shown periods of glacials and interglacials over the past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available.

Major ice ages[link]

Ice age map of northern Germany and its northern neighbours. Red: maximum limit of Weichselian glacial; yellow: Saale glacial at maximum (Drenthe stage); blue: Elster glacial maximum glaciation.

There have been at least five major ice ages in the Earth's past. Outside these ages, the Earth seems to have been ice-free even in high latitudes.^[30][31]

Rocks from the earliest well established ice age, called the Huronian, formed around 2.4 to 2.1 Ga (billion years) ago during the early Proterozoic Eon. Several hundreds of km of the Huronian Supergroup are exposed 10–100 km north the North Shore of Lake Huron extending from near Sault Ste. Marie to Sudbury NE of Lake Huron, with giant layers of now-lithified till beds, dropstones, varves, outwash, and scoured basement rocks. Correlative Huronian deposits have been found near Marquette, Michigan, and correlation has been made with Paleoproterozic glacial deposits from Western Australia.

The next well-documented ice age, and probably the most severe of the last billion years, occurred from 850 to 630 million years ago (the Cryogenian period) and may have produced a Snowball Earth in which glacial ice sheets reached the equator,^[32] possibly being ended by the accumulation of greenhouse gases such as CO₂ produced by volcanoes. "The presence of ice on the continents and pack ice on the oceans would inhibit both silicate weathering and photosynthesis, which are the two major sinks for CO₂ at present."^[33] It has been suggested that the end of this ice age was responsible for the subsequent Ediacaran and Cambrian Explosion, though this model is recent and controversial.

A minor ice age, the Andean-Saharan, occurred from 460 to 420 million years ago, during the Late Ordovician and the Silurian period.

The evolution of land plants at the onset of the Devonian period caused a long term increase in planetary oxygen levels and reduction of CO₂ levels, which resulted in the Karoo Ice Age. It is named after the glacial tills found in the Karoo region of South Africa, where evidence for this ice age was first clearly identified. There were extensive polar ice caps at intervals from 360 to 260 million years ago in South Africa during the Carboniferous and early Permian Periods. Correlatives are known from Argentina, also in the center of the ancient supercontinent Gondwanaland.

The current ice age, the Pliocene-Quaternary glaciation, started about 2.58 million years ago during the late Pliocene, when the spread of ice sheets in the Northern Hemisphere began. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales called glacial periods, glacials or glacial advances, and interglacial periods, interglacials or glacial retreats. The earth is currently in an interglacial, and the last glacial period ended about 10,000 years ago. All that remains of the continental ice sheets are the Greenland and Antarctic ice sheets and smaller glaciers such as on Baffin Island.

Sediment records showing the fluctuating sequences of glacials and interglacials during the last several million years.

Ice ages can be further divided by location and time; for example, the names Riss (180,000–130,000 years bp) and Würm (70,000–10,000 years bp) refer specifically to glaciation in the Alpine region. Note that the maximum extent of the ice is not maintained for the full interval. Unfortunately, the scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet does not achieve full coverage.

Glacials and interglacials[link]

Shows the pattern of temperature and ice volume changes associated with recent glacials and interglacials

Minimum (interglacial, black) and maximum (glacial, grey) glaciation of the northern hemisphere

Minimum (interglacial, black) and maximum (glacial, grey) glaciation of the southern hemisphere

Within the ice ages (or at least within the current one), more temperate and more severe periods occur. The colder periods are called glacial periods, the warmer periods interglacials, such as the Eemian Stage.

Glacials are characterized by cooler and drier climates over most of the Earth and large land and sea ice masses extending outward from the poles. Mountain glaciers in otherwise unglaciated areas extend to lower elevations due to a lower snow line. Sea levels drop due to the removal of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. Since the Earth has significant continental glaciation in the Arctic and Antarctic, we are currently in a glacial minimum of a glaciation. Such a period between glacial maxima is known as an interglacial.

The Earth has been in an interglacial period known as the Holocene for more than 11,000 years. It was conventional wisdom that the typical interglacial period lasts about 12,000 years, but this has been called into question recently. For example, an article in Nature^[34] argues that the current interglacial might be most analogous to a previous interglacial that lasted 28,000 years. Predicted changes in orbital forcing suggest that the next glacial period would begin at least 50,000 years from now, even in absence of human-made global warming^[35] (see Milankovitch cycles). Moreover, anthropogenic forcing from increased greenhouse gases might outweigh orbital forcing for as long as intensive use of fossil fuels continues.^[36]

Positive and negative feedbacks in glacial periods[link]

Each glacial period is subject to positive feedback which makes it more severe and negative feedback which mitigates and (in all cases so far) eventually ends it.

Positive feedback processes[link]

Ice and snow increase the Earth's albedo, i.e. they make it reflect more of the sun's energy and absorb less. Hence, when the air temperature decreases, ice and snow fields grow, and this continues until competition with a negative feedback mechanism forces the system to an equilibrium. Also, the reduction in forests caused by the ice's expansion increases albedo.

Another theory proposed by Ewing and Donn in 1956^[37] hypothesized that an ice-free Arctic Ocean leads to increased snowfall at high latitudes. When low-temperature ice covers the Arctic Ocean there is little evaporation or sublimation and the polar regions are quite dry in terms of precipitation, comparable to the amount found in mid-latitude deserts. This low precipitation allows high-latitude snowfalls to melt during the summer. An ice-free Arctic Ocean absorbs solar radiation during the long summer days, and evaporates more water into the Arctic atmosphere. With higher precipitation, portions of this snow may not melt during the summer and so glacial ice can form at lower altitudes and more southerly latitudes, reducing the temperatures over land by increased albedo as noted above. Furthermore, under this hypothesis the lack of oceanic pack ice allows increased exchange of waters between the Arctic and the North Atlantic Oceans, warming the Arctic and cooling the North Atlantic. (Current projected consequences of global warming include a largely ice-free Arctic Ocean within 5–20 years, see Arctic shrinkage.) Additional fresh water flowing into the North Atlantic during a warming cycle may also reduce the global ocean water circulation (see Shutdown of thermohaline circulation). Such a reduction (by reducing the effects of the Gulf Stream) would have a cooling effect on northern Europe, which in turn would lead to increased low-latitude snow retention during the summer. It has also been suggested that during an extensive glacial, glaciers may move through the Gulf of Saint Lawrence, extending into the North Atlantic ocean far enough to block the Gulf Stream.

Negative feedback processes[link]

Ice sheets that form during glaciations cause erosion of the land beneath them. After some time, this will reduce land above sea level and thus diminish the amount of space on which ice sheets can form. This mitigates the albedo feedback, as does the lowering in sea level that accompanies the formation of ice sheets^{[citation needed]}.

Another factor is the increased aridity occurring with glacial maxima, which reduces the precipitation available to maintain glaciation. The glacial retreat induced by this or any other process can be amplified by similar inverse positive feedbacks as for glacial advances^{[citation needed]}.

According to a BBC article on the BBC news website (09/01/2012) human emissions of carbon dioxide will defer the next ice age. Researchers used data on the Earth's orbit to find the historical warm interglacial period that looks most like the current one. According to the BBC article, the next ice age would usually begin with 1,500 years. However, due to incredibly high carbon emissions, this ice age will not occur.^{[citation needed]}

Causes of ice ages[link]

The causes of ice ages are not fully understood for both the large-scale ice age periods and the smaller ebb and flow of glacial–interglacial periods within an ice age. The consensus is that several factors are important: atmospheric composition, such as the concentrations of carbon dioxide and methane (the specific levels of the previously mentioned gases are now able to be seen with the new ice core samples from EPICA Dome C in Antarctica over the past 800,000 years^[38] ); changes in the Earth's orbit around the Sun known as Milankovitch cycles (and possibly the Sun's orbit around the galaxy); the motion of tectonic plates resulting in changes in the relative location and amount of continental and oceanic crust on the Earth's surface, which affect wind and ocean currents; variations in solar output; the orbital dynamics of the Earth-Moon system; and the impact of relatively large meteorites, and volcanism including eruptions of supervolcanoes.^{[citation needed]}

Some of these factors influence each other. For example, changes in Earth's atmospheric composition (especially the concentrations of greenhouse gases) may alter the climate, while climate change itself can change the atmospheric composition (for example by changing the rate at which weathering removes CO₂).

Maureen Raymo, William Ruddiman and others propose that the Tibetan and Colorado Plateaus are immense CO₂ "scrubbers" with a capacity to remove enough CO₂ from the global atmosphere to be a significant causal factor of the 40 million year Cenozoic Cooling trend. They further claim that approximately half of their uplift (and CO₂ "scrubbing" capacity) occurred in the past 10 million years.^[39][40]

Changes in Earth's atmosphere[link]

There is considerable evidence that over the very recent period of the last 100-1000 years, the sharp increases in human activity, especially the burning of fossil fuels, has caused the parallel sharp and accelerating increase in atmospheric greenhouse gases which trap the sun's heat. The consensus theory of the scientific community is that the resulting greenhouse effect is a principal cause of the increase in global warming which has occurred over the same period, and a chief contributor to the accelerated melting of the remaining glaciers and polar ice.

There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism.

The Snowball Earth hypothesis maintains that the severe freezing in the late Proterozoic was ended by an increase in CO₂ levels in the atmosphere, and some supporters of Snowball Earth argue that it was caused by a reduction in atmospheric CO₂. The hypothesis also warns of future Snowball Earths.

The August 2009 edition of Science provides further evidence that changes in solar insolation provide the initial trigger for the Earth to warm after an Ice Age, with secondary factors like increases in greenhouse gases accounting for the magnitude of the change.^[41]

William Ruddiman has proposed the early anthropocene hypothesis, according to which the anthropocene era, as some people call the most recent period in the Earth's history when the activities of the human species first began to have a significant global impact on the Earth's climate and ecosystems, did not begin in the 18th century with the advent of the Industrial Era, but dates back to 8,000 years ago, due to intense farming activities of our early agrarian ancestors. It was at that time that atmospheric greenhouse gas concentrations stopped following the periodic pattern of the Milankovitch cycles. In his overdue-glaciation hypothesis Ruddiman states that an incipient glacial would probably have begun several thousand years ago, but the arrival of that scheduled glacial was forestalled by the activities of early farmers.^[42]

At a meeting of the American Geophysical Union (December 17, 2008), scientists detailed evidence in support of the controversial idea that the introduction of large-scale rice agriculture in Asia, coupled with extensive deforestation in Europe began to alter world climate by pumping significant amounts of greenhouse gases into the atmosphere over the last 1,000 years. In turn, a warmer atmosphere heated the oceans making them much less efficient storehouses of carbon dioxide and reinforcing global warming, possibly forestalling the onset of a new glacial age.^[43]

Position of the continents[link]

The geological record appears to show that ice ages start when the continents are in positions which block or reduce the flow of warm water from the equator to the poles and thus allow ice sheets to form. The ice sheets increase the Earth's reflectivity and thus reduce the absorption of solar radiation. With less radiation absorbed the atmosphere cools; the cooling allows the ice sheets to grow, which further increases reflectivity in a positive feedback loop. The ice age continues until the reduction in weathering causes an increase in the greenhouse effect.

There are three known configurations of the continents which block or reduce the flow of warm water from the equator to the poles:^{[citation needed]}

• A continent sits on top of a pole, as Antarctica does today.
• A polar sea is almost land-locked, as the Arctic Ocean is today.
• A supercontinent covers most of the equator, as Rodinia did during the Cryogenian period.