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The Little Ice Age (LIA) was a period of cooling that occurred after the Medieval Warm Period (Medieval Climate Optimum). While it was not a true ice age, the term was introduced into the scientific literature by François E. Matthes in 1939. It has been conventionally defined as a period extending from the sixteenth to the nineteenth centuries, or alternatively, from about 1350 to about 1850, 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 AD 1550 and AD 1850 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. The Intergovernmental Panel on Climate Change Third Assessment Report considered the timing and areas affected by the LIA suggested largely independent regional climate changes, rather than a globally synchronous increased glaciation. At most there was modest cooling of the Northern Hemisphere during the period.
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.
Evidence from mountain glaciers does suggest increased glaciation in a number of widely spread regions outside Europe prior to the twentieth 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 twentieth century levels.
The IPCC Fourth Assessment Report (AR4) of 2007 discusses more recent research, giving particular attention to the Medieval Warm Period. It states that "when viewed together, the currently available reconstructions indicate generally greater variability in centennial time scale trends over the last 1 kyr than was apparent in the TAR." Considering confidence limits, "The result is a picture of relatively cool conditions in the seventeenth and early nineteenth centuries and warmth in the eleventh and early fifteenth centuries, but the warmest conditions are apparent in the twentieth century. Given that the confidence levels surrounding all of the reconstructions are wide, virtually all reconstructions are effectively encompassed within the uncertainty previously indicated in the TAR. The major differences between the various proxy reconstructions relate to the magnitude of past cool excursions, principally during the twelfth to fourteenth, seventeenth and nineteenth centuries."
There is no consensus regarding the time when the Little Ice Age began, although a series of events preceding the known climatic minima has often been referenced. In the thirteenth century, pack ice began advancing southwards in the North Atlantic, as did glaciers in Greenland. Anecdotal evidence suggests 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) state that cold summers and ice growth began abruptly between AD 1275 and 1300, followed by "a substantial intensification" from 1430 to 1455 AD.
For this reason, any of several dates ranging over 400 years may indicate the beginning of the Little Ice Age:
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-seventeenth century. Canals and rivers in Great Britain and the Netherlands were frequently frozen deeply enough to support ice skating and winter festivals. The first River Thames frost fair was in 1607 and the last in 1814; 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 attack 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 population of Iceland fell by half, but this was perhaps caused by fluorosis after the eruption of the volcano Laki in 1783. Iceland also suffered failures of cereal crops, and people moved away from a grain-based diet. The Norse colonies in Greenland starved and vanished (by the early fifteenth century), as crops failed and livestock could not be maintained through increasingly harsh winters, though Jared Diamond noted they had exceeded the agricultural carrying capacity before then. In North America, American Indians formed leagues in response to food shortages. In Lisbon, Portugal, snowstorms were much more frequent than today. Heavy snowfalls in the winters of 1665, 1744 and 1886 were reported.
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." Many springs and summers were cold and wet, but with 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). 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." Viticulture disappeared from some northern regions. Violent storms caused serious flooding and loss of life. Some of these resulted in permanent loss of large areas of land from the Danish, German and Dutch coasts.
The extent of mountain glaciers had been mapped by the late nineteenth 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. In Glacier National Park, the last episode of glacier advance came in the late 18th and early nineteenth centuries. In Chesapeake Bay, Maryland, large temperature excursions were possibly related to changes in the strength of North Atlantic thermohaline circulation.
In North America, the early European explorers and settlers reported exceptionally severe winters. For example, according to Lamb, Samuel Champlain reported bearing ice along the shores of Lake Superior in June 1608; both Europeans and indigenous peoples suffered excess mortality in Maine during the winter of 1607-1608; and extreme frost was reported in the Jamestown, Virginia settlement at the same time. 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.
Antonio Stradivari, the famous violin maker, produced his instruments during the Little Ice Age. The colder climate is proposed to have caused the wood used in his violins to be denser than in warmer periods, contributing to the tone of Stradivari's instruments. According to the science historian James Burke the period inspired such innovations in everyday life as the widespread use of buttons and button holes, knitting of custom-made undergarments to better cover and insulate the human body, and installation of fireplace hoods to make more efficient use of fires for indoor heating, as well as the development of the enclosed stove, in early versions often covered with ceramic tiles.
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 seventeenth century, writes Fagan, agriculture had dropped off so dramatically, "Alpine villagers lived on bread made from ground nutshells mixed with barley and oat flour."  Historian Wolfgang Behringer has linked intensive witch-hunting episodes in Europe to agricultural failures during the Little Ice Age.
William James Burroughs analyzes the depiction of winter in paintings, as does Hans Neuberger. Burroughs asserts that this occurred almost entirely from 1565 to 1665 and was associated with the climatic decline from 1550 onwards. Burrough claims that before this, there were almost no depictions of winter in art, and "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". In fact wintery scenes, which have technical difficulties in painting, had been regularly and very well handled since the early fifteenth century by artists in illuminated manuscript cycles showing the Labours of the Months, typically placed on the calendar pages of books of hours. January and February are typically shown as snowy, as in February in the famous cycle in the Les Très Riches Heures du duc de Berry, painted 1412-1416 and illustrated below. At this period independent landscape subjects had not developed as a genre in art, so the absence of other winter scenes is not remarkable.
The famous winter landscape paintings by Pieter Brueghel the Elder, most famously The Hunters in the Snow, are all thought to have been painted in 1565. His son Pieter Brueghel the Younger (1564-1638) also painted many snowy landscapes, though according to Burroughs as he "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...".
Burroughs say that snowy subjects return to Dutch Golden Age painting with works by Hendrick Avercamp from 1609 onwards. There is then a hiatus between 1627 and 1640, before the main period of such subjects from the 1640s to the 1660s, which relates well with climate records for this later period. However the subjects are less popular after about 1660, but this does not match any recorded reduction in severity of winters, and may just reflect changes in taste or fashion. In the later period between the 1780s and 1810s, snowy subjects again become popular.
Neuberger analyzed 12,000 paintings, held in American and European museums and dated between 1400 and 1967, for cloudiness and darkness. His 1970 publication shows an increase in such depictions that corresponds to the LIA, peaking between 1600 and 1649.
Paintings and contemporary records in Scotland demonstrate that curling and ice skating were popular outdoor winter sports, with curling dating back to the sixteenth century and becoming widely popular in the mid-nineteenth century. 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.
In China, warm-weather crops, such as oranges, were abandoned in Jiangxi Province, where they had been grown for centuries. 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).
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.
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.
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."
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."
In Ethiopia and Mauritania, permanent snow was reported on mountain peaks at levels where it does not occur today. 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.
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. 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. The authors note "other unexplained climatic events comparable in duration and amplitude to the LIA and MWP events also appear."
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. 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.
The MES stable isotope record suggests that the Ross Sea region experienced 1.6 ± 1.4 °C cooler average temperatures during the LIA in comparison to the last 150 yr 
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. Borehole reconstructions from Australia suggest that, over the last 500 years, the seventeenth century was the coldest in that continent, although the borehole temperature reconstruction method does not show good agreement between the Northern and Southern Hemispheres.
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.
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 eighteenth century, in one of the few places where a glacier thrust into a rain forest. 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.
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. 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. 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.
18O records from three Andean ice cores show a cool period from 1600-1800 
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." As of 2001, the border of the glacier has significantly retreated compared to the borders of 1675.
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. 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).
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, possibly of Mount Rinjani, 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. The cold summers can be maintained by sea-ice/ocean feedbacks long after volcanic aerosols are removed.
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. This trend could be extrapolated to continue into the future, possibly leading to a full ice age, but the twentieth-century instrumental temperature record shows a sudden reversal of this trend, with a rise in global temperatures attributed to greenhouse gas emissions.
There is still a very poor understanding of the correlation between low sunspot activity and cooling temperatures. 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. Other indicators of low solar activity during this period are levels of the isotopes carbon-14 and beryllium-10.
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."
Throughout the Little Ice Age, the world experienced heightened volcanic activity. 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).
Another possibility is that there was a slowing of thermohaline circulation. 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. There is some concern that a shutdown of thermohaline circulation could happen again as a result of the present warming period.
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 sixteenth century could have had a similar effect. Faust, Gnecco, Mannstein and Stamm (2005) and Nevle (2011) 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. The depopulation is linked to a drop in carbon dioxide levels observed at Law Dome, Antarctica.
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. Two studies have concluded that the demonstrated inherent variability is not great enough to account for the Little Ice Age.
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