Little Ice Age

The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. The period has been conventionally defined as extending from the 16th to the 19th centuries, but some experts prefer an alternative time-span from about 1300 to about 1850. The NASA Earth Observatory notes three particularly cold intervals. One began about 1650, another about 1770, and the last in 1850, all of which were separated by intervals of slight warming. The Intergovernmental Panel on Climate Change Third Assessment Report considered that the timing and the 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, variations in Earth's orbit and axial tilt (orbital forcing), inherent variability in global climate, and decreases in the human population (such as from the massacres by Genghis Khan, the Black Death and the epidemics emerging in the Americas upon European contact).

Areas involved The Intergovernmental Panel on Climate Change Third Assessment Report (TAR) of 2001 described the areas that were affected:

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 interval, 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 and gives particular attention to the Medieval Warm Period:

... 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. ... 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.

Dating

There is no consensus on when the Little Ice Age began, but a series of events before the known climatic minima have often been referenced. According to J. M. Lamb of Cambridge University, the Little Ice Age was already under way in Canada and Switzerland and in the wider North Atlantic region in the 13th and 14th centuries. In the 13th 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 that were collected from beneath ice caps on Baffin Island and Iceland, Miller et al. (2012) state that cold summers and ice growth began abruptly between 1275 and 1300, followed by "a substantial intensification" from 1430 to 1455. In contrast, a climate reconstruction based on the length of glaciers shows no great variation from 1600 to 1850 but a strong retreat thereafter. Therefore, 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, a cold period that was possibly triggered or enhanced by the massive eruption of the Samalas volcano in 1257 and the associated volcanic winter. 1275 to 1300 for when the radiocarbon dating of plants shows that they were killed by glaciation 1300 for when warm summers stopped being dependable in Northern Europe 1315 for when rains and the Great Famine of 1315–1317 occurred 1560 to 1630 for when the worldwide glacial expansion, known as the Grindelwald Fluctuation, began 1650, not the start of the Little Ice Age, but the start of the coldest years midway through, i.e., the First Climatic Minimum The Little Ice Age ended in the latter half of the 19th century or in the early 20th century.

The 6th report of the IPCC describes the coldest period in the last millennium as: a multi-centennial period of relatively low temperature beginning around the 15th century, with GMST averaging –0.03 [–0.30 to 0.06] °C between 1450 and 1850 relative to 1850–1900.The definitions for the beginning and end of the LIA differ considerably, depending on the region and dataset used. The LIA in the Northern Hemisphere started between 1200 and 1400 AD. In the Southern Hemisphere the beginning of the LIA was delayed by about two centuries.

By region

Europe

Central Europe

In 2021, historian Christian Pfister and climatologist Heinz Wanner published a reconstruction of seasonal temperatures in Central Europe using temperature indices (historical climatology) based on documentary data. After CE 1500, the reconstruction is based on an article by Czech geographer Petr Dobrovolny, which includes monthly, seasonal and annual temperature estimates for Germany, Switzerland and the Czech lands based on temperature indices up to 1759 and subsequent temperature measurements.

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