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Little Ice Age
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.〔 Matthes described glaciers in the Sierra Nevada of California that could not have survived the hypsithermal, in his opinion; his usage of "Little Ice Age" has been superseded by "Neoglaciation".〕 It has been conventionally defined as a period extending from the sixteenth to the nineteenth centuries,〔 (noted in Grove 2004:4).〕〔.〕 or alternatively, from about 1300〔 to about 1850,〔Grove, J.M., ''Little Ice Ages: Ancient and Modern,'' Routledge, London (2 volumes) 2004.〕〔Matthews, J.A. and Briffa, K.R., ("The 'Little Ice Age': re-evaluation of an evolving concept" ), ''Geogr. Ann., 87,'' A (1), pp. 17–36 (2005). Retrieved 17 July 2015.〕〔(【引用サイトリンク】title=1.4.3 Solar Variability and the Total Solar Irradiance - AR4 WGI Chapter 1: Historical Overview of Climate Change Science )〕 although 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. The NASA Earth Observatory 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.〔(【引用サイトリンク】title=Climate Change 2001: The Scientific Basis )〕
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.
==Areas involved==
The Intergovernmental Panel on Climate Change Third Assessment Report (TAR) of 2001 describes areas affected by the LIA:
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."〔(AR4 WG1 Section 6.6: The Last 2,000 Years ), 2007.〕
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