The Little Ice Age (LIA) was a period of cooling lasting approximately from the mid-14th to the mid-19th centuries. This cooling brought an end to an unusually warm era known as the Medieval climate optimum.
It was initially believed that the LIA was a global phenomenon. It is now less clear that this is true based on evidence about past temperature trends from IPCC-TAR. For example, some versions of the reconstruction of the temperature record of the past 1000 years in the northern hemisphere  do not show a pronounced period of cooling. See Medieval climate optimum for more on this. The IPCC describes the LIA as a modest cooling of the Northern Hemisphere during this period of less than 1°C, and says current evidence does not support globally synchronous periods of anomalous cold or warmth over this timeframe, 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.
The Little Ice Age brought bitterly cold winters to many parts of the world, but is most thoroughly documented in Europe and North America. In the mid-17th century, glaciers in the Swiss Alps advanced, gradually engulfing farms and crushing entire villages. The River Thames and the canals and rivers of the Netherlands often froze over during the winter, and people skated and even held fairs on the ice. 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 that island's harbors to shipping.
The severe winters affected human life in ways large and small. Famines became more frequent, the one in 1315 alone killing 1.5 million people, and deaths from disease increased. The population of Iceland fell by half, and the Viking colonies in Greenland died out. The Little Ice Age can be seen in the art of the time; for example, snow dominates many village-scapes by the Flemish painter Pieter Brueghel the Younger, who lived from 1564 to 1638.
Another famous person to live during the LIA was Antonio Stradivari, the reknowned violin maker. The colder climates of the time caused the wood from the trees he used to be more dense; the superb tone of Stradivari's creations has been attributed to this.
Scientists have identified two causes of the Little Ice Age from outside the ocean/atmosphere/land systems: decreased solar activity and increased volcanic activity. Research is ongoing on more ambiguous influences such as internal variability of the climate system, and anthropogenic influence (Ruddiman). Some have also speculated that depopulation of Europe during the Black Death, and the resulting decrease in agricultural output, may have prolonged the Little Ice Age.
During the period 1645–1715, right in the middle of the Little Ice Age, solar activity as seen in sunspots was extremely low, with some years having no sunspots at all. This period of low sunspot activity is known as the Maunder Minimum. The precise link between low sunspot activity and cooling temperatures has not been established. But the coincidence of the Maunder Minimum with the deepest trough of the Little Ice Age is suggestive of such a connection.  Other indicators of low solar activity during this period are levels of carbon-14 and beryllium-10. 
Throughout the Little Ice Age the world also 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 world-wide cooling that can last up to two years after an eruption. Also emitted by eruptions is sulfur in the form of SO2 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 the 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.
End of Little Ice Age
Beginning around 1850, the world's climate began warming again and the Little Ice Age may be said to have come to an end at that time. Some scientists believe that the Earth's climate is still recovering from the Little Ice Age and that this situation contributes to concerns over human-caused climate change.