The Little Ice Age was a period of cooling approximately from 1560 to 1890, which may also be called the Hypothermal Period (Dyson, 1962), but there are some signs that it could have lasted until 1930 (Aber, 2011a). This cooling period came after the Medieval Climatic Optimum, which was a time of a favorable, warm climate (Aber, 2011b). During this cooling period of the Little Ice Age, effects were seen throughout the Northern Hemisphere, but at varying times by location. During the Little Ice Age, there was cooling in average temperatures between 1-2 °C. Through climate and glacier study, there have been many discoveries to determine why this change occurred. Through research for evidence of the Little Ice Age in the United States, several have been discovered. The evidence includes: glacial deposition, pollen, climate changes, volcanic activity, and geomagnetism.
Image showing the temperature changes over the past 3000 years. (Image found in the public use section of creativecommons.org)
In James Dyson's book, glacial retreat has occurred over the past 150 years, in Glacier Bay, Alaska. As the glaciers melt, it has uncovered a forest that was destroyed due to the previous glacial advances (Dyson, 1962). Glaciers on Mount Rainier are well marked with moraines that have been dated by using Porter's lichen curve, which is a form of dating by using the size of the lichen to determine the age of an object (Grove, 1988).
The image above shows Glacier Bay, Alaska (NPS, 2011). For more pictures or information on Glacier Bay, Alaska or Glacier Bay National Park, please visit: Glacier Bay National Park .
Nisqually, the largest of the Rainier glaciers, has the longest records on the terminus of the glacier that date back to 1857. The maximum position of the terminus was determined by the growth of lichens on the andesite and granodiorite that became exposed by glacial movement in the past (Grove, 1988).
Image of Nisqually Glacier. This image is from commons.wikimedia.org
C.J. Heusser founded a datable record of the Little Ice Age on Blue Glacier in Washington state based on the growth of Alpine Fir and Hemlock on the moraines found in this area. Heusser dated these plants to start growing after seed travel in 1650 (Grove, 1988). In Marion Lake, Michigan, there is 3000 years of pollen records found in varves. These pollen records show a definite increase of Western Hemlock, spruce and pine between the years of 1430 and 1860, that show a cooling in this area. These vegetation assemblage changes can also found in Minnesota and New York (Grove, 2004)
Blue Glacier is a glacier in the Olympic Mountains (Glaciology Group, 2005). For more pictures and information, visit Blue Glacier.
The Olympic Glaciers of South Cascade and Thunder Creek tend to be extremely sensitive to climate change, which allows scientists to understand historic glaciers of this area by using mass balancing (Grove, 1988). H.H. Lamb found that in the 1600s there was a lack of monsoons, and 1630 and 1631 there was an extremely low amount of monsoons. The belief for the lowered amount of monsoons is based on a prolonged El Niño event during this time (Grove, 2004).
The above picture is of the South Cascade Glacier in 2006, looking to the southeast (USGS, 2011).
C.D. Miller, started dating Dome Peak in Washington state in the 1960s by using ash layers (Grove, 1988). Between February and March of 1600, the Huanyaputina volcano erupted in Peru, releasing 19.2 cubic kilometers or more of sediment into the atmosphere, darkening the sun and moon from the South Pole to Greenland. There were several more eruptions from 1641-1643, 1666-1669, 1675, and 1698-1699 that created lower than average temperatures throughout the continents. The 1641 eruption was from Mount Parker in the Philippines, but the other eruptions are unknown, but ash deposits are found during these time periods (Fagan, 2000).
1816 has been called The Year Without a Summer. The year before, 1815, the Tambora volcano in Indonesia erupted, releasing tons of ash into the atmosphere. During this time crop growth slowed and southern New England still had frost in late May. Average July temperatures that year were 3°C below normal. This is the coldest July in American meteorological history (Grove, 2004).
G. Wallin was the scientist that identified a correlation between the magnetic intensity and climate change; meaning when there is high magnetic intensity, there are cooler climates. Wallin found that when the pattern of the Earth's magnetic fields is over North America, the pattern of atmospheric pressure is similar. This means that the magnetic field will positively influence the average troposphere pressure (Grove, 2004).
While there are few historical reports in the United States (Mann, 2002), it can be deduced that the climate changed through the reasons above: glacial deposition, changes in the pollen record, climate fluctuations, volcanology, and geomagnetism. Many scientists debate when the beginning and end of the Little Ice Age occurred. This cold period lasted approximately 300 years, but varied depending on location.