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Global Warming
Editor: Steven I. Dutch, University of Wisconsin
December 2009 · 3 volumes · 1,212 pages · 8"x10"

Includes Online Database with Print Purchase


ISBN: 978-1-58765-563-0
Print List Price: $395


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Hunters in the Snow (1565)

Encyclopedia of Global Warming
Little Ice Age

Categories: Climatic events and epochs; cryology and glaciology

A brief cold period may have occurred during the seventeenth and eighteenth centuries. Some researchers think that the lower temperatures resulted from a reduced solar energy output related to the Maunder minimum in sunspot activity. The possibility that climate variations may be induced by solar activity complicates the conventional theory that human activity is causing global warming.

Key Concepts
luminosity of the Sun: The total energy output of the Sun every second,
    measured in watts
Maunder minimum: a period from about 1645 to about 1715, when very
    few sunspots were observed
sunspot cycle: also known as the solar activity cycle, an eleven-year
    cycle in the number of sunspots and the amount of other solar
    magnetic activity
sunspot maximum: the time during the eleven-year sunspot cycle when
    the number of sunspots is at its maximum
sunspot minimum: the time during the eleven-year sunspot cycle when
    the number of sunspots is at its minimum
sunspots: dark spots on the surface of the Sun caused by solar
    magnetic activity

Background
Indirect and anecdotal evidence indicates that it was colder than normal during the seventeenth century in Europe, and possibly worldwide as well. Without accurate weather records for this time period, the exact dates of the Little Ice Age are not known. The coldest period was the seventeenth century, but date estimates range from as early as about 1350 or 1400 to as late as 1850. The coldest portion of the Little Ice Age occurred during the time of the Maunder minimum, a period of virtually no sunspot activity. If the Sun's luminosity is slightly lower during periods of reduced sunspot activity, the Little Ice Age may have been caused by a temporary decrease in the Sun's luminosity.

Little Ice Age
The Little Ice Age was not an ice age. During the various ice ages, which occurred many millennia ago, glaciers invaded temperate midlatitude regions. The evidence for these ice ages is geologic, because they occurred before recorded history. The Little Ice Age occurred only a few centuries ago. It was colder that normal, but not nearly as cold as the ice ages. Historians know about the Little Ice Age from various anecdotal documents and indirect proxies. There are, however, no accurate weather records for this period, because the instrumentation needed to produce such records had for the most part not yet been invented. Therefore, scientific knowledge of the extent and severity of the Little Ice Age is not precise.

The exact dates of the start and end of the Little Ice Age are somewhat controversial. Authors generally agree that the Little Ice Age encompassed the seventeenth century, but there is some disagreement as to both how long afterward it lasted and how much sooner it started. This disagreement can be understood by examining reconstructed temperatures for the past millennium. The seventeenth century, which was about 0.5º Celsius cooler than the 1961-1990 average temperature, was both the longest-lasting cool period and the coldest period of the past one thousand years. Thus, nearly all researchers agree that this century was part of the Little Ice Age.

The entire period from about 1300 to the late nineteenth century was cooler than normal for the millennium. However, most of this time was not as cold as the seventeenth century, and there were some relatively warm periods during these centuries. The first half of the nineteenth century was colder than normal, but the eighteenth century was about as warm as the average for the millennium. Thus, the Little Ice Age may have lasted as late as 1850, but it may have ended in the early 1700's.

The period from 1000 to shortly after 1200, the Medieval Warm Period, was nearly as warm as the end of the twentieth century. Average temperatures then dropped fairly quickly during the late thirteenth century, making it possible to date the beginning of the Little Ice Age as early as about 1350 to 1400. The middle of the fourteenth century and the middle of the fifteenth century were nearly as cold as was the seventeenth century, but there were warmer periods around the turn of the fifteen century and during the first portion of the sixteenth century. Thus, the Little Ice Age may not have started until the seventeenth century.

The uncertainty of dating the beginning and end of the Little Ice Age results from the facts that the cool climate from approximately 1350 to 1850 was interspersed with relatively warm periods and that the longest-lasting and coolest period was the seventeenth century. The exact time period of the Little Ice Age is therefore fairly loosely defined.

Evidence for the Little Ice Age
Thermometers were not invented until the end of the sixteenth century, and widespread, systematic use of accurate thermometers did not occur until much later. There are therefore no accurate weather records to verify the Little Ice Age. Climate researchers must use other lines of evidence, including both various proxies and anecdotal evidence.

The most common proxy studies for climate involve tree rings. The thickness and density of the rings vary with various climatic conditions, including temperature and rainfall. In polar regions, studies of various properties of ice cores provide climate information. The properties include the rate at which ice accumulates, layers that have melted, and isotope ratios. Growth thicknesses and other properties of corals can also provide climate information.

For all of these proxy studies, climate researchers statistically analyze the relationship between the proxy and climate conditions during recent periods for which accurate weather records exist. The researchers then extrapolate the climate conditions back to the dates before accurate weather records were kept. The further back researchers extrapolate, the less accurate the proxy is in reconstructing climate conditions. Hence, climate estimates for the first half of the millennium are less accurate than are more recent estimates. Proxy studies are further complicated by the fact that multiple variables can affect the proxy. For example, tree rings are affected by both temperature and rainfall conditions.

In addition to proxy studies, there is anecdotal evidence for the Little Ice Age. Examples of this type of evidence include such things as diary entries and paintings. Diary entries might include reports of unusual freezings of various bodies of water, extreme snowfalls, and so forth. Paintings in eras when artists strove for realism can also depict frozen landscapes and bodies of water. If the paintings made during a particular time period show a large number of frozen landscapes of locations that seldom freeze now, researchers can conclude that the time period was cooler than normal. These lines of evidence are not scientific, but many reports of a particular time period being colder than normal strongly suggest that it actually was colder, even in the absence of scientifically reliable weather records. These lines of evidence apply primarily to Europe, so the Little Ice Age could have been either a strictly European phenomenon or a global phenomenon.

The Maunder Minimum and the Little Ice Age
There is fairly strong evidence that variations in the Sun's luminosity related to sunspot activity caused the Little Ice Age. Sunspots are dark regions on the surface of the Sun caused by solar magnetic activity. Solar magnetic activity also causes bright areas, or faculae, on the Sun's surface. The Sun undergoes an eleven-year cycle regulating the amount of sunspots, faculae, and related solar magnetic activity it experiences. Satellite measurements over the most recent solar cycles show that the Sun's luminosity is a very small amount higher during sunspot maximum that during sunspot minimum. The net effect of the bright areas on the Sun is slightly larger than the net effect of the dark areas, so the Sun is brighter during sunspot maximum.

There are also less well established, longer cycles in solar activity. Notably, the Maunder minimum was a period from about 1645 (possibly as early as 1620) to 1715 when there were very few sunspots. This period corresponds to the coldest portion of the Little Ice Age. If the observation that the Sun emits less energy during periods of minimal sunspot activity holds, then the Sun's lower luminosity during the Maunder minimum may have caused the coldest portions of the Little Ice Age.

Closer comparison of the sunspot activity and global temperatures over the past thousand years supports this hypothesis. The warm period from 1000 to 1200 corresponds to the Medieval Grand Maximum in sunspot activity: During sunspot maxima in this period, there were many more sunspots than is usual during such maxima. There were also extended periods of very few sunspots similar to the Maunder minimum, including the Spörer, Wolf, and Dalton minima. Like the Maunder minimum, these minima correspond to the cooler periods of the extended Little Ice Age. It is not proven that variations in the Sun's luminosity caused the Little Ice Age, but it seems to be the most likely explanation.

Context
Global warming, especially since the latter half of the twentieth century, has become a serious worldwide concern. Most climate researchers attribute this global warming to anthropogenic causes, particularly increased emissions of carbon dioxide and other greenhouse gasses. The extremely high surface temperatures on Venus clearly demonstrate that carbon dioxide can warm a planet.

If, however, the Little Ice Age resulted from solar luminosity variations related to long-term solar activity cycles, then there is the possibility that similar solar variations are contributing to current global warming. Some, but not all, late twentieth century sunspot maxima are higher than normal, suggesting the possibility that Earth is entering another sunspot grand maximum similar to the Medieval Grand Maximum. If this is the case, then global warming may have solar variations as well as increased greenhouse gasses as a component cause.

Paul A. Heckert

Further Reading
Eddy, J. A. "The Maunder Minimum." Science 192 (1976): 1189-1192. In this seminal article, Eddy demonstrates that the Maunder Minimum is real and argues both that the Sun's energy output varies with long-term cycles in the amount of sunspot activity and that this variation caused the Little Ice Age.

Foukal, P., C. Fröhlich, H. Spruit, and T. M. L. Wigley. "Variations in Solar Luminosity and Their Effect on Earth's Climate." Nature 443 (2006): 161-166. Reviews the state of knowledge of solar variability and its role in climate changes such as the Little Ice Age.

Golub, Leon, and Jay M. Pasachoff. Nearest Star: The Surprising Science of Our Sun. Cambridge, Mass.: Harvard University Press, 2001. This well-written book gives a detailed summary of scientists' knowledge of the Sun. Includes sections on climate variations and their relation to solar variability.

Hoyt, Douglas V., and Kenneth H. Schatten. The Role of the Sun in Climate Change. New York: Oxford University Press, 1997. Well-written, extensively documented, comprehensive account of solar variability and possible associated climate changes, including the Little Ice Age. The line of evidence for the reality of the Little Ice Age is well presented. Addresses both sides of controversial issues and attempts realistically to appraise the role that solar variability plays in climate changes.

Jones, P. D., T. J. Osborn, and K. R. Briffa. "The Evolution of Climate Over the Last Millennium." Science 292 (2001): 662-667. This review paper nicely summarizes the state of knowledge of Earth's temperature and climate changes from the years 1000 to 2000. Graphs show a temperature minimum during the seventeenth century.

Maunder, E. Walter. "A Prolonged Sunspot Minimum." Knowledge 17 (1894): 173-176.

_______. "The Prolonged Sunspot Minimum, 1645-1715." Journal of the British Astronomical Society 32 (1922): 140. These two papers are Maunder's original publications arguing that there was a prolonged minimum in sunspot activity in the late seventeenth century. Maunder offers evidence that the minimum is real and not an effect of few people observing the Sun. Hints at the possibility that this dearth of sunspot activity may have had some effect on Earth.

Soon, Willie Wei-Hock, and Steven H. Yaskell. The Maunder Minimum and the Variable Sun-Earth Connection. River Edge, N.J.: World Scientific. 2003. This well-documented book explores the Maunder Minimum and other long-term solar variability as well as the role this variability plays in Earth's climate changes including the Little Ice Age.

See Also: Facula; Medieval Warm Period; Solar (eleven-year) cycle; Sun; Sunspots.


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