They call it the Little Ice Age because it wasn't as big as major cooling periods on the earth, but it was cold enough to get people's attention. The period from roughly 1550 to 1850 was characterized by temperatures at least a couple of degrees Fahrenheit lower than today. Compared to major ice ages, which were up to 20 degrees colder than today, it was small potatoes. But it was enough to cause substantial dislocations in populated areas.
Ice Age Causes
Major ice ages are correlated with astronomical variables such as the eccentricity of the earth's orbit (how much it deviates from a circle), and changes in the tilt of the earth relative to the plane of its orbit. These changes take place on time scales of tens of thousands of years. The Little Ice Age is believed to be related to the sun's radiative output, though the mechanism is not clearly understood, and there were other possible contributing factors.
The Sunspot Cycle
As the number of sunspots varies over a 22 year cycle, the sun's radiative output has been measured to vary by about one-tenth of one percent. This actually comprises two 11-year cycles of reversed magnetic polarity, with minimum solar radiation correlated with sunspot minima. The 22 year sunspot cycle has been measured for a long enough time for scientists to be certain of its existence and its relation to magnetic changes in the sun. The mechanism, however, is still obscure. Longer term cycles of temperature variation of hundreds or thousands of years have been tentatively identified. There is, however, no historic record of sunspots with which to compare them.
Sunspots and the Little Ice Age
The Little Ice Age was a time of unusually low sunspot activity, with several extreme minima embedded, culminating in the Maunder Minimum, a 40 year period during which almost no sunspots were observed. After the invention of the telescope in 1608, there is a rather complete record of sunspots, and these data are not in doubt.
Decreased solar output was likely the primary cause of The Little Ice Age, but there are several candidates as contributing factors: the decline in population due to various plagues; increased volcanic activity; changes in ocean circulation.
Population Decrease
At the end of the Middle Ages, a number of plagues swept through Europe, cutting the population nearly in half. With a lack of people to work the land, deforested areas became re-vegetated. The result, theoretically, is that the new vegetation would soak up some of the carbon dioxide in the atmosphere, producing a reverse of greenhouse warming.
Volcanoes
The Little Ice Age was characterized by above normal volcanic activity, including some very massive volcanoes. The ash thrown into the atmosphere blocked sunlight from coming in.
Ocean Currents
The ocean's circulation and its relationship to the atmosphere are poorly understood at best. Factors such as the amount of fresh water coming from glacial melt and the forcing from surface winds are certainly important. It is clear that a minor shift of the Gulf Stream can significantly affect temperatures in North America and Europe.
It is highly likely that a variation in the sun's energy output was the major cause of the Little Ice Age. It is also a virtual certainty that the sun's energy output does not vary enough to cause major climate changes. It would be a mistake to downplay the importance of the current cycle of warming due to greenhouse gas increases just because climates have fluctuated in the past. Maybe a big mistake.
Sources
- Fire and Ice. Long Term Variations in the Earth's Climate
- The Maunder Minimum: Cold Temperatures and No Sunspots
- IPCC Third Assesment Report -- Climate Change 2001
- Evidence of Little Ice Age
Jon Plotkin - The author was a math major at Cornell and has a master's degree in meteorology from MIT.
