This winter, the continental U.S. has experienced such long periods of cold weather that some have started to wonder whether scientists have somehow gotten the signs wrong, and instead of global warming we are experiencing a mini ice age. This, of course, is not really the case; Europe and the Middle East have actually witnessed a warmer than usual winter. Ice ages are those relatively long time intervals in which the entire Earth experiences significantly colder temperatures, with glaciers covering continental-size regions. Formally speaking, therefore, we are still in an ice age that started about 2.6 million years ago, since the large Antarctic and Greenland ice sheets continue to exist (even though decreasing in size due to recent climate change).
Geological and fossil-based evidence suggests that the Earth has experienced at least five major ice ages, while being virtually ice-free in the periods between them. Within each ice age, ice sheets can significantly advance or retreat on timescales of tens to hundreds of thousands of years.
Why do ice ages occur at all? Researchers have identified a number of factors that undoubtedly contribute, but a comprehensive explanation is still lacking. The main causes appear to be (in no particular order): Small periodic changes in the Earth's tilt and wobble in its orbit around the Sun (called "Milankovitch cycles" after Serbian academic Milutin Milankovitch) or in the Earth-Moon dynamics; small variations in the Sun's luminosity; changes in the abundance of greenhouse gases (such as carbon dioxide) in the Earth's atmosphere; continental drift (which results in changes in atmospheric streams and oceanic currents); and increased volcanic activity, which can pour enormous amounts of smoky ash and dust into the atmosphere. The Tambora volcano provided a spectacular example of the last effect in 1815. Its eruption reduced the global, average Earth temperature by about one degree Fahrenheit, causing massive food shortages, but also spectacular sunsets, as captured by painter J. M. W. Turner (Figure 1). By the way, the weather extremes experienced by the Earth would be dwarfed in comparison with those occurring on an Earth-like planet in a very eccentric orbit (a very elongated ellipse) around another star.
Figure 1. J. M. W. Turner's painting "Chichester Canal," depicting a colorful sunset, from the period following the eruption of the Tambora volcano. (Image in the public domain.)
The fact that climate is affected by so many factors makes predictions difficult. Nevertheless, there is no doubt that we are currently witnessing an overall warming, associated with the melting of vast quantities of ice. The collapse of the West Antarctic ice sheet, which appears to be underway, could lead to a rise in sea levels by fifteen feet or more (Figure 2 shows the surface at Dome C Station, Antarctica). As odd as this may sound, therefore, we could use a micro ice age at this point.
Figure 2. The snow surface at Dome C Station, Antarctica. (Image in the public domain.)