It is clear from the fossil and geological record that the earth has periodically gone through times of much colder temperatures than today---and much hotter. Abrupt warming occurs at regular intervals, with more moderate cooling over a longer period of time to follow. The time scale of the most prominent of these events has varied from 41,000 to 100,000 years, and there are discernible cycles of other periodicities.
What causes an ice age?
The earth's atmosphere is in a precarious equilibrium, as radiation heats the equator and cools the poles. The oceans and atmosphere transfer heat poleward to keep the temperature in balance. The exact temperature of this balance can change with the process that leads to heating---the absorption of solar radiation. And the process that cools---the earth's radiation to space. These radiative regimes operate on different wavelengths. The incoming radiation is primarily in the visible band of the spectrum, while the outgoing radiation is in the microwave region. The whole system is sensitive to very minor changes in the composition of the earth's surface, it's cloud cover, and the composition of the gases in the atmosphere, as well as the variation of radiation from one place to another over the entire globe.
The many variables that affect the radiation balance
The earth's movement in the solar system is not set in stone. It is obvious, for example, that the position of the earth with respect to all the other objects in the solar system changes constantly, and very likely never repeats. The gravitational attraction of just three bodies in motion defies analytical solution, so it is not surprising that the motion of the earth cannot be predicted precisely. In fact, there is evidence that the whole system is unstable, and chaos theory indicates that the earth may some day experience wild changes in its orbit, or even be entirely ejected from the solar system. Stay calm! It won't happen soon.
The problem can be narrowed somewhat if we just consider the major interactions and keep the time scale under a billion years. The tilt of the earth's axis, now 23 and a half degrees, varies a degree on either side over 41,000 years. Similarly, the eccentricity of the orbit (the out-of-roundness) and the season at which perihelion (closest approach to the sun) occurs vary over long time scales. In addition, the earth feels the gravitational pull of Jupiter and Saturn, as well as the moon and sun.
All this tugging by celestial objects leads to variations in the distribution of radiation on a wide variety of time scales. From the temperature record (or a proxy thereof from ice cores and other measurements), the dominant time scales are 41,000 and 100,000 years. Other frequencies show up both shorter and longer, with lower amplitudes.
The very regular pattern of temperature is somewhat surprising. The current belief is that when some parameter changes sufficiently to begin the process, feedback mechanisms kick in, at first reinforcing and ultimately dampening, that cause the cycle to be rather pronounced and precise. Until a million years ago, the primary period of ice ages was 41,000 years, the period of the changing tilt of the earth. In the last million years, the predominant cycle has switched to 100,000 years, the time of the eccentricity cycle. It is not obvious why this change occurred.
How does historical climate change relate to the current global warming?
The earth is currently in an interglacial period. Experts disagree on how long it might last, but one thing is clear: the current intrusion of homo sapiens into atmospheric processes is of a scale never seen before. Any comparison of the climatic change initiated by the massive man-made injection of atmospheric pollutants with the conditions leading to climate change of any historical time scale is clearly misguided.
Sources
- http://www.ncdc.noaa.gov/paleo/milankovitch.html
- http://earthobservatory.nasa.gov/Features/Milankovitch/milankovitch_2.php
Jon Plotkin - The author was a math major at Cornell and has a master's degree in meteorology from MIT.
