Historical Geology/Milankovitch cycles
In this article we shall discuss what Milankovitch cycles are, their effect on climate, and how we know that this effect exists.
The reader should remember from high school that the reason why the Earth has seasons is that its axis of rotation is at an angle to the plane of its orbit. When the northern hemisphere is tilted towards the sun, then it is summer in the northern hemisphere and winter in the southern hemisphere; when the southern hemisphere is tilted towards the sun, it's the other way round.
The magnitude of this effect depends on how tilted the Earth's axis is, and this angle varies between 22.1° and 24.5° in a 41,000 year cycle.
The reader should also recall that the Earth's orbit is not perfectly circular: it is an ellipse with the Sun at one focus, meaning that the Earth is closer to the Sun during some months of the year than others. The effect of this is less than you might suppose: the Earth is five million kilometers closer to the Sun in January than in July, but this doesn't stop the Northern Hemisphere from undergoing winter.
The magnitude of this effect depends on how far the Earth's orbit deviates from being circular, and a number of factors affecting this figure add up to a cycle of about 100,000 years.
Finally, there is the precession of the Earth's axis. At present, as we have seen, the North Pole is tilted away from the Sun at the Earth's point of closest approach to the Sun. However, this too varies, in this case in a 21,000 year cycle.
These, then, are the three Milankovitch cycles; as they are of different lengths, their interaction will produce rather a complex pattern as they go in and out of phase with one another. Together, they will affect both the total annual insolation (the amount of solar radiation that reaches the earth's surface) and also season variations in insolation.
Although the idea of Milankovitch cycles as a factor in the Earth's climate was initially greeted with some suspicion by climate scientists, it is now generally accepted that Milankovitch cycles account for about 60% of past variation in climate.
How do we know?
First of all, how do we know that Milankovitch cycles exist? A short answer is that the physics of the Solar System require that they should exist; a longer answer would require an introduction to celestial dynamics which would be excessive in length and out of place in what is supposed to be an introduction to historical geology.
From a geological perspective, we can look for the effects of the cycles on the sediments and proxies used in paleoclimatology. As Milankovitch cycles can't be the only thing affecting the climate, this is not so simple as demonstrating that the climate fluctuates perfectly in synchrony with the cycles; rather, statistical analysis is necessary to sort out the "signal" of the cycles from the "noise" produced by (for example) variations in volcanic activity. Such an approach confirms that the cycles have a real effect on the climate.
One interesting effect of the cycles is that in places where the nature of sedimentation is sensitive to the climate, we can see rhythmites with a period dictated by the lengths of Milankovitch cycles, as shown in the photograph to the right.
There remains one outstanding puzzle. In principle, the 100,000-year cycle should have less of an effect than the 41,000-year cycle. But for the last million years or so, the 100,000-year cycle has predominated; whereas prior to that the 41,000-year cycle was indeed more important, in line with theory. This is known as the 100,000 year problem, and serves as a useful reminder that our understanding of long-term climatic change is still imperfect.