Milankovitch Cycles

Figure 41.

MILANKOVITCH CYCLES

A Serbian mathematician developed a theory that helps explain changes in global climate based on several astronomical factors. He was Milutin Milankovitch and he developed a long-term strategy for why we have had ice ages, etcetera. His cycles are only part of the cycles we currently live with and that determine our climates all the time.

What he looked at were three variations in our orbit that affect the sunlight we get in the amount of warmth the sun gives us in any given year. Out of these variations he created what are called the Milankovitch cycles that can change the incoming energy we received particularly in the middle latitudes. These three variations are related to the following:

1. The eccentricity of the earth's orbit.

2. The angle of the earth's tilt, called the obliquity.

3. The direction the earth's rotational axis is pointed, called precession.

Let's look at each one of these in turn.

You probably know that the earth rotates around the sun in an elliptical orbit. This orbit is not always the same ellipse. In fact, it changes slightly in a pattern that follows a one hundred thousand year cycle. The reason for the eccentricity of our orbit is that we occasionally come too close to Jupiter and Saturn, causing the earth to pull toward these planets and changing our elliptical orbit slightly.

The eccentricity of our orbit is why we do not have seasons of the same length. In the northern hemisphere summers are about 4.5 days longer than the winters and springs are three days longer than the autumns. When this eccentricity is less, the length of the seasons are more even. Overall, the difference in the length of the perihelion (where we are closest to the sun) and the aphelion (where we are furthest from the sun) is about 3.2 million miles. This is just 3.4 percent variation. Because we are closest to the sun on January 3, we get about 6.8 percent more sun rays than we do on July 4, when we are furthest from the sun.

Now, when we have our more elliptical orbits, we get almost 25 percent more of the sun's rays when we are close to the sun than we would when we are further from the sun. This would greatly affect the differences between summer and winter. Right now, the earth's orbit around the sun is quite circular so we would not see this pattern this is a cycle that spans nearly 100,000 years.

Next, we get to the changes in the tilt of the earth's axis relative to the orbital plane. The earth's axis is tilted, giving us our seasons. It turns out that this tilt ranges from 22.1 to 24.5 degrees. While this is slight, it can affect seasonal variations in our climate. The cycle created by this tilt change lasts forty-one thousand years. The greater the tilt of the axis, the more extreme the seasons will be.

Larger tilts mean that the glaciers will melt and retreat decreasing the chance of having an ice age. It makes sense then that smaller tilts would favor the development of an ice

age. The further away you go from the equator, the greater the effect of the tilt will be. Currently the earth's axis is 23.4 degrees. It is slowly decreasing, so that it will reach its minimum in about 9800 years. Theoretically, it would mean we would have warmer winters and cooler summers, favoring ice formation and cooling at the poles. Remember also that ice reflects the sun, so when we get more ice at the poles, the cooling effect is greater.

The final phenomenon affecting our climate in a cyclical fashion is called the axial precession. You can also call this a wobble of the earth's tilt. This cycle is approximately 28,000 years long. The wobbling is partly due to the tidal forces between the sun and the moon. These forces collectively make the earth bulge near the equator. It affects this phenomenon.

Axial precession tends to make the seasons more extreme in one of our hemispheres and less extreme in the other hemisphere. In our current time, our perihelion lies when we have winter in the northern hemisphere. What this means then is that the southern hemisphere will have more extremes and will have hotter summers, while the northern hemisphere does not have such extremes. This will change in about 13,000 years.

This wobbling our seasons so that they begin earlier over the course of time. Right now the two North stars are known as Polaris and Polaris australis. If you extended a line from the north pole out to the universe, it would run into the stars. Surprisingly, these were not are North stars a few thousand years ago.

Remember the effects of Jupiter and Saturn? These planets also affect the wobble of the earth on its axis. It makes the wobble slightly less predictable. This precession is called apsidal precession. The cycle of this is 112,000 years. When you combine the two types of precession, you get recessional cycles that are approximately 23,000 years, however, this is just an average.

When you put each of these cycles together and other cycles, you can see that there can be long fluctuations in our climate that have nothing to do with modern day activities. Because these are cyclical, you can factor them in to past climate events and possible future climate events. Milankovitch was able to predict that ice ages occur every 41,000 years. This was only partially true and valid between one and 3 million years ago.