This is the second of the three main Milankovitch cycles that I said we’d cover in my original post on Milutin Milankovitch. The first cycle was Earth’s orbital eccentricity, or how elliptical the orbit is. We found that this changing eccentricity lays down a base cycle of about 100,000 years. In this post we’ll look at the first of two cycles involving Earth’s axis — obliquity, or axial tilt. It turns out that, while orbital eccentricity might be the dominant cycle, its actual effect on our climate is not the strongest.
Everyone knows from their early school years that the tilt of Earth’s axis, 23.5 degrees relative to the plane of our orbit around the Sun, is the cause of our seasons. But how would this affect the climate? Surely it would just continue to put us through the seasons wherever we are on the other cycles, and not contribute anything to the great climatic cycles. That might be the case were it not for the fact that the tilt is continuously changing, oscillating between 22.1 and 24.5 degrees. We are currently about midway, and progressing toward the low end with the least tilt. The entire obliquity cycle takes about 41,000 years.
The effect of axial tilt on long term climate is somewhat complex, but the final outcome is that decreasing tilt leads to a cooler climate. In more detail, a greater angle means that summers are hotter due to more intense insolation, while winters are cooler because the Sun’s rays are more oblique. This effect is strongest at higher latitudes. That’s important because it’s at the higher latitudes where snow and ice accumulation can lead to glaciation, and hotter summers can prevent that. When summers are cooler, snow cover can persist and build up from season to season. That’s especially true in the northern hemisphere where most of the continental land masses are, where snow can increase the albedo.
The amount of tilt to the axis can either add to the effect of the shape of the orbit, or subtract from it. When high tilt (warmer summers) combines with lower eccentricity (generally warmer,) we are at our warmest. When low tilt combines with high eccentricity, we are at our coolest. Tilt was maximum just under 11,000 years ago, when we firmly left the last great glaciation behind. It will reach minimum just under 10,000 years from now, when we should be well into the next glaciation. As we are still very close to the least eccentric, and therefore warmest, part of our orbital oscillation, the deepest part of the next ice age should be one or two obliquities in the future. Even so, these two cycles alone can’t account for Earth’s long term climate, or its ice ages.
We’ll look at the third main Milankovitch cycle, axial precession, in the next post.