Milankovitch Cycles – Eccentricity


In the post on Milutin Milankovitch I said I would be discussing the Milankovitch cycles and their impact on Earth’s climate. There are three main cycles in Earth’s relationship to the Sun that have been shown to have an effect on the recurring cycles of the Ice Ages. Two of them involve Earth’s axis, and one its orbit around the Sun. In this post, we will look at the eccentricity of Earth’s orbit.

Here’s a graphical view of the cycles:

Credit Incredio – CC-BY-SA. Tap image for large original.

The first thing to realize about planetary orbits, as demonstrated by Johannes Kepler, is that they are not circular. As a first approximation, all orbits are elliptical. They orbit the central body on an oval path, so sometimes the orbiting body is closer, while sometimes it is farther away. In other words, for part of the year Earth is closer to the Sun than it is on the other half of the orbit. Presently, we are at our closest during the part of the orbit where it is winter in the northern hemisphere. It seems quite nice that things would be set up that way, doesn’t it? The part of our planet that has more land and more people is closer to its heat source during the coldest part of the year.

How does this affect our climate, and what is orbital eccentricity anyway? As there is more land in the north and more water in the south, the current regime keeps us a little warmer. Incoming sunlight is reflected more by snow-covered land than by open water. It has a higher albedo. If the great northern continents had more snow cover, as they would in longer, colder winters, they would reflect more sunlight and cool even more. One ingredient for a possible ice age. One component in our current lack of glaciation.

Orbital eccentricity, the Milankovitch cycle in this post, is the name for how elliptical an orbit is. A nearly circular orbit is less eccentric, and a more elliptical one is more eccentric.

The astounding fact is that Earth’s orbit oscillates between more and less eccentric in a 100,000 year cycle. That variability is one of the astronomical factors that Milankovitch calculated in his quest to see if such things could contribute to the ice ages discovered by geologists. Currently the eccentricity is near its minimum — our orbit is nearly circular — with a difference of only about three percent between the smallest and greatest distance from the Sun in a year. This tends to keep us a little warmer. However, the eccentricity is increasing, which should lead to a cooling trend. When the orbit is more elliptical, with the Sun at one focus of the ellipse, the Earth spends more time farther from the Sun, out on the long end of the ellipse. This leads to a gradual cooling trend, contributing to the possibility of accumulating snow and ice. On average during this latest Ice Age, comprising the last few million years, the approximately 100,000 year cycles are roughly 80% cold and glacial, and 20% warm and interglacial.

The 100,000 year cycle in Earth’s orbital eccentricity is the dominant cycle contributing to cooling and warming. The other, shorter cycles make their contribution when they ‘sync’ up with the big one. We’ll talk about those cycles in future posts, beginning with axial obliquity.

rjb

About arjaybe

Jim has fought forest fires and controlled traffic in the air and on the sea. Now he writes stories.
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7 Responses to Milankovitch Cycles – Eccentricity

  1. Richard says:

    So, the Earth warms and cools, relatively, over an approximately 100000 year cycle.
    And despite the fact that humans, like birds, consistently defecate in their nest, will
    not have any great effect on the life of the planet? Interesting.

    Do we humans consider ourselves the most important happening on the planet?
    Seems that way. And since we tend to feel guilty about our actions, we feel that
    humans are responsible for what we call global warming? Perhaps our point of view
    is distorted? Perhaps there is another way of looking at the world?

    There is so much that we explain by observing inferences from the world around us.
    Reminds me of the story of blind men describing and elephant by touch and
    limited point of view.

    Thanks for sharing the Green Comet.

    • arjaybe says:

      Stay tuned for more information. There will be two or three more posts on the Milankovitch cycles and you will see that it’s not as simple as it looks. Obviously, in the vastness of geological and astronomical time, our current warming trend is not alone. It might be uniquely abrupt and rapid, but similar things have happened before. In the Ice Age of the last three million years alone, glaciers have advanced and retreated more than twenty times, with only a five degree difference in global average temperature. Right now we should be cooling off as we head into another big freeze, if the 100,000 year cycle continues. But we’re warming instead. This should tell us that life on Earth can play a role in the climate. Plants can take up carbon dioxide, decreasing the greenhouse effect. Termites can release huge quantities of methane, a greenhouse gas even more effective than carbon dioxide. Humans can release carbon that was buried by plants hundreds of millions of years ago, bringing back the greenhouse of that time. Sure, things do seem to balance out eventually, if you’re willing to wait tens of millions of years, but it can be a big mess in the meantime. Humans uniquely have the ability to see what they’re doing and change it.

      • emmylgant says:

        I love the perspective and the added information in this séries of posts. Can’t wait for the next one. The balance in your comments is of much value also. Merci.

  2. Laird Smith says:

    Thanks for breaking up this most interesting and well explained subject into small bites. I look forward to the next segment.

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