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What is the Milankovitch Cycle?

July 28, 2010
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The inspiration for this article comes from an article called “Paleolimatologist studies sea levels in a desert” written by Seth Shulman for GRIST.

The article is interesting in that the climate scientist in question – Maureen Raymo, is studying Australia’s desert to better understand how fast and how high current sea levels will rise as today’s global warming trend melts the remaining ice sheets in Greenland and Antarctica.

In her research, Raymo tackles many aspects of paleoclimate and orbital forcings. In 2006, she hypothesized, in the journal Science, about why the 41,000 year Milankovitch cycle appears less pronounced over the past 500,000 years. “It was a question that I had literally been thinking about since graduate school some two decades earlier,” she says. No one knows the answer yet. But Raymo’s theory — that ice growth at one pole occurs simultaneously with ice decay at the other, thereby canceling out the signal of ice volume change in global climate records — has sparked a great deal of new research and debate. If she’s right, then earth’s climate is even more dynamic and complex, especially in the area of the Antarctic ice sheet.

While orbital forcings offer a fascinating window into the historical mechanisms of climate variation, Raymo emphasizes that they are overshadowed today by human-driven effects on the climate. “People sometimes ask me: ‘When will the next ice age be?'” she says. “The answer is that I’m pretty sure we have already prevented it. Like it or not, we are now the main drivers of the climate, even though so far we’ve been doing it completely by accident.” (View the full article here)

So the article speaks about orbital dynamics of Earth, which is simply lumped into what scientists call the Milankovitch Hypothesis. There are two simple ways that you can choose to understand this hypothesis. One is wikipedia, and the other… Youtube. I believe these two mediums provide the most accessible information there is today and why should climate science information be excluded? In any case, I’ve included both links for you to check out.

Milankovitch cycles – Wikipedia, the free encyclopedia


Still don’t get it? OK… let me try!

First, let me set the record straight. You DO know that Earth is tilted right? If you didn’t… YES, Earth is tilted slightly at about 23.5 degrees (angle). However, in certain years, Earth tilts more or less ranging from 22.1 to 24.5 degrees. The extent of this tilt has consequences on how much sunlight reaches Earth!

Earth's axial tilt

Currently the Earth is tilted at 23.44 degrees from its orbital plane, roughly half way between its extreme values. The tilt is in the decreasing phase of its cycle, and will reach its minimum value around the year 10,000 C.E.. This trend, by itself, would tend to make winters warmer and summers colder; however increases in greenhouse gases may overpower this effect.

Next, you DO know that the Earth orbits around the sun not in a circular fashion, but elliptical right?

Circular orbit, no eccentricity.

Orbit with 0.5 eccentricity.

The extent to which the Earth’s orbit is more OVAL-ish (or the deviation from being exactly circular) is called it’s eccentricity. The shape of the Earth’s orbit varies in time between being nearly circular (low eccentricity of 0.005) and being mildly elliptical (high eccentricity of 0.058) and has a mean eccentricity of 0.028. The present eccentricity is 0.017.

If the Earth were the only planet orbiting our Sun, the eccentricity of its orbit would not perceptibly vary even over a period of a million years. The Earth’s eccentricity varies primarily due to interactions with the gravitational fields of Jupiter and Saturn.

Now for the sciency bit…

The relative increase in solar irradiation at closest approach to the Sun (perihelion) compared to the irradiation at the furthest distance (aphelion) is slightly larger than 4 times the eccentricity. For the current orbital eccentricity this amounts to a variation in incoming solar radiation of about 6.8%, while the current difference between perihelion and aphelion is only 3.4% (5.1 million km). Perihelion presently occurs around January 3, while aphelion is around July 4. When the orbit is at its most elliptical, the amount of solar radiation at perihelion will be about 23% greater than at aphelion.

What this means is that the closer Earth is to the sun (because of the orbit shape), the warmer it gets.

OK so at this point you might ask… what has the Milankovitch Cycle got to do with climate change anyway?

Here’s the deal. Milankovitch studied changes in the orbital eccentricityobliquity, and precession of Earth’s movements and such changes in movement and orientation change the amount and location of solar radiation reaching the Earth.

This is known as solar forcing (an example of radiative forcing). Changes near the north polar area are considered important due to the large amount of land, which reacts to such changes more quickly than the oceans do. As such, changes in the Earth’s axial tilt/ obliquity (22.1-24.5 degrees), eccentricity (closeness to the sun based on it’s orbit) and precession (see gif image below) make a difference to how much sunlight gets to Earth!

Precession

Lastly, I must add that some had purported that the Milankovitch Theory/Hypothesis is incomplete. I’m unsure why… but here is a brief explanation:

The Milankovitch theory of climate change is not perfectly worked out; in particular, the largest observed response is at the 100,000-year timescale, but the forcing is apparently small at this scale, in regard to the ice ages. Various explanations for this discrepancy have been proposed, including frequency modulation or various feedbacks (from carbon dioxide, cosmic rays or from ice sheet dynamics).

In addition, the Milankovitch Cycle theory is by no means the only theory used to explain climate change or at least to supplement theories about Earth’s changing climate but it is useful to know. I hope you now better understand what the Milankovitch Cycle is!! If not, do check out these amazing websites for more information:

Milankovitch Cycle and Glaciation

Geography.about.com – Milankovitch Cycles

Milankovitch Cycles and the Age of the Earth

How the sun affects climate: Solar and the Milankovitch Cycles

The Encyclopedia of Earth – Milankovitch Cycles

Enjoy!! 🙂

Mel

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