The Ice Ages, Part 1: Milankovitch Cycles

Posted on October 23, 2013

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Father Theo goes to climate school 5.
Continued from parts 1, 2, 3 and 4.

Your house is on fire.  Down the block, the police have stopped a gentleman with matches in his pocket and a can of petrol suspended from his arm.  “Houses have burned down before,” he says.

Of course they have.  That’s reasonable.

Everything’s exactly the same as the times before except for the gasoline and the matches part.

And when confronted with the evidence of climate change (petrofuel fumes rising from their carbon footprints) the climate change denier crowd likes to say, “Climate has changed before.”

What they really mean to say, I suspect, is “Remember the Ice Ages.”

Yes, let’s.  Because we really need to see (as our house burns down) if everything’s indeed the same as every time before.  Or if the matches and the can of petrol have something to do with it.

Understanding the Ice Ages is pertinent to our study of climate change.

Milankovitch Cycles

milankovicThe current theory of  what caused the Ice Ages centres on the interlocking effects of the Milankovitch Cycles.  Serbian astronomer and geophysicist Milutin Milanković, finding himself with time on his hands during his internment in the First World War, came up the idea that these three astronomical cycles, each affecting Earth’s relationship to the Sun, had a powerful if slow-moving effect on the Earth’s climate.

What are the Milankovitch cycles?  They relate to 1) eccentricity, or how elliptical Earth’s orbit around the Sun is; 2) obliquity, or how tilted the Earth’s axis is in relation to the plane of the orbit, and 3) precession, or what direction the Earth is tilted in relation to where it is in its orbit.

Let’s look at these cycles one by one.

milankovitch cyclesEccentricity

A perfect circle has no eccentricity.  Any departure from the round is known as eccentricity.  We don’t want wheels to be eccentric because they’ll wobble when they roll.  When an orbit is eccentric, and all orbits are to some extent, it will produce variations in the intensity of the light reaching the planet.  On Earth, this can and will effect climate.

The Earth’s orbit is unlikely ever to be perfectly round, and sometimes the orbit tends to be a lot less perfect than others.  This is because our planet shares the solar system with other planets, and most pertinently the gas giant planets Jupiter and Saturn, which are big enough, especially acting in concert, to have significant gravitational effects despite being a long ways away from us.

All the planets orbit at different speeds and a Jupiter year and a Saturn year are much, much longer than an Earth year.  Sometimes the Jupiter year and the Saturn year will align themselves with each other for a while.  When that happens, each time Earth’s quicker orbit brings it near that part of solar system where Jupiter and Saturn are waltzing along, Earth’s orbit gets jounced slightly outward by their combined gravity.  Cycle by cycle, until Jupiter and Saturn’s alignment ceases, Earth’s orbit spirals out to maximum eccentricity.  What that means in practice is that for part of its orbit the Earth is a lot closer to the Sun than on other parts of its orbit.

This disturbance is not stable.  Eventually the orbit drifts back to nearly circular again.

This particular Milankovitch cycle is 100,000 years long.  It takes 100,000 years for Earth’s orbit to go from nearly circular to maximum eccentricity and back to nearly circular again.

Obliquity

Obliquity refers to the tilt of Earth’s axis in relation to the plane of the orbit.  Think of the plane of the orbit as being the floor and the Earth as being a top spinning on that floor.  Sometimes the top is standing more upright than others.  This is what we’re talking about with tilt.  Right now the tilt of the Earth’s axis is 23.5o relative to the plane of its orbit, and overall the tilt varies from 22.1o to 24.5o over a 41,000 year cycle.  Obliquity affects climate because the greater the tilt of the Earth’s axis, the greater the difference between seasons.  Less tilt means less extreme seasons.

Precession

The third Milankovitch cycle is probably the most difficult to visualize without a demonstration.  Fortunately, you know something about this idea already, although you may not have thought about it much since you learned it in school.

The tilt of the Earth’s axis is consistent in relation to its orbit.  That is, if we stood back and looked at Earth’s orbit from the outside, we’d see the northern axis tilting in towards the Sun on June 21st, and then, on December 21st, on the other side of its orbit, we’d see it tilting out from the Sun.  This phenomenon is accompanied by northern hemisphere summer and then by northern hemisphere winter.  When a particular axis is tilted towards the Sun, summer is happening in that hemisphere, and vice versa for winter.

The direction of the axis in relation to the orbit goes through a cycle of precession every 26,000 years owing to the gravitational effects of the Sun and Moon.  The direction of the axis matters because Earth’s northern hemisphere is different from its southern hemisphere, as will be explained.

Putting it all together (next time.)

You can’t expect a cycle of 100,000 years, another of 41,000 years, another of 26,000 years to produce the same results all the time.  But when certain conditions occur, Ice Ages happen, and when those conditions change, the Ice Ages go away.  We’ll examine those conditions in Part Two.

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Credit again to Coursera, UBC and the course Climate Literacy: Navigating Climate Change Conversations, taught by Sarah Burch and Sara Harris.  Any errors are my own. 

Posted in: Climate School