The Ice Ages, Part 2

Posted on November 13, 2013


Father Theo goes to climate school 6.

Continued from parts 1, 2, 3, 4 and particularly 5.

According to what scientists now believe, the Ice Ages, which have come and gone for the last million years or so, were caused by the coupling of Milankovitch cycles with certain natural feedback loops.  The Milankovitch cycles couldn’t have done it by themselves, without the feedbacks, because they don’t significantly alter the amount of energy entering the climate system.  That’s necessary to create an ice age.

The last ice age was 5o C cooler than the world has been since.  It takes a huge amount of energy—accumulating or leaking out—to change the temperature of a planet by 5 degrees.  Science accounts for that energy by invoking climate feedbacks, primarily but not exclusively involving albedo.

The three Milankovitch cycles, to describe them again briefly, are cycles in the eccentricity of Earth’s orbit, in the tilt of Earth’s axis, and in the shift of summer and winter in Earth’s hemispheres as related to Earth’s orbit.

The 100,000 orbital cycle is reflected in the Ice Ages, as this graph makes clear.

The 100,000 orbital cycle is reflected in the Ice Ages, as this graph makes clear.

The Earth’s orbit goes from being nearly circular to its most eccentric and back to nearly circular again every 100,000 years.  When the eccentricity of its orbit is at its most extreme, there is a difference of energy input of about ½ a Watt per metre squared.  That is the difference between Earth’s point of closest approach to the Sun, its perigee, and the point where it’s furthest away, its apogee.

Half a Watt per metre squared is an important difference, but it doesn’t really represent warming or cooling of Earth overall.  The warm and the cool parts of the orbit essentially cancel each other out and don’t significantly change the total amount of energy reaching Earth each year.  It doesn’t directly cause Earth’s stock of energy to grow or shrink.

What about the second cycle, orbital tilt?  An axis of maximum tilt will make the seasons more extreme, but again the warm and cool seasons cancel each other out.  An axis of minimum tilt means that seasonal differences are not as marked, but again the overall solar input remains the same.  Nothing about orbital tilt will by itself alter Earth’s energy budget.

The third Milankovitch cycle, precession, determines whether northern hemisphere or southern hemisphere summers, etc., occur when the Earth is closest to the Sun.  Again, which way the Earth is tilted on which part of its orbit has no effect on overall energy input.  The cycle is important to climate because of geography.  The Earth is different in the northern and southern hemispheres.  The majority of the land masses are clustered in the northern part of the globe.  The South Pole is a land mass surrounded by water.  The North Pole is an ocean hemmed about by land.  Thus the changes in distribution of energy input caused by Milankovitch cycles are going to have a different effect in the northern hemisphere than in the south.

How does geography effect the Ice Ages?  The Ice Ages are characterized by the growth of continental ice sheets.  They are both a result and a cause.  Now you can’t grow an ice sheet in Antarctica because it’s already full.  The continent is already 100% covered in ice.  It’s also really tough, being an extreme polar continent, to melt an ice sheet there even in the hottest summers.  The relatively mild slow effects of the Milankovitch cycles would have a difficult time, by themselves, triggering any changes there.  Thus, geography determines that most of the effects surrounding the coming and going of the Ice Ages involve the northern hemisphere.

So how are Milankovitch cycles involved in creating the Ice Ages?  Like the song says.  “It’s not what you do, it’s the way how you do it.”

Close orbit?  The world is warmer at perigee.  If the warmth occurs in summer in the northern hemisphere, you get really hot summers there, and extra chilly winters at its apogee.  If you want to chill the world for an ice age, and cover Canada with an ice sheet, for instance, an extra chilly winter seems the way to go.  Except that when a chilly winter is followed by an extra hot summer, then any effect of the chill is melted away by a blasting summer heat.

Nope.  Can’t grow a continental ice sheet that way.

What about the other way around?  In the northern hemisphere, warmish winters and coolish summers happen when Earth swings closest to the Sun during northern hemisphere winters.  Swinging closer to the Sun in December is not enough to cancel winter, or to stop snow from falling, because it’s not that powerful an effect.  It’s only a quarter Watt per metre squared (0.25 W/m2) different from no effect at all.  (For comparison, the Earth’s energy input at its surface is currently 494 W/m2.)  But even a quarter of a Watt can make a difference when multiplied by the surface of the Earth and extended over centuries.

In the short term, at the very beginning of the process, before the feedback effects kick in, the increase in energy may cause an increase of precipitation during that season, since more water evaporates in warmer temperatures, so there’s actually a chance for a net increase in snow in slightly warmer winters.  And when summer comes, a cool summer may allow the accumulated snow in the more northerly regions to stay or at least to remain longer on the ground than a warmer summer would.

Any snow lingering beyond its usual melt date has an immediate effect on Earth’s energy balance because of albedo.  Unmelted snow reflects light that otherwise would have been absorbed by the dark soil which the snow continues to cover.  Energy escapes into space which under previous conditions would have remained in Earth’s climate system.  In an accounting of Earth’s energy balance, the stock of energy in Earth’s system has gone down slightly.

Multiply that effect by centuries.  A continuing leakage of energy reduces the amount of energy in the system year by year, and gradually the Earth cools.  A cooling Earth in turn allows the winter snow to remain on the ground longer every year, and as time goes by, with more and more of the snow failing to melt when the summer comes, ice sheets begin to form, to thicken, to expand southward, increasing the albedo effects as they grow and accelerating the cooling as the centuries go by.

Bingo.  After a few centuries of that, we have a full blown ice age and Canada, our home and frozen land, could win a celebrity lookalike contest with Antarctica.  It is now covered with an ice sheet from sea to chilling sea.

Then, the turning of the cycles brings us back to the era of hot summers and cold winters again and the process goes into reverse.  The ice sheets melt at their edges with every hot summer.  The revealed soil at the edges of the ice sheet begin absorbing energy that the ice sheets used to reflect into space, changing the Earth’s energy balance again and gradually warming the climate.  With warming climate, some carbon dioxide which dissolved in the oceans in colder times begins to outgas like warm soda pop, increasing the amount of carbon dioxide in the atmosphere and recruiting greenhouse effects to aid with the warmup.

Goodbye, ice age.

This explanation of the coming and going of the Ice Ages over the last million years has the virtue of reconciling predictable astronomical cycles with Ice Age cycles, which they closely match in time.  The period of 100,000 years of the orbital eccentricity cycle coincides nicely with Earth’s largest climate cycles, which is part of the reason why Milutin Milanković’s ice age theories are so widely accepted by scientists.  The cycle of precession, at 26,000 years, also coincides nicely with the growth and waning of individual Ice Ages.

Not Like Today

The Ice Ages give us an example of how small scale climate forcings can have accumulative affects over time with the assistance of climate feedbacks.  But it’s important to note that what happened to create the Ice Ages is significantly different from what is happening today.  The time scale is greatly different.  The Earth entered and exited the Ice Ages over periods of thousands of years.  The present bout of climate change is accumulating changes on the scale of decades rather than millennia.  That’s bad.  The faster the changes, the more difficult it is to adapt.  Like trying to return a tennis volley that has been shot out of a gun.

The forcings that led to the Ice Ages were much weaker than the forcings represented by the carbon rapidly accumulating in our atmosphere.  As climate scientist James Hansen pointed out in Storms of My Grandchildren, the climate forcings leading to the Ice Ages could have be counteracted by the carbon emitted by a single large industrial plant.

And we’ve got more than one of those.

That’s what the “hockey stick” graph is telling us.  There is nothing normal about the current bout of climate change.   Climate may have changed before, but not at all like it’s changing now.

Somebody poured gasoline on that there fire.

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