Milankovitch Cycles and the Global Climate

               There are many opinions and debates about global warming.  I’m not really concerned with opinions, but fact is what science deals in.  Some say it’s a conspiracy, a scare tactic, or just a simple miscalculation.  Some are in denial, or just choose not to think about it.  Some are apathetic, assuming that their own small contribution couldn’t possibly make a difference.  Some say that these changes are just the natural cycles of the Earth, usually citing the Ice Age as their proof.  The truth is there are some uncertainties about how much we are affecting our planet’s climate, but there is absolutely no doubt that we are affecting it.  The Milankovitch Cycles are 3 very long cycles that the Earth goes through naturally.  There are 3 separate cycles that affect the Earth’s climate: Precession, Obliquity, and changes in eccentricity.

The first is called Precession, where the Earth’s axis of rotation shifts, creating a small circle in the sky with respect to the stars over 26,000 years.  For example, in about 4000 B.C., astronomers recorded that the axis of Earth’s rotation pointed toward the handle of the Big Dipper.  Now, our axis of rotation points toward the end of the handle of the Little dipper.  So what are the effects of this phenomenon? Well, I will talk about eccentricity later, but due to the eccentric orbit, the Earth will take 7 days longer to travel from vernal equinox to autumnal equinox than it will to travel from autumnal to vernal.  This is because of Kepler’s Second Law, the equal area law.  Due to the current orientation, the Northern Hemisphere has shorter winters than the Southern Hemisphere.  As the planet continues its cycle, this fact will slowly go toward longer winters in Southern Hemisphere, shorter in the Northern Hemisphere, and again every 26,000 years. 

There is also Obliquity (AKA Axial Tilt).  The Earth’s axis of rotation forms an angle with the plane of the ecliptic, and this is the angle, about 23.44 degrees, that causes the seasons.  Obliquity is the change of that angle over time, from about 22.1 to 24.5 degrees over 41,000 years.  This angle directly relates to the severity of the winter and summer months here on Earth.  The higher the tilt, the more extreme the seasons will be.  Severe winters can create large glaciers in the colder hemisphere.  But it’s not only which hemisphere you’re in, it’s also how far from the closest pole you are, or your latitude.  High latitudes experience high levels of insulation, letting the ice caps form, and lower latitudes have much lower insulation levels than average.  The Ice Age is theorized to have been produced by low obliquity because insulation during the summer increases.  Also, since most of the ice on the planet is at high latitude, the average insulation per year decreases with low obliquity.  This leads us to believe that perhaps there will be another ice age in the distance future due to this phenomenon.

Finally, the eccentricity of the Earth’s orbit around the sun also changes over time, ranging from almost circular, e=0.005, to much higher at e=0.058 over 413,000 years.  Overall, the average eccentricity is about 0.028.  Right now the eccentricity of Earth’s orbit is about 0.017.  The effect of this change in eccentricity is a definite change in climate as a consequence of orbital mechanics, as we learned.  The more eccentricity an orbit has, it travels further away and comes closer to the star within one orbital period, but overall stays further away longer.  The distance from a star is monumentally important to temperature and climate of a planet.

So is it really just natural cycles that are causing all these scientists to overreact?  No.  There is a mountain of evidence that condemns the way we are creating energy and our overall way of life today.  As the professor said in class, we cannot sustain this.  There are natural cycles that the planet goes through, but that doesn’t excuse us from acting the way we are.  Just because we don’t understand something all the way, doesn’t mean it doesn’t exist or that we can ignore it.  We all need the step up to the plate, and do our part to preserve our own little space ship: Earth.