Happy Aphelion! Wait, what’s that and how does it relate to the climate?

Well, today is one of the most interesting days of the year for me.  This year, 2012, July 5th is aphelion, the day that Earth is furthest from the sun in its orbit.

It is rare (in the tens of thousands of years scale) that the northern hemisphere summer solstice are so close to aphelion.

The time during the year that aphelion and perihelion (when we are closet to the sun) changes over a roughly 100,000 year cycle, known as the Milankovitch Cycle.  Our orbit around the sun is not a circle, it is an ellipse with an eccentricity of about 0.0167.  This orbit both changes shape and rotates around the sun much like a spirogram tracing out a flower-like shape.

Ellipse with 0.5 eccentricity. Earth has a much smaller eccentricity, making the orbit more nearly, but not exactly a circle. Aphelion and perihelion are the two ends of the egg shape.

Perihelion precession


It is summer in the northern hemisphere, a time when people often say things like, “We are closer to the sun than we are in winter.”  This is not really true.  Summer is a product of the angle at which Earth is tilted, right now Earth is tilted so that the northern regions lean toward the sun.  In terms of orbit we are actually at the furthest point Earth gets from the sun.

This has interesting implications in terms of the global climate.  This means that right now winters tend to be warm (the planet is closer to the sun) and summers cool (the planet further from the sun).  In the big picture this places us in the midst of a global cool cycle, the type of situation that tends to lead to ice ages, like the one we are emerging from.

The climate picture is not so simple, though.  Even in terms of celestial mechanics there are other factors that play into the climate picture.  Two large, cyclical factors are the precession and wobble of the Earth. One of these is the obliquity, or the angle at which Earth is tilted.  This is our Axial Tilt, and it is currently 23.4º, but the Axial Tilt changes between approximately 22º and 24º, over a 41,000 year cycle.  This affects where the tropics lie and how much solar energy different regions of Earth receive.

Range of Axial tilt – Wikipedia indicates 22.1º to 24.5º , other sources vary on the exact outer limits of the range, but they are all near 22º and 24º of tilt.

The final large scale cycle that comes into play is Axial Precession.  Spin a top, or a gyroscope; the handle by which it was spun precesses, or rotates around the axis of spin at a fixed angle.  In the case of Earth this cycle varies between 19,000 and 23,000 years.

Precession – just like a top or gyroscope, Earth’s axis of spin makes a slow circle

No single one of these factors leads to an ice age or global tropical forests, they must combine in just the right way to set the conditions.  These cycles are like waves in water, sometimes they cancel each other out, other times they reinforce each other.  Past climate records show clear evidence of the effects of these cycles.

NASA – Orbital Time Series showing the cyclic nature of the three major orbital cycles

NASA – Solar Insolation (energy received from the sun) and O18 (an heavy isotope of oxygen) records in sediment indicating temperatures at the time the O18 precipitated.  O18 is heavier, so it takes more energy to keep it aloft, and water made from this isotope tends to precipitate first when the temperature drops.

Once the orbital conditions are right then, other Earth surface factors come into play.  Where mountain ranges are, whether oceans are polar or tropical, how saline the oceans are, the arrangement and distribution of the continents , and volcanic activity, such as the Deccan and Siberian Trap.  These are immense lave flows in India and Siberia that pumped immense amount of greenhouse gasses into the atmosphere and seem to have strongly affected the climate.  Even short duration, one-off events such as the 1815 eruption of Tambora in the Dutch East Indies (now Indonesia), can have global climate effects.  Tambora is implicated in what is commonly known as The Year Without Summer when the global temperatures dropped by 0.4–0.7 °C , leading to massive famines across the northern hemisphere as  result of immense crop shortages.  This may have been a period of low solar activity as well, adding to the poor growing conditions.

Impacting bodies also can play a major role in the global climate.

The upshot of this is that we really should be paying close attention to the current rapid warming trend as it is happening at a time when it appears, from the large scale cycles, that conditions are not right for a warming trend of the magnitude we are experiencing.  This is a warning sign, one that we need to pay attention to.

The climate always changes, and that is as it should be.  The problem we now face is that our physical and social infrastructure has been built around the idea that the changes are small and seasonal rather than global and systemic.  Our cities, roads, fields, power generating systems, and economies are based on things pretty much staying as they are, which will not happen.  Even without us tipping the scale the climate would not stay the same.  We need to be aware of this and act accordingly, looking at the much larger picture.

If we don’t, well, life on Earth will be just fine in a few tens of millions of years, but we may not be around to see it.  If we continue on the path we are now taking, we many not even be around to see the northern summer occur at perihelion.

This is our planet.  Right now it is the only one we have.  Understand it, enjoy it, and please don’t break it.

The good folks at NASA have put together a great, short paleoclimate primer at the link below, which is where the last two charts come from.


The first three images from from the Wikipedia article on Milankovitch Cycles – they had the best diagrams.

I will return to the usual, smaller scale aspects of nature in the next post, which will be about Dogbane.


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