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The gravitational influence of the gas giants (plus general-relativistic effects) are known to cause the apses of the Earth's orbit to precess. For an illustration of that process, see this image. Now, it's clear that apsidal precession combined with changes in orbital eccentricity will affect climate - at minimum, this changes the mean (year-averaged) Earth-Sun distance, which one would of course expect to have an effect on climate.

I'm not really following how apsidal precession alone is supposed to affect climate, though. Let us suppose that the Earth's "rotational parameters" (e.g. axial tilt) are held constant, and that the orbit of the Earth is static except for apsidal precession (i.e. no changes in eccentricity, no orbital decay, etc.).

In this case, the total insolation experienced by the Earth over the course of a year should be independent of how much the apses have precessed - indeed, an observer looking at the Sun-Earth system from above the ecliptic plane should not be able to distinguish between, say, a 5-degree precession of the apses and a 5-degree rotation of the observer in the opposite direction. Since the latter cannot affect the Earth's climate, we should neither expect the former to.

Clearly, I'm making some sort of mistake here, but I'm not sure where exactly my thinking has gone wrong. Where is my error?

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During the 20th century, the global mean temperature in January was 12°C while it was 15.8°C in July. This is a bit paradoxical as insolation is currently about 7% higher during January than it is in July. So why is July warmer than January?

The solution to this apparent paradox and the answer your question (how does apsidal precession alone affect the climate) are one and the same: The uneven distribution of land and water between the northern and southern hemispheres. The northern hemisphere has about twice as much land as does the southern hemisphere. Continental climates exhibit much more seasonal variability than do maritime climates.

That worldwide insolation is greatest in January and least in July is a bit irrelevant. What is more relevant is the length of the seasons in the northern hemisphere. Right now, summer is longer than is winter in the northern hemisphere because perihelion is in early January. This situation will be reversed in ten thousand years when perihelion and northern hemisphere summer solstice coincide. Looking at precession only, glaciation is much more likely during periods where northern hemisphere summers are short and winters are long.

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    $\begingroup$ Let me double-check that I understand this - the reason that perihelion being in January makes northern-hemisphere winters shorter is simply that the Earth moves faster near perihelion and slower near aphelion, right? $\endgroup$ – senshin May 27 '14 at 11:19
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    $\begingroup$ That's correct. $\endgroup$ – David Hammen May 27 '14 at 12:59
  • $\begingroup$ Okay, and (just restating what you wrote for my own benefit) when the apses precess by 180 degrees (i.e. moving the sun from one focus of the orbit to the other), January will now be at aphelion, meaning that northern-hemisphere winters are longer, and so we expect a colder overall climate. Great, thanks! $\endgroup$ – senshin May 27 '14 at 13:03
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The precessional effect on climate is caused due to two factors:
(1) Axial Precession
(2) Apsidal Precession

The precession of the apses doesn't cause a change in climatic state by itself. However, this motion is in the same sense as the axial precession (gyroscopic motion of the earth's axes). This effect reduces the period of precession from 26kyr to ~21kyr.

Combined, the two effects cause what is known as the precession of the equinoxes. If you consider this a cyclic process, the two extreme precessional configurations are shown below: enter image description here

See NH++. In this configuration, NH summer is at perihelion. Thus, during NH summer, earth is closest to the sun, and therefore Northern hemisphere has a strong summer. During NH winter, earth is farthest from the sun, therefore Northern Hemisphere has a weak winter as well. Thus in this precessional configuration, the earth has extreme seasons in the Northern Hemisphere.

Now see SH++. In this configuration, SH summer is at perihelion. Thus, during SH summer, earth is closest to the sun, and therefore the southern hemisphere has a strong summer. During SH winter, earth is farthest from the sun, therefore southern Hemisphere has a weak winter as well. Thus in this precessional configuration, the earth has extreme seasons in the southern Hemisphere.

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  • $\begingroup$ This is incorrect. The current configuration has perihelion very close to summer solstice in the southern hemisphere. The Earth is very close to the "SH++" configuration in this answer. Yet southern hemisphere seasons are not extreme. Northern hemisphere summers are hotter than are southern hemisphere summers, and northern hemisphere winters are colder than are southern hemisphere winters. The timing of Earth's perihelion and aphelion has but a tertiary effect on the seasons. The uneven distribution of land vs ocean is more important than is the timing of perihelion and aphelion. $\endgroup$ – David Hammen Nov 1 '15 at 9:17
  • $\begingroup$ The answer here considers only the effect of orbital forcing on the earth, which is correct here, and has been accepted since the time of Milankovitch. The uneven distribution of land vs ocean created a different level of complexity on the climate response of the earth to a given orbital forcing, which is beyond the scope of this answer. $\endgroup$ – RRC Nov 3 '15 at 20:10

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