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Having read this answer about the precession of the equinoxes and how it affects Earth's climate, my understanding was that only because of the differing affect of continental and maritime insolation that precession of the equinoxes was a factor in ice-sheet growth.

So if all the continents magically switched to be reflected in the equator (i.e. now we have a Southern Hemisphere dominated by continents), would we now have ice-ages at the 'opposite' time (i.e. short SOUTHERN hemisphere summers)?

And if all the continents moved to be equally spread out about the globe, precession of the equinoxes would play no factor in the ice-ages at all?

In addition:

What is the mechanism that causes this effect? i.e. Why do continents determine the ice growth, and not the oceans? The response in the previous question that: "Continents show more seasonal variability" is a little vague.

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The Earth's climate switches between "hothouse Earth", where ice is nearly absent worldwide, and "icehouse Earth" where the Earth intermittently has ice that stretches far from the poles. Periods when the Earth is in the icehouse Earth phase are called ice ages. The most recent ice age began about 2.6 million years ago and has not yet ended. We are still in an ice age, as evidenced by the fact that Antarctica and Greenland are still covered by ice. Ice ages last many tens to a few hundred of millions of years.

Ice ages are characterized by glacial periods where ice reaches far from the poles separated by interglacial periods where the climate is a bit more moderate. The glacial and interglacial periods within an ice age last tens to hundreds of thousands of years. The current interglacial period began about 12,000 years ago. It's the Milankovitch cycles that are responsible for the short duration glacial and interglacial periods within an ice age.

These cycles are still present when the Earth is in a hothouse Earth phase. The Earth was in the hothouse phase for about 250-260 million years before the current ice age. That's 2000 or so multiples of the very longest Milankovitch cycle. The Milankovitch cycles are not responsible for the hothouse Earth / icehouse Earth phases. It's plate tectonics that dictates whether Earth is in a hothouse Earth or icehouse Earth phase.

So what aspects of plate tectonics govern whether the Earth is in a hothouse phase or icehouse phase? Below are a series of images depicting how the Earth looked over recent times.

http://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Blakey_35moll.jpg/640px-Blakey_35moll.jpg
35 million years ago, hardly any ice anywhere.

http://upload.wikimedia.org/wikipedia/commons/thumb/2/2c/Blakey_20moll.jpg/640px-Blakey_20moll.jpg
20 million years ago, just a tiny bit of ice in the very far north and very far south.

http://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Blakey_Pleistmoll.jpg/640px-Blakey_Pleistmoll.jpg Current ice age, (~2.6 million years ago to present), with ice sometimes reaching as far as 45° latitude.

Source: Ron Blakey, via wikimedia.org. (Blakey's maps are 1200x600, which is a bit wide. I used the 640x320 previews at wikimedia.org.)

There are two key differences at the boundaries of the Pacific Ocean between the pre-ice age Earth and our current ice age Earth. One is the Indonesian seaway. That closed up 3 to 4 million years ago thanks to New Guinea, Indonesia, and the Malay Peninsula. The other is the Panama seaway. That closed up about 3 million years ago thanks to the formation of the Isthmus of Panama. The closing off of the Pacific near the equator drastically changed the global ocean conveyor belt, which in turn drastically changed the climate of the Earth.

So which of these two seaway closures, if any, is the cause of the most recent switch from hothouse Earth to icehouse Earth? A number of researchers put the blame on the formation of the Isthmus of Panama (Bartoli 2005, Nie 2014). Others claim the closure of the Indonesian seaway was responsible (Cane and Molner 2001, Molner 2008). Yet others say "none of the above" (Lunt 2008). The jury is still out, but note that Smith and Pickering 2003 claim that plate tectonics are responsible for each of the four major icehouses in the last 620 million years.


Bartoli, G., et al. (2005). "Final closure of Panama and the onset of northern hemisphere glaciation." Earth and Planetary Science Letters, 237.1: 33-44.

Blakey, Ron. cpgeosystems.com/paleomaps.html (website)

Cane, Mark A., and Peter Molnar (2001). "Closing of the Indonesian seaway as a precursor to east African aridification around 3–4 million years ago." Nature 411.6834: 157-162.

Lunt, Daniel J., et al. (2008). "Late Pliocene Greenland glaciation controlled by a decline in atmospheric CO2 levels." Nature 454.7208: 1102-1105.

Molnar, Peter (2008). "Closing of the Central American Seaway and the Ice Age: A critical review." Paleoceanography 23.2.

Nie, Junsheng, et al. (2014). "Pacific freshening drives Pliocene cooling and Asian monsoon intensification." Scientific Reports 4.

Smith, Alan G., and Kevin T. Pickering (2003). "Oceanic gateways as a critical factor to initiate icehouse Earth." Journal of the Geological Society 160.3: 337-340.

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First, let us try to understand how precession affects climate:

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.

Now coming to your question of how landmasses affect climate. If the earth was symmetric, even then ice-ages would have occurred at same times in the earth's history. However, their intensity would have differed due to the Northern Hemisphere land bias.

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  • $\begingroup$ Welcome to Earth Science! I think you might have mistakenly posted this on two questions, as I'm not sure how this relates to this question. If it does, please edit your answer as it is the exact same as your other one. $\endgroup$ – hichris123 Jul 2 '14 at 16:44
  • $\begingroup$ Edited to add my actual answer! Don't know why it didn't get posted last time. $\endgroup$ – RRC Jul 2 '14 at 18:12
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I'm not sure of the HOW MUCH the effect of precession of equinoxes compares to the land-ocean distribution. So its hard to answer say what will happen if we reorder the land distribution. But i can try to answer the part of your question "Why continents show more seasonal variability".

The reason will be the huge difference between their heat capacities. The effective heat capacity of land is much less than that of the oceans on a seasonal time scale[1]. Hence land heats up and cools down rather quickly, whereas the ocean water does not vary in temperature so much and stays much closer to the mean values. So, with same amount of insolation, land will have much higher or lower temperature as compared to ocean. In case of a cooling trend, the lower temperatures tends to increase ice cover over land which results in increasing the albedo, and hence reducing the amount of solar energy absorbed, leading to more cooling. This positive feedback should be applicable to oceans as well, but on land its just much faster and thus leads to formation of ice sheets over it over extended cooling periods.

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