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I have been studying ocean currents and associated energy flows in the eastern Pacific and western Atlantic oceans. I also looked at El Nino cycles.

I also noted when the large glacial periods started 3 million years ago and noticed that these began when what is now Panama formed between North and South America by continental drift effectively separating the Gulf of Mexico from the Pacific Ocean. The ocean currents from the Atlantic and Pacific no longer mixed from then on. However, this article in Scientific American mentions some evidence that the isthmus of Panama actually formed enough to block ocean currents 10 million years ago.

I am not sure on the macroscopic thermodynamics on how this contributed to the start of the Ice Ages but this is the situation. Could be coincidental.

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    $\begingroup$ It would really be great if you could add some time estimates. $\endgroup$ – farrenthorpe Oct 9 '17 at 23:36
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    $\begingroup$ I have no knowledge of what is the current theory of the beginning of ice ages, but I can reason how this could affect. Ocean currents are a large factor balancing the temperature difference between equator and poles. The more restrained the oceans are the less effectively will they be able to form circulations that do this balancing. $\endgroup$ – Communisty Oct 10 '17 at 6:54
  • $\begingroup$ Here's a related article, a bit more recent (2015 vs 2012) but covers the same uncertainty. phys.org/news/… It's fairly obvious how changing ocean currents can trigger an ice age. That's why the Isthmus hypothesis was so neat and tidy and easy to believe and it may still be one of the key factors. But if it did form a few million years earlier, then more study is needed. $\endgroup$ – userLTK Oct 11 '17 at 8:46
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The closing of the Isthmus of Panama alone did not trigger the beginning of the Ice Age, but it is a potentially decisive step on the way of ice shield build up in the north.

The development that leads to the current Ice Age, per definition a phase in earth's history were polar and continental ice shield build up, starts much earlier. Cooling began after a greenhouse phase already 60 million years ago at the end of the Cretaceous, possibly because of declining atmospheric CO2 levels. Intense volcanism (flood basalts) and the Chicxulub impact surely have contributed to peak cooling at that time.

During the late Paleogene and Neogene, Around ~40 - 30 Million years ago Antarctica and Australia separated and the Drake Passage opened, enabling an Antarctic circumpolar current (ACC) that isolated Antarctica from global circulation. Though the role of the current is debated, it coincides at least partly with ice shield buildup in Antarctica, but declining CO2 may have played an equal or even bigger role. The uplift of the Tibetian plateau may have contributed to cooling by chaning continental circulation patterns. Reason for the CO2 carbon decline in the late Paleogene/early Neogene may be increased silicate weathering in the course of the Himalayas and the Alpine orogeny.

So, when the Isthmus of Panama closed (exact date debated), it was already pretty cold compared to the Cretaceous and an ice shield covered Antarctica. The closing is hypothesized to have steered the gulf stream farther north, enabling the transport of moisture and warm air to the north and thus ice shield build up there.

We also should keep in mind generally ice shield buildup has a feedback in itself by rising the albedo, thus reflecting more energy back to space.

It is a complicated path to the current Ice Age with a lot of facets, starting in the late Cretaceous.

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Pretty much, because the joining of the Americas caused a shift in the circulation of the oceans. Because before, there was no antarctic circumpolar current (ACC), but there was a current (maybe more) going north to south, distributing heat to the Antarctic. The closing of the isthmus caused that current to disappear, and create the ACC and the Weddel current. Thus making the Antarctic cold.

https://www.whoi.edu/oceanus/feature/how-the-isthmus-of-panama-put-ice-in-the-arctic/ (it's old this article but still..)

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