Most people are more concerned with sea level rise these days, but there have been times when the oceans dropped to alarmingly low levels. A new study calculates that the worst of those icy, low sea periods — what was called snowball Earth — saw the oceans drop more than 1,700 feet (525 meters). Nor is that number easy to come by. It turns out that a lot of tricky things come into play when you start shuffling around oceans and ice sheets.

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The two Princeton University researchers who worked out the numbers chose an arrangement of continents that was a reasonable match for the Earth 720 million years ago. The volume of the ice sheets on those continents during the snowball Earth period was the equivalent to a drop in sea level of about 2,500 feet (750 m).

But if you could have stood on the shore and measured the drop due to the ice forming and the locking-up of water on land, you would not have seen quite that big a drop. Why? Because taking all that water out of the oceans would lighten the load on the oceanic crust, which floats atop the elastic mantle of the Earth. The ocean floor would buoy up, what geologists call rebound.

Likewise all that weight of ice on the continents would push the land down, in an uneven pattern, depending on the thickness of the ice and quality of the crust and mantle in any given place. So that would result in an effective sea level drop along those ancient shorelines ranging from 920 to 1,700 feet (280 to 520 m).

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The work on this preliminary, theoretical estimate was made using models and geological records from rocks in Namibia (the same group of rocks that contains some of the oldest animal fossils known). The researchers, Yonggang Liu of the University of Toronto and Princeton University and W. Richard Peltier of Princeton University, just had their paper accepted for publication by the journal Geophysical Research Letters.

IMAGE: When Earth was covered in ice, how low did the sea levels get? Image credit: Stephen Hudson / Wikimedia Commons