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Sea Changes Bigger than Tsunamis

Posted by | March 10, 2010

In the mid-1800s there was a smart geologist with a face as sharp as flint who worked in the American Midwest. In those days, the “Midwest” was quite close to the frontier of the country. It took some guts and imagination to live out there, and maybe Charles Whittlesey had both in abundance – for he clearly saw evidence of dramatic climate change in riverbanks and hillsides around him. For Whittlesey, the Ice Age was evident in almost every field and ridge.

Many geologists of the time were still skeptical of the new theory that Earth’s climate could change at all. It wasn’t easy to think that Mother Nature had once put the whole globe into a deep freeze, but my hero got on-board with the program early. He argued (correctly) that much of the upper part of our country had once been buried under thick, glacial ice, and he did so by pointing to specific pieces of evidence he could describe and draw.

That alone would make Whittlesey commendable in my book. He looked at good evidence, published as widely as he could at the time, and argued for his views.

But this is what really impresses me. Despite the fact Whittlesey was living and working in the Midwest – pretty far from the ocean – he had the insight to see that the massive glaciers of the past must have changed global sea level drastically.

Here’s the picture:

During times of bitter cold in the past 2 million years, extensive ice sheets and major glaciers have formed in North America and Scandinavia. While those glaciers have been draped on the land, they have “locked up” a great deal of Earth’s waters.

More and bigger glaciers meant lower and lower sea level in the Ice Age, a point Whittlesey deduced early.  One of his estimates put sea level of the Ice Age as around 300 feet lower than today, a value that stands up well to current scientific data.

With sea level hundreds of feet lower than it is now, brown bears (and people) could walk from Siberia to Alaska – and they evidently did so, spreading down into North America.

But climate naturally evolves on Earth, and the Ice Age came to its end in due time. When global temperatures shot upward into the warmth we enjoy in this epoch, the massive glaciers and ice sheet started to melt. Water that had been on land (as ice), flowed down into the seas. Ocean levels rose, and rose, and rose some more, an increase totaling hundreds of feet.

But oddly enough, that’s not the end of the story. In some places – like Sweden and Hudson Bay – the land is rising out of the sea. In other words, in those places, local sea level has been falling even while global sea level has been rising.

In Scandinavia and Hudson Bay, the evidence the sea is falling compared to the land is the many old beaches that are high and dry on the land well above current sea level. These “raised beaches” show us the land is moving upward even faster than global sea level has been rising. But there are not raised beaches like these everywhere on Earth, only in places where (interestingly enough) major glaciers used to lie.

Our hero was one geologist who had some insight on this issue, too. The ice sheets of the Ice Age were literally a couple miles thick and covered whole regions. When the Ice Age glaciers melted, their staggering weight was removed. Gradually, the land under the ice has moved upward – and it’s still doing so.

The land in Hudson Bay and Scandinavia is headed higher and higher at a faster rate than global sea level. So local sea level where my herring-eating ancestors live in southern Sweden is dropping relative to land.

Climate change on Earth guarantees that sea level can go up a whole lot in some places and down in others. Those are the realities with which people have adapted for a long time, and will doubtless have to do so again.

Like climate itself, the only constant we geologists can see when it comes to sea level is change.