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Greenhouse gas and the Gamburtsev Mountains

A recent study, summarized here, described the Gamburtsev Mountains in the heart of East Antarctica. This buried landscape exhibits many of the classical results of alpine glaciation, including cirques – great bowl-shaped hollows carved out of mountainsides by glaciers – and overdeepened valleys.

The work may have begun as early as 34 million years ago, when records from elsewhere show that ice began to accumulate in Antarctica in significant amounts. But the bowl-shaped hollows are still there. Ice sheets are about 2000 km across, and they don’t carve bowl-shaped hollows that are only about 5 km across. So the cirques were probably shaped more than 14 million years ago, which is when we think the ice in Antarctica grew to continental proportions. If this is right, the ice must have shifted from carving up the bedrock surface to protecting it.

We don’t know when the Gamburtsev Mountains were first lifted up. The dates just given are from indirect reasoning. On the other hand, the glaciation of Antarctica had to start sometime, somewhere, and a preglacial mountain range not far from the South Pole sounds like a good nucleus. But there is more missing from the story than just the age of the Gamburtsevs.

First, whether mountainous or not, there has been an extensive landmass over the South Pole for much longer than 34 million years. Motions due to plate tectonics brought Antarctica to roughly its present position almost 100 million years ago, yet it seems to have enjoyed a benign climate for the first 60 or more million years of that span. The switch from benign to cool and then frigid could well have been triggered by the uplift of the Gamburtsevs, or possibly of the more extensive Transantarctic Mountain Range, but in the one case we have no evidence as yet and in the other the uplift has been going on for even longer than 100 million years.

Second, this is a good excuse for me to tell you about the widely-unread paper I published 25 years ago about the subglacial topography of Antarctica. Developments since then have not altered the main conclusion: if you take away the ice that now covers East Antarctica, and allow the bed to rebound from the load of 3 to 4 km of ice, you get a rather unusual preglacial continent. This ice-free East Antarctica of the geomorphologist’s imagination is a full 700 m higher than all of the continents we know today (except that it is only 500 m higher than Africa – but that is another story). We are not talking about a single mountain range here. This is the whole continent, or in other words a broad plateau cooler than a normal continent would have been by perhaps 4°C.

Unfortunately, we don’t know when Antarctica became an elevated plateau, any more than we know when the Gamburtsev Mountains first appeared. There are far too many ifs in the story of Antarctic topography and glaciation. That is a strong argument for reducing the number of ifs, but lurking in the background there is a familiar friend: the greenhouse effect.

Less greenhouse gas in the atmosphere would account for all of the evidence that Antarctica has been getting colder for several tens of millions of years. The evidence that the greenhouse effect has been diminishing for a long time is in fact extremely good. One, or to be more accurate Bob Berner of Yale University, does a detailed accounting of all the carbon in the rocks, and uses the book-keeping to drive calculations of how the carbon would have passed to and fro between the various stores, such as the Earth’s deep interior, the biosphere and the atmosphere. The atmospheric stock, nearly all of it carbon dioxide, was about five times its present size 100 million years ago. (Why so? That is yet another other story.)

The more diverse the facts that a hypothesis succeeds in explaining, the more do we respect it. The long-term cooling of Antarctica is not as remote from our 21st-century concerns as it sounds. In fact the same explanation holds for the climate of Antarctica over the past 100 million years as for the temperatures we have measured over the past 100 years and the temperatures we expect over the next 100 years. Greenhouse gas makes our home warmer.

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