Climate: A Very Short Introduction by Mark Maslin

Author:Mark Maslin [Maslin, Mark]
Language: eng
Format: epub
ISBN: 9780199641130
Publisher: Oxford University Press
Published: 2013-12-15T06:00:00+00:00

30. Global climate over the last 65 million years

What caused the big freeze?

In 1988 Professor Bill Ruddiman and his then graduate student Maureen Raymo while at the Lamont-Doherty Earth Observatory wrote an extremely influential paper. They suggested that global cooling and the build up of ice sheets in the Northern Hemisphere were caused by uplift of the Tibetan-Himalayan and Sierran-Coloradan regions. As we saw in Chapter 5 huge plateaus can alter the circulation of the atmosphere and they argued this cooled the Northern Hemisphere, allowing snow and ice to build up. However, what they did not realize at the time was most of the Himalayan uplift occurred much earlier between 20 and 17 million years ago and thus it was too early to have been the direct cause of the ice in the north. But Maureen Raymo then came up with a startling suggestion that this uplift may have caused a massive increase in erosion that uses up atmospheric carbon dioxide in the process. This is because when you make a mountain range you also produce a rain shadow. So, one side of the mountain has a lot more rain on it as the air is forced up and over the mountain. This is also why mountains erode much faster than gentle rolling hills. She argued that this extra rainwater and carbon dioxide from the atmosphere form a weak carbonic acid solution, which dissolves rocks. But interestingly only the weathering of silicate minerals makes a difference to atmospheric carbon dioxide levels, as weathering of carbonate rocks by carbonic acid returns carbon dioxide to the atmosphere. As much of the Himalayas is made up of silica rocks there was a lot of rock that could lock up atmospheric carbon dioxide. The new minerals dissolved in the rainwater are then washed into the oceans and used by marine plankton to make shells out of the calcium carbonate. The calcite skeletal remains of the marine biota are ultimately deposited as deep sea sediments and hence lost from the global carbon cycle for the duration of the lifecycle of the oceanic crust on which they have been deposited. It’s a fast track way of getting atmospheric carbon dioxide out of the atmosphere and dumping it at the bottom of the ocean. Geological evidence for long-term changes in atmospheric carbon dioxide does support the idea that it has dropped significantly over the last 20 million years. The only problem scientists have with this theory is what stops this process. With the amount of rock in Tibet that has been eroded over the last 20 million years all the carbon dioxide in the atmosphere should have been stripped out. So there must be other natural mechanisms which help to maintain the balance of carbon dioxide in the atmosphere as the long-term concentration of carbon dioxide in the atmosphere is the result of a balance between what is removed by weathering and deposition in the deep ocean and the amount recycled by suduction zones and emitted by volcanoes.

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