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WEATHERING AND CLIMATE CHANGE
Earth has two important carbon cycles. One is the biological one, wherein living organisms, mostly plants, consume carbon dioxide from the atmosphere to make their tissues, and then, after they die, that carbon is released back into the atmosphere when they decay over a period of years or decades. A small proportion of this biological-cycle carbon becomes buried in sedimentary rocks: during the slow formation of coal, as tiny fragments and molecules in organic-rich shale, and as the shells and other parts of marine organisms in limestone. This then becomes part of the geological carbon cycle, a cycle that involves a majority of Earth’s carbon, but one that operates only very slowly.
The geological carbon cycle below shows the various steps in the process (not necessarily in this order):
During much of Earth’s history, the geological carbon cycle has been balanced, with carbon being released by volcanism at approximately the same rate that the other processes store it. Under these conditions, the climate remains relatively stable.
During some periods of Earth’s history, that balance has been upset. This can happen during prolonged periods of higher than average volcanism. One example is the eruption of the Siberian Traps at around 250 Ma, which appears to have led to strong climate warming over a few million years.
A carbon imbalance is also associated with significant mountain-building events. For example, the Himalayan Range was formed between about 40 and 10 Ma and over that period, and still today, the rate of weathering on Earth has been enhanced because those mountains are so high and the range is so extensive. The weathering of these rocks, most importantly the hydrolysis of feldspar, has resulted in the consumption of atmospheric carbon dioxide and transfer of the carbon to the oceans and ocean-floor carbonate minerals. The steady drop in carbon dioxide levels over the past 40 million years, which led to the Pleistocene glaciations, is partly attributable to the formation of the Himalayan Range.
Another, non-geological form of carbon-cycle imbalance is happening today on a very rapid time scale. We are in the process of extracting vast volumes of fossil fuels (coal, oil, and gas) that were stored in rocks over the past several hundred million years, and converting these fuels to energy and carbon dioxide. By doing so, we are changing the climate faster than has ever happened in the past.
Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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