Carbon cycling on Earth operates on different timescales depending on the components of the Earth system that are involved. Over the short term, biological processes are important. In particular, living organisms — mostly plants — consume carbon dioxide from the atmosphere to make their tissues. After they die, the carbon is released back into the atmosphere over years to decades as the plant matter decays.
Over the longer term, geological processes drive the carbon cycle. Geological carbon-cycle processes operate very slowly, but they affect much more of Earth’s carbon than the biological component. Carbon can move from the biological cycle to the geological cycle if it is buried in sedimentary rocks. The biological carbon could be fragments of plant material or organic molecules that are preserved as coal or in organic-rich shale. It could also be calcium carbonate body parts of marine organisms that are preserved in limestone.
The geological component of the carbon cycle is shown in Figure 8.27. The various steps in the process (not necessarily in this order) are as follows:
|Organic matter from plants is stored in peat, coal, and permafrost for thousands to millions of years.
|Weathering of silicate minerals converts atmospheric carbon dioxide to dissolved bicarbonate, which is stored in the oceans for thousands to tens of thousands of years.
|Dissolved carbon is converted by marine organisms to calcite, which is stored in carbonate rocks for tens of millions to hundreds of millions of years.
|Organic carbon compounds are stored in sediments for tens to hundreds of millions of years; some end up in petroleum deposits.
|Carbon-bearing sediments are transferred to the mantle, where the carbon may be stored for tens of millions to billions of years.
|During volcanic eruptions, carbon dioxide is released back to the atmosphere, where it is stored for years to decades.
At some times in Earth’s history, the geological carbon cycle has been balanced, with carbon being released to the atmosphere by some processes at approximately the same rate as other processes store it. Under these conditions, the climate can remain relatively stable.
At other times, the balance is upset. Prolonged periods of greater than average volcanism can cause an imbalance. The eruption of the Siberian Traps at around 250 Ma warmed the climate significantly over a few million years, leading to a mass extinction.
Mountain-building events may also cause an imbalance. The formation of the Himalaya range between about 40 Ma and 10 Ma ago exposed rocks to weathering over a large region. The over-all rate of weathering on Earth increased because the mountains were so high, and the range was so extensive. The weathering of these rocks — most importantly the hydrolysis of feldspar — consumed atmospheric carbon dioxide and transferred carbon to the oceans and to ocean-floor carbonate minerals. Decreasing carbon dioxide levels contributed to climate cooling that culminated in the Pleistocene glaciations.
Today, burning fossil fuels is causing an imbalance in the carbon cycle. Burning coal, oil, and gas releases in a geological instant carbon that was stored by the biological carbon cycle over hundreds of millions of years. Scientists who study Earth’s past climate tell us that today carbon dioxide is being added to the atmosphere faster than during some of the most extreme climate change events in Earth history. Eventually, higher carbon dioxide levels will accelerate chemical weathering, and that will help to remove some of the carbon dioxide from the atmosphere. However, weathering is part of the geological carbon cycle, and operates over long timescales. If humans stopped burning all fossil fuels today, it could still take thousands of years for balance to be restored.