The "carbon cycle," as most people understand it, is represented in figure 1.1. Carbon dioxide is taken up via photosynthesis by green plants on the continents or phytoplankton in the ocean. On land carbon is transferred to soils by the dropping of leaves, root growth, and respiration, the death of plants, and the development of soil biota. Land herbivores eat the plants, and carnivores eat the herbivores. In the oceans the phytoplankton are eaten by zooplankton that are in turn eaten by larger and larger organisms. The plants, plankton, and animals respire CO2. Upon death the plants and animals are decomposed by microorganisms with the ultimate production of CO2. Carbon dioxide is exchanged between
Figure 1.1. The short-term carbon cycle. (Adapted from Berner, 1999.)
the oceans and atmosphere, and dissolved organic matter is carried in solution by rivers from soils to the sea. This all constitutes the short-term carbon cycle. The word "short-term" is used because the characteristic times for transferring carbon between reservoirs range from days to tens of thousands of years. Because the earth is more than four billion years old, this is short on a geological time scale.
As the short-term cycle proceeds, concentrations of the two principal atmospheric gases, CO2 and CH4, can change as a result of perturbations of the cycle. Because these two are both greenhouse gases—in other words, they adsorb outgoing infrared radiation from the earth surface— changes in their concentrations can involve global warming and cooling over centuries and many millennia. Such changes have accompanied global climate change over the Quaternary period (past 2 million years), although other factors, such as variations in the receipt of solar radiation due to changes in characteristics of the earth's orbit, have also contributed to climate change. Over the past century human perturbation of the short-term carbon cycle, from activities such as deforestation and biomass burning (for CO2), and rice cultivation and cattle raising (for CH4), have contributed to a rise in atmospheric levels of these gases. However, the major perturbation of the level of atmospheric CO2, and consequently an overall rise in global temperature over the past century, is due to a process of the long-term carbon cycle. This is the burning of fossil fuels. Organic matter in sedimentary rocks, which would otherwise be slowly exposed to the atmosphere by erosion and oxidized by weathering, is instead being rapidly removed from the ground, oxidized by burning, and given off to the atmosphere as CO2.
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