Atmospheric Carbon Dioxide Inputs

We have identified numerous natural processes that put significant amounts of carbon dioxide into the atmosphere, much larger amounts than are contributed by burning fossil fuels or deforestation. And the uncertainty in the magnitudes of these natural sources is large, perhaps much larger than the amounts of anthropogenic (human) sources. It is then tempting to attribute the increase to some error in our estimate of natural sources and not blame humans.

Figure 5 shows convincing evidence that the source of much of the increase in carbon in the atmosphere is fossil fuels. Carbon has three isotopes, C¹², C¹³, and C¹⁴. Carbon 12 is the most abundant, and C¹³ and C¹⁴ are produced from C¹² in the biosphere by cosmic radiation. Once produced, the C¹³ or C¹⁴ will slowly decay back to C¹². Fossil fuels represent carbon that has been removed from the biosphere for centuries and buried under the surface of the earth where it is shielded from cosmic radiation. The C¹³ and C¹⁴ of such carbon stores have had a long time to decay back to C¹² without production of new amounts of C¹³ and C¹⁴. Therefore, fossil fuels are almost pure C¹². Combustion of fossil fuels then adds C¹² to the atmosphere but not C¹³ or C¹⁴. This means that the relative amount of C¹³ and C¹⁴ should decrease as the level of C¹² is increased. The accompanying graph shows measurements of the partial fraction of C¹⁴ in the atmosphere over the last 130 years. The data clearly show a decrease in the relative abundance of C¹⁴ during the last few decades. These data provide strong evidence implicating fossil fuels as a major contributor to the increase in atmospheric carbon dioxide. The data for C¹³ provide additional confirming evidence.

The fate of carbon dioxide put into the atmosphere by burning fossil fuels can be summarized as shown in Figure 6 which shows the number of petagrams of carbon per year taken up by the ocean and atmosphere and ocean together between 1800 and 1990. The difference in these two curves, labeled as the "residual", is the estimated amount that must be accounted for by changes in vegetation uptake and changes in land use (e.g., deforestation, carbon or carbon-sequestering capability lost due to urbanization). The figure on the right gives the best estimate of the amount due to land use. The remainder, labeled "missing sink", suggests that there is some unaccounted-for loss of carbon from the atmosphere/ocean system. Speculation is that the boreal forest or high-latitude oceans may be responsible, but more data are needed to confirm the identity of the missing sink. A recent paper suggests this missing sink is over the central part of the US.

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