Carbon Dioxide

Carbon dioxide is a friendly gas: at atmospheric concentrations, even double the present amount, it is not harmful to humans, since it is odorless, colorless, and does not react in the human body. And plants grow more vigorously in enriched CO₂ environments, so why do we raise the concern about its increase? A significant characteristic of carbon dioxide is that it has a very long effective lifetime in the atmosphere. Figure 3 shows atmospheric concentration excess as a function of time. This graph provides an answer to the following question: If we put one extra kilogram of carbon dioxide into the atmosphere, how quickly will the atmospheric CO₂ level return to its original level? This curve shows that 60 years have elapsed before half of the initial kilogram is removed and about 200 years elapsed before two-thirds of the initial amount it lost. The lifetime of carbon dioxide in the atmosphere is obviously very long. This means that large amounts of carbon dioxide presently being put into the atmosphere by burning fossil fuels and deforestation will, on average, be around for many decades.

Figure 4 gives the global distribution of carbon as best known in 2001.

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