The CO2 in the atmosphere is only the tiniest fraction of the carbon on Earth (Fig. 8.1). There is also carbon stored at the land surface, in the oceans, in sedimentary rocks, and the deep interior of the Earth. These other carbon reservoirs all "breathe" CO2, causing atmospheric CO2 to vary, naturally, on all sorts of timescales, from one year to millions of years.
The atmosphere contains about 700 gigatons of carbon. A gigaton, abbreviated Gton, is a billion (109) metric tons, equivalent to 1015 g. The amount of CO2, relative to the other gases, is about 380 ppm, or 0.038% (percent means "parts per hundred"). For every CO2 molecule in the air, there are roughly 2500 molecules of other gases, mainly nitrogen and oxygen. If we were able to gather up all of the CO2 in the atmosphere and
bring it down to the ground to build a column of pure CO2 gas at sealevel pressure, that column would be about 3 m high, reaching perhaps some second-story windows. If we were to precipitate the CO2 into a snowfall of dry ice (solid CO2), we would end up with about 7 cm of snow on the ground. The atmosphere is a very small, very thin carbon reservoir.
There are two forms of carbon that we associate with the landscape that we live in. The actual living carbon, trees and camels and all the rest of it, is called the terrestrial biosphere. There is about 500 Gton C in the terrestrial biosphere, similar in size to the atmosphere. It turns out that there is more carbon in soils than there is in the living biosphere. This is the soil carbon pool: you can see in Fig. 8.1 that it is larger than the terrestrial biosphere carbon pool by about a factor of about two. The soil carbon pool is largely dead carbon, decomposing leaves and other plant material. The amount of carbon stored in soils is highly variable from place to place, depending on the climate, the forestry, and the history of the land. Deserts do not have much carbon in their soils whereas grasslands tend to have quite a bit. There is more soil carbon in colder climates than in warmer climates because organic carbon decomposes more quickly when it is warm. Farming tends to decrease the amount of carbon in soils, but a technique called no-till avoids this problem to some extent.
The terrestrial biosphere inhales CO2 from the atmosphere during the growing season, and exhales it during winter. You can see a seasonal cycle in atmospheric CO2 in Fig. 8.2. The seasons in the southern hemisphere are the reverse of those in the northern hemisphere, and you can see that the CO2 cycles are reversed also. The cycles are smaller in the southern hemisphere than they are in the north because there is less land in the south. One interesting observation, foreshadowing Chapter 10, is that the annual cycles in atmospheric CO2 have been getting larger as atmospheric CO2 has risen. It's not obvious by eyeballing Fig. 8.2, but the trend emerges when the data are treated statistically. The terrestrial biosphere has been breathing more deeply with increasing atmospheric CO2.
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.