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Stratospheric water vapor acts as a significant greenhouse gas. About one-quarter of the total global warming caused by increased methane emissions is not brought about directly, but rather is due to this effect in the stratosphere whereby the region's water content is increased. Due to decreased levels of ozone and increased levels of carbon dioxide, the stratosphere has undergone cooling in recent decades; the increase in water vapor has reduced the amount of this cooling, thus contributing to the warming of the atmosphere overall.

Per molecule, increasing the amount of methane in air causes a much larger warming effect than does adding more carbon dioxide, since each CH4 molecule is much more likely to absorb a thermal IR photon that passes through it than does a C02 molecule. The effect of the methane is restricted to the first decade or two after its release, however, because it is highly likely to be oxidized during this period. When considered over the century after its emission, a kilogram of methane is still about 23 times more effective in raising air temperature than the same mass of carbon dioxide; the ratio is about three times that value over the first 20 years. However, because C02 is so long-lived in air and its concentration has increased 80 times more, methane has been less important in heating the atmosphere. To date, methane is

FIGURE 6-15 Changes over the twentieth century in the annual emission (black line) and annual growth in atmospheric concentration (green line) of methane. [Source: "Global Methane Rise Slows," Atmosphere (CSIRO Newsletter) (April 1999).]

1900

1920

1940 1960 Year

1900

1920

1940 1960 Year estimated to have produced about one-third as much global warming as has carbon dioxide.

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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