Methane

Methane is another atmospheric constituent whose concentration has increased in recent years. Methane is also a greenhouse gas that is about twenty times as effective on a molecule for molecule basis as is CO₂. One methane molecule will absorb 20 times as much infrared radiation as CO₂. Its lifetime is much shorter than carbon dioxide, however, so this partially compensates for its higher absorption.

Actually, methane is the most rapidly increasing greenhouse gas. Figure 8 shows the recent concentration of methane, in parts per million by volume (ppm), to be about 1.7 ppm. Updated values are given by the Carbon Dioxide Information Analysis Center. Sometimes methane concentration is given in parts per billion by volume (ppb), and then would have a value of 1700 ppb. The numerical values show that methane is much less abundant than carbon dioxide which has a present concentration of about 360 ppm. However, the curve shows that the concentration increased by more than 1% per year from 1978 - 1987. If we look at a longer term, as shown in figure 8, we see that concentrations have increased substantially since the Industrial Revolution. Estimate of atmospheric methane from a thousand years ago suggest values around 0.7 ppm (700 ppb) which were constant until about the late 1700s. Since that time, concentrations have more than doubled. If we examine the Antarctic ice core data going back 160,000 years, we see that methane levels fluctuated between about 300 parts per billion and 700 parts per billion until the Industrial Revolution when it began its climb to near 1700 parts per billion (Figure 9).

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