What are the important greenhouse gases

The most important greenhouse gases in Earth's atmosphere are water vapor and carbon dioxide, followed by ozone, nitrous oxide, methane, and various other gases (see table 1.1). Water vapor (and clouds) and carbon dioxide have the dominant effect, although their respective levels are maintained in different ways. The level of C02 is almost constant throughout the atmosphere, and year by year its level is maintained by a balance between emissions (e.g., respiration by animals and bacteria) and natural sinks, including photosynthesis in plants and absorption by the ocean. On longer timescales, geological processes such as volcanic outgassing, weathering of rocks, and changes in the ocean circulation come into play, and the co2 level may change by natural causes on timescales of thousands of years and longer. As we discuss more in chapter 7, the burning of fossil fuels has led to increased emissions of C02 in the past few centuries and the level itself has increased from about 280 ppm in preindustrial times to 390 ppm today.

Water vapor, on the other hand, comes and goes on a daily timescale, as we know from our everyday experience. The source of water vapor in the atmosphere is evaporation from the ocean, lakes, and wet land, and the sink is condensation and rainfall. (clouds are made mainly of small water droplets that remain suspended in the air.) The level of water vapor varies both spatially (deserts are dry, the tropics are humid) and temporally (some days are dry, others are rainy). But one factor above all determines the average level of water vapor in the atmosphere, and that is temperature. The higher the temperature, the more water vapor a given volume can hold, and so if the atmosphere warms then the amount of water vapor in the atmosphere, on average, increases. Since water vapor is a greenhouse gas, this increase leads to a bit more warming still, and so on, and this process is known as positive feedback.

As a rough rule of thumb, in the present climate about half the greenhouse effect comes from water vapor, about a quarter from clouds, and a fifth from carbon dioxide. However, these numbers are approximate because the effects of the greenhouse gases are not additive: if the atmosphere is dry, then adding co2 makes a big difference to the greenhouse effect. On the other hand, if we have a lot of water vapor in the atmosphere already so that we already have a significant greenhouse effect, then adding co2 does not make nearly as much difference:

Table 1.2

Effect of the main longwave absorbers in the atmosphere.1

Table 1.2

Effect of the main longwave absorbers in the atmosphere.1

Absorber

Just the absorber

Everything but the absorber

Range of contribution

Water vapor

62

61

39-62

Clouds

36

85

15-36

Water vapor and clouds

81

33

67-85

carbon dioxide

25

86

14-25

All others

9

95

5-9

Ozone

5.7

97.3

2.7-5.7

Nitrous oxide

1.6

99

1-1.6

Methane

1.6

99.3

0.7-1.6

Aerosols

1.8

99.7

0.3-1.8

cFcs

0.5

99.9

0.1-0.5

The first two columns of numbers give the approximate percentage of the present greenhouse effect that would remain if either just the absorber or everything but the absorber were present, with temperatures fixed; the third column summarizes the percentage range of the contribution of the absorber. "All others" refers to the combined effects of all other absorbers, which are then listed individually. To obtain radiative fluxes, multiply the percentages by 1.55 W/m2.

the longwave radiation that C02 would absorb has already been absorbed by water vapor. Table 1.2 shows the numbers, which we may interpret as follows. If the only greenhouse gas in the atmosphere were co2, then we would have 25% of the greenhouse effect that we have now (that is, a quarter of the long-wave radiation would be absorbed in the atmosphere, if the temperature were held fixed). On the other hand, if we were to remove CO2 entirely from the atmosphere, the greenhouse effect (longwave absorption) would be reduced by about 14%, to 86% of its current value. If we had only water vapor (and no clouds), we would have about 62% greenhouse effect, and if we remove water vapor but keep everything else, we reduce the present-day greenhouse effect by about 39%. If we had only water vapor and clouds, then we would have 81% of the greenhouse effect, and if we remove water vapor and clouds, then we lose about 67% of the present greenhouse effect.

However, these figures do not provide a good measure of the real importance of the dry or noncondensing greenhouse gases (e.g., co2, ozone, or methane) because the calculation assumes that the temperature stays the same. If we did not have any of these gases, the temperatures would fall, leading to a reduction in the absolute humidity and a significantly reduced greenhouse effect from water vapor, causing a further reduction of temperature, and so on. In fact, without the dry greenhouse gases, the terrestrial greenhouse effect would likely completely collapse and the temperature would fall so much that Earth would become completely frozen over, forming a "snowball Earth" (Lacis et al., 2010). Thus, ultimately almost all of the greenhouse effect stems from the dry greenhouse gases. The amount of water vapor adjusts to the level of the other greenhouse gases, and so we usually regard water vapor as a feedback and not a primary forcing.

We'll come back to radiative effects in the last chapter, but let's now shift our attention to the ocean and its role in climate, beginning with a descriptive overview of the oceans themselves.

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