Other Greenhouse Gases

Ozone (O3). Ozone plays a number of important roles in the atmosphere, depending on location, and its concentration varies substantially, both vertically and horizontally. The highest concentrations of ozone are found in the stratosphere—the layer of the atmosphere extending from roughly 10 to 32 miles (15 to 50 km) in height (Figure 6.8)— where it is produced naturally by the dissociation of oxygen molecules by ultraviolet light. This chemical reaction, along with the photodissociation of ozone itself, plays the beneficial role of absorbing the vast majority of incoming ultraviolet radiation, which is harmful to most forms of life, before it reaches the Earth's surface. Levels of ozone in the stratosphere have been declining over the past several decades, especially over Antarctica. Scientific research has definitively shown that CFCs, along with a few other related man-made halogenated gases (see above), are responsible for these ozone losses in the stratosphere; thus, halogenated gases contribute to both global warming and stratospheric ozone depletion. The Montreal Protocol, which was originally signed in 1987 and has now been revised several times and ratified by 196 countries, has resulted in a rapid phase-out of these gases (see Figure 6.7). Recent evidence suggests that ozone levels in the stratosphere are starting to recover as a result, although it may be several more decades before the ozone layer recovers completely (CCSP, 2008a).

Near the Earth's surface, ozone is considered a pollutant, causing damage to plants and animals, including humans, and it is one of the main components of smog (see Chapter 11). Most surface ozone is formed primarily when sunlight strikes air that

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FIGURE 6.7 Atmospheric concentrations of the two halogenated gases with the largest individual climate forcings, CFC-11 and CFC-12, from 1979 to 2008. The Montreal Protocol limited the production of these and other compounds, and so their atmospheric concentrations are now slowly declining. SOURCE: NOAA/ESRL (2009).

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contains nitrogen oxides (NOx) in combination with carbon monoxide (CO) or certain volatile organic compounds (VOCs). All of these substances have natural sources, but their concentrations have increased as a result of human activities. Much of the NOx and CO in the troposphere comes from man-made sources that involve burning, including automobile exhaust and power plants, while sources of VOCs include vegetation, automobiles, and certain industrial activities.

Ozone is also found in the upper troposphere, where its sources include local formation, horizontal and vertical mixing processes, and downward transport from the stratosphere. In general, tropospheric ozone levels show a lot of variability in both space and time, and there are only a few locations with long-term records, so it is difficult to estimate long-term ozone trends. Observational evidence to date shows increases in ozone in various parts of the world (e.g., Cooper et al., 2010). Models that include explicit representations of atmospheric chemistry and transport have also been used to estimate long-term ozone trends. These models, which are generally able to simulate observed ozone changes, indicate that tropospheric ozone levels have increased appreciably during the 20th century (Forster et al., 2007).

In addition to its role in near-surface air pollution and absorbing ultraviolet radiation in the stratosphere, ozone is a GHG, and so changes in its concentration yield a climate forcing. The losses of ozone in the stratosphere are estimated to yield a small negative forcing (cooling) of -0.05 ± 0.10 W/m2, while increases in tropospheric ozone, which

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Ozone Concentration -►

FIGURE 6.8 The vertical distribution of ozone with height, showing the protective layer of ultraviolet-absorbing ozone in the stratosphere, the harmful ozone (smog) near the Earth's surface, and the lesser—but still important—amounts of ozone in the upper troposphere. SOURCE: UNEP et al. (1994).

are comparatively larger, are estimated to yield a positive forcing of between 0.25 and 0.65 W/m2, with a best estimate of 0.35 W/m2 (Forster et al., 2007) (see Figure 6.4). Thus, in total, the changes in atmospheric ozone are responsible for a positive forcing that is on par with the halogenated gases and possibly as large as or slightly larger than the forcing associated with CH4. However, the exact ozone forcing is more uncertain than for the well-mixed GHGs.

Water vapor (H2O). Water vapor is technically the most abundant GHG and also the most important in terms of its contribution to the natural greenhouse effect (see Figure 2.1). A number of human activities (primarily agricultural irrigation but also through cooling towers, aircraft exhaust, and other sources) can influence local water vapor levels. However, on a global basis the concentration of water vapor in the lower atmosphere is controlled by the rate of evaporation and precipitation, which are processes that occur on a relatively fast time scale and are much more strongly influenced by changes in atmospheric temperature and circulation than by human activities directly. Thus, water vapor is usually considered to be part of the climate system—and indeed, it is involved in a number of important climate feedback processes, as described below—rather than a climate forcing agent.

Ozone Concentration -►

FIGURE 6.8 The vertical distribution of ozone with height, showing the protective layer of ultraviolet-absorbing ozone in the stratosphere, the harmful ozone (smog) near the Earth's surface, and the lesser—but still important—amounts of ozone in the upper troposphere. SOURCE: UNEP et al. (1994).

In the stratosphere, on the other hand, water vapor is relatively rare and somewhat isolated from the hydrological cycle in the lower atmosphere. Processes that influence water vapor concentrations at these high altitudes can thus lead to a small but discernible climate forcing. The largest such forcing is associated with the oxidation of CH4 into water vapor and CO2: as CH4 concentrations have increased, so has this source of water vapor in the stratosphere, leading to a small positive climate forcing estimated to be 0.05 ± 0.05 W/m2 (Hansen et al., 2005).5 Recent satellite-based observations reveal that stratospheric water vapor levels have actually declined since 2000 (Solomon et al., 2010); the causes and possible implications of this decline are still being studied.

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