Climate Change 2001: Working Group I: The Scientific Basis

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6.6.3 Indirect Forcing by NO_x Emissions

Through production of tropospheric O₃, emissions of nitrogen oxides (NO_x = NO + NO₂) lead to a positive radiative forcing of climate (warming), but by affecting the concentration of OH they reduce the levels of CH₄, providing a negative forcing (cooling) which partly offsets the O₃ forcing. Due to non-linearities in O₃ photochemical production together with differences in mixing regimes and removal processes, the O₃ and OH changes strongly depend on the localisation of the NO_x surface emission perturbation, as calculated by Hauglustaine and Granier (1995), Johnson and Derwent (1996), Berntsen et al. (1996), Fuglestvedt et al. (1996, 1999) and Gupta et al. (1998). The CH₄ and O₃ forcings are similar in magnitude, but opposite in sign, as calculated by Fuglestvedt et al. (1999). Due to differences in CH₄ and O₃ lifetimes, the NO_x perturbation on the CH₄ forcing acts on a global scale over a period of approximately a decade, while the O₃ forcing is of regional character and occurs over a period of weeks. Based on three-dimensional model results, Fuglestvedt et al. (1999) have calculated that the O₃ radiative forcing per change in NO_x emission (10-2 Wm^-2 per TgN/yr) is 0.35 and 0.29 for the USA and Scandinavia, respectively, and reaches 2.4 for Southeast Asia. The CH₄ forcing per change in NO_x emission ranges from -0.37 (Scandinavia) and -0.5 (USA) to -2.3 (Southeast Asia) in the same units. Additional work is required to assess the impact of NO_x on the radiative forcing of climate.

Deep convection can remove pollutants from the lower atmosphere and inject them rapidly into the middle and upper troposphere and, occasionally, into the stratosphere. Changes in convective regimes associated with climate changes have therefore the potential to significantly modify the distribution and the photochemistry of O₃ in a region where its impact on the radiative forcing is the largest. Based on the tropospheric O₃ column derived from satellite during the 1997 to 1998 El Niño, Chandra et al. (1998) reported a decrease in O₃ column of 4 to 8 Dobson Units (DU) in the eastern Pacific and an increase of 10 to 20 DU in the western Pacific, largely as a result of the eastward shift of the tropical convective activity. Lelieveld and Crutzen (1994) showed that convective transport can change the budget of O₃ in the troposphere. Berntsen and Isaksen (1999) have also indicated that changes in convection can modify the sensitivity of the atmosphere to anthropogenic perturbations as aircraft emissions. Their study indicates that reduced convective activity leads to a 40% increase in the O₃ response to aircraft NO_x emissions due to modified background atmospheric concentrations. In addition to that, lightning is a major source of NO_x in the troposphere and thus contributes to the photochemical production of O₃ (Huntrieser et al., 1998; Wang et al., 1998; Hauglustaine et al., 2001; Lelieveld and Dentener, 2000). In the tropical mid- and upper troposphere, modelling studies by Lamarque et al. (1996), Levy et al. (1996), Penner et al. (1998a), and Allen et al. (2000) have calculated a contribution of lightning to the NO_x levels of 60 to 90% during summer. Direct observations by Solomon et al. (1999) of absorption of visible radiation indicate that nitrogen dioxide can lead to local instantaneous radiative forcing exceeding 1 Wm^-2. These enhancements of NO₂ absorption are likely to be due both to pollution and to production by lightning in convective clouds. Further measurements are required to bracket the direct radiative forcing by NO₂ under a variety of storm and pollution conditions. On the basis of two-dimensional model calculations, Toumi et al. (1996) calculated that for a 20% increase of lightning the global mean radiative forcing by enhanced tropospheric O₃ production is about 0.1 Wm^-2. Based on an apparent correlation between lightning strike rates and surface temperatures, Sinha and Toumi (1997) have suggested a positive climate feedback through O₃ production from lightning NO_x in a warmer climate. Improved modelling and observations are required to confirm this hypothesis.

Aircraft emissions also have the potential to alter the composition of the atmosphere and induce a radiative forcing of climate. According to IPCC (1999), in 1992 the NO_x emissions from subsonic aircraft are estimated to have increased O₃ concentrations at cruise altitudes in the Northern Hemisphere mid-latitude summer by about 6% compared with an atmosphere without aircraft. The associated global mean radiative forcing is 0.023 Wm^-2. In addition to increasing tropospheric O₃, aircraft NO_x are expected to decrease the concentration of CH₄ inducing a negative radiative forcing of -0.014 Wm^-2 for the 1992 aircraft fleet. Again, due to different O₃ and CH₄ lifetimes, the two forcings show very different spatial distribution and seasonal evolution (IPCC, 1999). The O₃ forcing shows a marked maximum at northern mid-latitudes, reaching 0.06 Wm^-2, whilst the CH₄ forcing exhibits a more uniform distribution, with a maximum of about -0.02 Wm^-2 in the tropical regions. Based on IPCC (1999), the O₃ forcing calculated for 2050 conditions is 0.060 Wm^-2 and the CH₄ forcing is -0.045 Wm^-2.