As global warming increases in the next century, the first-order atmospheric changes that impact tropospheric chemistry are the anticipated rise in temperature and water vapour. For example, an early 2-D model study (Fuglestvedt et al., 1995) reports that tropospheric O3 decreases by about 10% in response to a warmer, more humid climate projected for year 2050 as compared to an atmosphere with current temperature and H2O. A recent study based on NCAR (National Center for Atmospheric Research) CCM (Community Climate Model) projected year 2050 changes in tropospheric temperature and H2O (Brasseur et al., 1998a) finds a global mean 7% increase in the OH abundance and a 5% decrease in tropospheric O3, again relative to the same calculation with the current physical climate.
A 3-D tropospheric chemistry model has been coupled to the Hadley Centre Atmosphere-Ocean General Circulation Model (AOGCM) and experiments performed using the SRES preliminary marker A2p emissions (i) as annual snapshots (Stevenson et al., 2000) and (ii) as a 110-year, fully coupled experiment (Johnson et al., 1999) for the period 1990 to 2100. By 2100, the experiments with coupled climate change have increases in CH4 which are only about three-quarters those of the simulation without climate change and increases in Northern Hemisphere mid-latitude O3 which are reduced by half. The two major climate-chemistry feedback mechanisms identified in these and previous studies were (1) the change of chemical reaction rates with the average 3°C increase in tropospheric temperatures and (2) the enhanced photochemical destruction of tropospheric O3 with the approximately 20% increase in water vapour. The role of changes in the circulation and convection appeared to play a lesser role but have not been fully evaluated. These studies clearly point out the importance of including the climate-chemistry feedbacks, but are just the beginning of the research that is needed for adequate assessment.
Thunderstorms, and their associated lightning, are a component of the physical climate system that provides a direct source of a key chemical species, NOx. The magnitude and distribution of this lightning NOx source controls the magnitude of the anthropogenic perturbations, e.g., that of aviation NOx emissions on upper tropospheric O3 (Berntsen and Isaksen, 1999). In spite of thorough investigations of the vertical distribution of lightning NOx (Huntrieser et al., 1998; Pickering et al. 1998), uncertainty in the source strength of lightning NOx cannot be easily derived from observations (Thakur et al., 1999; Thompson et al., 1999). The link of lightning with deep convection (Price and Rind, 1992) opens up the possibility that this source of NOx would vary with climate change, however, no quantitative evaluation can yet be made.
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