Working Group I: The Scientific Basis

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4.5.2 Impacts of Physical Climate Change on Atmospheric Chemistry

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.

4.5.3 Feedbacks through Natural Emissions Natural emissions of N2O and CH4 are currently the dominant contributors to their respective atmospheric burdens, with terrestrial emissions greatest in the tropics. Emissions of both of these gases are clearly driven by changes in physical climate as seen in the ice-core record (Figure 4.1e). Soil N2O emissions are sensitive to temperature and soil moisture and changes in rates of carbon and nitrogen cycling (Prinn et al., 1999). Similarly, methane emissions from wetlands are sensitive to the extent of inundation, temperature rise, and changes in rates of carbon and nitrogen cycling. Natural emissions of the pollutants NOx, CO, and VOC play an important role in production of tropospheric O3 and the abundance of OH; and these emissions are subject to similar forcings by both the physical and chemical climates. Terrestrial and aquatic ecosystems in turn respond to near-surface pollution (O3, NO2, acidic gases and aerosols) and to inadvertent fertilisation through deposition of reactive nitrogen (often emitted from the biosphere as NO or NH3). This response can take the form of die back, reduced growth, or changed species composition competition that may alter trace gas surface exchange and ecosystem health and function. The coupling of this feedback system between build-up of greenhouse gases, human-induced climate change, ecosystem responses, trace gas exchange at the surface, and back to atmospheric composition has not been evaluated in this assessment. The variety and complexity of these feedbacks relating to ecosystems, beyond simple increases with rising temperatures and changing precipitation, argues strongly for the full interactive coupling of biogeochemical models of trace gas emissions with chemistry and climate models.

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