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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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