Climate Change 2001:
Working Group I: The Scientific Basis
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14.2.6 Trace Gases, Aerosols, and the Climate System

The goal is a completely interactive simulation of the dynamical, radiative, and chemical processes in the atmosphere-ocean-land system with a central theme of characterising adequately the radiative forcing in the past, in the present, and into the future (See Chapter 6, Sections 6.1 and 6.2; see also Chapter 9, Section 9.1). Such a model will be essential in future studies of the broad question on the role of the oceans, terrestrial ecosystems, and human activities in the regulation of atmospheric concentrations of CO2 and other radiatively active atmospheric constituents. It will be required for understanding tropospheric trace constituents such as nitrogen oxides, ozone, and sulphate aerosols. Nitrogen oxides are believed to control the production and destruction of tropospheric ozone, which controls the chemical reactivity of the lower atmosphere and is itself a significant greenhouse gas. Tropospheric sulphate aerosols, carbonaceous aerosols from both natural and anthropogenic processes, dust, and sea salt, on the other hand, are believed to affect the Earth's radiation budget significantly, by scattering solar radiation and through their effects on clouds. Systematic observations of different terrestrial ecosystems and surface marine systems under variable meteorological conditions are needed along with the development of ecosystem and surface models that will provide parametrizations of these exchanges.

Models that incorporate atmospheric chemical processes provide the basis for much of our current understanding in such critical problem areas as acid rain, photochemical smog production in the troposphere, and depletion of the ozone layer in the stratosphere. These formidable problems require models that include chemical, dynamical, and radiative processes, which through their mutual interactions determine the circulation, thermal structure, and distribution of constituents in the atmosphere. That is, the problems require a coupling of the physics and chemistry of the atmosphere. Furthermore, the models must be applicable on a variety of spatial (regional-to-global) and temporal (days-to-decades) scales (see Chapter 6). A particularly important and challenging issue is the need to reduce the uncertainty on the size and spatial pattern of the indirect aerosol effects (see Chapter 6, Section 6.8).

Most of the effort in three-dimensional atmospheric chemistry models over the last decade has been in the use of transport models in the analysis of certain chemically active species, e.g., long-lived gases such as nitrous oxide (N2O) or the chlorofluorocarbons (CFCs). In part, the purpose of these studies was not to improve our understanding of the chemistry of the atmosphere, but rather to improve the transport formulation associated with general circulation models and, in association with this improvement, to understand sources and sinks of CO2. The additional burden imposed by incorporating detailed chemistry into a comprehensive general circulation model has made long-term simulations and transient experiments with existing computing resources challenging. Current three-dimensional atmospheric chemistry models which focus on the stratosphere seek a compromise solution by employing coarse resolution (both vertical and horizontal dimensions); incorporating constituents by families (similar to the practice used in most two-dimensional models); omitting or simplifying parametrizations for tropospheric physical processes; or conducting "off line" transport simulations in which previously calculated wind and temperature fields are used as known input to continuity equations including chemical source/sink terms. This last approach renders the problem tractable and has produced much progress towards understanding the transport of chemically reacting species in the atmosphere. The corresponding disadvantage is the lack of the interactive feedback between the evolving species distributions and the atmospheric circulation. Better descriptions of the complex relationship between hydrogen, nitrogen, and oxygen species as well as hydrocarbons and other organic species are needed in order to establish simplified chemical schemes that will be implemented in chemical/transport models. In parallel, better descriptions of how advection, turbulence, and convection affect the chemical composition of the atmosphere are needed. (See Chapter 4, Section 4.5.2.)

We also need improved understanding of the processes involving clouds, surface exchanges, and their interactions with radiation. The coupling of aerosols with both the energy and water cycles as well as with the chemistry components of the system is of increasing importance. Determining feedbacks between the land surface and other elements of the climate system will require careful attention to the treatments of evapotranspiration, soil moisture storage and runoff. All of these occur on spatial scales that are small compared with the model meshes, so the question of scaling must be addressed. These improvements must be paralleled by the acquisition of global data sets for validation of these treatments. Validation of models against global and regional requirements for conservation of energy is especially important in this regard. (See Chapter 4, Section 4.5.1.)

The problems associated with how to treat clouds within the climate system are linked to problems associated with aerosols. Current model treatments of climate forcing from aerosols predict effects that are not easily consistent with the past climate record. A major challenge is to develop and validate the treatments of the microphysics of clouds and their interactions with aerosols on the scale of a general circulation model grid. A second major challenge is to develop an understanding of the carbon components of the aerosol system. Meeting this challenge requires that we develop data for a mechanistic understanding of carbonaceous aerosol effects on clouds as well as developing an understanding of the magnitude of the anthropogenic and natural components of the carbonaceous aerosol system. (See Chapter 6, Sections 6.7 and 6.8; see also Chapter 4, Section

As attention is turned toward the troposphere, the experimental strategy simply cannot adopt the stratospheric simplifications. The uneven distribution of emission sources at the surface of the Earth and the role of meteorological processes at various scales must be addressed directly. Fine-scaled, three-dimensional models of chemically active trace gases in the troposphere are needed to resolve transport processes at the highest possible resolution. These models should be designed to simulate the chemistry and transport of atmospheric tracers on global and regional scales, with accurate parametrizations of sub-scale processes that affect the chemical composition of the troposphere. It is therefore necessary to pursue an ambitious long-term programme to develop comprehensive models of the troposphere system, including chemical, dynamical, radiative, and eventually biological components. (See Chapter 4, Sections 4.4 to 4.6.)

The short-lived radiatively important species pose an observational challenge. The fact that they are short-lived implies that observations of the concentrations are needed over wide spatial regions and over long periods of time. This is particularly important for aerosols. The current uncertainties are non-trivial (see again Chapter 6, Figure 6.7) and need to be reduced.

In sum, there needs to be an expanded attack on the key contributors to uncertainty about the behaviour of the climate system today and in the future. As stated in Chapter 13, Section 13.1.2, "Scenarios should also provide adequate quantitative measures of uncertainty. The sources of uncertainty are many, including the trajectory of greenhouse gas emissions in the future, their conversion into atmospheric concentrations, the range of responses of various climate models to a given radiative forcing and the method of constructing high resolution information from global climate model outputs (see Chapter 13, Figure 13.2). For many purposes, simply defining a single climate future is insufficient and unsatisfactory. Multiple climate scenarios that address at least one, or preferably several, sources of uncertainty allow these uncertainties to be quantified and explicitly accounted for in impact assessments."

In addition to this needed expansion in the attack on uncertainties in the climate system, there is an important new challenge that should now be addressed more aggressively. It is time to link more formally physical climate-biogeochemical models with models of the human system. At present, human influences generally are treated only through emission scenarios that provide external forcings to the climate system. In future comprehensive models, human activities will interact with the dynamics of physical, chemical, and biological sub-systems through a diverse set of contributing activities, feedbacks, and responses. This does not mean that it is necessary or even logical to attempt to develop prognostic models of human actions since much will remain inherently unpredictable; however, the scenarios analysis could and should be more fully coupled to the coupled physical climate-biogeochemical system.

As part of the foundation-building to meet this challenge, we turn attention now to the human system.

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