1.1 Aims

This Technical Paper is intended as a primer on the climate system and SCMs, and has two objectives: (a) to explain how SCMS work, the processes that are included in them, what their strengths and weaknesses are in relation to more complex models, the purposes to which they are applied, and why they have been used extensively in the SAR WGI; and (b) to fully document the procedures and assumptions used to generate the trace gas concentration, global mean temperature change, and global mean sea level rise projections presented in the SAR WGI (Section 6.3) and in IPCC TP STAB (1997).

1.2 Climate Models as Tools for Scientific and Policy Analysis

Understanding the climate system is a problem of great intrinsic scientific interest. Our growing understanding of interactions between the atmosphere, oceans, biosphere, cryos phere and land surface is revolutionizing the Earth sciences. Moreover, in recent years, a sense of urgency has infused research on modelling the climate system. The prospect of human activities altering atmospheric composition, affecting climate globally and regionally, and ultimately affecting human economies and natural ecosystems, has stimulated the development of models of the climate system.

Clearly, it is important to have useful and credible tools for policy analysis before the climate itself changes. Thus, climate system models employed by researchers contributing to the SAR WGI are motivated, at least in part, by the desire to make timely predictions of anthropogenic climatic impacts from greenhouse gas and aerosol emissions across the chain of causality from emissions to impacts.

An important concept in climate system modelling is the notion of a hierarchy of models of differing levels of complexity, dimensionality and spatial resolution, each of which may be optimum for answering different questions. It is not meaningful to judge one level as being better or worse than another, independent of the context of analysis.

Ideally, one seeks a balance whereby each component of the climate system is represented at an appropriate level of detail. How to do this is the modeller's "art". There is no methodological crank to turn, although some overall principles are clear; for example, it would be an inefficient use of computer resources to couple a detailed model for some part of the system with little effect on the particular area of concern to one with crudely represented physical processes that dominates the model output. Einstein once quipped that, "everything should be as simple as possible, but no simpler". Generations of modellers have agonized over what "no simpler" means. This has been a

particularly important issue for assessments of anthropogenic climate change conducted by the IPCC.

The most general computer models for climate change employed by the IPCC are the coupled AOGCMs (see Section 3.1), which solve the equations of the atmosphere and oceans approximately by breaking their domains up into volumetric grids, or boxes, each of which is assigned an average value for properties like velocity, temperature, humidity (atmosphere) and salt (oceans). The size of the box is the models' spatial resolution. The smaller the box, the higher the resolution. An assumption of research involving general circulation models (GCMs) is that the realism of climate simulations will improve as the resolution increases.

In practice, computing limitations do not allow models of high enough resolution to resolve important sub-grid processes. Phenomena occurring over length scales smaller than those of the most highly resolved GCMs, and that cannot be ignored, include cloud formation and cloud interactions with atmospheric radiation; sulphate aerosol dynamics and light scattering; ocean plumes and boundary layers; sub-grid turbulent eddies in both the atmosphere and oceans; atmosphere/biosphere exchanges of mass, energy and momentum; terrestrial biosphere growth, decay and species interactions; and marine biosphere ecosystem dynamics -to cite a few examples. Mismatches between the scale of these processes and computationally realizable grid scales in global models is a well-known problem of Earth system science.

To account for sub-grid climate processes, the approach has been to "parametrize" - that is, to use empirical or semi-empirical relations to approximate net (or area-averaged) effects at the resolution scale of the model (see Section 3 for further discussion). It is important to stress that all climate system models contain empirical parametrizations and that no model derives its results entirely from first principles. The main conceptual difference between simple and complex models is the hierarchical level at which the empiricism enters.

It is essential, for example, to account for the heat and carbon that enter the oceans as the climate warms from the greenhouse effect of CO2 emitted by fossil fuel burning. The internal mixing and transport in the oceans of this energy and mass invading at the air-sea interface are key processes that must be represented in any model used to project future CO2, climate and sea level variations. The rate at which heat and dissolved carbon penetrate the thermocline (roughly the first kilometre of ocean depth) controls how much global warming is realized for a given radiative forcing, and how much CO2 remains in the atmosphere. In principle, these processes could be computed by AOGCMs, but AOGCMs are presently too time-consuming to run on computers for a wide range of emission scenarios. For this reason, the global mean CO2, temperature, and sea level projections for the IS92 emission scenarios and the CO2

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stabilization calculations presented in the SAR WGI, and similar calculations in IPCC TPSTAB (1997), were carried out with simple models.

The choice of the most appropriate level of parametrization for climate system modelling is a qualitative judgement based on the best scientific knowledge and computer limitations. Consider the one-dimensional upwelling-diffusion ocean introduced by Hoffert, et al. (1980, 1981) and subsequently developed by many other researchers (Section 3.1), used to parametrize the world's oceans in several IPCC carbon cycle, climate and sea level calculations. In this paradigm, the three-dimensional world oceans are replaced by a single horizontally-averaged column in which carbon concentration and temperature vary with depth. The column exchanges mass and energy at its top with a well-mixed ocean surface layer; at its bottom, the column is fed by cold water from a downwelling polar sea. This one-dimensional paradigm works well at simulating historical climate and carbon cycle variations. To simplify further by replacing the column with a single well-mixed box or a purely diffusive ocean would make it too simple. A well-mixed box cannot account for the fact that the mixing time of the oceans is long compared to the rates at which carbon emissions and radiative forcing at the surface are changing. The result would be incorrect rates of heat and mass uptake over time. Things are already "as simple as possible" with a one-dimensional upwelling-diffusion ocean, so we stop there.

Another frequently asked question is: "how do we know if model predictions are credible"? Science today recognizes that there is no way to prove the absolute truth of any hypothesis or model, since it is always possible that a different explanation might account for the same observations. In this sense, even the most well-established physical laws are "conditional". Rather, the test should be whether a theory or model is false. The more independent challenges that a theory or model passes successfully, the more confidence one can have in it. Indeed, the testability of a conjecture has become a necessary condition for it to be considered in the domain of science. As Sir Karl Raimund Popper, philosopher of science and developer of the doctrine of falsifiability, put it, "Our belief in any particular natural law cannot have a safer basis than our unsuccessful critical attempts to refute it" (Popper, 1969).

The application of the falsifiability rule can be seen in the values of the climate sensitivity (Section 2.3), equivalent to the

equilibrium temperature change for a CO2 doubling, estimated by the SAR WGI to lie, most probably, in the range of 1.5 to 4.5°C (SAR WGI: Technical Summary, Section D.2). Climate sensitivity is computed in AGCMs based on a combination of physical laws and sub-grid scale model parametrizations, but is directly specified as an input in simple ocean/climate models. At least four independent methods have been used to estimate the climate sensitivity: (a) from simulations with three-dimensional AGCMs (Cess, et al., 1989); (b) from direct observations, at the relevant temporal and spatial scales, of the key processes that determine radiative damping to space and hence climate sensitivity (e.g., Soden and Fu, 1995); (c) from reconstructions of radiative forcing and climate response of ancient (palaeo-) climates (Hoffert and Covey, 1992); and (d) from comparisons of ocean/climate model runs with historical global temperature records (see Section 4.2 and Figure 10). Each method has unique disadvantages and uncertainties. However, all of these independent methods give results that are consistent with the SAR WGI range 1.5 to 4.5°C, and are inconsistent with values substantially lower or higher.

Finally, simple climate system models appear to have the drawback of dealing only with global or zonal averages, whereas regional variations of temperature and precipitation change are needed to complete the link in integrated assessments from emissions to impacts. Again, in practice, many present-day integrated assessments are conducted with models whose core transient climate calculations are done with simple ocean/climate models using regional distributions of temperature and precipitation (typically produced by AOGCMs) that have been scaled to the global mean temperature change (Santer, et al., 1990, Hulme, et al., 1995).

The foregoing considerations are meant to explain the rationale underlying the use of simplified models of the climate system in the SAR, and do not suggest that a particular modelling methodology or level of complexity is inherently superior for climate system analysis for all time. Indeed, the consensus of the climate modelling community is that detailed threedimensionally resolved models of atmosphere and ocean dynamics, and correspondingly highly resolved models of the Earth's terrestrial and marine biota, are the long-term goals of Earth system science. These modelling efforts need to proceed in parallel with, and mutually reinforce, the more idealized models of the climate system used in work relating to scenario analysis and climate policy, as the IPCC process evolves.

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Humans are altering the concentration of greenhouse gases and aerosols, both of which influence, and are influenced by, climate. The greenhouse gases reduce the net loss of infrared heat to space, while having little impact on the absorption of solar radiation, thereby causing the surface temperature to be warmer than it would be otherwise and producing the so-called greenhouse effect (see SAR WGI: Sections 1.2.2 and 1.3.1). Aerosols, on the other hand, are important largely because of their impact on solar radiation, and have a predominantly cooling effect (see SAR WGI: Section 1.3.2).

Some greenhouse gases occur naturally but are influenced either directly or indirectly by human activity, whereas others are purely anthropogenic. The main naturally-occurring greenhouse gases are water vapour (H2O), carbon dioxide (CO2), ozone (O3), methane (CH), and nitrous oxide (NO). The main groups of purely anthropogenic greenhouse gases are the CFCs, HCFCs, and HFCs (collectively known as halocarbons), and fully fluorinated species such as sulphur hexafluoride (SF6) (see SAR WGI: Chapter 2).

plants, soils, ocean water and ocean sediments). The sources of natural greenhouse gases, and the removal processes of all greenhouse gases, are themselves influenced by climate (see SAR WGI: Sections 1.2 and 2.2).

Aerosols are suspensions of small particles in the air which influence climate primarily through their role in reflecting a portion of the incoming solar energy back to space (a direct effect) and in regulating to some extent the amount and optical properties of clouds (an indirect effect). Aerosols also absorb infrared radiation to some extent. Aerosols are produced both naturally and through human activity; natural aerosols include sea salt, dust, and volcanic aerosols, while anthropogenic aerosols are produced from burning of biomass and fossil fuels, among other sources. Some aerosols, such as dust, are directly emitted into the atmosphere. The majority of aerosols, however, are not directly emitted but, like tropospheric O3, are produced through chemical transformation of precursor gases. All tropospheric aerosols have a short lifespan in the atmosphere due to the fact that they are rapidly washed out with rain. For this reason, and because emission source strength varies strongly from one region to another, the amount of aerosols in the atmosphere varies considerably from one region to another. The nature, amount and distribution of atmospheric aerosols are themselves influenced by climate (see SAR WGI: Sections 2.3 and 2.4).

Apart from the composition of the Earth's atmosphere, a number of processes involving clouds, surface properties, and atmospheric and oceanic motions are also important to regional and global scale climate.

With the exception of ozone, all of the greenhouse gases that are directly influenced by human emissions are well mixed within the atmosphere, so that their concentration is almost the same everywhere and is independent of where emissions occur. Ozone also differs from the other greenhouse gases in that it is not directly emitted into the atmosphere; rather, it is produced through photochemical reactions involving other substances — referred to as "precursors" — which are directly emitted. With regard to removal processes, all of the non-water vapour greenhouse gases except CO2 are removed largely by either chemical or photochemical reactions within the atmosphere. Carbon dioxide, in contrast, continuously cycles between a number of "reservoirs" or temporary storage depots (the atmosphere, land

Clouds

The amount, location, height, lifespan, and optical properties of clouds exert important controls on the Earth's climate, and changes in these properties might play an important role in climatic change. The radiative impact of a given change in cloud properties, cloud amount, or cloud height depends on the location and time of year and day when the changes occur. Such changes in clouds as do occur will depend on the threedimensional temperature and moisture fields and on atmospheric dynamical processes (ie., those related to winds). For these reasons, three-dimensional models with high spatial resolution and a diurnal cycle hold the only prospect of correctly simulating the net effect on climate of cloud changes. However, most key cloud processes occur at scales well below the resolution of global models, so that simple area-average representations ("parametrizations") of cloud processes are required, thereby introducing the potential for substantial error in the simulated cloud changes (see SAR WGI: Sections 4.2 and 5.3.1.1.4 and Section 3 of this paper).

The physical characteristics of the land surface, including the vegetation cover, have a strong effect on the absorption of solar energy and on the fluxes of heat, water vapour and momentum between the surface and atmosphere. These fluxes at any given location strongly influence the local surface climate and have effects on the atmosphere which, in some cases, extend globally. Of particular importance are changes in the extent of highly reflective ice and snow cover, as climate warms, the area of ice and snow will decrease, leading to greater absorption of solar energy and further warming. However, concurrent changes in cloud cover induced by the changes in ice and snow extent complicate the picture considerably. Correct simulation of landsurface changes and their net effect requires models with high spatial and temporal resolution on account of potential interactions with clouds and because of the spatial heterogeneity of the surface (see SAR WGI: Sections 1.4.3 and 4.4). On a time-scale of decades to centuries, changes in the vegetative cover and soil properties will also alter the exchanges of heat, moisture and momentum between the surface and atmosphere, as well as the sources and sinks of a number of greenhouse gases.

The oceans play a number of important roles in the climate system and in climatic change. First, they are a major storehouse of carbon, and have played an important role in absorbing a portion of the anthropogenic CO2 emitted up to the present. This role will continue to some extent in the future. Second, ocean currents transport substantial amounts of heat, thereby exerting a strong influence on regional climates. Changes in oceanic heat transport could significantly affect regional climatic changes, possibly causing some regions to cool temporarily and others to warm by considerably more than the global mean as the global climate warms. Third, the absorption and downward mixing of heat by the oceans considerably slows down the rate of surface warming. This reduces those impacts which depend on the rate of climatic change, but also implies that, until some time after greenhouse gas concentrations have been stabilized, there will be an irreversible commitment to more climatic change than has already occurred. Ocean currents and the rate of absorption of heat by the oceans depend on wind patterns and the exchange of heat and freshwater (through precipitation and evaporation) between the ocean and the atmosphere. At high latitudes, the presence of sea ice has a very strong effect on these exchanges, so the satisfactory simulation of sea ice is of considerable importance (see SAR WGI: Sections 1.4.2, 4.3, and 6.2; and SAR WGI: Chapter 10).

2.2.4 Atmospheric Motions

Atmospheric motions (winds) are important for transporting heat and moisture and moderating temperatures in both polar and equatorial regions. Atmospheric motions exert a strong

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control over the formation, nature and lifespan of clouds, thereby providing a direct coupling to both solar and infrared radiation budgets. Atmospheric heat transport and changes therein will also influence the response of sea ice and land snow cover to global mean temperature changes, thereby providing another link to the Earth's overall radiative balance. Changes in atmospheric winds, or in evaporation and precipitation due in part to changes in atmospheric winds, could also lead to significant and possibly abrupt changes in the oceans' circulation (see SAR WGI: Sections 4.2, 4.3. and 6.2).

The temperature of the Earth tends to adjust itself such that there is a balance between the absorption of energy from the Sun and the emission of infrared radiation from the surfaceatmosphere system. If, for example, there were to be an excess of absorbed solar energy over emitted infrared radiation (as occurs with the addition of greenhouse gases to the atmosphere), temperatures would increase but, in so doing, the emission of infrared radiation to space would increase. This would reduce the initial imbalance, and eventually a new balance would be achieved, but at a new, warmer temperature (see SAR WGI: Sections 1.2 and 1.3.1).

Anthropogenic greenhouse gases and aerosols affect the climate system by altering the balance between absorbed solar radiation and emitted infrared radiation, as discussed in the SAR WGI (Section 2.4). The imbalance is quantified as the "radiative forcing", which is defined as the change in net downward radiation (combined solar and infrared) at the tropopause when, for example, greenhouse gas or aerosol amounts are altered, after allowing for the adjustment of stratospheric temperatures only. The surface climate responds to the initial change in net radiation at the tropopause rather than at the surface itself or at the top of the atmosphere because the surface and troposphere are tightly coupled through heat exchanges, and respond as a unit to the combined heating perturbation. The adjustment of the stratosphere is included in the radiative forcing because the stratosphere responds quickly and independently from the surface-troposphere system. Non-anthropogenic radiative forcings relevant at the decade to century time-scales include variations in solar luminosity and volcanic eruptions, the latter producing reflective sulphate aerosols which are effective for several years if injected into the stratosphere.

The radiative forcing for a CO2 doubling is 4.0-4.5 W m2 before adjustment of stratospheric temperatures (Cess, et al., 1993); allowing for stratospheric adjustment reduces the forcing by about 0.5 W m2 to 3.5-4.0 W m2. If temperature were the only climatic variable to change in response to this