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The most talked-about models of recent years have been those relating air temperature to emissions of Carbon Dioxide (see Greenhouse Gas ). These models predict an upward trend in the Surface Temperature Record , as well as a more rapid increase in temperature at higher altitudes. Models can range from relatively simple to quite complex:
This is not a full list; for example "box models" can be written to treat flows across and within ocean basins. ZERO-DIMENSIONAL MODELS It is possible to obtain a very simple model of the radiative equilibrium of the Earth by writing : where
and
The constant ''πr''2 can be factored out, giving : which yields a value of 246 to 248 Kelvin s — about -27 to -25 °C — for the Earth's average temperature T. This is approximately 35 kelvins colder than the average surface temperature of 282 K. This is primarily because the above equation attempts to represent the ''radiative'' temperature of the Earth, and the average radiative level is well above the surface. The difference between the radiative and surface temperatures is the natural Greenhouse Effect . This very simple model is quite instructive, and the only model that could fit on a page. But it produces a result we are not really interested in — the radiative temperature — rather than the more useful surface temperature. It also contains the albedo as a specified constant, with no way to "predict" it from within the model ... RADIATIVE-CONVECTIVE MODELS The zero-dimensional model above predicts the temperature of an imaginary layer where long wave radiation is emitted to space. This can be extended in the vertical to a one dimensional radiative-convective model, which simplifies the atmosphere to consider only two processes of energy transport:
The radiative-convective models have advantages over the simple model: they can tell you the surface temperature, and the effects of varying Greenhouse Gas concentrations on the surface temperature. But they need added parameters, and still represent by one point the horizontal surface of the earth. Links:
ENERGY BALANCE MODELS Alternatively, the zero-dimensional model may be expanded horizontally to consider the energy transported horizontally in the atmosphere. This kind of model may well be zonally averaged. This model has the advantage of allowing a plausible dependence of albedo on temperature - the poles can be allowed to be icy and the equator warm - but the lack of true dynamics means that horizontal transports have to be specified.
EMIC'S (EARTH-SYSTEM MODELS OF INTERMEDIATE COMPLEXITY Depending on the nature of questions asked and the pertinent time scales, there are, on the one extreme, conceptual, more inductive models, and, on the other extreme, General Circulation Model s operating at the highest spatial and temporal resolution currently feasible. Models of intermediate complexity bridge the gap. One example is the Climber-3 model. Its atmosphere is a 2.5-dimensional statistical-dynamical model with 7.5° × 22.5° resolution and time step of 1/2 a day; the ocean is MOM-3 ( Modular Ocean Model ) with a 3.75° × 3.75° grid and 24 vertical levels.
GCMS (GLOBAL CLIMATE MODELS OR GENERAL CIRCULATION MODELS) See Also: general circulation model Three (or more properly, four) dimensional GCM's discretise the equations for fluid motion and integrate these forward in time. They also contain parametrisations for processes - such as convection - that occur on scales too small to be resolved directly. Atmospheric GCMs (AGCMs) model the atmosphere and impose sea surface temperatures. Coupled atmosphere-ocean GCMs (AOGCMs, e.g. HadCM3 , EdGCM ) combine the two models. AOGCMs represent the pinnacle of complexity in climate models and internalise as many processes as possible. However, they are still under development and uncertainties remain. Most recent simulations show "plausible" agreement with the measured temperature anomalies over the past 150 years, when forced by observed changes in "Greenhouse" gases and aerosols, but better agreement is achieved when natural forcings are also included [http://www.hadleycentre.gov.uk/research/hadleycentre/pubs/talks/sld017.html . SEE ALSO CLIMATE MODELS ON THE WEB
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