Transient climate simulation
Encyclopedia
A transient climate simulation is a mode of running a global climate model
Global climate model
A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...

 (GCM) in which a period of time (typically 1850-2100) is simulated with continuously-varying concentrations of greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

es so that the climate of the model represents a realistic mode of possible change in the real world.

Related models

This may be contrasted with an equilibrium climate simulation in which greenhouse gas concentrations are suddenly changed (typically from pre-industrial values to twice pre-industrial values) and the model allowed to come into equilibrium with the new forcing.

History

Early GCMs used "swamp" or "slab" ocean models for reasons of computational simplicity. Such models cannot simulate the heat take up that occurs in the real ocean and were thus unable to run transient simulations: instead, the response to equilibrium change was investigated. There are problems with this approach, which were well appreciated at the time: the deep ocean has a very long time constant and in some areas (most notably the southern ocean) the transient and equilibrium responses are very different.

Model interpretation issues

In an equilibrium simulation, time is merely a label and a given year or decade does not represent the simulation of a calendar year or decade.
  • Thus (by definition) a portion of a simulation labelled "2000-2030" represents 30 years, but not any particular 30 years.
  • In a transient simulation, instead of a sudden change in greenhouse gases and other forcings, the forcings are changed gradually, either in an idealised way (1% increase, for example) or a more realistic fashion (one of the scenarios described in the Special Report on Emissions Scenarios
    Special Report on Emissions Scenarios
    The Special Report on Emissions Scenarios was prepared by the Intergovernmental Panel on Climate Change in 2000, based on data developed at the Earth Institute at Columbia University. The emissions scenarios described in the Report have been used to make projections of possible future climate...

    ).


If the simulation is partly of the past, observed levels may be used (and perhaps solar variation, and volcanic forcing).

The transient simulation is intended to be a physically plausible path for the climate system to follow.

Although (given natural variability) even a perfect model would not simulate the year-to-year variations seen in the real world, in an ideal model the variation from decade to decade would track that of the real world.





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