Transient Climate Simulation
The present unit describes a slightly different kind of global climate simulation, called a transient simulation. The same model is used for both equilibrium and transient simulations, but for the transient runs, external forcing (e.g., greenhouse gases, solar irradiation) are allowed to gradually change according to known (e.g, past measured greenhouse gas concentrations) or assumed future projections of external forcing. Also, internal components of the climate system (e.g., oceans, ice masses, atmosphere) are allowed to change at their natural rates in response to external forcing and feedback processes. For instance, a sudden increase in heat energy to the climate system (for whatever reason) would lead rather quickly to a rise in air temperature, but rises in land temperatures would take much longer and oceans and ice masses even longer. Similarly the gradual rate of redistribution of thermal energy (warming at the surface and cooling in the stratosphere) due to increases in greenhouse gases allows the atmosphere to keep up with these changes but the oceans and ice likely do not keep up because their time scales for change are so much longer than that of the atmosphere.
On the basis of these simple concepts we can speculate on how a transient simulation of global climate will differ from an equilibrium simulation. As greenhouse gases build up to the equivalent of twice the pre-industrial level (2xCO2 level) in a transient simulation, the ocean and ice masses will not be in equilibrium with the rate of heating of the lower atmosphere: they will be lagging behind the rate of forcing due to their longer time scales. We can then speculate that the global-mean temperature at the time the transient simulation reached 2xCO2 greenhouse levels would not be as high as that for an equilibrium 2xCO2 simulated climate.
Characteristics and results of some coupled ocean-atmosphere global climate models run in transient mode are tabulated in Figure 1 for runs having various levels of increase of CO2 (scenarios). The flux adjustment indicates how the ocean and atmosphere are coupled together. The "Warming at doubling" column gives the global mean temperature when the CO2 passes the 2xCO2 level (twice pre-industrial level of CO2) in the transient run. The "Equilibrium warming" gives the global mean temperature when the model is run in equilibrium mode, and the final column gives the ratio of the previous two columns. Note that our speculation of the previous paragraph is confirmed by the last column.