To simulate vegetation climate feedbacks, it is necessary to pass back and forth between a climate model and the vegetation cover. Initially, a particular climate and a vegetation distribution are set up together. The vegetation distribution can be whatever the modeler is interested to try out, and does not need to be anything that corresponds to the actual present-day vegetation, or anything that is in balance with the climate. The purpose of the exercise is to see how the two of them vegetation and climate get along together. The vegetation is allowed to modify the climate (using such feedbacks as albedo, roughness and evaporation), and the modified climate is allowed to modify the vegetation (using the sort of bioclimatic relationships mentioned in Chapter 2). The two are allowed to interact, until they eventually settle down into some sort of steady state. The state that is arrived at can then be compared with what happens with a different starting point for vegetation for example, more desert or less desert. Or it can be compared with a world in which vegetation only responds passively to climate and does not feed back to change the climate. Making such comparisons allows us to find how important vegetation is in making climate.
In the early days of modeling vegetation climate feedbacks, this back-and-lorth interaction was worked out as many separate steps. The first run of the computer would give a particular climate, and a particular vegetation distribution would now be added in. Adding vegetation would modify the climate. Then, the simulation would be stopped, and the vegetation distribution would now be changed to something which corresponded to this altered climate. The simulation would be started again exactly where it left off except with a new modified vegetation, which now had a chance to modify the climate further. The process would be repeated again and again, until eventually vegetation and climate reached a balance with one another.
Now, this rather clumsy process of stopping and re-starting the model has been replaced by interactive models. The climate and vegetation respond to one another smoothly and continuously. The key to this is to have a vegetation scheme that in effect has the plants dynamically growing or dying off as the climate around them changes. One example of such a scheme is CLIMBER, which seems an appropriate acronym because the vegetation pulls itself along in its interaction with climate.
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