The path the models take from the observed and continuing increase in the CO2 content of the atmosphere to the geographical distribution of forests 100 years in the future is a tortuous one, with a number of pitfalls. Further, no single person is normally responsible for or even familiar with all of the steps. Because of this, it seems appropriate to review these steps, with particular emphasis on those known to be poor approximations of reality.
The full paper includes:
Beyond this, foresters need to push for improvement of the climate models in three ways.
First, models are needed that do a better job of parameterizing the processes that are related to both the energetics of the models and the relationship of the modeled surface climate to the modeled circulation. Probably the most important of these processes at the moment are:
Clouds and condensation processes. This includes not only clouds and their effect on the radiation balance (both shortwave and longwave), but also precipitation processes and the role of latent heat.
Boundary layer processes involving transport of energy and moisture through the boundary layer. These are the processes that control both the climate derived from the modeled circulation and much of the energy flow driving the circulation.
The second major change needed is to include in the climate models processes currently treated as boundary processes, such as the circulation of the oceans. Although this is a formidable challenge mathematically, it may well be essential if we are going to be able to predict the results of our unthinking and to a large extent uncontrollable manipulation of the composition of our atmosphere.
Finally, foresters need to interact with modellers in determining what are the most useful model outputs for their particular applications. Models work by creating their own internal weather, and almost any aspect of that interal weather can be captured and recorded if the modeller so desires. If a forester needs degree days above 30 degrees C, frost days in the summer months, number of hours with surface winds above a given threshold, consecutive days without precipitation, or a similar output, this information is generated by general circulation models. Its accuracy is subject to the same caveats is are the standard climate statistics, but getting it out of the model is more a matter of communicating its importance to the modellers than of any real programming difficulties.
Ideally, future generations of climate models will be improvements on today's not only in the accuracy of their depiction of the complex, chaotic system we call climate, but in improvement in the availability of a broad range of kinds of climate data. It is up to the users of non-standard measures of climate, however, to work with the modellers in developing these outputs.