Climate Change as Seen by Trees and by Climate Modelers

Sue Ann Bowling


Over the last few years several attempts have been made to model how CO2 warming will affect forest distribution. One of the questions raised about such studies, that of whether forests can migrate fast enough to keep up with modeled climate change, is probably not as serious as it seems -- in our human-dominated world, if forests are considered desirable, trees will be planted in tundra that has warmed enough for them to survive, even if the nearest seed source is several thousand kilometers away. The best species and provenance of trees is addressed in a number of the other papers in this Proceedings. The question examined in this paper is more basic: how trustworthy are the results of climate change models? In particular, can the results of the models be used as a guide for planting seedings that will mature 50 to 100 years in the future?

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:

The physical basis of climate
Climate as seen by a tree
Climate from the point of view of the climate modeller

I think it is clear from the above that predicting future forest distribution from the output of a single climate model is a rather questionable enterprise. This does not, however, mean that the models are of no use.
So far as the present models are concerned, a key factor is consensus among models. There are some results on which most or all the current models seem to be in agreement. A global temperature rise in response to increasing greenhouse gasses, for instance, is predicted by most models and also indicated by past correlations between carbon dioxide content and global temperature. There is considerably more disagreement among models about predicted changes in precipitation and regional climate change, while none of the present models can claim to predict precipitation with any accuracy in areas of strong relief. In general, current models can reasonably be used to predict qualitative forest boundary changes in regions where most of the state-of-the-art models are in agreement.

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.

The original paper was presented at the NATO Advanced Research Workshop on Forest Development in Cold Climets held June 18-23, 1991 in Laugarvain, Iceland. It was published by Plenum in their ASI Series Series A: Live Sciences Vol 244, 1993. The version given here is being updated somewhat from the original paper to reflect advances in modeling over the last several years.