the dynamics as we can in comprehensive numerical models. On the other hand, we try to understand by simplifying and capturing the essence of a phenomenon in idealized models, or even with qualitative pictures. As our comprehensive models improve in quality, they more and more often become the primary tools by which theory confronts observations. The study of global warming is an especially good example of this trend. A handful of major modeling centers around the world compete in creating the most convincing climate simulations and the most reliable forecasts of climate change, while large observational efforts are mounted with the stated goal of improving these comprehensive models.

Due to the great practical value of simulations, and the opportunities provided by the continuing increases in computational power, the importance of understanding is occasionally questioned. What does it mean, after all, to understand a system as complex as the climate, when we cannot fully understand idealized nonlinear systems with only a few degrees of freedom?

Without attempting an all-encompassing definition, it is fair to say that we typically gain some understanding of a complex system by relating its behavior to that of other, especially simpler, systems. For sufficiently complex systems, we need a model hierarchy on which to base our understanding, describing how the dynamics change as key sources of complexity are added or subtracted. Our understanding benefits from appreciation of the interrelationships among all elements of the hierarchy.

The importance of such a hierarchy for climate modeling has often been emphasized. See Hoskins (1983) for a particularly eloquent discussion. But despite notable exceptions in a few subfields, climate theory has not, in my opinion, been very successful at hierarchy construction.

Consider by analogy another field that must deal with exceedingly complex systems—molecular biology. How is it that biologists have made such dramatic and steady progress in sorting out the human genome and the actions and interactions of the thousands of proteins of which we are constructed? Without doubt, one key has been that nature has provided us with a hierarchy of biological systems of increasing complexity amenable to experimental manipulation, ranging from bacteria to fruit fly to mouse to man. Furthermore, the nature of evolution assures us that much of what we learn from simpler organisms is directly relevant to deciphering the workings of their more complex relatives. What good fortune for the biological sciences to be presented with precisely the kind of hierarchy needed to understand a complex system! Imagine how much progress would have been made if one were limited to studying man alone.

Unfortunately, nature has not provided us with simpler climate systems that form such a beautiful hierarchy. Planetary atmospheres provide us with some insights into the range of behaviors possible, but they are few in number, and each planet has its own idiosyncrasies. While their study has connected to



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