Throughout this book, a number of common themes reappear. Some of these common themes were apparent to us before we began preparing the first edition of the book, and in fact, they were a major motivation for us to write the book. Others became more and more apparent as we performed the literature survey and analysis, and prepared the written text. These issues have become even better defined in the years since our first edition. Some of these common themes are:

• the importance and underappreciation of natural variability,

• the dangers of overselling,

• the capricious administration of science,

• scientific credibility and advocacy, and

• politics and science.

While these subjects have been discussed in depth by others including science policy experts (Pielke Jr., 2007), let us briefly overview each of these common themes. A very effective summary of several of these issue are presented at http://sciencepolicy.colorado.edu/prometheus/archives/climate_change/index.html

The importance and underappreciation ofnatural variability

We have seen that our ability of determining if human activity demonstrably causes some observed or hypothesized effect, such as changes in local rainfall or global climate, is strongly dependent upon the natural variability of the system. This has certainly been true of determining if cloud seeding actually is capable of causing increases of rainfall in specified target areas. However, it is also true in assessing the role of such human climate forcings as anthropogenic greenhouse gas emissions, or deforestation, or release of CCN on global climate.