Grand Challenges in Earth and Environmental Science

Recognizing the Signal Within the Natural Variability This first challenge is, of course, at the crux of the global warming conundrum. Are steady increases in global temperature (and accompanying climate changes) in the past 150 years simply an expression of natural variability, or are they a direct result of mankind’s activities that have resulted in an increase in greenhouse gases? The weight of the scientific evidence suggests the latter, however the debate on global warming has turned into a high-stakes, international issue with potentially multibilliondollar implications. Documenting and understanding natural variability is a vexing topic in almost every environmental problem: How do we recognize and understand changes in natural systems if we don’t understand the range of baseline levels? Our geologic perspective allows us to view the short interval of historical records with a healthy skepticism. Figure 1 shows a 350 year record of precipitation in California (1600–1950), determined using 52 treering chronologies as proxies for precipitation (Fritts, 1984). I have added two important historical dates to this chronology: 1769, the founding of the first of the Spanish missions in California and the inception of written records; and 1849, the beginning of the California gold rush that within years increased the number and 2000 Presidential Address: Grand Challenges in Earth and Environmental Sciences: Science, Stewardship, and Service for the Twenty-First Century