“We buy fire insurance for a house and health insurance for our bodies. We need planetary sustainability insurance.”

In April 1968, Columbia University (New York) was the center of student protests so violent that faculty were assaulted and the entire university ground to a halt for the academic quarter. Hundreds of students were arrested, but Stephen H. Schneider, now a prominent climatologist, watched the riots from the sidelines as he pursued his engineering doctoral work. Yet when desperate administrators and trustees agreed to negotiate with a few elected students, Schneider felt compelled to come forward and make himself heard. As he learned on the fly to mediate between opposing groups, he unknowingly found a training ground for his future role as a controversial climate change spokesman. Here, Schneider says, he first learned that each value system has its own merits, and that some decisions boil down to a simple trade-off between value alternatives.

Thirty-four years later, in 2002, Schneider was elected to the National Academy of Sciences for his research work in climate modeling and policy applications. One of the earliest researchers in global warming, he developed models to describe the role of cloud height in climate systems and determined the need for time-evolving coupled atmosphere–ocean models. Schneider is now Melvin and Joan Lane Professor for Interdisciplinary Environmental Sciences and a professor in the Department of Biological Sciences at Stanford University (Stanford, CA), in addition to being a senior fellow in the Stanford Institute for the Environment and Professor by Courtesy in the Department of Civil and Environmental Engineering.

In his Inaugural Article in this issue of PNAS (1), Schneider and postdoctoral fellow Michael D. Mastrandrea use probabilistic models to connect economics and technology to climate change outcomes. Through their models, they attempt to quantify the value systems of various groups affected by climate change and to estimate the likelihoods of exceeding “dangerous” climate impact thresholds given alternative …