1.0: Introduction

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Considerably more than 90% percent of climate scientists attribute the bulk of the increase in global mean temperature over the past three to four decades to the anthropogenic increase in atmospheric greenhouse gases that commenced with the Industrial Revolution.^* The great majority of these scientists hold that continued warming presents significant risks to humankind over the coming centuries. What scientific evidence led the scientific community to these conclusions? How robust is that evidence? To what extent should we trust uncertain projections of future climate change based on complicated global climate models? How do we deal with climate change as a problem of risk assessment and management?

This essay summarizes the most important lines of evidence for anthropogenic climate change, confronts some of the stickier questions behind uncertainty in climate projections, and concludes with a discussion of the particular risks entailed by climate change and how they are being quantified.

The essay is structured as follows. First, a brief history of climate science reveals that the most important principles underlying the field were established more than a century ago. A brief tutorial on the greenhouse effect follows and provides evidence that we are altering the concentrations of important greenhouse gases and consequently altering our climate. Trends in global mean temperature over the last century are then set in the context of climate change over much of earth’s history, and the causes of past and present climate change are reviewed, addressing the question of whether some of the mechanisms underlying past climate change—such as changing sunlight and orbital forcing—could explain the extraordinarily rapid warming of the past several decades. The concept of a climate model is explained, and the use of such models to estimate future climate change is illustrated. Finally, climate change is presented as a problem of risk assessment and management, and quantitative estimates of individual climate risks to the United States are given.

But, first, a brief note about science itself.

Footnote

Cook et al., 2016: Consensus on consensus: A synthesis of consensus estimates on human-caused global warming. Environ. Res. Lett. 11
http://iopscience.iop.org/article/10.1088/1748-9326/11/4/048002.