The chapter begins by referencing the “harrowing” media coverage of the threat of global warming in 2009. However, in the 1970s, equally harrowing headlines covered the effects of global cooling. The global temperature dropped between 1945 and 1968, and experts feared that the drop could result in global crop shortages that could destroy humanity. While there is now a consensus that human activities contribute significantly to climate change, the specific sources are often misunderstood. A striking example presented is the significant impact of ruminant animals such as cows on greenhouse gas emissions. The authors highlight that livestock, and especially cows, are substantial contributors to emissions of methane, a potent greenhouse gas. This piece of information challenges the more common focus on cars and transportation as the primary culprit of greenhouse gas emissions. The authors therefore suggest that human dietary changes—such as substituting beef with less methane-intensive meats like kangaroo—could have a more substantial environmental benefit than the widespread adoption of hybrid vehicles.

Next, Levitt and Dubner introduce the economic concept of externalities—costs or benefits incurred by third parties due to the actions of an individual or group. Environmental pollution is a classic negative externality in which the broader population bears the costs of environmental degradation caused by certain activities.