Climate is a topic of great concern to students, but it remains difficult to discuss scientifically. We are bombarded daily by emotional messaging for and against the proposition that human activities are altering the climate in ways that may have catastrophic effects. But it often seems that those predictions rest entirely on massive computer simulation models, which the listener is not in a position to evaluate. We wished to find an aspect of the puzzle that could be directly experienced, and that would therefore give us a solid starting point for discussion. Here we describe a dramatic lecture demonstration that scales well for large audiences and that requires as little as 10 classroom minutes to show an important difference between room air and carbon dioxide gas. Beyond its significance for public policy, the demonstration can serve as a springboard to motivate discussion of important physics concepts such as molecular rotation and vibration spectra.