We are concerned about too much of something without which we could not be alive: the greenhouse effect. This existential imperative is the regulation of the Earth’s atmospheric temperature by a few trace gases—above all by carbon dioxide (CO2) and methane (CH4). Compared to the two gases that make the bulk of the atmosphere (nitrogen at 78 percent, oxygen at 21 percent), their presence is negligible (small fractions of a percent) but their effect makes the difference between a lifeless, frozen planet and a blue and green Earth.[35]
