Carbon dioxide is the chief offender, accounts for about 380 of every million molecules in the air, or 380 parts per million (ppm). That number has been climbing by 1–3ppm, or about one-quarter to three-quarters percent per year. The worldwide emissions of CO₂ are increasing at several percent per year, but that annual ramp-up becomes a smaller percentage when it joins the large amount of CO₂ already in the air. Both a pollutant and a natural part of the atmosphere, carbon dioxide is produced when fossil fuels are burned as well as when people and animals breathe and when plants decompose.
Plants and the ocean soak up huge amounts of carbon dioxide, which is helping to keep CO₂ levels from increasing even more rapidly. Because the give-and-take among these processes is small compared to the atmospheric reservoir of CO₂, a typical molecule of carbon dioxide stays airborne for over a century. Atmospheric levels of CO₂ held fairly steady for centuries – at around 270–280ppm – until the Industrial Revolution took off. In past geological eras, the amount of CO₂ has risen and fallen in sync with major climate changes, although there’s chicken-or-egg uncertainty in whether CO₂ led or lagged some of these transitions.