than preindustrial conditions (usually taken as 280 ppm). As discussed in further detail in the next section, data from even longer ice cores extracted from the hearts of the Greenland and Antarctic ice sheets—the bottoms of which contain ice that was formed hundreds of thousands of years ago—indicate that the current CO2 levels are higher than they have been for at least 800,000 years.
Collectively, the in situ measurements of CO2 over the past several decades, ice core measurements showing a sharp rise in CO2 since the Industrial Revolution, and detailed estimates of CO2 sources and sinks provide compelling evidence that CO2 levels are increasing as a result of human activities. There is, however, an additional piece of evidence that makes the human origin of elevated CO2 virtually certain: measurements of the isotopic abundances of the CO2 molecules in the atmosphere—a chemical property that varies depending on the source of the CO2—indicate that most of the excess CO2 in the atmosphere originated from sources that are millions of years old. The only source of such large amounts of “fossil” carbon are coal, oil, and natural gas (Keeling et al., 2005).