FIG. 25.4

What is the major source of the CO₂ that has accumulated in Earth’s atmosphere over the past two centuries?

BACKGROUND Chemical analyses show that CO₂ levels in the current atmosphere are about 40% higher than they were at the time of the American Revolution. This rise coincides with major advances in manufacturing and transportation, which are driven by the burning of fossil fuels. These coincidences in timing suggest that human activities are responsible for increasing CO₂ levels.

HYPOTHESIS The burning of fossil fuels, central to the emergence of modern industrial societies, has been the principal cause of the measured increase of CO₂ in the atmosphere.

EXPERIMENT AND RESULTS Hans Suess measured the abundance of the two stable isotopes of carbon, ¹³C and ¹²C, in air samples and demonstrated that the ratio of ¹³C to ¹²C in atmospheric CO₂ has decreased over the second half of the twentieth century. Additional measurements of coral skeletons and wood show that the ratio of ¹³C to ¹²C in air has been decreasing over the entire period during which atmospheric CO₂ levels have been increasing.

DISCUSSION The ratio of ¹³C to ¹²C in CO₂ added to the atmosphere over the past 200 years is lower than that in CO₂ that was already in the air more than 200 years ago. The ratio of ¹³C to¹²C in CO₂ emitted by volcanoes is too high to account for the data, as is the ratio in CO₂ released from the oceans. In contrast, organic matter formed by photosynthesis has just the right ratio of ¹³C to ¹²C to account for Suess’s measurements. By itself, the changing abundance of ¹³C in the air could reflect the conversion of plant carbon to CO₂ by burning, or it could reflect the burning of fossil fuels formed by the burial of plant and algal materials in the geologic past.

FOLLOW-UP WORK Further studies, using ¹⁴C, the third, unstable isotope of C, also show a strong decrease in the proportional abundance of ¹⁴C in atmospheric CO₂ over time. While the burning of vegetation to clear land for agriculture results in CO₂ with the right ratio of ¹³C to ¹²C, modern vegetation has far too much ¹⁴C to account for observed changes in the ¹⁴C of air. Only fossil fuels have the right ratios of all three carbon isotopes—¹²C, ¹³C, and ¹⁴C—to account for the pattern of isotopic change in atmospheric CO₂ measured by Suess and others. In fact, ¹⁴C data from the high Rocky Mountains indicate that if fossil fuels were the principal source of added CO₂, CO₂ levels should have increased by a little less than 2 ppm per year between 2004 and 2008—which is very close to the increase actually measured.

CONCLUSION Fossil fuel burning by industrialized societies has been, and continues to be, a principal source of CO₂ buildup in Earth’s atmosphere.

SOURCES Revelle, R., and H. E. Suess. 1957. “Carbon dioxide exchange between atmosphere and ocean and the question of an increase of atmospheric CO₂ during the past decades.” Tellus 9:18–27; Turnbull, J. C., et al. 2007. “A new high precision ¹⁴CO₂ time series for North American continental air.” Journal of Geophysical Research: Atmospheres 112, no. D11. Article Number: D11310. doi: 10.1029/2006JD008184.