The history of atmospheric CO₂ on Earth has been one of slow but constant changes, far slower than the rate of today’s atmospheric CO₂ changes. The last million year’s worth of history of atmospheric carbon dioxide is trapped in the ice core bubbles.

Web Figure 9.5.A  Air bubbles trapped inside frozen ice. (Courtesy of NewScientist.com: http://www.newscientist.com/data/images/ns/cms/dn8369/dn8369-1_500.jpg)

As snow is compressed to form ice, air is trapped within the snow, forming bubbles that will persist for thousands of years. In Antarctica near the South Pole, scientists at the Vostok Research Station are drilling into the ice to recover the atmospheric history of the past 600,000 years.

Web Figure 9.5.B  An aerial view of the Vostok Field Station in Antarctica near the South Pole. At this international station, scientists are extracting ice cores containing bubbles of ancient air that were trapped more than 600,000 years ago. (Courtesy of Subglacial Antarctic Lake Exploration: http://salegos-scar.montana.edu/images/Vostok%20Station.jpg)

These ice core data show that atmospheric CO₂ levels changed between 180 ppm (glacial periods) and 280 ppm (interglacial periods) as Earth moved in and out of ice ages. These repeated glacial cycles reflect the absorption of carbon dioxide into the oceans during glacial periods and its release to the atmosphere during interglacial periods.

Web Figure 9.5.C  A time series of ancient air trapped as ice bubbles for the past 600,000+ years. On the top line, we see the methane content of the air bubble; on the lower line we see the carbon dioxide patterns over this same time period. The middle line represents the hydrogen isotope ratio of the ice which is a good proxy for the air temperature record. (Courtesy of RealClimate: http://www.realclimate.org/epica.jpg)

As a result of burning fossil fuels, atmospheric carbon dioxide levels have increased. Fossil-fuel consumption rates have accelerated since the 1950s, resulting in atmospheric carbon dioxide levels today that greatly exceed the levels recorded in the ice core records. The ice core records also show that temperature and atmospheric carbon dioxide levels are tightly correlated with each other; the global temperature is reflected in the hydrogen isotope ratios of the ice δD) in Web Figure 9.5.C above.