Figure 30
The record of changes in atmospheric concentration of the greenhouse gases carbon dioxide (CO2) and methane (CH4) over the last four glacial cycles from the Vostok ice core in Antarctica. This data comes from the direct assay of air bubbles sealed in glacial ice on an annual basis over the last 400,000 years. The upper line graph is carbon dioxide, the lower one methane. The middle graph is for Oxygen 18, a heavy isotope that serves as a proxy for temperature variation. The timeline reads from right to left. Note how closely temperature and greenhouse gas concentration have covaried over the last 400,000 years. Also note that the current levels of carbon dioxide at 365 ppmv and methane at 1600 ppbv are without precedent in this extraordinarily lengthy record. From Raynaud et al. 2000.