Past atmospheric composition and greenhouse gases
Past temperatures can be estimated from the isotopic composition of the water molecules in ice cores (read more about the isotope measurements here), and the past atmospheric composition can be determined from bubbles of old atmospheric air preserved in the ice cores (read more about the gas analysis techniques here). The results of such analyses are shown below.
The figure presents results from the Antarctic EPICA Dome C ice core showing the development of temperature in Antarctica and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO2, in blue) and methane (CH4, in red) over the past 800 thousand years. To the right, modern values are indicated by arrows (CO2 concentration 386 ppm, CH4 concentration 1790 ppb, data from 2009). The temperature record shows the temperature anomaly from present day temperature, so the zero point of the temperature scale is the present day mean annual temperature at the drill site. The greenhouse gas concentrations are given in units of parts per million by volume (ppmv) and parts per billion by volume (ppbv), respectively.
Past temperatures and greenhouse gas concentrations are seen to show a strong degree of co-variation over the entire time period, except for the most recent period. This natural variability is related to the glacial cycles. In recent times, the greenhouse gas concentrations are seen to show a very abrupt rise as compared to the natural levels of the past 800 thousand years. The rise is caused by mankind's extended use of fossil fuels since the industrial revolution.