The current global levels of CO2 are probably higher than have occurred during the last few ice ages and warm interglacial periods, as shown in Figure 6
, which is based on data from the Vostok Ice Core in Antarctica. It shows a maximum level in the ice core of around 300 ppm. However, present levels in the atmosphere are higher, at over 380 ppm. Over time the gas trapped in ice migrates to adjacent layers and the difference in the age of the ice and the gas trapped in it can be hundreds of years: this means the concentration of a gas in the ice is the average level over several hundred years and a rise like the recent one over a period of a few decades would not be noticed. The graph also includes temperature, which shows that temperature and CO2 rise at the same time but the CO2 lags behind temperature during the falling phase. It is now generally accepted that temperature rises as a result of the Milankovitch cycles
and CO2 responds to this increase but may be an amplifying factor.