Additional info from Skeptical Science
The following graph shows atmospheric CO2 levels over the last 10,000 years. It includes ice core data for CO2 levels before 1950. For values after 1950, direct measurements from Mauna Loa, Hawaii were used.
Figure 1: CO2 levels (parts per million) over the past 10,000 years. Blue line from Taylor Dome ice cores (NOAA). Green line from Law Dome ice core (CDIAC). Red line from direct measurements at Mauna Loa, Hawaii (NOAA). © John Cook.
Mauna Loa is often used as an example of rising carbon dioxide levels because it’s the longest continuous series of directly measured atmospheric CO2. The reason it's acceptable to use Mauna Loa as a proxy for global CO2 levels is because CO2 mixes well throughout the atmosphere. Consequently, the trend in Mauna Loa CO2 is statistically indistinguishable from the trend in global CO2 levels (Figure 2). If global CO2 was used in Figure 1 above, the resulting "hockey stick" shape would be identical.
Figure 2: Global atmospheric CO2 versus Mauna Loa CO2 (NOAA data). © John Cook.
Adapted from © John Cook and Skeptical Science