Figure 21.5 Atmospheric concentrations of CO2 over time. Using measurements of air bubbles from ice cores and, in modern times, direct measurements, researchers have documented that for more than 400,000 years CO2 concentrations never exceeded 300 ppm. During the past 200 years the concentration of CO2 in the atmosphere has increased rapidly and is now approaching 400 ppm.
Data from http://climate.nasa.gov/evidence.