This record, from the NOAA-operated Mauna Loa Observatory, near the top of Mauna Loa on the big island of Hawaii, shows the steady increase in atmospheric carbon dioxide since the year 1700. The graph is called the Keeling Curve, named for Charles D. Keeling who began current measurements in 1958. The level then was around 316 parts per million. We are near 400 ppm today. While the most recent numbers are just below 400 ppm, recent readings have exceeded that level.
Taken from the Scripps Institution of Oceanography web site: scripps.ucsd.edu:
"Scientists make CO2 measurements in remote locations to obtain air that is representative of a large volume of Earth’s atmosphere and relatively free from local influences that could skew readings. The quality of data is verified before daily average values are determined.
"Concentrations are currently approaching the symbolically important value of 400 parts per million. The continued rapid rise in CO2 ensures that levels will rise far beyond 400 ppm before they stabilize. If the pace of the last decade continues, carbon dioxide will reach 450 ppm by the year 2040. Carbon dioxide is the most important man-made greenhouse gas, produced mainly by the burning of fossil fuels such as coal, oil and natural gas. The pace of rise depends strongly on how much fossil fuel is used globally."