Careful and precise measurements from Hawaii of CO2 in the atmosphere.

Since 1957, there has been a steady increase in CO2.

Note the annual fluctuation with the change of seasons in the Northern Hemisphere. Click image to enlarge.

These  measurements were begun in the International Geophysical Year in 1957.  A group of nations organized an intensive study of the earth that year.  The monitoring station for CO2 was set up on a dormant volcano in the big island of Hawaii.  The site was chosen for it's ocean location far from industrial air pollution.  Mauna Loa is about 13,000 feet high and the air up there is about as pure as it gets anywhere.