The longest-running effort to track atmospheric CO2 began in 1958

In 1958, Charles David Keeling began making daily measurements of the concentration of atmospheric carbon dioxide (CO2) at the Mauna Loa Observatory on the Big Island of Hawaii. These measurements were begun as part of a one-year initiative, the International Geophysical Year, but because of Keeling's persistence, the daily record has continued through the present, almost without interruption.

 Mauna Loa is an ideal place to take such a record. The CO2 sensors at Mauna Loa are located at an altitude of over 11,000 feet, and positioned to sample winds blowing in directly from the ocean. That means they are not affected by local sources of either CO2 emission or CO2 absorption, such as cars or factories or forests. Because Hawaii is isolated in the middle of the Pacific Ocean, the CO2 they detect is well-mixed and parallels global averages. The volcano itself emits CO2, but Charles Keeling took this into account when choosing the site for his sensors. Now known as the Keeling Curve, it continues to be collected under the direction of Keeling's son, Ralph.   

Keeling’s observations helped establish the fact that in the short term, CO2 levels fluctuate up and down somewhat with the seasons, but that over the past 50-plus years, the overall level of CO2 in the atmosphere has risen steadily.

Since Keeling began his work, other scientists have been able to indirectly reconstruct a much longer-term record of CO2 in the atmosphere, revealing an uptick that began in the eighteenth century, with the start of widespread coal use, and that has been accelerating ever since. One way of reconstructing these longer records has been by analyzing air bubbles trapped in ancient ice.