Mauna Loa Observatory.
Once upon a time, scientists didn't know how to measure atmospheric carbon dioxide. Which is a strong and alarming signal that the atmosphere is changing. Then in the fifties a guy figured it out, using Big Sur trees that have served as California's lungs for centuries. That scientist set up some instruments at Mauna Loa Observatory in Hawaii, the best place the US could think of at the time to measure undisturbed air. Over decades he was able to show the progressive buildup of carbon dioxide in the atmosphere, and his observations yielded a noticeable curve. In 1958, he measured 315 parts per million of carbon in the atmosphere. In December, Mauna Loa Observatory measured 391 parts per million.
Charles David Keeling died on my birthday in 2005, and when he did, Scripps Institute of Oceanography hailed him as a pioneer. To a climate reporter like me, he's something of a legend. The Keeling Curve is iconic. Scripps articulates the reasons why pretty well: