The Control Knob

A little over fifty years ago, Charles David Keeling, then a postdoctoral student at the California Institute of Technology, began measuring levels of carbon dioxide in the atmosphere. As Spencer Weart notes in The Discovery of Global Warming, at the time this was a field of research that "had little to recommend itself to an ambitious scientist," but Keeling, frankly, wanted to work in the outdoors rather than be stuck in the lab. 

His work did not go unnoticed, however, and in 1958 Keeling accepted an invitation from Roger Revelle, Director of the Scripps Institution of Oceanography, to continue and expand his work there. Keeling established a monitoring station on top of Mauna Loa volcano in Hawaii; the data that has been collected every year since then has provided a vivid picture of the steady increase in concentrations of atmospheric carbon dioxide over the past half century.

The so-called Keeling Curve has become something of an iconic and defining image in the annals of climate change science. We know, thanks to Keeling, that the concentration of atmospheric carbon dioxide has increased - from approximately 315 parts per million by volume (ppmv) in 1958 to just shy of 390 ppmv now. We know also, primarily from ice core studies, that contemporary levels are more than 100 million ppmv higher than they were at the start of the Industrial Revolution. And this all matters, of course, because we know that carbon dioxide is a greenhouse gas.

But what about atmospheric carbon dioxide much longer ago? What can we determine about its levels in prehistory and the possible effect on climate? Earth has been much warmer than it is now, and much colder too. What role, if any, did changes in levels of CO2 in the atmosphere have on those climatic variations?

If you have a little under an hour to spare - and if you are at all interested in this subject, I really would encourage you to find the time if possible - then click on the picture of the Keeling Curve (or, if you prefer, here) and strap yourself in as Richard B. Alley of Pennsylvania State University takes us on a tour-de-force journey through Earth's climate history, and the part CO2 has likely played. There is, of course, a multiplicity of factors involved in our planet's climate history - changes in solar output, plate tectonics, vulcanism, and Milankovitch cycles among them - but, as Alley explains, in a lecture he gave at the meeting of the American Geophysical Union last week, "if you leave CO2 out, nothing makes sense; if you put CO2 in, a whole lot of it makes sense."