An Iconic Image of Science Turns 50

A few weeks ago we wished Darwin a happy 199th, so here's three cheers for the Keeling Curve passing the big 5-0.​

Described by the Scripps Institution of Oceanography, in only a bit of a stretch, as “one of the iconic images of science, rivaling the double helix or Darwin’s sketches of finches,â€? the Keeling Curve is an unassuming sawtooth tracing a steepening path up a piece of graph paper. It’s also the longest continuous record of atmospheric carbon dioxide levels we have.

In 1958, Charles David Keeling began taking extremely precise measurements from an observatory 11,000 feet up on Mauna Loa, Hawaii. He had recently developed a new measurement method accurate to within 1 part per million (ppm). (Watch the pioneering scientist gleefully pouring liquid nitrogen barehanded into his equipment. He mentions he got interested in his field because it was a chance to build gadgets.)

The work is still going strong, and the Curve now charts a slow and unflinching rise in the carbon dioxide levels in the air, from an already-elevated 315 ppm in 1958 to some 380 ppm today. Keeling's equipment was so precise, he later said, that the rise was already detectable with just 2 or 3 years of data.

As a representation of the natural world, the Keeling Curve is remarkable for its decorum. Pretty much any other historical record, from the temperature at your local airport to the vicissitudes of Wall Street, is a hysterical EKG of peaks and valleys, as each irregular day passes into the next. By contrast, the Keeling Curve looks like the work of an obsessive with an Etch-a-Sketch. The instrument’s location helps, stuck high into the atmosphere in the middle of the world’s biggest ocean, far from smokestacks and tailpipes.

In the absence of noise, any variation on the graph means something. That sawtooth pattern reflects passing seasons in the Northern Hemisphere, where most of the world’s vegetated land is. During summer, plants take up carbon dioxide to grow, putting a roughly 6-ppm dent in atmospheric CO2 levels. During northern winters, decaying matter releases carbon dioxide back into the atmosphere, and the Keeling Curve peaks again.

Notice anything else? The line is getting steeper. That means that carbon dioxide isn't just accumulating - we're adding more upon more each year. You could seek out appendices full of statistics on car ownership and megawatt production to calculate this, but the gist is right here on this graph.

And as Keeling noticed while plotting results a decade or so ago, the size of the sawtooths is getting bigger. That’s an ominous indication of a subtle shift: slightly more plant growth each year, a result of longer growing seasons stemming from earlier springs and later falls.

So while we're at it, let's save a birthday cheer for Keeling, who died in 2005. This year would have marked his 80th birthday.

(Scripps atmospheric scientist Ralph Keeling - Dave Keeling's son)