It looks, at first glance, like a doodle, a zig-zag pattern squiggled absent-mindedly on a notepad during an uninspiring meeting. In fact, it has been called one of the most important scientific works of the 20th century and its emergence in the 1950s offered one of the first key readings of the health of planet Earth.

The seemingly innocuous squiggle of the Keeling Curve is actually a meticulous record of the amount of carbon dioxide in our atmosphere, the result of daily readings that have continued almost uninterrupted for more than 60 years. Its importance lies in the fact that, over those six decades, the zig-zag has trended steadily upward.

Science historian Spencer Weart describes the Keeling Curve as “the central icon of the greenhouse effect.” It was, he writes in his book, The Discovery of Global Warming, “not quite the discovery of global warming. It was the discovery of the possibility of global warming.”

Its origins can be traced to a campsite in Big Sur, California. In 1953, Charles David Keeling was a young postgraduate geochemist embarking on a study to compare the relative abundances of carbon dioxide in water and air. To do that, he first had to measure the level of CO2 in the atmosphere, which, to that point, nobody had done to any great precision. And because nobody had done it, there was no off-the-shelf equipment readily available to do so. So, Keeling made his own instrument, working from instructions for a prototype he found in a 1916 journal article, and he undertook the day’s drive to Big Sur. Unsure whether the CO2 even in pristine air next to the Pacific Ocean would be constant, he decided to take air samples every few hours over a full day and night, a meticulousness that would characterize his career.

“He lived by a kind of moral code that looked at there being a right way and a wrong way to do things, and the right way was always the thorough way,” explains Ralph Keeling, his son and the Director of the Scripps CO2 Program at the Scripps Institution of Oceanography.

Keeling soon determined that the level of atmospheric CO2 was approximately 310 parts per million (ppm)—that is, for every million molecules of gas in the atmosphere, 310 of them were carbon dioxide. That figure remained the same whether he was at his campsite in Big Sur, in the rainforests of the Pacific Northwest or the mountain deserts of Arizona. At the time, this was considered primarily an answer to a question that not too many people were asking, but his efforts would soon assume greater significance.

The young geochemist's work attracted the attention of other scientists, and in 1957, Roger Revelle, the head of the Scripps Institution of Oceanography, approached Keeling with a plan to measure CO2 concentrations from a series of locations around the world. Revelle’s initial idea was to conduct a “snapshot” of CO2 levels and then to come back and do it again in, say, a decade. But Keeling insisted on a more thorough approach: taking measurements not once a decade but daily.

“Keeling’s a peculiar guy,” Revelle later said. “He wants to measure CO2 in his belly. And he wants to do it with the greatest precision and the greatest accuracy that he can.”

The U.S. Weather Bureau had recently built an observation station in the clear air atop the Mauna Loa volcano in Hawaii, and its Director of Meteorological Research, Henry Wexler, proposed it as the perfect spot from which daily measurements could be made. In the end, Keeling would conduct two projects in parallel: he would work out of Scripps and lead Revelle’s global effort, while simultaneously running daily monitoring at Mauna Loa under the direction of Wexler.

The first Mauna Loa measurement, in March 1958, produced a reading of 313 ppm. But, to Keeling’s surprise, the following month it increased by 1 ppm, and increased again in May, before declining steadily until October, at which point it rose again. The same thing happened in the first full year of measurement, in 1959. The reason? There is far greater land mass, and thus much more vegetation, in the northern hemisphere than the southern. And so, as the trees above the equator grew their leaves in spring and summer, they drew carbon dioxide out of the atmosphere, the level increasing anew with the return of northern fall and winter. The Keeling Curve had, in effect, captured Planet Earth “breathing.”

But there was more to come. Although the seasonal variation created a zig-zag, it soon became clear that the zig zag was trending upward. Year-on-year, the amount of carbon dioxide in the atmosphere was increasing.

Several 19-century scientists had established that carbon dioxide in the atmosphere warms the planet—creating a “greenhouse effect.” The greenhouse effect is essential for life on Earth; without it, Earth would be a frigid, barren place. But those same scientists speculated that burning fossil fuels such as coal (which contain carbon from ancient plants) might increase CO2 levels and cause global temperatures to increase. Such concerns had, however, largely remained hypothetical. But now Keeling’s work established that the first part of that theoretical equation—an increase in carbon dioxide in the atmosphere—was real.

The clear and vivid illustration provided by the Keeling Curve spurred other researchers to begin looking at the possible impacts. By 1967, a team led by Syukuro Manabe of the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration (NOAA) had devised the first comprehensive model of the response of climate to an increase in atmospheric CO2 extrapolated from the Keeling Curve. It predicted that a doubling of carbon dioxide in the atmosphere would cause an increase in global temperature of around 3 to 4 degrees Fahrenheit.

“I do have the impression that Manabe was motivated to jump in on this because of the evidence from my father that CO2 levels were actually changing,” says Ralph Keeling. “My father was shaping the agenda in a way that influenced people. I know that there were researchers, perhaps half a generation older than me, who were attracted to the field because they saw this curve.”

Further studies and modeling over the years refined the predictions and confirmed the underlying theses. By drilling deep into polar ice sheets and examining tiny pockets of air that had been trapped hundreds and even thousands of years ago, scientists were even able to measure CO2 levels from far before Keeling. By the time the Mauna Loa measurements began, those levels had already increased by almost 12 percent since preindustrial times, and they have continued to grow. By 2016, the annual low was above 400 ppm for the first time in several million years.

The Keeling Curve has since been joined by another iconic climate graph: the so-called “hockey stick,” first published by Michael Mann of Penn State University and colleagues in 1998, which presents relatively stable global temperatures from the year AD 1000 and a sharp spike upward during the 20 century that closely tracks the Mauna Loa measurements.

“Like the ‘Hockey Stick’ curve that my co-authors and I published two decades ago, the Keeling Curve is truly iconic because it tells a simple story,” Mann says. “You don’t need to understand the complexities of climate science to understand what either of these curves tell us: that human activity is having a profound impact on Earth’s environment.”

As humanity grapples with how to respond to the changes on Earth’s environment, the curve continues to tick upward.

“He was cognizant that the curve would be looked at and scrutinized into the future,” says Ralph Keeling of his father, who died in 2005. “And the curve is almost like an oracle speaking. It’s the oracle on the mountain.”