More than 100 years ago, scientists were concerned about global warming. What they forecast is happening, only faster.

We tend to think of man-made global warming as a modern concept, something that has come into vogue in the last twenty years or so, but in reality this idea is more than one hundred years old. The notion that the global climate could be affected by human activities was first put forth by Svante Arrhenius in 1896. He based his proposal on his prediction that emissions of carbon dioxide from the burning of fossil fuels (i.e., coal, petroleum, and natural gas) and other combustion processes would alter atmospheric composition in ways that would lead to global warming. Arrhenius calculated how much the temperature of the Earth would drop if the amount of CO2 in the atmosphere was halved; he also calculated the temperature increase to be expected from a doubling of CO2 in the atmosphere—a rise of about eight degrees Fahrenheit.


More than a century later, the estimates from state-of-the-art climate models doing the same calculations to determine the increase in temperature due to a doubling of the CO2 concentration show that the calculation by Arrhenius was in the right ballpark. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) synthesized the results from eighteen climate models used by groups around the world to estimate climate sensitivity and its uncertainty. They estimated that a doubling of CO2 would lead to an increase in global average temperature of about 5.4°F, with an uncertainty spanning the range from about 3.6°F to 8.1°F. It’s amazing that Arrhenius, doing his calculations by hand and with very few data, came so close to the much more detailed calculations that can be done today.

Nobody thought there was any reason to worry about Arrhenius’s hypothetical future warming, which he suggested would be caused by humans and their burning of fossil fuel.

Arrhenius’s calculations, however, did have some shortcomings. For example, in estimating how long it would take for the CO2 concentration in the atmosphere to double, he assumed that it would rise at a constant rate. With about 1.6 billion people on the planet in 1895 and with relatively small use of fossil fuels, Arrhenius predicted that it would take about three thousand years for the atmospheric CO2 concentration to double. Unfortunately, when scientists today factor in the quadrupling of world population since then and the increasing demand for energy, doubling is now projected before the end of this century unless substantial cutbacks in emissions are adopted by nations around the world. So, technically, Arrhenius was off by about two thousand eight hundred years. (Another of his doubtful predictions was that he firmly believed a warmer world would be a good thing.)

In Arrhenius’s time, the impacts of global warming was mainly left to future investigation—the majority of scientists still needed to be convinced that the concentration of CO2 in the atmosphere could vary, even over very long timescales, and that this variation could affect the climate. Scientists at the time were focused more on trying to understand the gradual shifts that took place over periods a thousand times longer than Arrhenius’s estimate: those that accounted for alternating ice ages and warm periods and, in distant times (more than sixty-five million years ago), for the presence of dinosaurs. They couldn’t even begin to wrap their minds around climate change on a human time scale of decades or centuries. Nobody thought there was any reason to worry about Arrhenius’s hypothetical future warming, which he suggested would be caused by humans and their burning of fossil fuel. It was an idea that most experts at the time dismissed. Most scientists of the era believed that humanity was simply too small and too insignificant to influence the climate.

Fast-forward to the mid-nineteen fifties, and enter Charles David Keeling, a brilliant and passionate scientist who was then beginning his research career at Cal Tech. Keeling had become obsessed with carbon dioxide and wanted to understand what processes affected fluctuations in the amount of CO2 in the atmosphere. Answering this question required an instrument that didn’t exist, the equivalent of an ultra-accurate “atmospheric Breathalyzer.” So Keeling built his own instrument and then spent months tinkering with it until it was as close to perfect as he could get at measuring the concentration of CO2 in canisters with a range of values of known concentration.

Keeling tried his instrument out by measuring CO2 concentrations in various locations around California and then comparing these samples in the lab against calibration gases. He began to notice that the samples he took in very pristine locations (i.e., spots where air came in off the Pacific Ocean) all yielded the same number. He suspected that he had identified the baseline concentration of CO2 in the atmosphere; a clear signal that wasn’t being contaminated by emissions from factories, farms, or uptake by forests and crops.

With this instrument, called a gas chromatograph, Keeling headed to the Scripps Institution of Oceanography to begin what is perhaps the single most important scientific contribution to the discovery of global warming. Keeling was on a mission to find out, once and for all, if CO2 levels in the atmosphere were increasing. He would spend the next fifty years carefully tracking CO2 and building, data point by data point, the finest instrumental record of the CO2 concentration in the atmosphere, generating a time history that is now known by scientists as the Keeling curve.

The Keeling curve is a monthly record of atmospheric carbon dioxide levels that begins in 1958 and continues to today. The instrument Keeling built, the gas chromatograph, works by passing infrared (IR) light through a sample of air and measuring the amount of IR absorbed by the air. Because carbon dioxide is a greenhouse gas, Keeling knew that the more IR absorbed by the air, the higher the concentration of CO2 in the air. Because CO2 is found in very small concentrations, the gas chromatograph measures in terms of parts per million (ppm).

This was the first instance when a document discussing global warming ended up in front of the president of the United States. It would not be the last.

Keeling knew from his travels around California that he needed to make his measurements at a remote location that wouldn’t be contaminated by local pollution. That’s why he settled on Hawaii. Hawaii’s big island is the site of the volcano Mauna Loa, and Keeling set up his CO2 instrument near the top of Mauna Loa. Isolated in the middle of the Pacific Ocean and at more than eleven thousand feet above sea level, the top of the Mauna Loa volcano is an ideal location to make measurements of atmospheric carbon dioxide that reflect global trends, but not local influences such as factories or forests that may boost or lower the carbon dioxide level within their vicinity. The sensors were positioned so that they sampled the incoming ocean breeze well above the thermal inversion layer; thus the air was not affected by nearby human activities, vegetation, or other factors on the island. Obviously, volcanoes are potentially a big source of CO2, but Keeling took this into account when positioning his instrument, locating it upwind of Mauna Loa’s vent and installing sensors to give alerts if the winds shift.

What he found was both disturbing and fascinating, creepy and profound. Keeling, using his Mauna Loa measurements, could see that with each passing year CO2 levels were steadily moving upward. As the years passed and the Mauna Loa data accumulated, Keeling’s CO2 record became increasingly impressive, showing levels of carbon dioxide that were noticeably higher year after year after year. The first instrumental measurements indicated a CO2 concentration of three hundred and fifteen ppm in 1958. The slow rise in its concentration over the first several years was enough to prompt a report from a panel of the President’s Science Advisory Council to President Johnson in 1965, indicating that the early prediction that an increase in CO2 could occur was correct and that global warming would indeed be expected to occur. This was the first instance when a document discussing global warming ended up in front of the president of the United States. It would not be the last.

In 2008, just over fifty years after Keeling started his observations, the concentration at Mauna Loa had reached 385 ppm. Keeling’s measurements thus provided solid evidence that the atmospheric CO2 concentration was increasing. If anything proved that Arrhenius had been on to something, it was these data.

One of the most striking aspects of the Keeling curve is a small CO2 wiggle that takes place every year. For every little jump up, there is a little dip back down, so that the whole curve looks sawtoothed. This wiggle happens like clockwork and is timed with the seasons. In the northern hemisphere during fall and winter, plants and leaves die off and decay, releasing CO2 back into the atmosphere and causing a small spike. And then during the spring and summer, when plants are taking CO2 out of the atmosphere in order to grow, carbon dioxide levels drop. Hawaii, along with most of the planet’s landmass, is situated in the northern hemisphere, so the seasonal trend in the Keeling curve is tracking the seasons in the northern hemisphere. The Keeling curve proved many important things at once. It proved that CO2 levels in the atmosphere can indeed change and that they can change on very short timescales.

Unfortunately, scientific discoveries are not always good news. And there is a nagging fear among scientists that we’ll prove ourselves to be not so different from the woolly mammoth, the symbol of a climate that no longer exists.

Keeling’s record was the icing on the cake, and he rightly stands with Louis Agassiz, John Tyndall, and Arrhenius among the giants of climate science. He helped prove the reality of global warming by providing the data upon which the pioneering theories of Tyndall and Arrhenius could finally rest. As is the case in research science, Keeling’s painstaking measurements have been verified and supplemented by many others. Measurements at about one hundred other sites have confirmed the long-term trend shown by the Keeling curve, although no sites have a record as long as Mauna Loa. Other scientists have also extended the Keeling curve farther back in time, using measurements of CO2 in air trapped in bubbles in polar ice and in mountain glaciers. Ice cores collected from Antarctica and Greenland can be used to reconstruct climate hundreds of thousands of years ago, showing that the preindustrial amount of CO2—the level from A.D. 1000 to 1750—in the atmosphere was about two hundred and eighty ppm, about one hundred and five ppm below today’s value. The record indicates that the concentration of CO2 has increased about 36 percent in the last one hundred and fifty years, with about half of that increase happening in the last three decades. In fact, the CO2 concentration is now higher than any seen in at least the past eight hundred thousand years—and probably many millions of years before the earliest ice core measurement.

Over the past century, the evidence has piled up in support of Arrhenius’s explanation of global warming. As the evidence accumulates with each passing year, what was once a fringe theory that sprang from the mind of a single scientist in Sweden is now part of the bedrock of scientific accomplishments. Unfortunately, scientific discoveries are not always good news. And there is a nagging fear among scientists that we’ll prove ourselves to be not so different from the woolly mammoth, the symbol of a climate that no longer exists.

Heidi Cullen is a senior research scientist with Climate Central, a nonprofit research organization through which she reports on climate change for news outlets, including _PBS NewsHour_,, and The Weather Channel. She is a visiting lecturer at Princeton University, a member of the American Meteorological Society, and an associate editor of the journal _Weather, Climate, and Society_. She holds a BS in engineering and a PhD in climatology from Columbia University, and lives with her husband and two dogs in Princeton, New Jersey.

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