The Paris climate summit may go down in history as the singular moment nations decided to tackle the threat of anthropogenic climate change. But few of us appreciate the fact that it’s taken over a century to arrive at a global consensus on the science.

Decades before global warming became a buzzword in the ‘90s, radical scientists suspected that human activity could be messing with the planet’s thermostat. Early climate change soothsayers were ignored and derided by their peers—but their findings turned out to be remarkably prescient. Here are four people who believed that human activity would warm the planet, long before the world had a clue.

Svante Arrhenius

The first person to suggest industrial activity could heat up the Earth was Svante Arrhenius, a Swedish chemist famous for his discoveries about the temperature dependence of reaction rates. Arrhenius preferred pen and paper to real-world observations, and his predictions about Earth’s future climate drew on the work of several of his contemporaries.


In particular, Arrhenius made use of data collected by astronomer Samuel Pierpoint Langley to calculate the absorption of infrared radiation by CO2 and water vapor. After months of painstaking calculations, Arrhenius managed to produce crude estimates of the energy balance for each latitudinal band on the Earth. “I should certainly not have undertaken these tedious calculations,” Arrhenius wrote in his seminal 1895 paper on the greenhouse properties of CO2 (at the time, carbonic acid) “if an extraordinary interest had not been connected with them.”

That extraordinary interest paid off: Arrhenius discovered that even a small change in the concentration of CO2 in the atmosphere could have global impacts, and that cutting CO2 by half would be sufficient to produce an ice age—a finding that stacks up pretty well with paleoclimate data on Earth’s geologic past.

Svante Arrhenius, 1909. Image: Wikimedia


In his 1895 paper, Arrhenius was primarily concerned with global cooling. But shortly thereafter, a colleague—the Swedish geologist Arvid Högbom—put a strange idea in Arrhenius’ mind. Högbom had calculated that coal burning and other industrial activities were adding CO2 to the atmosphere at a rate comparable to natural processes. Arrhenius realized that human carbon emissions might, in the future, have the capacity to warm the planet.

But due to the relatively low carbon emissions at the turn of the century, Arrhenius thought the process of anthropogenic global warming would take thousands of years to manifest. A handful of scientists took an interest in his ideas, but by the early 1900s, they were widely discredited. Simply put, people didn’t see how humans could ever be a force of nature powerful enough to influence the climate. Prevailing wisdom held that nature would always balance itself out—and it would be decades before that notion was rattled once again.

Guy Stewart Callendar

Arrhenius may have been the first to suggest that fossil carbon emissions could warm the planet, but British steam engineer Guy Stewart Callendar was the first to show that they already had in 1938.


A back-of-the-envelope scientist in the truest sense, Callendar spent his free time compiling global temperature records and carbon dioxide measurements. And he came to a remarkable discovery: the two datasets appeared to be correlated.

In a paper published in the Quarterly Journal of the Royal Meteorological Society in April of 1938, Callendar not only showed that the Earth’s land surface had warmed over the past 50 years, he argued that the combustion of fossil fuels was responsible. For a brief period of time thereafter, global warming came to be known as the “Callendar effect.”

Guy Stewart Callendar, 1934. Image: Wikimedia


Here’s the synopsis that Callendar wrote at the beginning of his paper over 75 years ago:

By fuel combustion man has added about 150,000 million tons of carbon dioxide to the air during the past half century. The author estimates from the best available data that approximately three quarters of this has remained in the atmosphere.

The radiation absorption coefficients of carbon dioxide and water vapour are used to show the effect of carbon dioxide on “sky radiation.” From this the increase in mean temperature, due to the artificial production of carbon dioxide, is estimated to be at the rate of 0.003°C. per year at the present time.

The temperature observations at zoo meteorological stations are used to show that world temperatures have actually increased at an average rate of 0.005°C. per year during the past half century.

Callendar’s calculations were remarkably accurate given his simple methods, his limited CO2 data, and our incomplete understanding of atmospheric radiative physics in the 1930s. His observations of global temperature change not only map neatly onto modern reconstructions of the early 20th century, he correctly observed that more warming was taking place at high latitudes. Callendar also took steps to account for the urban heat island effect, a phenomenon that was not widely known at the time.

Observed global temperature departures for the atmosphere over western Europe and New York state. Image: Callendar (1938)

Comparison of modern reconstructions of 19th—early 20th century temperature departures (black) with Callendar 1938 (red) and Callendar 1961 (blue). Image: Hawkins & Jones (2013)



There are a few parallels between Callendar and Arrhenius. For one, both mens’ conclusions were pretty much dismissed by the scientific community. Callendar, at least, seemed to derive some satisfaction from punching holes in the conventional wisdom of purported experts. “Few of those familiar with the natural heat exchanges of the atmosphere, which go into the making of our climates and weather, would be prepared to admit that the activities of man could have any influence upon phenomena of so vast a scale,” he wrote. Undeterred by naysayers, the engineer would go on to author dozens more papers on global warming over the next thirty years.

Second, both Arrhenius and Callendar thought global warming would be a good thing for the planet. Arrhenius—clearly speaking from experience as a Swede—envisioned that global warming would make the north a much more pleasant place to live. Callendar believed man-made climate change would “indefinitely” delay the return of “deadly glaciers,” in addition to boosting agricultural productivity at high latitudes. “[I]t may be said that the combustion of fossil fuel, whether it be peat from the surface or oil from 10,000 feet below, is likely to prove beneficial to mankind in several ways, besides the provision of heat and power,” Callendar wrote.

Hey, nobody’s right about everything.

Roger Revelle and David Keeling

During the first half of the 19th century, barely anyone had heard of anthropogenic climate change. Most who had considered the notion preposterous. But following the advent of nuclear energy in 1945, thinking about man’s relationship with nature began to change. Suddenly, people had the technological power to destroy civilization, if not wipe out all life on planet Earth. So why shouldn’t we control the weather, too? At the same time, advances in digital computing and radiative physics offered scientist the tools they needed to dust off Callendar’s papers and take a modern look at the issues he raised.

Among those scientists: American oceanographer Roger Revelle, who served as director for the Scripps Institute of Oceanography (SIO) from 1950 to 1964. One argument that Earth scientists were kicking around in the 1950s was that the oceans effectively soaked up any and all carbon humans put in the air. The timescales for this process, however, were unknown.


Roger Revelle. Image: San Diego History Center

Revelle did the math, and in 1957, he co-authored a paper showing that Earth’s oceans weren’t absorbing CO2 very quickly at all. This led Revelle to conclude that the accumulation of atmospheric CO2 “may become significant during future decades if industrial fuel combustion continues to rise exponentially.” Revelle was well-aware of the—albeit limited—global warming research of his time, and he soon realized that Earth’s climate could be in for a dramatic change in the not-too-distant-future.


Given Revelle’s position as a respected scientist, his claims attracted the notice of reporters and politicians. Revelle did not shy away from the attention. Rather, he was among the first scientists to issue public warnings that humans were “conducting a great experiment” with the atmosphere and Earth’s climate. He went on the record making predictions that carbon emissions could turn parts of the southwest into “real deserts,” and that Arctic melting could cause the Soviet Union to become a maritime power by the 21st century.

While Revelle could not have predicted the dramatically altered global geopolitical landscape by the end of the century, his comments have a disturbing air of prescience in light of the California’s ongoing 500 year drought, and the recent scramble to prospect newly opened Arctic waters for oil, which some have dubbed “a new Cold War.”

The Keeling Curve, developed by Charles Keeling in 1960. Image Credit: Scripps


Still, many of Revelle’s predictions were little more than speculation. To drill deeper into the science of global warming, Revelle recruited geochemist Charles David Keeling to Scripps to head up a new Atmospheric Carbon Dioxide program. Keeling began taking atmospheric CO2 measurements at Mauna Loa, Hawaii, and in Antarctica. After a few years of laborious data collection, Keeling announced that he had detected a rise in the atmospheric CO2 concentration, yielding the earliest version of his famous “Keeling curve.” Year after year, that curve would be extended, the trend becoming impossible to ignore. To this day, the Keeling curve remains a powerful symbol of the impact of human society and technology on the Earth.

Charles David Keeling receiving a medal of science from President Bush in 2001. Image: Wikimedia

Fifty years ago last month, Revelle, Keeling, and three other prominent climate scientists authored a report for President Lyndon Johnson, warning him that fossil carbon emissions were having a significant impact on Earth’s climate. In addition to laying out the mechanisms of climate change with remarkable accuracy, the report calls carbon dioxide an “invisible pollutant”—a classification that wasn’t officially recognized until this year.



It also made a number of predictions about the consequences of climate change, including sea level rise, Antarctic melting, and an increase in the acidity of fresh water. Sound familiar? Finally, the scientists used UN data on fossil fuel growth to estimate the concentration of carbon dioxide in the atmosphere by the year 2000—350 parts per million. The actual figure? 370.


Many view the development of the Keeling curve as the turning point at which global warming stepped out of the shadowy fringes to take its place as an important topic of scientific discourse. But it would take another thirty years, hundreds more scientific papers, and some nasty cover-ups for the idea to seep into general public’s consciousness. And public acceptance of climate change is far from universal today.


Arrhenius, Callendar, Revelle and Keeling were all ahead of their time, using simple tools and limited data to see the future with remarkable clarity. As nations around the world finally start heeding the council of scientists, it’s worth taking a moment to appreciate just how far back this paper trail goes.

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Top: The Mauna Loa observatory, Hawaii, 1965. Image Credti: NCAR/HAO