19
Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
—Robert Frost, “Fire and Ice”
If one were to ascribe a specific date to the dawn of modern climate science, a strong case could be made for August 23, 1856—though its significance went unrecognized for 150 years. Eunice Newton Foote was an artist, inventor, citizen scientist, and early suffragist from upstate New York whose singular contribution to climate science was lost in plain sight until a retired petroleum geologist stumbled over it in 2010. Without knowing it, Foote conducted and described what could be called the first modern climate change experiment. It was a variation on Saussure’s chambered box idea consisting of a pair of sealed glass cylinders equipped with thermometers. After filling one with “ordinary air” and the other with “carbonic acid gas” (aka carbon dioxide), Foote took readings at room temperature before exposing them to direct sunlight. While both cylinders heated up, the one filled with CO2 grew twice as hot in a matter of minutes. “An atmosphere of that gas would give to our earth a high temperature,” Foote wrote in a brief article entitled “Circumstances affecting the Heat of the Sun’s Rays,” “and if as some suppose, at one period of its history the air had mixed with it a larger proportion than at present, an increased temperature from its own action…must have necessarily resulted.”
It was on that late summer day in 1856, in Albany, New York, that Foote’s account of her experiment was presented at the Annual Meeting of the American Association for the Advancement of Science, where it was read aloud by a male colleague. Attendees would have assumed correctly that Mrs. Foote’s experiment was an investigation into climates of the distant past—a topic of growing interest at the time. Few, if any, could have imagined that they were being given a glimpse of the future, too. Foote’s conclusions included an ominous warning that would have gone unnoticed by an audience still inhabiting a horse-drawn, steam-driven world: “The receiver containing the gas,” Foote wrote, “became itself much heated—very sensibly more so than the other—and on being removed, it was many times as long in cooling.”
Humanity is now re-creating Eunice Foote’s groundbreaking experiment in real time, only “the receiver containing the gas” is our atmosphere.
Climate science came of age in tandem with the oil and automotive industries, and all of them are products of the Petrocene Age. In 1859, just three years after Eunice Foote’s landmark CO2 experiment, “Colonel” Edwin Drake drilled his first oil well in Pennsylvania. That same year, Étienne Lenoir built a prototype of the first commercially viable internal combustion engine. In an uncanny coincidence, 1859 was also the year an Irish physicist named John Tyndall proved once and for all that concentrations of certain gases in the atmosphere had the potential to alter Earth’s climate. His findings, which he presented at the June 10th Evening Meeting of the Members of the Royal Institution, effectively confirmed discoveries made by others over the previous century:
When the heat is absorbed by the planet, it is so changed in quality that the rays emanating from the planet cannot get with the same freedom back into space. Thus the atmosphere admits of the entrance of solar heat, but checks its exit; and the result is a tendency to accumulate heat at the surface of the planet.
This was the greenhouse effect described in every respect but name. While the concept may sound familiar, Tyndall was the first to prove it, and also to explain in unambiguous terms that water vapor and carbon dioxide are strong absorbers of reflected solar heat in the form of infrared radiation. As similar as Tyndall’s findings may have been to Eunice Foote’s, he appeared to have no awareness of her research. In fairness, Tyndall was working at a much more advanced level with the most sophisticated equipment available, and doing so in London, a scientific hub a world away from upstate New York. Tyndall was a remarkable man by any measure; in addition to being a dedicated scientist with a long-standing interest in the physics of heat, he was a successful commercial surveyor, an inventor, and a popular writer and speaker with several influential texts to his credit, including Heat: A Mode of Motion, which stayed in print for more than fifty years. Like Saussure before him, he was also a pioneering alpinist, logging one of the first ascents of the Matterhorn.
It would take another forty years, but in 1896 (the same year Henry Ford built his first car), a Nobel Prize–winning Swedish chemist named Svante Arrhenius published new research on global warming (and cooling) driven by changes in atmospheric CO2. At the time, there was growing consensus that epochal climate change in the form of ice ages and interglacial warming periods was driven by fluctuations in carbon dioxide, and that these fluctuations were due, primarily, to volcanic activity. After making tens of thousands of calculations—by hand—in which he accounted for Earth’s elliptical orbit, diurnal and seasonal changes, and variations in “nebulosity” (cloud cover), among many other factors, Arrhenius determined that a 50 percent increase in CO2 from 1890s levels would raise the mean surface temperature of Earth by roughly 6°F. Considering that climate modeling did not exist then, his calculations were astonishingly accurate. He even foresaw that the warming effects of increased CO2 would vary depending on latitude, a fact now painfully clear to those of us living at higher latitudes.
In that same prescient article, which was inspired by “very lively discussions” at the Physical Society of Stockholm, the thirty-seven-year-old professor also speculated on the impacts of carbon dioxide from coal burning. This was an extraordinary thing to be considering at that time—as intuitively brilliant, in its way, as Joseph Priestley recognizing that mice and plants could alter the quality of air inside a bell jar. Svante Arrhenius often gets credit for this leap of imagination, but he was following the lead of a colleague, the geologist Arvid Högbom, whom he quotes extensively: “This quantity of [CO2], which is supplied to the atmosphere chiefly by modern industry,” Högbom wrote in 1894, “may be regarded as completely compensating the quantity of [CO2] that is consumed in the formation of limestone.”
In other words, Högbom had already determined that industrial CO2 emissions were replacing any CO2 being removed by natural processes—chemical weathering, uptake by forests, and absorption by oceans (a process called “buffering”).
At the turn of the nineteenth century, the dawn of the automobile age, coal was the dominant industrial energy source (as it would be until the 1960s), but oil and gas were gaining ground rapidly. Even at this early date, new language was being found to articulate the effects of carbon dioxide. “The atmosphere may act like the glass of a green-house,” wrote the meteorologist Nils Ekholm, a friend and colleague of Arrhenius, in 1901, “letting through the light rays of the sun relatively easily, and absorbing a great part of the dark rays emitted from the ground, and it thereby may raise the mean temperature of the earth’s surface.”
Like Högbom and Arrhenius, Ekholm believed that increased industrial emissions would have a warming effect on the climate. By this time, “Rockefeller” was already a metonym for extreme oil wealth, and, in 1906, John D. Rockefeller’s Cleveland-based Standard Oil Company, which wielded near-total control over North America’s petroleum industry, was challenged in what would become a yearslong antitrust lawsuit. It would end with the massive corporation being broken up into dozens of “smaller” companies, including the future energy giants Esso, Mobil, Amoco, Chevron, and Texaco. By then, it was clear that the automobile—powered by gasoline (as opposed to coal oil, steam, or electricity, all of which were viable at the time)—was here to stay.
In 1907, while the Standard Oil antitrust suit was before the court, the British physicist John Poynting self-consciously coined two terms that would inform discussions of climate for the foreseeable future. They appeared in the opening sentences of a response to an article by the wealthy Bostonian travel writer and citizen scientist Percival Lowell. “[Lowell] takes into account the effect of planetary atmospheres in a much more detailed way than any previous writer,” Poynting wrote in a prominent British scientific journal. “But he pays hardly any attention to the ‘blanketing effect,’ or, as I prefer to call it, the ‘greenhouse effect’ of the atmosphere.”
The following year, 1908, Henry Ford introduced his multimillion-selling Model T, and Svante Arrhenius published the English-language edition of his magnum opus, Worlds in the Making: The Evolution of the Universe. This audacious work, conceived (like the Model T) with a general audience in mind, was the first popular science book to examine the possibility of anthropogenic warming. After describing the “hot-house” theory of Earth’s climate, demonstrated by Saussure in the 1770s and expanded on by Fourier, Foote, Tyndall, and others, Arrhenius went a step further: “The enormous combustion of coal by our industrial establishments suffices to increase the percentage of carbon dioxide in the air to a perceptible degree.”
At the time, few in the scientific community approached this issue with any great urgency. To most people who gave the matter any thought at all (including Arrhenius), a tilt toward warmer winters sounded like a wonderful idea.
Neither Arrhenius nor his visionary colleague Arvid Högbom could have imagined a new warming period arriving as quickly as the one currently upon us, nor could they have foreseen that anthropogenic emissions could so rapidly overtake volcanic eruptions, the primary source of excess carbon dioxide. But when it comes to fire, heat, and CO2, we—Homo flagrans—are a “volcanic eruption,” one whose intensity has only grown since the Petrocene Age began. In the time since Arrhenius’s landmark 1896 article was published (almost, but not quite, within living memory), industrial fossil fuel emissions have increased by twenty-five times, far outstripping Nature’s ability to absorb or neutralize them in the short term.
Following Arrhenius’s and Högbom’s early projections, the pace of atmospheric science began to accelerate in parallel with the rapid adoption of the gas-powered automobile and the concomitant expansion of petroleum use across the globe. In an astonishingly short time, what had once been merely speculative became measurable. One of the first people to calculate the impact of industrial CO2 in a systematic way was a Canadian-born and British-raised steam engineer and amateur meteorologist named Guy Callendar. By the 1930s, there was already anecdotal evidence that the climate was warming, but Callendar was the first to actually track and graph it. His inquiry arose from an old-fashioned impulse: curiosity. The son of a successful (and wealthy) physicist, Callendar, like the natural philosophers before him, was free to pursue science for its own sake. He had doubts about carbon dioxide’s influence on Earth’s climate, and he wished to test it. After analyzing a hundred years of temperature records from two hundred weather stations around the globe, Callendar detected a trend. His results were published in 1938, when he was forty years old, just as the automobile was achieving true ubiquity on North American and European roads. In his paper, entitled “The Artificial Production of Carbon Dioxide and Its Influence on Climate,” Callendar foretold a future no one was ready to see:
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. In the following paper I hope to show that such influence is not only possible, but is actually occurring at the present time.
Because there were no computers, and no precedent for this kind of long-term climate analysis, Callendar made his calculations, and his graphs, by hand:
After a painstaking review of thousands of weather records, Callendar determined that the mean global temperature had risen 0.9°F between 1890 and 1935.
While many scientists and civilians alike acknowledged the occurrence of temperature fluctuations (cooling as well as warming), most attributed them to sunspots or other natural cycles. Callendar, like Högbom and Arrhenius before him, was an outlier when he attributed the fifty-year warming trend to anthropogenic CO2. It was a lonely position to be taking on the eve of World War II and the petroleum-driven explosion of growth and prosperity that was to follow. Although Callendar’s work was published in a respected British meteorological journal, it garnered scant attention outside that niche. At the time, with most climate studies still focused on past glaciations, such a rapid temperature increase would have been seen more as an anomaly than an augury. Who was to say such a trend might not reverse itself in the coming decades?
But it didn’t, and, more than a century on, it hasn’t. In fact, the similarities to NASA’s current data are uncanny—as was Callendar’s confident prediction: “The course of world temperatures during the next twenty years,” he wrote more than eighty years ago, “should afford valuable evidence as to the accuracy of the calculated effect of atmospheric carbon dioxide. In any case, the return of the deadly glaciers should be delayed indefinitely.”
NASA’s graph of temperature variation, using state-of-the-art data and equipment, shows a nearly identical pattern.
The significance of Guy Callendar’s proposition, all but ignored at the time, was enormous: he was saying, unequivocally, that humans, due specifically to their preoccupation with fire, had become a force of Nature. The term “anthropogenic” did not exist in Callendar’s day, but if it had, he likely would have used it. That we have entered the Anthropocene Epoch (the geologic era of humankind’s global influence on weather and ecosystems) is now generally accepted among scientists. Precisely when this period began is a matter of debate. Was it 50,000 years ago, with the first evidence of our ability to extirpate populations of Pleistocene megafauna? Was it 12,000 years ago, with the onset of the Holocene Epoch, generally considered to be the age of modern humans? Was it 10,000 years ago, with the appearance of agriculture and city-states? Was it 2,000 years ago, with the accumulation of global pollution residue from Roman smelting operations? Was it 160 years ago, with the introduction of the internal combustion engine? Was it 75 years ago, with the introduction of the atom bomb? Or was it a million years ago, with the first evidence for the controlled use of fire?
That is for scientists to decide.
What is clear is that geologic epochs are typically divided into ages, and Guy Callendar was the first to document and chart the atmospheric changes being wrought by the Petrocene Age. Like so many other groundbreaking scientists, he was on his own and vindication was not guaranteed. At the time of its publication, Callendar’s work was largely ignored, but it could have been so much worse. He could have met the same fates as Joseph Priestley, the oxygen pioneer, driven from Britain and buried in exile, or Ignaz Semmelweis, the advocate for handwashing, harassed out of Switzerland and killed in a mental institution, or, more recently, Alfred Wegener, who, in 1912, forcefully advanced the theory of continental drift, only to be publicly attacked and ridiculed and then perish on the Greenland ice cap before he could be vindicated. Callendar, who died in 1964, would live to see his work accepted, if not universally. In his lifetime, the link between CO2 and temperature would come to be known as the Callendar Effect.
The “next twenty years” that Guy Callendar believed would vindicate his global warming theory ended in 1958. During this time, a Canadian-born geophysicist named Gilbert Plass had been using infrared spectroscopy to challenge and, ultimately, reconfirm discoveries made over the previous two centuries: long-wave radiation—aka infrared radiation, aka solar heat—is retained by water vapor, and also by industrial CO2. Starting in the early 1950s, a number of prominent papers and magazines began reporting on Plass’s work. The Washington Post covered it on May 5, 1953, employing as they did so some now-familiar similes: “Releases of carbon dioxide from coals and oils…blanket the earth’s surface ‘like glass in a greenhouse.’ ”
The New York Times followed with a similar story a couple of weeks later, using the same imagery, and so did Time magazine: “In the hungry fires of industry, modern man burns nearly 2 billion tons of coal and oil each year. Along with the smoke and soot of commerce, his furnaces belch some 6 billion tons of unseen carbon dioxide into the already tainted air…This spreading envelope of gas around the earth serves as a great greenhouse.”
In June 1953, Life, one of the most popular weekly magazines of the day, ran a twenty-page article entitled “The Canopy of Air,” which addressed the suspected link between warming temperatures, rapid glacial retreat, and industrial CO2. Three years later, in 1956, Plass would discuss his findings in American Scientist: “It is not usually appreciated,” he wrote in the July issue, “that very small changes in the average temperature can have appreciable influence on the climate. For example,…a rise in the average temperature of perhaps only 4°C would bring a tropical climate to most of the earth’s surface.”
What is refreshing, but also unnerving, about scientists is the way they relay their findings—mundane or devastating—in the same measured tones. An increase in average temperature of 4°C, or roughly 7°F,[*1] would end life as we know it. In the twenty years since Guy Callendar published his CO2 and temperature graphs, the picture had already changed noticeably. After citing Callendar, Plass wrote, “Today man by his own activities is increasing the percentage of carbon dioxide in the atmosphere by thirty percent a century.”[*2]
Even though we emit carbon dioxide constantly—from our fires and engines, and from our very mouths—it remains, for most, an abstraction. In recent years, its role has become an article of faith that one can choose, or refuse, to believe. But its impacts are less subjective: “In the last fifty years,” wrote Plass in 1956, “virtually all known glaciers in both hemispheres have been retreating.” He continued: “There can be no doubt that this will become an increasingly serious problem as the level of industrial activity increases” (italics mine).
In his article, which he titled “Carbon Dioxide and Climate,” Plass went on to explain how the full exploitation of known coal reserves would drive up CO2 levels by a factor of ten, pushing the average global temperature into uncharted territory. More disturbing than Plass’s conclusions, or his confidence, is the fact that his article was published almost seventy years ago. In this landmark work, we see for the first time the suggestion, albeit oblique, that in order to maintain any semblance of atmospheric equilibrium, humanity would need to moderate its use of fossil fuels (i.e., “keep it in the ground”). Throughout the 1950s, Plass’s work continued to be written up in scholarly journals and in well-regarded popular magazines and newspapers. And yet, as graphic, reasoned, and alarming as his message was, these articles passed through the news cycle and into the library stacks, leaving behind the barest ripple.
But unlike Guy Callendar, Gilbert Plass wasn’t alone. In addition to researchers in Europe, several scientists at the Scripps Institution of Oceanography in California were also doing cutting edge research on atmospheric CO2. One of them, Roger Revelle, an oceanographer, former Navy man, and the director of Scripps, became the first person to formally raise the topic of anthropogenic climate change before members of the U.S. Congress.
On March 8, 1956, Revelle was called before the House Committee on Appropriations to discuss research and funding for the upcoming International Geophysical Year. The IGY (1957–58) was as much a diplomatic effort as a scientific one; it signaled a partial easing of Cold War hostilities that saw the archenemies Soviet Russia and the United States working cooperatively with dozens of other countries on a multipronged effort to better understand Earth’s atmospheric, marine, and terrestrial systems. The IGY’s objectives were magnificently ambitious and included programs for studying everything from polar auroras to the deepest ocean trenches, from the jet stream to the Gulf Stream. The vast array of planned experiments would showcase the latest technology from satellites to bathyspheres, and samples would be taken from anything remotely measurable—from the most ancient glacial ice to the most ephemeral atmospheric gases. In many ways, the International Geophysical Year showed humanity at its best, and it was a great honor to be included in such a historic endeavor.
When Revelle addressed a subcommittee of the Appropriations Committee on that balmy March morning, their questions were focused on polar research. In the U.S., in the 1950s, “polar research” included where to dump rapidly growing quantities of nuclear waste, and how the ice caps might be used to hide nuclear submarines and launch missile attacks. More relevant to Revelle’s expertise was how long those ice caps would even last.
“Human beings during the next few decades may, almost in spite of themselves, be doing something that will have a major effect on the climate of the earth,” Revelle said to the committee. “I refer to the combustion of coal, oil, and natural gas by our worldwide civilization, which adds carbon dioxide to the atmosphere. In this way we are returning to the air and the sea the carbon stored in sedimentary rocks over hundreds of millions of years. From the standpoint of meteorologists and oceanographers we are carrying out a tremendous geophysical experiment of a kind that could not have happened in the past or be reproduced in the future. If all this carbon dioxide stays in the atmosphere, it will certainly affect the climate of the earth, and this may be a very large effect. The slight general warming that has occurred in northern latitudes during recent decades may be greatly intensified.”
Representative Albert Thomas, a right-leaning Texas Democrat, weighed in: “Didn’t I read, from what Dr. Gould says, we have been warming up for the last fifty years?”
Dr. Laurence Gould was chairman of the U.S. National Committee’s Antarctic Committee, and one can see why his introductory remarks would have caught the Texas lawmaker’s attention: “Glacier studies,” Gould wrote,
have given clear indications that we are now in a cycle of warming which began about 1900. It is estimated that if the indicated warming continues for another 25 to 50 years (c. 2000), the ice will melt out of the Arctic Ocean in the summer making it navigable. In addition, the warming cycle, if continued, may melt enough ice tied up in glaciers to add to the sea level sufficiently to affect the lives of millions of people living along low coastal lands. It is conceivable within 20 or 25 years (c. 1980) a peninsula such as Florida might become inundated. Whether this actually happens or not, the slow change of climate has already begun to show a change of storm paths and redistribution of rainfall.
“The reason [for this new warming cycle],” Revelle explained, “may be because we have been adding carbon dioxide to the atmosphere…One of the aspects of this IGY oceanographic program is to try to find out what proportion of the total carbon dioxide produced by the burning of fossil fuels goes into the ocean and how much stays in the atmosphere.”
“This gas that you speak of, has that had any effect on human life, do you know?” asked New York Democratic representative Harold Ostertag.
“It may be having an effect already,” Revelle answered, “primarily through the effect on the weather…The increase in the number of hurricanes on the east coast, however, is certainly tied in one way or the other with the general northward movement of the warm air.”
It bears noting that these observations and exchanges were entered into the Congressional Record in 1956, fifteen years before the first Earth Day, and thirty-five years before the Intergovernmental Panel on Climate Change released its first report. There was no such thing as “climate denial” then, and there were no “climate skeptics.” Those present, all of whom were white men, and most of whom were churchgoers born at the very dawn of the automotive age, were open-minded about this alarming new information, and they discussed it with intelligent interest.
In May of the following year, Revelle was summoned to Washington, D.C., again for a progress report on America’s participation in the International Geophysical Year. He opened the afternoon session with extensive remarks on what he referred to as “heat balance,” and he began in a way that seems surprisingly holistic for a Cold War–era scientist and former military man. The fact that he was also a sailor from Southern California might have had something to do with it. “I think the best way to introduce this subject,” he began, “is to point out to you gentlemen something which is not often thought of, and that is that the earth itself is a space ship…We have lived here on this space ship of our earth for a good many hundred thousand years, and we human beings are specifically adapted to it…Our whole physiology and psychology really depend upon the characteristics of the earth.”
Albert Thomas, the congressman from Texas, wasn’t impressed. “You are talking like an environmentalist,” he said. “I thought that you believed in heredity.”[*3]
Revelle, whose affinity for his subject was as soulful as it was scholarly, continued undeterred. “We are certainly shaped by the earth on which we live,” he said. “A simple example is that we breathe oxygen. This is the only planet in the solar system we know of that has free oxygen on it—”
“That will not be free long,” growled the congressman. “The Federal Government will tax it.”
Revelle persevered: “—and this is the only planet that has large bodies of liquid water on it. The fact that water has such a great capacity for storing heat means that it can absorb a great deal of radiation and not change its temperature very much.”
Thomas, a veteran of World War I and a forceful politician credited with bringing the Johnson Space Center to his home district of Houston, may have presented like a crusty Texas oilman, but he was listening. “You have announced,” he said, “a very fundamental principle of weather when you said that these vast bodies of water are reservoirs for tremendous heat loads.”
“That is correct, sir,” said Revelle.
After a lengthy exchange about drought and water shortages, particularly in Thomas’s and Revelle’s home states, Revelle moved on to short- and long-term weather forecasting. “You used the word ‘climate’ there for the long range,” Thomas said, “and ‘weather’ for the day-to-day basis?”
“That’s right,” said Revelle.
Eisenhower was president, nuclear Armageddon was a preoccupying concern, and Thomas would soon be voting against the Civil Rights Act of 1957,[*4] but on this exceptionally warm May Day afternoon this conservative Texan was talking climate science with a West Coast progressive on Capitol Hill. Revelle went on to say that if industrial CO2 increased by 20 percent, as predicted, “It would mean that…southern California and a good part of Texas, instead of being just barely livable as they are now, would become real deserts.”
There followed some back-and-forth about the causes and impacts of drought in ancient Greece and Mesopotamia, and then this remarkably prescient conversation continued.
“From a weather point of view,” Thomas asked, “how did that happen?”
“No one knows,” Revelle said.
“There is no theory behind it or anything?”
“This carbon dioxide thing that I was talking about,” said Revelle, “is in fact a way to test some of these theories.”
Thomas tried to grasp it: “Carbon dioxide absorbs the infrared rays that are bouncing back from the earth, and when they are absorbed, that absorbs the heat and therefore, what?”
“It raises the temperature,” Revelle said. “It is like a greenhouse…If you increase the temperature of the earth, the north latitude belt, which covers most of the western part of the United States and the Southwest, would move to the north. Does this make any sense?”
“Yes,” said Thomas.
Then, the discussion turned to ocean currents.
Revelle’s warnings to those long-dead congressmen have turned out to be surgically accurate. The 20 percent increase in industrial CO2 that Revelle predicted in 1957 was achieved in 2004, along with the anticipated atmospheric changes. The kind of disruption Revelle alluded to in the context of drought and rainfall is now referred to as a “phase shift”: a dramatic, effectively irreversible change in a region’s climate regime. There is abundant evidence that phase shifts are now under way across much of the planet. Fire behavior is just one indicator, but it is a graphic one, and Revelle’s home state of California offers a good example: in the 1950s, the state’s fire season lasted about four months; today, it is effectively year-round, and the acreage burned during the most severe seasons (1950 versus 2020) has increased eightfold (to say nothing of lives and property lost).[*5] Meanwhile, the drought Revelle predicted has become a serious and persistent condition—winter and summer, threatening the viability of mountain forests, agricultural lands, and the waterways that link them. As for fire tornadoes, those exceeded even Revelle’s powers of prediction.
These historic exchanges between men of such different backgrounds and philosophies were a wonderful by-product of the International Geophysical Year (as well as a reminder of how the U.S. Congress is capable of functioning). While the crucial link between fossil fuel burning and carbon dioxide garnered neither the attention nor the action it deserved, Roger Revelle and his colleagues did secure funding to study it. Given where things stand today, it is sobering to consider that Revelle addressed these matters, accurately and emphatically, more than thirty years before the NASA scientist James Hansen gave his own historic testimony before Congress. Since Revelle’s presentations on Capitol Hill, three generations, amounting to 5 billion people, have been added to the world’s population, along with billions of fuel-burning vehicles, engines, stoves, generators, and power plants of all sizes. In that time, annual CO2 emissions have increased fivefold from their already-climate-altering 1950s levels.