Climate change is the long-term alteration in Earth’s climate and weather patterns. It took nearly a century of research and data to convince the vast majority of the scientific community that human activity could alter the climate of our entire planet. In the 1800s, experiments suggesting that human-produced carbon dioxide (CO2) and other gases could collect in the atmosphere and insulate Earth were met with more curiosity than concern. By the late 1950s, CO2 readings would offer some of the first data to corroborate the global warming theory. Eventually an abundance of data, along with climate modeling would show not only that global warming was real, but that it also presented a host of dire consequences.
Early Inklings That Humans Can Alter Global Climate
Dating back to the ancient Greeks, many people had proposed that humans could change temperatures and influence rainfall by chopping down trees, plowing fields or irrigating a desert.
One theory of climate effects, widely believed until the Dust Bowl of the 1930s, held that “rain follows the plow,” the now-discredited idea that tilling soil and other agricultural practices would result in increased rainfall.
Accurate or not, those perceived climate effects were merely local. The idea that humans could somehow alter climate on a global scale would seem far-fetched for centuries.
WATCH: How the Earth Was Made on HISTORY Vault.
The Greenhouse Effect
In the 1820s, French mathematician and physicist Joseph Fourier proposed that energy reaching the planet as sunlight must be balanced by energy returning to space since heated surfaces emit radiation. But some of that energy, he reasoned, must be held within the atmosphere and not return to space, keeping Earth warm.
He proposed that Earth’s thin covering of air—its atmosphere—acts the way a glass greenhouse would. Energy enters through the glass walls, but is then trapped inside, much like a warm greenhouse.
Experts have since pointed out that the greenhouse analogy was an oversimplification, since outgoing infrared radiation isn’t exactly trapped by Earth’s atmosphere but absorbed. The more greenhouse gases there are, the more energy is kept within Earth’s atmosphere.
But the so-called greenhouse effect analogy stuck and some 40 years later, Irish scientist John Tyndall would start to explore exactly what kinds of gases were most likely to play a role in absorbing sunlight.
Tyndall’s laboratory tests in the 1860s showed that coal gas (containing CO2, methane and volatile hydrocarbons) was especially effective at absorbing energy. He eventually demonstrated that CO2 alone acted like sponge in the way it could absorb multiple wavelengths of sunlight.
By 1895, Swedish chemist Svante Arrhenius became curious about how decreasing levels of CO2 in the atmosphere might cool Earth. In order to explain past ice ages, he wondered if a decrease in volcanic activity might lower global CO2 levels. His calculations showed that if CO2 levels were halved, global temperatures could decrease by about 5 degrees Celsius (9 degrees Fahrenheit).
Next, Arrhenius wondered if the reverse were true. Arrhenius returned to his calculations, this time investigating what would happen if CO2 levels were doubled. The possibility seemed remote at the time, but his results suggested that global temperatures would increase by the same amount—5 degrees C or 9 degrees F.
Decades later, modern climate modeling have confirmed that Arrhenius’ numbers weren’t far off the mark.
Welcoming a Warmer Earth
Back in the 1890s, however, the concept of warming the planet was remote and even welcomed.
As Arrehenius wrote, “By the influence of the increasing percentage of carbonic acid [CO2] in the atmosphere, we may hope to enjoy ages with more equable and better climates, especially as regards the colder regions of the earth.”
By the 1930s, at least one scientist would start to claim that carbon emissions might already be having a warming effect. British engineer Guy Stewart Callendar noted that the United States and North Atlantic region had warmed significantly on the heels of the Industrial Revolution.
Callendar’s calculations suggested that a doubling of CO2 in Earth’s atmosphere could warm Earth by 2 degrees C (3.6 degrees F). He would continue to argue into the 1960s that a greenhouse-effect warming of the planet was underway.
While Callendar’s claims were largely met with skepticism, he managed to draw attention to the possibility of global warming. That attention played a part in garnering some of the first government-funded projects to more closely monitor climate and CO2 levels.
Most famous among those research projects was a monitoring station established in 1958 by the Scripps Institution of Oceanography on top of Hawaii’s Mauna Loa Observatory.
Scripps geochemist Charles Keeling was instrumental in outlining a way to record CO2 levels and in securing funding for the observatory, which was positioned in the center of the Pacific Ocean.
Data from the observatory revealed what would become known as the “Keeling Curve.” The upward, saw tooth-shaped curve showed a steady rise in CO2 levels, along with short, jagged up-and-down levels of the gas produced by repeated wintering and greening of the Northern Hemisphere.
The dawn of advanced computer modeling in the 1960s began to predict possible outcomes of the rise in CO2 levels made evident by the Keeling Curve. Computer models consistently showed that a doubling of CO2 could produce a warming of 2 degrees C or 3.6 degrees F within the next century.
Still, the models were preliminary and a century seemed a very long time away.
1970s Scare: A Cooling Earth
In the early 1970s, a different kind of climate worry took hold: global cooling. As more people became concerned about pollutants people were emitting into the atmosphere, some scientists theorized the pollution could block sunlight and cool Earth.
In fact, Earth did cool somewhat between 1940-1970 due to a postwar boom in aerosol pollutants which reflected sunlight away from the planet. The idea that sunlight-blocking pollutants could chill Earth caught on in the media, as in a 1974 Time magazine article titled “Another Ice Age?”
But as the brief cooling period ended and temperatures resumed their upward climb, warnings by a minority of scientists that Earth was cooling were dropped. Part of the reasoning was that while smog could remain suspended in the air for weeks, CO2 could persist in the atmosphere for centuries.
1988: Global Warming Gets Real
The early 1980s would mark a sharp increase in global temperatures. Many experts point to 1988 as a critical turning point when watershed events placed global warming in the spotlight.
The summer of 1988 was the hottest on record (although many since then have been hotter). 1988 also saw widespread drought and wildfires within the United States.
Scientists sounding the alarm about climate change began to see media and the public paying closer attention. NASA scientist James Hansen delivered testimony and presented models to congress in June of 1988, saying he was “99 percent sure” that global warming was upon us.
One year later, in 1989, the Intergovernmental Panel on Climate Change (IPCC) was established under the United Nations to provide a scientific view of climate change and its political and economic impacts.
As global warming gained currency as a real phenomenon, researchers dug into possible ramifications of a warming climate. Among the predictions were warnings of severe heat waves, droughts and more powerful hurricanes fueled by rising sea surface temperatures.
Other studies predicted that as massive glaciers at the poles melt, sea levels could rise between 11 and 38 inches (28 to 98 centimeters) by 2100, enough to swamp many of the cities along the east coast of the United States.
Kyoto Protocol: United States In, Then Out
Government leaders began discussions to try and stem the outflow of greenhouse gas emissions to prevent the most dire predicted outcomes. The first global agreement to reduce greenhouse gases, the Kyoto Protocol, was adopted in 1997.
The protocol, which was signed by President Bill Clinton, called for reducing the emission of six greenhouse gases in 41 countries plus the European Union to 5.2 percent below 1990 levels during the target period of 2008 to 2012.
In March 2001, shortly after taking office, President George W. Bush announced the United States would not implement the Kyoto Protocol, saying the protocol was “fatally flawed in fundamental ways” and citing concerns that the deal would hurt the U.S. economy.
An Inconvenient Truth
That same year, the IPCC issued its third report on climate change, saying that global warming, unprecedented since the end of the last ice age, is “very likely,” with highly damaging future impacts. Five years later, in 2006, former Vice President and presidential candidate Al Gore weighed in on the dangers of global warming with the debut of his film An Inconvenient Truth. Gore won the 2007 Nobel Peace Prize for his work on behalf of climate change.
Politicization over climate change, however, would continue, with some skeptics arguing that predictions presented by the IPCC and publicized in media like Gore’s film were overblown.
Among those expressing skepticism over global warming was future U.S. president Donald Trump. On November 6, 2012, Trump tweeted “The concept of global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive.”
Paris Climate Agreement: United States In, Then Out
The United States, under President Barack Obama, would sign onto another milestone treaty on climate change, the Paris Climate Agreement, in 2015. In that agreement, 197 countries pledged to set targets for their own greenhouse gas cuts and to report their progress.
The backbone of the Paris Climate Agreement was a declaration to prevent a global temperature rise of 2 degrees C (3.6 degrees F). Many experts considered 2 degrees C of warming to be a critical limit, which, if surpassed will lead to increasing risk of more deadly heat waves, droughts, storms and rising global sea levels.
The election of Donald Trump in 2016 led to the United States declaring it would withdraw from the Paris treaty. President Trump, citing the “onerous restrictions” imposed by the accord, stated that he could not “in good conscience support a deal that punishes the United States.”
That same year, independent analyses by NASA and the National Oceanic and Atmospheric Administration (NOAA) found Earth’s 2016 surface temperatures to be the warmest since modern record keeping began in 1880. And in October 2018, the U.N.'s Intergovernmental Panel on Climate Change issued a report that concluded "rapid, far-reaching" actions are needed to cap global warming at 1.5 Celsius (2.7 Fahrenheit) and avert the most dire, irreversible consequences for the planet.
Greta Thunberg and Climate Strikes
In August 2018, Swedish teenager and climate activist Greta Thunberg began protesting in front of Swedish Parliament with a sign: “School Strike for Climate.” Her protest to raise awareness for global warming caught the world by storm and by November 2018, over 17,000 students in 24 countries were participating in climate strikes. By March 2019, Thunberg was nominated for a Nobel Peace Prize. She participated in the United Nations Climate Summit in New York City in August of 2019, famously taking a boat across the Atlantic instead of flying to reduce her carbon footprint.
The UN Climate Action Summit reinforced d that “1.5℃ is the socially, economically, politically and scientifically safe limit to global warming by the end of this century,” and set a deadline for achieving net zero emissions to 2050.
The Discovery of Global Warming, by Spencer R. Weart. (Harvard University Press, 2008).
The Thinking Person’s Guide to Climate Change, by Robert Henson. (AMS Books, 2014).
“Another Ice Age?” Time.
“Why we know about the greenhouse gas effect” Scientific American.
The History of the Keeling Curve, Scripps Institute of Oceanography.
Remembering the Drought of 1988, NASA Earth Observatory.
Sea Level Rise, National Geographic/reference.
“Guy Stewart Callendar: Global warming discovery marked,” BBC News.
President Bush Discusses Global Climate Change, The White House, President George W. Bush.
“Why the Paris talks won’t prevent 2 degrees of global warming,” PBS News Hour.
Statement by President Trump on the Paris Climate Accord, The White House.
“Trump Will Withdraw U.S. From Paris Climate Agreement,” The New York Times.
“NASA, NOAA Data Show 2016 Warmest Year on Record Globally,” NASA.