In 1958, Charles David Keeling began plotting levels of CO2 in the atmosphere from atop Mauna Loa in Hawaii, producing the famous Keeling Curve, which updates daily. (When he began, levels of CO2 in the atmosphere were 313 parts per million (ppm); now they are around 420 ppm.)

Keeling’s measurements provided the first, unequivocal irrefutable proof that levels of carbon dioxide in the atmosphere were increasing. According to Spencer Weart in The Discovery of Global Warming, his eponymous curve “became the central icon of the greenhouse effect.” It spurred other scientists to conduct corroborating research, including Syukuro Manabe of the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration (NOAA).

Manabe then led a team that devised the first comprehensive model of the response of climate to an increase in atmospheric CO2 extrapolated from the Keeling Curve. In 2021, Manabe’s work earned him a share of the Nobel Prize for Physics.

In 1965, scientists on the U.S. President’s Science Advisory Committee first put forward concerns about greenhouse warming, arguing that the continued release of CO2 into the atmosphere would “almost certainly cause significant changes” and “could be deleterious from the point of view of human beings.” And in 1983, back-to-back reports from the National Academy of Sciences and the Environmental Protection Agency sounded the alarm about rising greenhouse gas levels, with the EPA report warning that “Substantial increases in global warming may occur sooner than most of us would like to believe.”

As a result, notes the American Institute of Physics, “climate scientists found themselves in demand to give tutorials to journalists, government agency officials, and even groups of senators, who would sit obediently for hours of lecturing on greenhouse gases and computer models.”

In the wake of the EPA and NAS reports, and other growing evidence of the reality of greenhouse warming, Congress held a number of hearings on the issue and invited the testimony of outside experts. The most impactful came on June 23, 1988 when, on a hot day in Washington, D.C., James Hansen of the NASA Goddard Institute of Space Studies told senators that, “The earth is warmer in 1988 than at any time in the history of instrumental measurements,” and that there was “only a 1 percent chance of an accidental warming of this magnitude ... The greenhouse effect has been detected, and it is changing our climate now.”

The New York Times declared that Hansen’s testimony “sounded the alarm with such authority and force that the issue of an overheating world has suddenly moved to the forefront of public concern.” Weart wrote that coverage of Hansen’s speech was so extensive that, “according to a 1989 poll, 79 percent of Americans recalled having heard or read about the greenhouse effect”—a huge jump from 31 percent in 1981.

Two months later, George H.W. Bush, campaigning for president, proclaimed that, “'Those who think we are powerless to do anything about the greenhouse effect forget about the 'White House effect'; as President, I intend to do something about it … We will talk about global warming, and we will act.”

Following his defeat in the 2000 U.S. Presidential election, Al Gore began presenting a slide show on the science and policy issues of climate change, which producer Laurie David and director David Guggenheim transformed into the 2006 documentary An Inconvenient Truth.

The documentary, and an accompanying book, laid out the science behind global warming and visualized potential climate change impacts on the polar ice caps, sea level rise, and extreme weather, among other areas, before closing with an appeal for collective action. A 2017 study, noting that several countries had proposed using the film as an educational tool in schools, found that viewing it increased awareness of global warming and a willingness to take action to prevent it (even as it arguably helped accentuate a growing partisan divide on the issue).

Ten years after its release, it was widely credited with inspiring a new generation to think about global warming and even become climate activists. The movie grossed more than $50 million worldwide at the box office and won the 2007 Academy Award for Best Documentary Feature. That same year, Gore shared the Nobel Peace Prize with the Intergovernmental Panel on Climate Change.