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Scientists first began to study climate change 200 years ago, but most of the events and discoveries that have driven conversation about the planet’s temperature came in more recent years. Here are some key moments that drew attention to the issues:

1. Early Evidence

The Keeling Curve tracks changes in the concentration of CO2 in the Earth's atmosphere using data from a research station on Mauna Loa in Hawaii.

The Keeling Curve tracks changes in the concentration of CO2 in the Earth's atmosphere using data from a research station on Mauna Loa in Hawaii.

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.”

2. Jim Hansen Testifies

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.” 

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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.” 

3. ‘An Inconvenient Truth’

An Inconvenient Truth, Al Gore and climate change

Clockwise from lower left Davis Guggenheim, director, and producers Laurie David, Lawrence Bender, and Scott Burns, posing next to an image on the screen of Al Gore from their documentary, "An Inconvenient Truth."

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.

4. Extreme Weather

Scientists have long predicted that a major consequence of global warming would be an increase in the amount and severity of extreme weather events, and a series of them in the 21st century called attention to that prediction in very dire ways.  

In 2005 Hurricane Katrina flooded 80 percent of New Orleans, and killed an estimated 1,800 people. Superstorm Sandy struck the coast from Maryland north to Manhattan in 2012, causing $70.2 billion worth of damage, destroying 650,000 homes, and causing the deaths of at least 72 Americans

Following Katrina, engineers rebuilt levees at a greater height to account for increased storm surges as a result of sea-level rise. Such measures may have protected New Orleans from disaster when Hurricane Ida struck in 2021. Similarly, in New York City, planners consulted NOAA data on rising sea levels to develop their response to Sandy, and used “the most current climate science to influence their decisions regarding the city's future plans for everything from infrastructure to community preparedness.”

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