Global warming first became a major public issue in the summer of 1988, when NASA scientist James Hansen told the U.S. Senate that human-produced greenhouse gases were warming the planet and that the effect was already measurable. But the scientific foundations go back more than 160 years before that moment, and the path from laboratory curiosity to global political crisis unfolded in distinct stages.
The 19th-Century Science Behind It
The basic physics of global warming were understood surprisingly early. In the 1820s, French mathematician Joseph Fourier proposed that Earth’s atmosphere acts as an insulating layer, trapping heat that would otherwise escape into space. He published his ideas in 1824 and again in 1827, though his descriptions were cryptic enough that later authors sometimes stretched their meaning.
The real experimental proof came in 1859, when Irish physicist John Tyndall built the first instrument capable of measuring how individual gases absorb heat radiation. He discovered that water vapor, carbon dioxide (then called “carbonic acid”), ozone, and hydrocarbons are powerful absorbers of heat, while oxygen, nitrogen, and hydrogen are nearly transparent to it. This was the first demonstration that specific gases in the atmosphere are responsible for keeping the planet warm.
Swedish chemist Svante Arrhenius took the next leap in 1896. He sat down with pencil and paper and calculated what would happen if the amount of carbon dioxide in the atmosphere doubled. His answer: Earth’s average temperature would rise by 5 to 6 degrees Celsius. That estimate was remarkably close to the upper range of modern projections, though Arrhenius himself saw potential warming as a slow, possibly beneficial process that might stave off future ice ages. At this stage, global warming was a theoretical prediction, not a perceived threat.
Tracking CO2 in Real Time
For decades, no one had reliable measurements of how much carbon dioxide was actually in the atmosphere or whether the concentration was changing. That gap closed on March 29, 1958, when American scientist Charles Keeling recorded his first reading from an observatory atop Mauna Loa in Hawaii: 313 parts per million of CO2.
Keeling kept measuring. Within a few years, his data showed an unmistakable upward trend, with a sawtooth pattern reflecting seasonal plant growth. The “Keeling Curve” became one of the most important datasets in climate science because it transformed the greenhouse effect from a theoretical concern into a documented, ongoing change. Carbon dioxide levels were rising year after year, and the source was clear: fossil fuel combustion and deforestation.
The First Warnings to Governments
By the late 1970s, enough evidence had accumulated that the scientific community felt compelled to speak directly to world leaders. In 1979, the First World Climate Conference brought hundreds of scientists together in Geneva for two weeks. At its close, participants issued an urgent “appeal to the nations,” warning of the consequences of human-caused climate change and calling on the world community to act quickly. This was the first time the global scientific establishment collectively told governments that the problem was real and serious.
The warning didn’t generate much public attention at the time. Climate science remained a niche topic through most of the 1980s, familiar to researchers and a handful of policymakers but largely invisible to ordinary people.
1988: The Year It Went Public
That changed dramatically on June 23, 1988. James Hansen, head of NASA’s Goddard Institute for Space Studies, testified before the U.S. Senate during a brutal heat wave that had parched crops across the Midwest. Hansen presented data showing that the surface of Earth had warmed over the past century and stated with high confidence that humans were a significant contributor to that warming. He detailed how carbon dioxide was the largest driver of the warming effect, with methane contributing roughly half as much forcing when its indirect effects on other gases were included.
Hansen’s testimony landed on front pages across the country. The combination of sweltering temperatures outside and a respected scientist speaking in plain terms inside the Senate chamber made global warming tangible to the American public for the first time. Within months, the United Nations established the Intergovernmental Panel on Climate Change (IPCC) to formally assess the science. From that point on, global warming was no longer just a scientific issue. It was a political one.
From Awareness to International Action
The 1990s brought the first attempts at a coordinated global response. The 1992 Earth Summit in Rio de Janeiro produced a framework convention on climate change, but it set no binding limits on emissions. That came five years later with the Kyoto Protocol in 1997, which committed 37 industrialized countries to reduce greenhouse gas emissions by an average of 5 percent below 1990 levels during the period from 2008 to 2012. A second commitment period raised the target to at least 18 percent below 1990 levels through 2020, though with a different set of participating countries.
The 2015 Paris Agreement broadened participation further, with nearly every nation setting its own reduction targets and collectively aiming to hold warming well below 2 degrees Celsius above pre-industrial levels. The long-term goal of limiting warming to 1.5 degrees became a rallying point for climate advocacy worldwide.
Where Things Stand Now
The CO2 concentration Keeling first measured at 313 parts per million in 1958 has now climbed past 420 ppm. According to the European Union’s Copernicus Climate Change Service, global surface temperature in 2025 was 1.47 degrees Celsius above the pre-industrial baseline, following 2024 at 1.60 degrees, the warmest year on record. The three-year average from 2023 through 2025 exceeded 1.5 degrees above pre-industrial levels for the first time, and the current level of long-term warming sits around 1.4 degrees Celsius.
In other words, the warming Fourier theorized about in the 1820s, Tyndall demonstrated in a lab in 1859, and Arrhenius calculated in 1896 is no longer a projection. It is the measured temperature of the planet. The science preceded the public awareness by more than a century, and the political response has lagged behind both.