The Keeling Curve, Measured on Top of Mauna Loa Since 1958

In March 1958, an instrument began breathing the air two miles above the Pacific Ocean, on the northern flank of Mauna Loa, the largest volcano on Earth. The station had been built by the U.S. Weather Bureau, and it sat well above the island's clouds and far from cities, factories and forests. The instrument belonged to a chemist not yet thirty years old named Charles David Keeling, who worked at the Scripps Institution of Oceanography in La Jolla, California, and who had a habit his colleagues found either admirable or exhausting, depending on the day. He wanted to measure carbon dioxide in the air more precisely than anyone ever had, and he refused to cut corners to do it. The first readings that spring came in at a little above 310 parts per million. He did not know yet that within a few years his numbers would show something nobody had proved: that the amount of carbon dioxide in the atmosphere was rising, steadily and measurably, year after year. The line he started drawing that spring has never stopped climbing.

In short: Charles David Keeling's measurements of carbon dioxide on Mauna Loa, begun in 1958, produced the Keeling Curve, a rising sawtooth line that showed humans were changing the composition of the whole atmosphere. It nearly died for lack of money more than once. In 2026 it remains the backbone of climate science, and it depends on federal agencies whose budgets are under pressure.

A chemist who measured things properly

Keeling grew up in the Chicago area, studied chemistry, and came to the California Institute of Technology in the 1950s as a postdoctoral researcher. He became interested in carbon dioxide almost by accident, while working on how carbonate in water interacts with air. He built his own equipment, based on a precise gas measuring technique, and went camping in the forests and mountains of California, taking air samples at all hours. He discovered something that other researchers had missed because their measurements were sloppier: away from local sources, such as cities or plants breathing at night, the carbon dioxide level in the air was remarkably consistent, around 310 parts per million.

That was a big deal. Earlier scientists had reported wildly varying numbers and concluded that measuring a global background level was hopeless. Keeling showed that the variation was mostly in the samples, not the atmosphere. If you were careful, you could measure the planet's carbon dioxide level, and if you kept doing it, you might see it change.

Roger Revelle and the year of geophysics

Keeling's precision caught the attention of Roger Revelle, the director of Scripps. Revelle and a colleague, Hans Suess, had published a paper in 1957 arguing that the ocean would not absorb carbon dioxide from fossil fuels as quickly as many assumed, which meant the gas would build up in the air. In a line that became famous, they described humanity as carrying out a large-scale geophysical experiment. Revelle wanted someone to measure what was happening.

The timing was lucky. The International Geophysical Year of 1957 and 1958 brought a burst of government funding for global science, from satellites to Antarctic research. Revelle hired Keeling, and the IGY money helped pay for instruments at Mauna Loa, in Antarctica and elsewhere. Mauna Loa was ideal: high, remote, surrounded by thousands of miles of ocean, with winds that brought well-mixed air from across the Pacific.

The sawtooth

The first surprise was a wiggle. Keeling found that carbon dioxide at Mauna Loa rose to a peak each spring, in about May, and fell to a low each autumn. He worked out why. The Northern Hemisphere holds most of the planet's land and forests. In spring and summer, leaves grow and photosynthesis pulls carbon dioxide out of the air; in fall and winter, leaves drop and decay and plants respire, and the gas comes back. Keeling was watching the planet breathe.

The second finding mattered more. Underneath the seasonal wiggle, the average level was going up. By the early 1960s, after just a few years of data, the rise was clear. The line on the chart is a sawtooth climbing a hill: every year the spring peak is higher than the last one, and every autumn low is higher too. That sawtooth line is the Keeling Curve, and it is probably the most widely reproduced graph in climate science.

Over the decades the climb has also gotten steeper. In the early years, the yearly increase was less than one part per million. In recent years, it has typically been well over two. Scientists measuring air trapped in ancient ice have shown that before the industrial era, carbon dioxide stood around 280 parts per million and had not been much higher for hundreds of thousands of years. Daily readings at Mauna Loa first crossed 400 parts per million in May 2013. The record set each spring has kept rising since.

How we know the carbon is ours

A rising line alone does not prove where the extra carbon dioxide comes from. Volcanoes release the gas, oceans breathe it in and out, forests store and release it. Scientists have answered the question of the source with several independent lines of evidence, and some of them come from the same kind of patient measurement Keeling started.

The first clue is in the carbon atoms themselves. Carbon comes in slightly different forms, called isotopes. Plants prefer the lighter form, and fossil fuels, made from ancient plants, carry that same light signature. As fossil carbon has poured into the air, the share of the lighter isotope in atmospheric carbon dioxide has gone up in a way that matches the burning of coal, oil and gas. Fossil fuels are also so old that they contain none of the radioactive carbon that living things carry, and the air's radioactive carbon has been diluted accordingly. The chemist Hans Suess, Revelle's collaborator, first spotted that dilution in the 1950s, and it bears his name.

The second clue is oxygen. Burning fossil fuels consumes oxygen as well as producing carbon dioxide. Beginning around 1990, Ralph Keeling developed instruments precise enough to detect the tiny decline in atmospheric oxygen, and the measurements show oxygen falling in step with fossil fuel combustion. The amount is far too small to matter for breathing, but it is a fingerprint.

The third clue is the ocean. Roughly a quarter or more of the carbon dioxide humans release each year dissolves into seawater, which makes the oceans more acidic. A long-running station near Hawaii, run by University of Hawaii scientists, has recorded seawater acidity rising alongside the Mauna Loa curve. Volcanoes, for their part, release only a small fraction of the carbon dioxide that human activity does each year, according to the U.S. Geological Survey.

Put together, the evidence leaves no serious doubt. Each line was measured by different people with different instruments, and they point the same way. The extra carbon in the air comes from us.

Reading the wiggles

Scientists who study the curve closely see more than a rising trend. The rate of increase varies from year to year, faster during strong El Niño years, when heat and drought in the tropics reduce how much carbon forests absorb and fires release more. The size of the seasonal swing has grown over time in the far north, a sign that plant growth in high latitudes has changed as the climate has warmed.

And sometimes the curve teaches by what it does not show. In 2020, when pandemic lockdowns sharply reduced driving, flying and industry around the world, global emissions fell by several percent. Many people expected to see a dip at Mauna Loa. There was barely a visible change. The reason is that the curve tracks the total amount of carbon dioxide accumulated in the atmosphere, not the yearly flow. A smaller annual addition is still an addition. To make the line level off, emissions have to fall much further, toward the point where what humans add is balanced by what land and oceans take up. That is what the phrase net zero means, and the curve shows why nothing less will do.

The money kept running out

The record exists because Keeling fought for it, and he had to fight constantly. In the mid-1960s, funding problems forced cuts, and there were gaps when measurements at the South Pole stopped. Government grant officers sometimes regarded the work as routine monitoring rather than real science, the sort of thing that produced no new discovery each year. Keeling's argument was that the discovery was the long, unbroken record itself. A measurement interrupted loses its value, because the whole point is to compare this year with the last fifty.

He won that argument, mostly. Over time, the federal government's role grew. The National Oceanic and Atmospheric Administration, created in 1970, took over the Mauna Loa Observatory and began its own independent carbon dioxide measurements alongside the Scripps program, so that the two could check each other. Keeling received the National Medal of Science in 2002. He died in 2005, and his son, Ralph Keeling, a Scripps geochemist, continues the Scripps measurements.

There have been other interruptions. In late 2022, Mauna Loa itself erupted, and lava cut the road and the power lines to the observatory. Scientists moved temporary instruments to the summit of nearby Maunakea and kept the record going, an act of improvisation the elder Keeling would have recognized.

The record and the budget

The Keeling Curve depends on public money, and in 2025 and 2026 that money became a political fight. The Trump administration's budget for 2026 proposed eliminating NOAA's main research office, which runs climate monitoring, including the global network of greenhouse gas measurements. Agencies cut staff, and there were reports that the observatory's support facilities in Hilo were on lists of leases to be terminated. Whatever Congress finally funds in a given year, that kind of uncertainty is itself a threat to programs whose whole value lies in never stopping.

The argument for cutting deserves to be stated. Supporters said climate research had become politicized, that the government should focus on weather forecasting that directly serves the public, and that universities and private funders could pick up anything worth keeping. Our answer is that a measurement like this is a public good in the purest sense. It serves farmers, pilots, insurers, energy companies and governments around the world, it has value only if it is never broken, and nobody can profit from selling it. That is exactly the kind of work markets neglect and governments exist to do. Ending it would not make the carbon dioxide go away. It would only stop us from watching.

What one line made possible

It is hard to overstate how much the Keeling Curve changed the debate. Before it, the idea that humans could alter the whole atmosphere was a theory. After it, the change was a measured fact, visible to anyone who could read a graph. It underlies every major climate assessment, the U.S. government's own national climate reports, and the international negotiations that began in the 1990s. People who reject climate policy rarely dispute the curve; the numbers are too clean and are confirmed by stations around the world.

The curve has also had a long public life outside the journals. Scripps posts the daily reading online, and teachers use the sawtooth to show students how seasons, forests and fossil fuels connect. It turns an invisible gas into something anyone can watch change. Keeling himself was reserved about advocacy and preferred to let the numbers speak, but he understood that a measurement the public can see and check for itself carries a kind of authority that no press release can match. In a period when trust in experts is contested, a single honest line that has been drawn the same way for nearly seventy years is worth defending for that reason alone.

The curve is also a lesson about patience and public institutions. One careful scientist, a university that backed him, and federal agencies that paid for the work year after year built something no single company or donor would have funded. Progressives who argue for a capable government should tell this story more often. The Keeling Curve is what government competence looks like: unglamorous, consistent and essential.

And there is a note of hope in it. The curve measures what we put into the air. If emissions fall, the slope will bend. The same instruments that recorded the rise will record the turn, whenever the world manages to make it.

What you can do

• Look at the curve for yourself. The climate dashboard charts carbon dioxide measured at Mauna Loa alongside the global temperature record and the U.S. power mix. Watch for the springtime peak.
• Defend the people who take the measurements. NOAA's research budget is decided by Congress. Use the lookup for your senators and representative and ask them to protect long-term climate monitoring by name.
• Comment when rules change. Federal changes to climate reporting and greenhouse gas rules go through public comment; the open dockets are on the federal comments page, closing soonest first.
• Read what Congress and the states are doing. Our climate policy page follows the fights over emissions, clean power and the science budget.