Keeling Curve

• 1. Introduction

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  The Keeling Curve is a line on a graph that changed how humanity thinks about its own future. It records the concentration of carbon dioxide in Earth's atmosphere, measured continuously at the Mauna Loa Observatory on the island of Hawaii, from 1958 right up to the present day. When Charles David Keeling began taking those readings in March 1958, atmospheric CO2 stood at 313 parts per million by volume. By November 2018, that number had climbed to 406 ppm. The curve is still rising.

  Naomi Oreskes, Professor of History of Science at Harvard University, calls it one of the most important scientific works of the twentieth century. Many scientists credit it with first bringing the world's attention to the mounting accumulation of CO2 in the atmosphere. But how did a single monitoring station on a Hawaiian volcano come to carry that weight? And who was the man who built it, fought to keep it running, and gave it his name?

• 2. Before the Curve

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  In 1938, engineer and amateur meteorologist Guy Stewart Callendar compared two sets of atmospheric measurements decades apart. Data gathered at Kew between 1898 and 1901 showed an average of 274 parts per million by volume. Readings from the eastern United States between 1936 and 1938 averaged 310 ppm. Callendar concluded that concentrations were rising because of human-caused emissions.

  The scientific community largely dismissed his finding. The measurements were too scattered, taken at different places and times, with no consistent methodology binding them together. What Callendar had done was suggestive, not conclusive. The problem was not the idea but the data. Filling that gap would take a researcher willing to build a system from scratch, starting in the field.

• 3. Keeling's Method

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  Charles David Keeling, working at the Scripps Institution of Oceanography at UC San Diego, became the first person to take frequent, regular measurements of atmospheric CO2 in Antarctica and on Mauna Loa. Before setting up those stations, he tested his measurement techniques at Big Sur near Monterey, in the rain forests of the Olympic Peninsula in Washington state, and in high mountain forests in Arizona. That fieldwork revealed something striking: CO2 levels swung sharply through each day, rising at night as plants and soils released it through respiration, then falling during afternoon hours to values representative of what Keeling called the "free atmosphere" over the Northern Hemisphere.

  That discovery pointed him toward a practical solution. To capture a true baseline reading, he needed to measure above the messy biological noise of forests and soils. He needed a remote location, high up and surrounded by ocean. Mauna Loa fit precisely.

• 4. The Mauna Loa Station

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  During the International Geophysical Year of 1957-1958, Keeling secured funding from the Weather Bureau to install infrared gas analyzers at remote sites, including the South Pole and Mauna Loa. The volcano was chosen for two reasons: its distance from continents and its complete absence of vegetation. Keeling and his collaborators measured the incoming ocean breeze above the thermal inversion layer, a technique that minimized contamination from the volcanic vents below. The raw data was then normalized to strip out any remaining local interference.

  The instrument at the heart of this work was a type of infrared spectrophotometer, now called a nondispersive infrared sensor. That class of instrument traces its lineage to a capnograph first invented by John Tyndall in 1864, which recorded readings as pen traces on a strip chart recorder. Keeling's 1960 article in the journal Tellus presented the first monthly records from Mauna Loa and Antarctica, covering 1957 to 1960, and described what he cautiously called "a distinct seasonal cycle...and possibly, a worldwide rise in CO2 from year to year."

  Funding cuts in the mid-1960s forced Keeling to abandon continuous monitoring at the South Pole. He scraped together enough money to keep the Mauna Loa station running, and those measurements have continued without interruption to the present day.

• 5. What the Numbers Say

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  The data from Mauna Loa shows two patterns running simultaneously. The first is a long, relentless rise. Mean atmospheric CO2 climbed from 313 ppm in March 1958 to 406 ppm in November 2018, and it is currently increasing at a rate of 2.48 ppm per year. Ice cores drilled from polar glaciers show that during the Holocene epoch, from around 9,000 BCE onward, CO2 stayed between 275 and 285 ppm. It began rising sharply at the start of the nineteenth century, when industrial burning of fossil fuels took hold.

  The second pattern is a seasonal wave about 6 ppm wide. From a peak in May, CO2 drops through the northern spring and summer as new plant growth draws carbon out of the atmosphere through photosynthesis. It bottoms out in September, then climbs again through fall and winter as leaves and plants die and decay, releasing CO2 back into the air. Because most of the world's land vegetation sits in the Northern Hemisphere, that hemisphere's seasonal rhythm drives the entire planet's annual pulse. On the 9th of May, 2013, that long-term rise crossed a threshold: the daily mean concentration at Mauna Loa surpassed 400 ppm. Estimates for earlier geologic eras suggest CO2 had not reached that level since the mid-Pliocene, somewhere between 2 and 4 million years ago.

• 6. A Family Record

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  Charles David Keeling supervised the monitoring program until his death in 2005. Responsibility then passed to his son, Ralph Keeling. On the fiftieth anniversary of the project's start, Ralph wrote an article in Science magazine about his father's life and work, describing how the program had grown and evolved. He wrote about his pride in continuing it.

  The continuity matters more than it might seem. No other monitoring site anywhere in the world has a record as long as Mauna Loa's. In the 1970s, NOAA began expanding the effort, and today the Global Greenhouse Gas Reference Network tracks atmospheric CO2 at roughly 100 sites around the globe. Every one of those stations has confirmed the long-term trend the Keeling Curve first revealed. The Integrated Carbon Observation System runs a parallel network across 38 monitoring sites in Europe, producing what is called the ICOS Curve. That European average runs slightly higher than the global figure, reflecting the continent's dense population and high emission levels.

  In 2015, the American Chemical Society designated the Keeling Curve a National Historic Chemical Landmark. Commemorative plaques were installed at both Mauna Loa Observatory and at the Scripps Institution of Oceanography at the University of California, San Diego.