On May 18, 1955, Charles David Keeling, a young postdoctoral researcher, set up camp near a coastal river in California’s Big Sur State Park.

Keeling was a young chemist and outdoor enthusiast interested in learning how carbon dioxide moved between water and air, as part of a geological study of limestone. There in the sheltering redwoods, Keeling was having such a good time, he took water and air samples every few hours throughout the day and night, far more than he needed. He went back to his university, the California Institute of Technology, with some surprising findings—not just about limestone, but about Earth’s atmosphere itself.

First, he found that the CO₂ concentration in the forest air changed on a daily cycle. It decreased during the day, as plants took in CO₂ to make sugar during photosynthesis, and increased during the night due to respiration by plants and other organisms. Second, he determined that CO₂ concentrations in air were about the same everywhere he made measurements in the afternoon: 310 parts per million (ppm). No one had ever measured CO₂ in the atmosphere so consistently and so precisely before.

Word of Keeling’s groundbreaking work soon found its way to Harry Wexler at the U.S. Weather Bureau. Wexler offered him a chance to measure atmospheric CO₂ around the world, including at a new observatory located at 3,397 meters (11,138 feet) on Mauna Loa, a volcano on the island of Hawaii (Figure 14.14).

At the time Keeling began his measurements, there was widespread disagreement in the scientific community about the atmospheric effects of fossil fuel burning. The prevailing idea was that the vast oceans, which actively exchange CO₂ with the overlying air, would be capable of rapidly absorbing excess atmospheric CO₂. Scientists saw Keeling’s precise measurements as a way to settle the debate.

Keeling’s first two years at Mauna Loa produced a sawtooth plot showing CO₂ rising and falling with each season. The reason for this sawtooth pattern is that the concentration of CO₂ reaches a maximum during each Northern Hemisphere winter as respiration by all the organisms that make up ecosystems pump CO₂ into the atmosphere. Then it declines during the Northern Hemisphere growing season as plants come out of dormancy and rates of photosynthesis increase across the northern continental landmasses. As Keeling continued taking these measurements over the next 50 years, he saw the sawtooth curve gradually rising at Mauna Loa. By 2012 atmospheric CO₂ had increased by approximately 25%. This striking graph is what we now call the Keeling Curve (Figure 14.15).

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The dramatic modern increase in atmospheric CO₂ becomes especially apparent when we look back in time, again using the ice record. As shown in Figure 14.16, atmospheric CO₂ oscillated around an average of about 280 ppm for approximately 1,800 years and then began rising with increased fossil fuel use during the Industrial Revolution, which began in the late 18th century, just as Svante Arrhenius predicted it would—except it has risen even faster than Arrhenius anticipated.

Because fossil fuel emissions are not the only source of carbon dioxide in Earth’s atmosphere, alternative hypotheses are that the rise in CO₂ concentrations might be due to volcanic eruptions or might result from emissions from the oceans. Volcanic eruptions can be discounted as the source of increased atmospheric carbon dioxide since fossil fuel burning releases over 100 times more CO₂ to the atmosphere. How about the oceans as the source? Scientists can answer this question by looking at the ratio of two carbon isotopes in the atmosphere, ¹³C to ¹²C, which Keeling had also measured in Big Sur.

Because plants preferentially take in the lighter ¹²C isotope from the air, plant biomass, and therefore the CO₂ emitted by plants during respiration, has a higher relative concentration of ¹²C than occurs in the atmosphere. At Big Sur, Keeling could see that the ¹³C isotope decreased in relative concentration at night, when the plants respired and weren’t absorbing carbon dioxide during photosynthesis, and increased in relative concentration during the day when the plants photosynthesized, removing ¹²C-rich carbon dioxide from the atmosphere. These carbon isotope measurements proved to Keeling that respiration by plants was primarily responsible for the buildup of CO₂ in Big Sur during the forest night.

The relative concentrations of ¹³C and ¹²C can also be used to determine the source of increased atmospheric CO₂ over the past two centuries. Because fossil fuels formed over millions of years out of compressed plant biomass, the carbon dioxide released when fossil fuels combust also contains relatively low concentrations of the ¹³C isotope compared with that in the atmosphere. In contrast, carbon dioxide in the oceans has the same ratio of ¹³C to ¹²C as the atmosphere. If atmospheric CO₂ increases are the result of fossil fuel burning, which is relatively rich in ¹²C, the relative concentration of ¹³C should begin to decline after the Industrial Revolution in 1800—and that is exactly what has happened (Figure 14.17).

Meanwhile, other greenhouse gases—specifically, methane, CH₄, and nitrous oxide, N₂O—have also increased since the Industrial Revolution (Figure 14.18). Some of these gases come from agricultural sources in addition to fossil fuel burning. For instance, flooded rice paddies and livestock such as cattle and buffalo produce significant quantities of climate-warming methane.

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