The (Ralph) Keeling Curve

In March, one of the most significant research efforts in recent science history will celebrate its 50th anniversary. The record of rising carbon dioxide levels in the atmosphere known as the Keeling Curve marks for many people the point when society became aware that human activities can cause long-term changes to climate.

The Keeling Curve, named for the late Charles David Keeling, a geochemist at Scripps Institution of Oceanography at UC San Diego, has resolved a number of debates in its 50-year history. It helped establish that fossil fuel burning would lead to greater concentrations of atmospheric CO2. It helped make clear to climate scientists that that rise in carbon dioxide would trigger large climate changes. And it helped determine that the climate changes caused by that rise were already taking place.

But the Keeling Curve only describes what happens to a portion of the carbon dioxide produced by natural and human activities. Other parts of the story are now being told by a number of researchers, including Keeling’s own son Ralph. Some 20 years after he entered the family business, Ralph Keeling has produced a record of atmospheric levels of oxygen, yin to the yang of CO2, that is helping scientists understand what to expect from climate in the future. This part of the story started with a conversation between a father and a son at the kitchen table.

Finite Reservoir

It took the Keeling Curve to prove false the explanation by early twentieth century scientists for where all the emissions of a new industrial era were going. As the age of the automobile ramped up, researchers at the time understood that human activities were producing extra carbon dioxide beyond the amount created by nature. They assumed, however, that the vast oceans would have the capacity to absorb that extra amount with little consequence to people.

The curve showed, however, that that was far from the case. Beginning in March 1958 with an hourly measurement of CO2 taken at a National Weather Bureau station atop Hawaii’s Mauna Loa, Charles David Keeling’s measurements revealed a sharp increase in the concentration of carbon dioxide in the atmosphere. It has risen from roughly 315 parts per million (ppm) for that month to 383 ppm in the most recent measurement. The upward trajectory of the last century is nearly perpendicular to the flat line that existed in the centuries prior to the industrial age. As dramatized by former Vice President Al Gore in his film “An Inconvenient Truth,” present-day levels are, in fact, a wild departure from anything seen in the last 480,000 years. Through several ice ages and interglacial periods, ice core records show that those concentrations have never risen above 300 ppm until now.

But the measurement of rapid increase of CO2 in the atmosphere still doesn’t account for all the CO2 that people have produced through fossil fuel burning, a quantity relatively easy to calculate from energy industry records from the past century. Where is the rest going? 

Scripps marine chemist Andrew Dickson has helped to chip away at the answer from one angle. Since 1990, he has taken part in efforts to make ultraprecise measurements of concentrations of carbon levels in the oceans that show an upward trend consistent with uptake of carbon dioxide from the atmosphere.

That uptake is part of a natural exchange of gases between the ocean and sky; the oceans are a carbon "sink" or repository, absorbing it through a direct mixing of gases that takes place through the actions of winds and waves at the surface and through the photosynthetic activity of phytoplankton in the surface ocean.

But even though there was more carbon dioxide in the air and extra being taken up by the oceans, scientists have found these quantities in the two spheres still aren’t sufficient to account for all that is produced by fossil fuel burning.

Where Does CO2 Go?

A kitchen table conversation with his father, a man whose work Ralph Keeling didn’t fully appreciate until he was in college, set the younger Keeling to begin making a measurement of oxygen levels in the atmosphere.  At the time, in the late 1970s, a debate was brewing between oceanographers and terrestrial ecologists who estimated that people were emitting even more CO2 on land through activities such as urbanization and increased timber use than oceanographers could account for. The gap between what people produced and what scientists measured was even greater than previously thought, said the ecologists.

Ralph Keeling recalls that during his father’s account of what was happening with his research, the elder Keeling mentioned how an accurate measure of oxygen in the atmosphere could provide an answer to the discrepancy because CO2 uptake in the oceans is discernibly different from how land plants take up carbon dioxide.

With that seed planted in his mind, the younger Keeling devised methods for measuring extremely minute variations in atmospheric oxygen. His first readings were taken in 1989, about the same time that Dickson began his ocean carbon work. The O2 readings, taken at stations stretching north from Antarctica to points in and around the Pacific and Atlantic oceans, showed what Ralph Keeling expected to see in a world filled with an ever-growing number of internal combustion engines that take in oxygen and emit greenhouse gases: Just as carbon dioxide levels were going up, oxygen levels were going down about 400 ppm from 1985 to the present day.

The downward trend in oxygen levels was no surprise but the rate of decline was. It is not declining as fast as it should be. With the amounts of carbon dioxide in the oceans and the air well-estimated, Ralph Keeling postulates that increased photosynthesis on land must account for the rest of the CO2 produced by society.

Exactly how all the terrestrial trees and plants in the world achieve this is an open topic, however. In an era of clear-cut forests and the replacement of wilderness with human development, it would seem at first glance that there would be fewer plants available to somehow do more work. The act of chopping down trees and burning them releases the carbon dioxide stored in the trees, making extra absorption all the more unlikely.

There are a variety of theories out there about why there must be more plant biomass on land, said Keeling and Dickson, and each of them probably accounts for part of the answer. One basic premise is that there is available to plants more carbon dioxide and nitrogen, two natural fertilizers produced through fossil fuel burning. Additionally, Dickson notes, there is a body of evidence suggesting that, as farming practices in the last century have become more efficient and less land-intensive, places such as the northeastern United States have become reforested. Such new growth creates temporary surges in CO2 uptake before plateauing as forests mature.

Out of Mother Nature’s Hands

But however Mother Nature is working to mitigate the effects of rising CO2 levels on land and in the oceans, Ralph Keeling is quick to point out that the apparent ability of land plants to slow down drops in atmospheric oxygen levels is only slight. It cannot be read as a sign that nature will prevail somehow in the face of increasing greenhouse gas concentrations. In fact, Ralph Keeling's findings suggest to him a dire future in which, as he puts it,  "instability will be the norm."

“What we’re talking about isn’t a world that’s a little bit warmer but a world that’s constantly warming,” said Keeling. “The period of instability plays out for centuries. That’s why it’s so important to stabilize carbon dioxide.”

The younger Keeling points out that the use of remaining fossil fuel reserves on the planet will create a store of carbon dioxide at least five times that of the store of carbon stored in the atmosphere or on land. If all those reserves are used, his father’s famous curve could register CO2 readings as high as 1,500 ppm within a few hundred years. Concentrations of CO2 will remain inflated for thousands of years, overwhelming natural controls over climate.

“Ralph’s work with oxygen is taking us to the next depth of understanding,” said Dickson. “We need to know that to make plausible assertions about what likely future we have.”

Charles David Keeling will be remembered as a scientist who made it clear that carbon dioxide levels are rising because of human activities, ending what had been only speculation before the Keeling Curve. Son Ralph's legacy might be that he among many others will have made the nature of the solution equally clear. Nature’s own fate will essentially be beyond its own control, leaving society only two options. It can keep on expanding fossil fuel use at current rates and almost certainly face catastrophic climate change. Its other option is drastically curtail fossil fuel use and learn to adapt to a new world in which climate changes significantly but hopefully not as severely.

“We are seeing the beginnings of what will almost certainly be a rapidly changing planet,” Keeling said.


By Robert Monroe

About Scripps Oceanography

Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.

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