Keeling Curve

Introduction

The Keeling curve, named after American scientist Charles David Keeling (1928–2005), shows the steady rise of atmospheric CO₂ from 1958 to the present. It has become an iconic image, an easily recognized representation of facts that are relevant to all life on Earth. In the graph of the curve, atmospheric CO₂ is the vertical axis and time is the horizontal axis: as time proceeds from 1958 (at the far left) to later dates (toward the right), the line of the graph, signifying atmospheric CO₂,ascends. The CO₂ causing the increase is released by human activities, especially the burning of fossil fuels.

Superimposed on the main curve of the graph are spikes like saw teeth. These minor ups and downs record the annual wave of CO₂ uptake by green plants flourishing in the growing season of the Northern Hemisphere, which causes a temporary dip in CO₂ levels every spring and summer. The Southern Hemisphere has its growing season while the Northern Hemisphere is in winter, but the two do not cancel each other because the Northern Hemisphere has about three times as much land as the Southern.

The Keeling curve makes a strong visual argument for the anthropogenic origin of global climate change, especially when paired with the famous hockey-stick graph (developed in the late 1990s) showing that global average air temperatures have risen sharply in recent decades, most likely surpassing any warming period for at least 1,000 years.

Historical Background and Scientific Foundations

By the early 2000s, atmospheric CO₂ was being monitored at more than 100 sites worldwide. Subtle variations were recorded, even allowing scientists to identify which continents upwind were emitting most of the CO₂ that was added to the atmosphere. In the early 1950s, however, measuring atmospheric CO₂ was difficult, and scientists were still debating the possibility that the CO₂ being added to the atmosphere by fossil-fuel usage was being completely absorbed by the oceans. Some calculations seemed to show that the oceans would quickly absorb as much CO₂ as human beings were adding to the atmosphere.

In 1955, measuring atmospheric CO₂ was a central goal of climate science. In 1955 and 1956, a group of Scandinavian scientists set up fifteen CO₂ measuring stations to track atmospheric CO₂, but found that measurements were so variable as to be useless: CO₂ seemed to vary by as much as a factor of two from day to day. Because of these results, scientific opinion veered toward the view that to make any statements about CO₂ concentrations, it would be necessary to make measurements at a large number of widely dispersed stations over a great many years.

Charles David Keeling, then working on atmospheric gas measurements in California, disagreed. With new instrumentation (infrared spectrophotometers), he accepted that it was possible to make high-precision measurements of atmospheric CO₂. But one would have to intercept a flow of pristine wind, a well-mixed sample of the world's air rather than downwind gusts from cities or other sources of CO₂. Austrian chemist Hans Suess (1909–1993) and U.S. scientist Roger Revelle (1909– 1991), with funding from the International Geophysical Year, hired Keeling to make the new measurements.

In 1957, Keeling set up measuring stations on top of Hawaii's Mauna Loa volcano (upwind of the vent) and in Antarctica. A year later the Antarctic station had to shut down because of a lack of funds, but due to the zealous fund-raising of Keeling and his colleagues, there was only a short interruption in data-taking in Hawaii.

Impacts and Issues

When Keeling began his measurements, CO₂ was present in Earth's atmosphere at about 315 parts per million (i.e., 315 molecules of CO₂ in every 1 million molecules of air). Soon his records covered enough time so that he could observe that every year, from April to October, CO₂ actually decreases by about 5 parts per million. This is due to the absorption of CO₂ by green plants in the Northern Hemisphere during that hemisphere's growing season. From October to March, CO₂ increases again, always to a higher level than the previous Spring—and so global atmospheric CO₂ zigzags steadily upward. By 2006, atmospheric CO₂ was over 380 parts per million, a 20% increase over 1958 and a 35% increase over levels before about 1750, when the Industrial Revolution began.

By 1960, Keeling had enough data to draw the earliest portions of what became known as the Keeling curve, the record of increasing atmospheric CO₂ concentrations, and published his first results. These results, as well as longer CO₂ records published in later years, were quickly recognized by scientists as crucial to understanding the effects of human activities on global climate. By the early 2000s, measurements similar to Keeling's were being conducted at over 100 stations worldwide and supplemented by measurements from satellites.

WORDS TO KNOW

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

CLIMATE CHANGE: Sometimes used to refer to all forms of climatic inconsistency, but because Earth's climate is never static, the term is more properly used to imply a significant change from one climatic condition to another. In some cases, climate change has been used synonymously with the term, global warming; scientists, however, tend to use the term in the wider sense to also include natural changes in climate.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

The increase in atmospheric CO₂ recorded by the Keeling curve is the single largest contribution by human activity to the causes of global climate change.

See Also Carbon Dioxide Concentrations.

BIBLIOGRAPHY

Books

Weart, Spencer. The Discovery of Global Warming. Cambridge, MA: Harvard University Press, 2004.

Periodicals

Keeling, C. D., et al. “Inter-annual Extremes in the Rate of Rise of Atmospheric Carbon Dioxide since 1980.” Nature 375 (1995): 666–670.

Keeling, Ralph F., et al. “Global and Hemispheric CO₂ Changes Deduced from Changes in Atmospheric O₂ Concentration.” Nature 381 (1996): 218–221.

Quay, Paul. “Ups and Downs of CO₂ Uptake.” Science 298 (2002): 2344.

Web Sites

Briggs, Helen. “The Keeling Curve Legacy.” BBC News, December 1, 2007. <http://news.bbc.co.uk/1/hi/sci/tech/7120770.stm> (accessed December 1, 2007).

“The Keeling Curve.” National Aeronautics and Space Administration (NASA), 2005. <http://earthobservatory.nasa.gov/Newsroom/ NewImages/images.php3?img_id=16954> (accessed December 1, 2007).

“Obituary Notice: Climate Science Pioneer: Charles David Keeling.” Scripps Institution of Oceanography, June 22, 2005. <http://scrippsnews.ucsd.edu/Releases/?releaseID=687> (accessed December 1, 2007).

Larry Gilman