Local View: Humans responsible for increase in atmospheric carbon dioxide

A recent Yale poll found that only 57 percent of Americans believe accelerating changes in climate are caused primarily by humans. Scientific findings that should be more widely known may help to convince the remaining 43 percent.During the ice a...

David Gerhart

A recent Yale poll found that only 57 percent of Americans believe accelerating changes in climate are caused primarily by humans. Scientific findings that should be more widely known may help to convince the remaining 43 percent.
During the ice ages, carbon dioxide in the atmosphere varied between about 180 and 280 parts per million. It is now about 400 ppm, having risen dramatically during the past century.
But how do we know this increase is caused by humans burning fossil fuels and is not derived from other natural sources? For instance, erupting volcanoes are a known source of atmospheric carbon dioxide. And what about the oceans? Could they be contributing carbon dioxide to the atmosphere?
The scientific reasoning on this issue is fairly simple.
Because we know how many tons of coal, oil and gas are burned each year globally, it is possible to calculate the carbon dioxide emissions from this activity. And when scientists also calculate the amount of carbon dioxide emitted by volcanoes and compare it to the amount from fossil-fuel combustion, the volcanic source turns out to be very small (1 percent to 2 percent) relative to the human contribution.
As for the oceans, they are becoming more acidic, indicating that oceans are currently net absorbers of carbon dioxide. An analogy would be pumping carbon dioxide into water to create a fizzy drink. When you do this, the water becomes more acidic because of the formation of carbonic acid. That’s one reason ocean animals such as corals and mollusks that rely on a calcium carbonate skeleton or shell are threatened.
Furthermore, a variety of methods can be used to identify the different sources of atmospheric carbon dioxide. When volcanoes release carbon dioxide into the atmosphere, no oxygen is used in the process. In contrast, when humans burn fossil fuels, oxygen is used in the process of combustion.
Therefore, if most carbon dioxide emissions are from humans burning fuels, oxygen in the atmosphere should be declining. In fact, oxygen in the air has declined about as much as is predicted by the human combustion of fossil fuels.
The three isotopes of carbon also can help us find the source of increased atmospheric carbon dioxide. Carbon-12 is especially common in plants, whereas carbon-13 is relatively more abundant in volcanic gases. In contrast, carbon-14 is radioactive and mostly disappears after about 50,000 years. Because of this radioactive decay, carbon-14 is not present in fossil fuels.
So which isotopes are actually changing as atmospheric carbon dioxide increases? Of the three, only carbon-12 is increasing (the Suess Effect), ruling out a significant contribution from volcanoes. And the fact that carbon-14 is not increasing indicates that most of the carbon dioxide is not coming from living (or recently living) plants.
However, coal, oil and gas are derived from plants that have been dead for a very long time and have no carbon-14. Burning them creates the exact changes in atmospheric carbon isotopes that scientists have been reporting.
These observations have convinced the scientific community that the dramatic increase in atmospheric carbon dioxide over the past century is a result of humans burning fossil fuels.
Other energy sources are abundant, particularly solar. The task of people living today is to figure out how to tap into these sources so that energy remains affordable. Equally important is for us to accept and embrace the changes that will accompany a new energy future.

David Gerhart of Duluth has a Ph.D. in aquatic ecology from Cornell University and has published and reviewed manuscripts for scientific journals in the fields of ecology and biochemistry. This commentary, he said, was reviewed and edited by Byron Steinman, a climatologist and assistant professor at the Large Lakes Observatory in Duluth. Some information in this letter is from Penn State University climatologist Richard Alley’s October 2011 lecture at the Science Museum of Minnesota in St. Paul.

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(Mike Keefe / Cagle Cartoons)

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