How We Know Global Warming Is Real: The Science Behind Human-Induced Climate Change
Schneider, Tapio, Skeptic (Altadena, CA)
ATMOSPHERIC CARBON DIOXIDE concentrations are higher today than at any time in at least the past 650,000 years. They are about 35% higher than before the industrial revolution, and this increase is caused by human activities, primarily the burning of fossil fuels.
Carbon dioxide is a greenhouse gas, as are methane, nitrous oxide, water vapor, and a host of other trace gases. They occur naturally in the atmosphere. Greenhouse gases act like a blanket for infrared radiation, retaining radiative energy near the surface that would otherwise escape directly to space. An increase in atmospheric concentrations of carbon dioxide and of other greenhouse gases augments the natural greenhouse effect; it increases the radiative energy available to Earth's surface and to the lower atmosphere. Unless compensated for by other processes, the increase in radiative energy available to the surface and the lower atmosphere leads to warming. This we know. How do we know it?
How do we know carbon dioxide concentrations have increased?
The concentrations of carbon dioxide and other greenhouse gases in atmospheric samples have been measured continuously since the late 1950s. Since then, carbon dioxide concentrations have increased steadily from about 315 parts per million (ppm, or molecules of carbon dioxide per million molecules of dry air) in the late 1950s to about 385 ppm now, with small spatial variations away from major sources of emissions. For the more distant past, we can measure atmospheric concentrations of greenhouse gases in bubbles of ancient air preserved in ice (e.g., in Greenland and Antarctica). Ice core records currently go back 650,000 years; over this period we know that carbon dioxide concentrations have never been higher than they are now. Before the industrial revolution, they were about 280 ppm, and they have varied naturally between about 180 ppm during ice ages and 300 ppm during warm periods (Fig. 1). Concentrations of methane and nitrous oxide have likewise increased since the industrial revolution (Fig. 2) and, for methane, are higher now than they have been in the 650,000 years before the industrial revolution.
How do we know the increase in carbon dioxide concentrations is caused by human activities?
There are several lines of evidence. We know approximately how much carbon dioxide is emitted as a result of human activities. Adding up the human sources of carbon dioxide--primarily from fossil fuel burning, cement production, and land use changes (e.g., deforestation)--one finds that only about halt" the carbon dioxide emitted as a result of human activities has led to an increase in atmospheric concentrations. The other half of the emitted carbon dioxide has been taken up by oceans and the biosphere--where and how exactly is not completely understood: there is a "missing carbon sink."
Human activities thus can account for the increase in carbon dioxide concentrations. Changes in the isotopic composition of carbon dioxide show that the carbon in the added carbon dioxide derives largely from plant materials, that is, from processes such as burning of biomass or fossil fuels, which are derived from fossil plant materials. Minute changes in the atmospheric concentration of oxygen show that the added carbon dioxide derives from burning of the plant materials. And concentrations of carbon dioxide in the ocean have increased along with the atmospheric concentrations, showing that the increase in atmospheric carbon dioxide concentrations cannot be a result of release from the oceans. All lines of evidence taken together make it unambiguous that the increase in atmospheric carbon dioxide concentrations is human induced and is primarily a result of fossil fuel burning. (Similar reasoning can be evoked for other greenhouse gases, but for some of those, such as methane and nitrous oxide, their sources are not as clear as those of carbon dioxide.)
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Publication information: Article title: How We Know Global Warming Is Real: The Science Behind Human-Induced Climate Change. Contributors: Schneider, Tapio - Author. Magazine title: Skeptic (Altadena, CA). Volume: 14. Issue: 1 Publication date: Spring 2008. Page number: 31+. © 2009 Skeptics Society & Skeptic Magazine. COPYRIGHT 2008 Gale Group.
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