Practical Focus
Carbon dioxide – Where does it come from? – Where does it go? Greenhouse effect – Fiction and reality
gas aktuell No. 39, 1990
Photo: Some 750 billion metric tons of carbon are stored in plants, and tropical rain forests, in particular, contribute significantly to the CO₂ balance of the Earth.
Practical Focus
Carbon dioxide – Where does it come from? – Where does it go? Greenhouse effect – Fiction and reality
gas aktuell No. 39, 1990
Photo: Some 750 billion metric tons of carbon are stored in plants, and tropical rain forests, in particular, contribute significantly to the CO₂ balance of the Earth.
The carbon dioxide content of air is vitally important for life on Earth. But the increase from 280 vpm to 350 vpm observed since the start of the industrial revolution is cause for concern. If the current trend continues, the carbon dioxide concentration in the air will double by the middle of the next century (350 vpm ≤ 0.035 %). Carbon dioxide in combination with other trace gases in the atmosphere causes the so-called greenhouse effect. According to computer model-based predictions, a doubling of the current CO₂ concentration would cause the Earth’s temperature to increase by between 1.5 °C and 4.5 °C. As a result, sea levels would rise by 0.2 to 1.7 m. Experts disagree, however, as to whether computer models developed up till now accurately reflect reality.
The carbon cycle in nature
The carbon dioxide content of the atmosphere is basically determined by two cycles (see Fig. below).
- A biological cycle: Plants use photosynthesis to convert carbon dioxide into sugar. During respiration and during the decay of biomass, carbon dioxide is released again.
- Through absorption and desorption of carbon dioxide in the oceans: the oceans play a major role in nature’s carbon dioxide budget. Recent estimates suggest that only about 1 to 2 percent of the total supply of carbon dioxide is contained in the atmosphere.
Huge quantities are tied up in cold, deep ocean waters. Some 11 metric gigatons of carbon dioxide are removed from the natural cycle every year through transport from the surface layers into the deep water. Due to their extremely slow mixing, however, the oceans’ ability to absorb additional carbon dioxide is limited. In the figure above, note that the data listed in metric gigatons (for reservoirs) and metric gigatons/year (for the flows) refer to carbon and not carbon dioxide. One metric ton of carbon in this context corresponds to about 3.7 metric tons of carbon dioxide. The net balance shows that anthropogenic emissions are currently raising the carbon dioxide content of the atmosphere by about 7.5 to 15 metric gigatons per year. Roughly 25 % of that increase is due to the destruction of tropical rain forests and 75 % is due to the combustion of fossil fuels.
Global carbon flows and reservoirs: All figures in billions of metric tons of carbon (one metric ton of carbon corresponds to 3.7 metric tons of CO₂)
Anthropogenic carbon dioxide emissions
Due to our need for both energy and living space, and not least as a result of our respiration, humans are the source of a substantial quantity of carbon dioxide emissions. Western industrialized countries account for about 49 % of the world’s consumption of fossil fuels, while the Eastern Bloc’s share represents about 20 %. The remainder is attributable to the so-called “Third World.” On average, each human on Earth produces 4 metric tons of carbon dioxide per year, whereby the regional distribution is by no means uniform.
Industrial use of carbon dioxide
Carbon dioxide is marketed under the trade name “Carbonic acid” and used for industrial purposes. It forms as a by-product of chemical processes (“process carbon dioxide”) or originates from natural sources, mainly developed in connection with water extraction (the share of carbonic acid from natural springs in the Federal Republic of Germany is approx. 45 %). Carbon dioxide has a very wide variety of applications, primarily in food technology, the metal industry and the chemical industry. The carbon dioxide industry worldwide produces about 5 metric megatons per year. Of that total, Messer-Griesheim Group (MGI, KSW R. Buse, Distillers MG, SIAC, MAG) accounts for about 350,000 metric tons per year. That is less than 0.0014 % of the total quantity released annually as a result of human activity.
Regions of origin and primary sources of CO₂ emissions (The Federal Republic of Germany produces 4 % of the global emissions)
Greenhouse effect by trace gases
The atmosphere contains a number of trace gases such as carbon dioxide, ozone, nitrous oxide, methane, ammonia and halogenated hydrocarbons, which contribute to the heat balance of the Earth. Both water vapor and those trace gases allow visible light to pass largely unimpeded, but absorb radiation reflected from the Earth’s surface in the infrared range. Were it not for this so-called “greenhouse effect,” the Earth would be uninhabitable, with a mean surface temperature of -18 °C instead of the actual +15 °C. With its broad absorption bands, water vapor determines the radiation reflected from the Earth’s surface into space. Heat emission is possible only in limited spectral ranges – the so-called windows – where those particular remaining trace gases absorb. So, an increase in the concentration of these gases closes this window more and more. Not least for that reason, the absorption effect of trace gases varies in magnitude. If we doubled the carbon dioxide content of the air from 300 to 600 vpm, for example, only 10 % more of the total heat emissions would be held back. The same effect already results from an additional concentration of 9.3 vpm of methane or 0.02 vpm of the chlorofluorocarbon CFCl₃. One important benchmark for assessing the harmfulness of a trace gas is the so-called “greenhouse potential.” This index is a measure of the absorption effect of a molecule additionally entering the atmosphere. Today, methane, nitrous oxide, ozone and halogenated hydrocarbons already account for half of the so-called “additional greenhouse effect.” Moreover, the growth rate for methane is estimated to be three times that of carbon dioxide.