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THE GREENHOUSE EFFECT

The earth is warmed by the sun's radiation (including visible light) that strikes it. The earth, in turn, radiates energy back into outer space, but this outgoing radiation differs from that of the sun: it has a longer wavelength and is invisible to the human eye. Furthermore, some of this outgoing, long-wave radiation is absorbed by various gases in the air, thereby warming the atmosphere. This warming is referred to as the "greenhouse effect" (though the warming effect inside glasshouses is really quite different!). The warming from the greenhouse effect is highly beneficial; without it, the average temperature on our planet would be about 33°C colder, making the earth inhospitable.

Image: Figure 2: Pasture (improved and unimproved) as a proportion of farmland in 1996. (F. Wang and D.B. Gleig, AAFC)

Figure 2: Pasture (improved and unimproved) as a proportion of farmland in 1996. (F. Wang and D.B. Gleig, AAFC)

Image: Figure 3: Annual crops as a proportion of farmland in 1996. (F. Wang and D.B. Gleig, AAFC)

Figure 3: Annual crops as a proportion of farmland in 1996. (F. Wang and D.B. Gleig, AAFC)

Image: Figure 4: Distribution of livestock in Canada in 1996. One animal unit is the quantity of liestock that produces manure containing 170 kg. of N per year. For example, approximately 2 dairy cows are equivalent to 1 animal unit. (F. Wang and D.B. Gleig, AAFC).

Figure 4: Distribution of livestock in Canada in 1996. One animal unit is the quantity of liestock that produces manure containing 170 kg. of N per year. For example, approximately 2 dairy cows are equivalent to 1 animal unit. (F. Wang and D.B. Gleig, AAFC).

Table 1:
The area of farmland in Canada occupied by annual crops (million ha)
  1981 1986 1991 1996
Total farmland 65.9 67.9 67.7 68.0
Croplands 31.0 33.2 33.5 35.0
Summer fallow 9.7 8.5 7.9 6.3
Improved pasture 4.4 3.6 4.1 4.4
Nonimproved pasture 20.8 22.6 22.2 22.3

The gases causing the warming of the atmosphere are known as "greenhouse gases." The most important are water vapor, CO2, CH4, N2O, and CFCs. Foremost among these is water vapor because it is a powerful absorber of long-wave radiation and is present in relatively high concentration. This gas, however, is already present in high enough concentration in the lower atmosphere that further increases in its concentration would have minimal effect on temperature.

The "greenhouse effect"

Short-wavelength radiation emitted from the sun is absorbed by the earth and re-radiated at longer wavelengths. Carbon dioxide, CH4, and N2O account for 90% of this "greenhouse effect." In the long term, incoming radiation is balanced by outgoing radiation. Because of the greenhouse effect, the average surface temperature of the Earth is about 15ºC, instead of ­18ºC.

Image:The "greenhouse effect"

Much of the current concern about greenhouse gases has arisen from the recent recognition that the concentration of other greenhouse gases-CO2, CH4, N2O, and CFCs-has been increasing steadily since the industrial revolution, almost certainly because of human activity. By 1992, CO2 had increased by 30%, CH4 by 145%, and N2O by 15%. Current rates of increase are 0.5% per year for CO2, 0.6% for CH4, and 0.3% for N2O. The CFCs were not even present in the atmosphere until a few decades ago. If the current rates of increase continue, many scientists expect significant impact on the world's climate. For example, the Intergovernmental Panel on Climate Change predicts that the doubling of the CO2 concentration, likely to happen in the 21st century, would increase average global temperatures by 1 to 3oC-a rate of warming unprecedented in the last 10 000 years. As well, the enhanced greenhouse effect could amplify climate variability.

In short, greenhouse gases have a desirable effect, as they warm the atmosphere and create favorable conditions for biological activity. Further increases in these gases, however, may lead to an "enhanced greenhouse effect" with uncertain, possibly disruptive, consequences.

The Intergovernmental Panel on Climate Change

In 1988, the World Meteorological Organisation and the United Nations Environment Programme created the Intergovernmental Panel on Climate Change (IPCC). The IPCC evaluates research and policy options and publishes reports on climate change and the risk of global warming.

The latest synthesis report, based on 1995 science, includes the following conclusions:

  • The balance of evidence from observed changes suggests a discernible human influence on global climate
  • Human-induced climate change represents an important additional stress, particularly to the many ecological and socioeconomic systems already affected by pollution, and nonsustainable management practices
  • Significant reduction in net greenhouse gas emissions are technically possible and can be economically feasible . . . in all sectors including . . . agriculture

The assessment report now being planned will provide the major science input to the future evolution of the UN Framework Convention on Climate Change and the Kyoto protocol.

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Date Modified: 2003-08-27
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