[Figure10] :The greenhouse effect.
Thirty percent of incoming solar energy (left) is reflected away from the earth, either by clouds and particles in the atmosphere, or by the earth's surface. The remaining 70 percent is absorbed, that absorbed energy is reemitted at infrared wavelengths by the earth's surface (right) and by the atmosphere (which is also heated by other atmospheric processes such as updrafts and cloud formation). Most of the outgoing surface radiation is trapped by clouds and greenhouse gases, and returned to earth. This heat trapping is called the greenhouse effect, which causes the earth;s surface to be about 33oC warmer than it otherwise would be.
Source : Adapted from Schneider, 1989.

Prior to the rise of human civilization, the concentration of these various greenhouse gases fluctuated very slowly over time. Increases in their concentration were associated with increases in global temperature, while decreases were correlated with cooling periods. At least on the geologic time scale, past evidence has shown that fluctuations in greenhouse gases are associated with fluctuations in global average temperatures.

The most important greenhouse gas, in terms of volume in the atmosphere and annual emissions, is carbon dioxide (CO2). It is exchanged between the earth's surface and the atmosphere as part of the carbon cycle (see Figure 11). The actual concentration of carbon in the atmosphere is the result of a balance between the rate of carbon released into the atmosphere and the rate of carbon taken from the atmosphere. Although carbon is constantly moving between the compartments of the carbon cycle, the proportion of carbon in each compartment does not change significantly in the short term. The situation for other greenhouse gases is the same in that their atmospheric concentration is the result of a balance between input and output rates.

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Since the beginning of the industrial revolution in the mid to late 18th century, the natural balance of carbon dioxide and other greenhouse gases going in and out of the atmosphere has been altered by human activity, most notably through the combustion of fossil fuels. For many thousands of years, oil, coal and natural gas represented a geological store of carbon and nitrogen that did not circulate actively in the carbon or nitrogen cycle. By burning these fossil fuels, we have effectively added a dose of new carbon and nitrogen to the circulating portion of the system. This has resulted in a 25% increase in the concentration of CO2 in our atmosphere (see Figure 12) and an 8% increase in the concentration of N2O over the past 200 years. More significantly, the concentrations of these gases are continuing to rise at an annual rate of 0.5% and 0.2%, respectively. At this rate, atmospheric concentrations of CO2 will double from preindustrial levels by the year 2075.

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Methane (CH4) is another extremely powerful greenhouse gas (approximately 50 times more powerful than carbon dioxide) which has increased in atmospheric concentration in step with rising world population. Its concentration is increasing at between 0.75 and 1.0% each year.

Methane is produced through the breakdown or rotting of vegetation in the absence of oxygen, through digestion and through the burning of plant material. Rice paddies and land fills are responsible for tremendous releases of methane as plant material decomposes in water logged soils. The digestive systems of cattle also produce large amounts of this greenhouse gas. Today, these three sources are thought to account for at least half of the global methane emissions.

Extensive burning of wood, both as a fuel and to clear land, also releases significant amounts of methane, particularly in the developing world. Minor amounts come from the exploitation of coal and natural gas resources, and naturally from wetland areas.

Chlorofluorocarbons (CFCs) and other related man made chemicals are also potent greenhouse gases, but their concentrations are low in the atmosphere. International agreements have now been established to reduce their use and release into the atmosphere.

Scientists believe that these increases in greenhouse gases will be accompanied by an increase in atmospheric temperatures. Computer programs designed to model the earth's climate predict that if CO2 concentrations were to reach twice the preindustrial level, global average temperatures could eventually rise by between 1.5° and 4.5°C. These same models also predict that the amount of future warming will be greater in the higher northern latitudes than near the equator, and greater in winter than in summer. While atmospheric concentrations of CO2 are not expected to double until after the middle of the next century, the combined effect of increases in all greenhouse gases could cause a warming equivalent to that of a doubled CO2 environment several decades earlier (see Figure 13 showing the overall contribution of gases other than water to the greenhouse effect).

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However, despite the sophistication of these computer models, a great deal of uncertainty remains over the precise amount of global warming we can expect, since there are many feedbacks to the greenhouse effect that can alter the extent of this warming. Most notably, the effect of oceans, ocean currents and clouds have not yet been dealt with adequately in the current climate models. These uncertainties, which are being vigorously addressed by scientists throughout the world, suggest that global warming could be significantly less, or alternatively greater, than currently believed.

What are the likely impacts on Canada and the world?

The uncertainty surrounding the actual impacts of global warming is even greater than that surrounding the predicted degree of global warming, partly because the relationship between climate and other systems such as lakes and forests is not completely understood, and partly because the computer models cannot predict regional changes with any reliability.

Nonetheless, reasonable scientific consensus is emerging, through important institutions such as the Intergovernmental Panel on Climate Change (IPCC), around the following potential impacts:

• Sea levels could rise between 30 cm and 1.0 m before the end of the next century. This would have implications for many coastal Canadian cites including Charlottetown, Saint John and Vancouver. Internationally, densely populated coastline countries such as Bangladesh would be flooded, and entire island states such as the Maldives, in the Indian Ocean, could find themselves submerged.

• The length of the growing season in cooler, temperate countries such as Canada is expected to increase. However, the expansion of agriculture into Canada's north as a result of this warmer climate is likely to be limited since the vast majority of Canada's northern soils are unsuitable for agriculture.

• Changes in the global pattern of winds, precipitation and ocean currents are expected, and extreme events such as droughts, floods, tropical cyclones and heat waves will likely occur with greater frequency.

• Over much of Canada, precipitation is expected to rise slightly, but evaporation rates are expected to rise even more. This is expected to result in an overall reduction in soil moisture in southern Canada, which could seriously affect crop yields in parts of Saskatchewan and Alberta where dry conditions already affect yields.

• Lakes, rivers, wetlands, forests and other natural ecosystems will all be affected in some way by the change. Plant species are very sensitive to climate and usually exist within a narrow range of average temperatures and moisture conditions. In the past, plant and animal species that inhabit terrestrial ecosystems such as tropical or boreal forests have slowly migrated with changing global environmental conditions such as temperatures and moisture. There is now a fear, however, that anticipated changes will be so rapid that at least some species will be unable to move quickly enough into new areas and that dieback along the warm dry margins of the ecosystem will occur. Related to this is the concern that for those species which could keep up with changing temperature and moisture conditions, other necessary elements for survival, such as appropriate soils, may not be present in the new areas.