XI.

Human Induced Global Warming

In 1988, the United Nations Environment Program and the World Meteorological Organization established the Intergovernmental Panel on Climate Change (IPCC) to assess the environmental, social, economic, and scientific information available on climate change. The IPCC consists of more than 200 leading earth scientists. Their Second Assessment Report, published in 1995, concluded that the earth’s average surface air temperature has increased by between 0.3 and 0.6 Celsius degrees (between 0.5 and 1.1 Fahrenheit degrees) in the past 100 years. Their report states that this warming should continue and that global average surface temperature will increase by between 1.0 and 3.5 Celsius degrees (between 1.8 and 6.3 Fahrenheit degrees) by the year 2100 (see Global Warming). If such a warming should occur, sea level should rise by between 15 cm and 95 cm (6 in and 37 in) by the year 2100, with the most likely rise being 50 cm (20 in). Such a rise in sea level might have a damaging effect on coastal ecosystems. Other changes brought on by this warming might include a shift in the world’s wind and rainfall patterns, which might put added stress on important agricultural areas, especially those in the western United States that depend on irrigation water from reservoirs and streams.

Many climate scientists believe that human activity is responsible for global warming. They attribute the main cause of global warming to the burning of fossil fuels, which increases the concentration of carbon dioxide (CO₂) gas in the atmosphere. Carbon dioxide levels, presently about 360 parts per million (ppm), have increased 28 percent in the past century. The IPCC estimates that the concentration of CO₂ in the atmosphere will surpass 500 ppm, an increase of another 40 percent, before the end of the 21st century.

Carbon dioxide warms the atmosphere through a process known as the atmospheric greenhouse effect. The atmospheric greenhouse effect is caused by certain gases in our atmosphere, called greenhouse gases, selectively absorbing and emitting infrared radiation, or heat energy. The two most plentiful greenhouse gases are water vapor (H₂O) and carbon dioxide (CO₂). Other less plentiful (and hence less important) greenhouse gases include nitrous oxide (N₂O), methane (CH₄), and chlorofluorocarbons (CFCs).

A greenhouse gas is like a filter; it allows the shorter wavelengths of radiant energy (such as visible light) to pass through it, but it absorbs some of the longer wavelengths of radiant energy (such as infrared radiation). Visible sunlight readily passes through the greenhouse gases to reach the earth’s surface, where it warms the surface. The earth’s surface, which is much cooler than the sun, emits radiant energy in the form of longer infrared waves. The greenhouse gases absorb some of these infrared waves emitted by the earth’s surface. When greenhouse gases absorb infrared energy, they share this energy with other gases and the atmosphere warms. The greenhouse gases also emit infrared radiation. Some of the emitted radiation travels back to the earth’s surface, where it warms the earth again. By preventing the rapid escape of infrared energy to space, greenhouse gases act as an insulating layer around the earth, keeping its surface much warmer than it would be if these gases were not present.

The atmospheric greenhouse effect is a natural effect that has been occurring for billions of years. Indeed, without it, the earth would be a frozen planet with an average temperature of about -18° C (about 0° F). Due to the greenhouse effect, the earth’s average surface temperature is a comfortable 15° C (about 59° F).

It is not the greenhouse effect that concerns scientists, but the enhancement of the greenhouse effect by human induced increases in the levels of greenhouse gases. Climate models predict that the world’s average surface temperature should rise by between 1 and 3.5 Celsius degrees (1.8 and 6.3 Fahrenheit degrees) by the year 2100. However, these models show that increasing the concentration of carbon dioxide to 500 ppm and keeping everything else constant only accounts for a global warming of less than 1 Celsius degree (1.8 Fahrenheit degrees). This slight warming, however, would increase the air’s capacity for holding water vapor. The added water vapor, the most plentiful greenhouse gas, would enhance the atmospheric greenhouse effect by producing a positive feedback on the climate system. A positive feedback occurs when an initial change is reinforced by another process. In this situation, the increase in temperature causes an increase in water vapor, which absorbs more of the earth’s infrared energy, thus accounting for the rest of the warming.

The interactions between the earth and its atmosphere are complex. There are many uncertainties in the climate system, especially with regard to clouds (which tend to cool the earth by reflecting sunlight) and the oceans (which act as a huge storehouse of heat energy). It is difficult to prove that increasing concentrations of greenhouse gases are responsible for the recent global warming. Most climate scientists contend, however, that at least part of the warming is due to human induced greenhouse gases.