The greenhouse effect is a naturally occurring process that helps warm the Earth's surface and atmosphere. It results from the fact that certain atmospheric gases, such as water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), can alter the planet's energy balance by absorbing longwave radiation emitted by the Earth's surface. Without the greenhouse effect, much of the life found on our planet probably would perish as the average temperature of the Earth would be a chilly -18°C (0°F) rather than the present 15°C (59°F).
The greenhouse effect process begins with the heating of the Earth's surface by incoming sunlight. This heating causes the ground surface to become an energy radiator in the longwave band. This energy emission is generally directed into space (Figure 5.34); however, only a small portion of this initial energy returns to space. About 90% of the outgoing infrared radiation is intercepted and absorbed bygreenhouse gases. Absorption of longwave radiation by these gases adds additional heat to the Earth's atmosphere. The now warmer atmospheric greenhouse gas molecules begin radiating longwave energy in all directions. Most of this longwave radiation is directed back to Earth's surface, where it is absorbed again. The heating of the ground by the longwave radiation causes the ground surface to radiate again, repeating the cycle described above until no more longwave is available for absorption. The net result of this complex process is an average global temperature 33°C (59°F) warmer than without the greenhouse effect.
The amount of heat energy added to the atmosphere by the greenhouse effect is controlled by the concentration of greenhouse gases in the Earth's atmosphere. All the major greenhouse gases have increased in concentration since the beginning of the Industrial Revolution (Table 1). Most scientists believe that these higher gas concentrations are enhancing the greenhouse effect. One piece of supporting evidence for this theory is that Earth's average global temperature has risen by about 15°C (2.7°F) between 1900 and 2025. This greenhouse effect related warming is commonly referred to as global warming.
Global warming, driven by enhanced greenhouse gas concentrations, is predicted to continue throughout the 21st century. This forecast is being made because human activity will most likely continue to increase the concentrations of greenhouse gases in our planet's atmosphere. Of the gases that enhance the Earth's greenhouse effect, the most important is carbon dioxide (CO2). It alone accounts for about 63% of the recent change in the intensity of the greenhouse effect process. The enhancement contributions of the other gases are 18% for methane (CH4), 10% for chlorofluorocarbons (CFCs), and 6% for nitrous oxide (N2O) (Forster et al., 2007). The greenhouse effect is a naturally occurring process that helps warm the Earth's surface and atmosphere. It results from the fact that certain atmospheric gases, such as water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), can alter the planet's energy balance by absorbing longwave radiation emitted by the Earth's surface. Without the greenhouse effect, much of the life found on our planet probably would perish as the average temperature of the Earth would be a chilly -18°C (0°F) rather than the present 15°C (59°F).
Predicting additional global warming in the immediate future is accomplished by sophisticated computer climate modeling. Computer models suggest that doubling the concentration of just the main greenhouse gas, carbon dioxide, could raise the average global temperature by 1 to 3°C (1.8 to 5.4°F) above today's levels. A few scientists have voiced concern about the accuracy of these forecasts. These objections are based on the fact that the numerical equations of computer models may not precisely simulate the effects of positive and negative feedback mechanisms across the various components of the Earth's climate system. One component of the climate that is not simulated very well in global climate models is the effect of clouds. Yet, clouds are important to our global climate.
Increasing Earth's temperature due to global warming should cause the oceans to evaporate more water into the atmosphere. More water vapor in the atmosphere would cause the climate to warm further because it is a greenhouse gas. But higher concentrations of water vapor would also lead to more cloud formation. These extra clouds would probably reflect a greater proportion of the Sun's energy back to space, reducing the amount of solar radiation absorbed by the atmosphere and the Earth's surface. With less solar energy being absorbed at the surface, the Earth's temperature would cool. Current global climate models do not simulate the interrelated dynamics of these two opposing processes very well. Thus, researchers are not entirely sure whether increased atmospheric moisture will cause net warming or cooling of the Earth.
FIGURE 5.34 The diagram above illustrates the greenhouse effect. This process begins with the absorption of shortwave radiation from the Sun. Absorption converts solar energy into sensible heat at the Earth's surface. Some of this heat is transferred to the lower atmosphere by conduction and convection. After the ground and lower atmosphere are heated, these surfaces become radiators of infrared, or longwave, radiation and begin to cool. This energy is directed into space. However, only a portion of this energy actually reaches the atmosphere. About 90% of the longwave radiation emitted from the Earth's surface is absorbed by the atmosphere's greenhouse gases. Absorption of this energy adds heat to the Earth's atmospheric system by warming greenhouse gas molecules. The greenhouse gas molecules then begin radiating longwave energy primarily back to the Earth's surface, where it once again generates heat. The heating of the ground by longwave radiation causes the surface to radiate again, repeating the cycle described above, until no more infrared radiation is available for surface absorption. In conclusion, the net result of the greenhouse effect is an increase in the creation and storage of heat energy in the Earth's atmosphere and at the ground surface. Image Copyright: Michael Pidwirny.