Page 134 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

98 part I The Energy–Atmosphere System
northern Indian Ocean causes a reduction in average summer monsoon precipitation. The increased pres- ence of aerosols in this region increases cloud cover, causing surface cooling. This in turn causes less evapo- ration, which leads to less moisture in the atmosphere and in the monsoonal flow (read more about monsoons in Chapter 6). A weakening of the seasonal monsoon will have a negative impact on regional water resources and agriculture in this densely populated area. Re- cent research by an international team confirmed con- nections between aerosols and rainfall patterns, and suggests that increased aerosols make some regions more prone to extreme precipitation events. (An over- view of the effects of aerosols is at earthobservatory.nasa .gov/Features/Aerosols/page1.php; information about aerosol monitoring is at www.esrl.noaa.gov/gmd/aero/.)
Energy Balance
in the Troposphere
The Earth–atmosphere energy system budget naturally balances itself in a steady-state equilibrium. The inputs of shortwave energy to Earth’s atmosphere and surface from the Sun are eventually balanced by the outputs of shortwave energy reflected and longwave energy emit- ted from Earth’s atmosphere and surface back to space. Think of cash flows into and out of a chequing account, and the balance that results when cash deposits and withdrawals are equal (see Figure GIA 4).
Certain gases in the atmosphere effectively delay longwave energy losses to space and act to warm the lower atmosphere. In this section, we examine this “greenhouse” effect and then develop an overall, detailed energy budget for the troposphere.
The Greenhouse Effect
and Atmospheric Warming
In Chapter 2, we characterised Earth as a cooler black- body radiator than the Sun, emitting energy in longer wavelengths from its surface and atmosphere toward space. However, some of this longwave radiation is ab- sorbed by carbon dioxide, water vapour, methane, nitrous oxide, chlorofluorocarbons (CFCs), and other gases in the lower atmosphere and then emitted back, or reradiated, toward Earth. This process affects the heating of Earth’s atmosphere. The rough similarity between this process and the way a greenhouse operates gives the process its name—the greenhouse effect. The gases associated with this process are collectively termed greenhouse gases.
The “Greenhouse” Concept In a greenhouse, the glass is transparent to shortwave insolation, allowing light to pass through to the soil, plants, and materials inside, where absorption and conduction take place. The ab- sorbed energy is then emitted as longwave radiation, warming the air inside the greenhouse. The glass physi- cally traps both the longer wavelengths and the warmed
air inside the greenhouse, preventing it from mixing with cooler outside air. Thus, the glass acts as a one-way fil- ter, allowing the shortwave energy in, but not allowing the longwave energy out except through conduction or convection by opening the greenhouse-roof vent. You experience the same process in a car parked in direct sunlight. Opening the car windows allows the air inside to mix with the outside environment, thereby removing heated air physically from one place to another by con- vection. The interior of a car gets surprisingly hot with the windows closed, even on a day with mild tempera- tures outside.
Overall, the atmosphere behaves a bit differently. In the atmosphere, the greenhouse analogy does not fully apply because longwave radiation is not trapped as in a green- house. Rather, its passage to space is delayed as the long- wave radiation is absorbed by certain gases, clouds, and dust in the atmosphere and is reradiated back to Earth’s sur- face. According to scientific consensus, today’s increasing carbon dioxide concentration in the lower atmosphere is ab- sorbing more longwave radiation, some of which gets rera- diated back toward Earth, thus producing a warming trend and related changes in the Earth–atmosphere energy system.
Clouds and Earth’s “Greenhouse” As discussed ear- lier, clouds sometimes cause cooling and other times cause heating of the lower atmosphere, in turn affecting Earth’s climate (Figure 4.7). The effect of clouds is depen- dent on the percentage of cloud cover, as well as cloud type, altitude, and thickness (water content and density). Low, thick stratus clouds reflect about 90% of insolation. The term cloud-albedo forcing refers to an increase in al- bedo caused by such clouds, and the resulting cooling of Earth’s climate (albedo effects exceed greenhouse effects, shown in Figure 4.8a). High-altitude, ice-crystal clouds reflect only about 50% of incoming insolation. These cir- rus clouds act as insulation, trapping longwave radiation from Earth and raising minimum temperatures. This is cloud-greenhouse forcing, which causes warming of Earth’s climate (greenhouse effects exceed albedo effects, shown in Figure 4.8b).
Jet contrails (condensation trails) produce high cir- rus clouds stimulated by aircraft exhaust—sometimes called false cirrus clouds, or contrail cirrus (Figure 4.8c and d). Contrails both cool and warm the atmosphere, and these opposing effects make it difficult for scientists to determine their overall role in Earth’s energy budget. Recent research indicates that contrail cirrus trap out- going radiation from Earth at a slightly greater rate than they reflect insolation, suggesting that their overall effect is a positive radiative forcing, or warming, of climate. When numerous contrails merge and spread in size, their effect on Earth’s energy budget may be significant.
The three-day grounding of commercial air traf- fic following the September 11, 2001, terrorist attacks on the World Trade Center provided researchers with an opportunity to assess contrail effects on tempera- tures. Dr. David Travis and his research team compared