Page 133 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 4 atmosphere and Surface Energy Balances 97
Shortwave reflection
Cloud
Earth’s surface
Sun
(a) Clouds reflect and scatter shortwave radiation, returning a high percentage to space.
in particular—are good absorbers of longwave radiation emitted by Earth. These gases absorb heat in the lower tro- posphere, a process that explains why Earth’s atmosphere is warmer at the surface, acting like a natural greenhouse.
Clouds, Aerosols, and Atmospheric Albedo Clouds and aerosols are unpredictable factors in the tropospheric en- ergy budget. The presence or absence of clouds may make a 75% difference in the amount of energy that reaches the surface. Clouds reflect shortwave insolation, so that less insolation reaches Earth’s surface, and they absorb long- wave radiation leaving Earth (Figure 4.7). Longwave ra- diation trapped by an insulating cloud layer can create a warming of Earth’s atmosphere called the greenhouse ef- fect (discussed in the next section).
Air pollutants from both natural and anthropo- genic sources affect atmospheric albedo. The 1991 erup- tion of Mount Pinatubo in the Philippines, introduced in Chapter 1, injected approximately 15–20 megatons of sulfur dioxide droplets into the stratosphere. Winds rapidly spread these aerosols worldwide (see the im- ages in Figure 6.1), resulting in an increase in global
◀Figure 4.7 The effects of clouds on shortwave and longwave radiation.
atmospheric albedo and a temporary average cooling of 0.5 C°. Scientists have correlated similar cooling trends with other large volcanic eruptions throughout history.
In rapidly developing regions such as China and India, industrial pollutants such as sulfate aerosols are increasing the reflectivity of the atmosphere. These aerosols act as an insolation-reflecting haze in clear-sky conditions, cooling Earth’s surface. However, some aerosols (especially black carbon) readily absorb radiation and reradiate heat back toward Earth—with warming effects.
Global dimming is the general term describing the pollution-related decline in insolation to Earth’s surface. This process is difficult to incorporate into climate mod- els, although evidence shows that it is causing an under- estimation of the actual amount of warming happening in Earth’s lower atmosphere. One recent study estimates that aerosols reduced surface insolation by 20% during the first decade of this century.
The connections between pollution and cooling of Earth’s surface provide an example of how components of the Earth energy budget affect other Earth systems. Scientists have discovered that air pollution over the
Cloud
Earth’s surface
(b) Clouds absorb and reradiate longwave radiation emitted by Earth; some longwave energy returns to space and some toward the surface.
Georeport 4.2 Aerosols Cool and Warm Earth’s Climate
Ground and satellite measurements indicate that global stratospheric aerosols increased 7% from 2000 to 2010. The effect of aerosols on climate depends on whether they reflect or absorb insolation, which in turn depends in part on the composi-
tion and colour of the particles. in general terms, brightly coloured or translucent particles such as sulfates and nitrates tend to reflect radiation, with cooling effects. Darker aerosols such as black carbon absorb radiation, with warming effects (although black carbon also shades the surface, with slight cooling effects). The effects of dust are variable, depending in part on whether it is coated with black or organic carbon. One recent climate model showed that removing all aerosols over the eastern United States could lead to a slight average increase in warming and an increase in the severity of annual heat waves. Research into the complex role of these tiny atmospheric particles in energy budgets is critical for understanding climate change.