Visualizing Environmental Science

224 CHAPTER 9 Global Atmospheric Changes

PROCESS DIAGRAM

are causing an enhanced greenhouse effect

(ˆ}ÕÀiʙ°Ç).

Radiative forcing is the term used to de-

scribe how gases affect heat in the atmosphere.

When the atmosphere contains more heat, its

temperature is greater. While radiative forc-

ing by some gases can be negative (causing

temperature to decrease), the greenhouse

gases released as we burn fossil fuels cause

positive radiative forcing, and atmospheric

temperature rises (ˆ}ÕÀiʙ°n). For example,

atmospheric aerosols tend to cool the atmo-

sphere. Aerosols, which come from both nat-

ural and human sources, are particles so small

they remain suspended in the atmosphere for days,

weeks, or even months. Sulfur haze is an aerosol that re-

flects sunlight back into space, reducing the amount of

solar energy reaching Earth’s surface, and thereby cool-

ing the atmosphere. Sulfur haze significantly moderates

warming in industrialized parts of the world.

Sulfur emissions come from the same smoke-

stacks that emit CO 2. Volcanic eruptions also

eject sulfur particles into the atmosphere

(ˆ}ÕÀiʙ°™).

This cooling effect, however, is much

weaker than the en hanced greenhouse ef-

fect. Human-produced sulfur emissions typi-

cally remain in the atmosphere for less than a

year and rarely disperse globally. Greenhouse

gases can remain in the atmosphere for

hundreds of years. And carbon dioxide and

other greenhouse gases help warm the planet

24 hours a day, whereas sulfur haze cools the

planet only during the daytime. In addition, sulfur emis-

sions are a respiratory irritant and cause acid deposi-

tion (discussed later in this chapter). Most nations are

trying to reduce their sulfur emissions, not maintain or

increase them.

enhanced green-

house effect

Additional atmo-

spheric warming

produced as human

activities increase at-

mospheric concentra-

tions of greenhouse

gases.

radiative forcing

For greenhouse

gases, the capacity to

retain heat in Earth’s

atmosphere.

˜…>˜Vi`Ê}Àii˜…œÕÃiÊivviVÌÊUʈ}ÕÀiʙ°Ç

The buildup of carbon dioxide (CO 2 ) and other greenhouse gases warms the atmosphere by absorbing

some of the outgoing infrared (heat) radiation. Some of the heat in the warmed atmosphere is

transferred back to Earth’s surface, warming the land and ocean.

✓✓THE PLANNER

Sunlight (insolation)

is absorbed at

surface.

1

Some heat radiated

from Earth escapes

directly to space.

(^2) Some heat radiated
from Earth is absorbed
by greenhouse gases.
(^3) Some of this heat
is transferred back to
Earth’s surface.
4
Year
1880
1957
1967
1977
1987
1997
2007
Temperature (°C)
13.75
13.97
14.00
14.13
14.26
14.39
14.44
CO 2 concentration (ppm)
280
315
322
335
350
365
385
~