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CHAPTER 18 • Causes of Warming over the Last 125 Years 333

N

60°

30°

0°

30°

3
Sulfate (mg/m ) 2–6 6–10 >10
60°
S 120° 60° 0° 60° 120°
W E
FIGURE 18-10 Sulfate aerosol plumes Sulfur dioxide emissions from industrial smokestacks in
the North American Midwest, Eastern Europe, and China produce sulfate aerosols that prevailing
winds carry eastward in large plumes. (Adapted from R. J. Charlson et al., “Perturbation of the
Northern Hemisphere Radiative Balance by Backscattering from Aerosols,” Tellus 43 [1991]: 152–63.)

Part of the history of SO emissions is recorded in
2
Greenland ice (Figure 18–11). Throughout the 1800s,
industrial SO emissions had been smaller than those
2
from natural sources, but by the middle 1900s, industry Volcanic
emissions became the dominant SO source. The sul- explosions
2 Sulfate concentration
fate concentration in Greenland ice rose sharply during
World War II industrialization. Concentrations in
Greenland ice began to drop sharply after 1980, when
the United States (and then other nations) acted to limit
0
SO emissions. 1700 1800 1900 2000
2
Climate scientists have inferred that the large plumes A Sulfate concentration in ice cores
of sulfate aerosols cause regional cooling downwind from 100
smokestack sources. Like sulfate aerosols created by vol-
canic explosions, industrial sulfate particles reflect and
scatter some of the incoming solar radiation back to
space and keep it from reaching Earth’s surface. The
reduction in radiation cools climate regionally, and these SO 2 (gigatons/year) 50 Human
effects show up in tabulations of global mean tempera- emissions
ture change.
A second potential climatic effect of sulfate aerosols Range of natural emissions
is less well understood. The tiny particles form natural
centers (nuclei) around which water vapor can condense, 0
forming droplets and then clouds. Clouds can have two 1700 1800 1900 2000
Year
opposing effects on climate: the surfaces of thick clouds
B Estimated global SO emissions
can reflect more incoming solar radiation and cool 2
climate, but thinner, higher clouds can also absorb more FIGURE 18-11 Preindustrial and anthropogenic sulfates
outgoing radiation from Earth’s surface and increase the (A) Measured sulfate concentrations in Greenland ice
greenhouse effect. Because sulfate aerosols mainly affect reveal a significant regional increase since the nineteenth
low-level clouds, their indirect effects are thought to century. (B) Estimated global SO emissions from
2
cool climate, but the amount is highly uncertain. smokestacks now exceed natural emissions. (A: Adapted from
P. A. Mayewski et al., “An Ice-Core Record of Atmospheric
IN SUMMARY, the net global effect of increased Responses to Anthropogenic Sulphate and Nitrate,” Atmospheric
sulfate aerosols is thought to be a cooling, but Environment 27 [1990]: 2915–19. B: Adapted from R. J. Charlson
estimates of the size of the cooling vary widely. et al., “Climate Forcing by Anthropogenic Aerosols,” Science 255
[1992]: 423–30.)