Page 358 - Earth's Climate Past and Future

334 PART V • Historical and Future Climate Change

18-8 Brown Clouds of the brown clouds have also been found to extend thou-
sands of kilometers downwind from source regions.
In the late 1980s and 1990s, scientists (and anyone travel-
ing on airplanes) could see brown hazes hanging over
many regions, particularly Eurasian megacities with mil- IN SUMMARY, these findings indicate that part of the
lions of people. These layers of haze contain a wide range true greenhouse-gas warming effect has been
of chemical constituents, including carbon-rich organic masked in recent decades in regions where brown
aerosols generated by small stoves used for cooking and clouds occur. The actual greenhouse warming effect
heating, by biomass burning, and by other activities. in these areas (and for the planet as a whole) has
Because dark carbon-rich aerosols absorb the Sun’s been larger than previously thought.
radiation, scientists initially suspected that these hazes
heated the lower atmosphere and added to the net During the late 1900s and early 2000s, satellite observa-
amplitude of global warming on a regional basis. Inves- tions detected a related phenomenon called global
tigations in the late 1990s and early 2000s led, however, dimming. The brightness of Earth’s surface viewed
to a surprisingly different interpretation of these hazes, from space decreased by roughly 7% over four decades.
called brown clouds. The carbon-rich aerosols do Part of this trend is connected to the buildup of carbon
absorb radiation, but they heat the layer of air 2–3 km aerosols in brown clouds. With the brown clouds inter-
above the surface. As a result, the clouds block a portion cepting more solar radiation, the land received less
of the incoming solar radiation and prevent it from solar radiation and became dimmer (less bright). Other
reaching Earth’s surface, which cools. One important factors include blocking of solar radiation by contrails
consequence of this ongoing decrease in solar radiation emitted from jets and other emissions from urban areas.
has been a reduction of the intensity of the hydrological In several older industrial regions where aerosol
cycle. With less heating of the land, evaporation has emissions have been reduced, such as the closing of
decreased, contributing to greater sub-Saharan drought polluting industries in the former Soviet Union, the
and a weakened Indian monsoon. long-term dimming trend has reversed since the early
In regions of severe brown-cloud hazes, the reduction 1990s and the surface is brightening. The dimming
in solar radiation during peak seasons is almost an order trend continues over Southeast Asia, Africa, and other
of magnitude larger than the global average increase from regions where emissions from urbanization and other
the greenhouse-gas effect (Figure 18–12). The effects sources are still growing.

60°N

30°N

EQ

FIGURE 18-12 Brown clouds
30°S
Brown-cloud hazes in the lower
atmosphere shroud regions
downwind of large populations in
India and China. (Adapted from C.
60°S
E. Chung and V. Ramanathan, “South
Asian Haze Forcing: Remote Impacts
with Implications to ENSO and AO,”
Journal of Climate 16 [2003]:
0 60°E 120°E 180°E 1791–1806.)