Climate Modeling - Beyond Smoke and Mirrors - page 38

Technical Note . : (cont.)
concern at the time was about ozone as the screen that
prevents intense solar ultraviolet radiation from reaching
the ground. The

role as greenhouse gases
was little appreciated then. The contrast between the ease
of obtaining an international agreement on

uorocarbons '

uorocarbons

and the dif culty of getting an international agreement on

CO is an interesting example of the reluctance of govern-

ments to enter into treaties that they do not know how to
implement. For fluorocarbons, substitutes were available and

the economic implications were small. For greenhouse gases,
what to do is not so clear and the economic implications
are large.
Aerosols are next on the list. These are not the stuff that
propels spray out of cans, but substances made by the inter-
action of chemicals with water vapor in the atmosphere.
There are many different ones, but the largest effect comes
from sulfate aerosols, which are produced by the sulfur
emitted by coal- red power plants. There are large uncer-

tainties about the effect of these aerosols, but one thing is
clear; their effect on climate is to reduce the temperature.
They do this by increasing cloud formation, thereby increas-
ing the re ection of incoming sunlight back into space (the

re is albedo). Efforts to

stop acid rain are reducing the sulfate aerosols produced in
the industrialized countries, but increasing coal use in the
developing world means that their actual amount is increas-
ing globally. It is not clear how the balance comes out in the
long term.
Black carbon (soot) is also an aerosol, but it has a
warming effect. Mainly derived from combustion pro-
cesses, it increases temperature by absorbing more solar
radiation than would normally be absorbed by the surfaces
which it coats. There remain large uncertainties about
its impact.

ected light is called the Earth '