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aerosols come largely from power plants and from burning biomass.
The best estimates suggest that these contributed a negative forcing of
about 0.7 W/m 2 in 2011. In other words, aerosols tend to cool the earth
and mask the warming forces.
Most projections indicate that by 2100, CO 2 will be the major con-
tributor to global warming. The contribution of other factors, parti-
cularly aerosols, is highly speculative. One of the diffi culties of projecting
aerosols is that we do not know how much power will be generated
by coal, nor how much of the emissions of coal-burning plants will be
cleaned up in the future.
In the present analysis, I generally simplify the story by concentrat-
ing on CO 2 emissions, which captures the essential issues. I discuss other
factors when they enter the picture. 1
WHERE DO CO 2 EMISSIONS COME FROM?
The task of reducing CO 2 emissions is simple in principle but diffi cult
in practice. It “simply” requires the world either to use less fossil fuel, or
to fi nd a way to remove the CO 2 emissions if fossil fuels are burned.
Figure 23 shows the major contributors to CO 2 emissions. 2 Coal and oil
are each 35-40 percent of global energy CO 2 emissions; natural gas con-
tributes about one-fi fth of the total. Percentages for the United States
differ little from those of the world as a whole. There are other sectors
that emit CO 2 , such as cement production, but it is most useful to fo-
cus on fossil fuels, where the economic stakes are largest and the con-
tribution to warming the greatest.
Figure 23 shows the physical volumes of CO 2 emissions. We can
also examine the relative economic value of the CO 2 emissions. By this
is meant the dollar value the market places on the fuel to which the CO 2
is attached. Some fuels are more expensive than others. For example,
when you use gasoline in your car, the amount of CO 2 released is low
per dollar of spending. By contrast, when an electric utility burns coal,
the amount of CO 2 released is high per dollar.
Here are the estimates of the tons of CO 2 emissions per $1,000 of ex-
penditures on fuel:
 
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