Geoscience Reference
In-Depth Information
2.8
2.4
2.0
1.6
1.2
0.8
Emissions per unit output
Trend (-1.8% per year)
0.4
1900
1920
1940
1960
1980
2000
Figure 3.
Carbon intensity of U.S. economy, 1900-2010.
7 billion in 2012. In most countries, output of goods and services (gross
domestic product) per person has grown as well. Fortunately, world
CO
2
emissions have not been growing as rapidly as world output be-
cause of what is called decarbonization. This means simply that over
time we are using less CO
2
-rich energy to produce a given amount of
output. This is seen in the trend of the “carbon intensity” of economic
activity, which is measured as the ratio of CO
2
emissions to output.
The reasons for decarbonization are many, but three factors explain
most of it. One is that, for most products, we use less energy per unit
of output today than in earlier years; this is true whether the output is
a shirt or a gallon of milk or a telephone call. Another source of decar-
bonization is that our most rapidly growing economic sectors, such as
electronics and health care, tend to use less energy per unit of output
than sectors that are growing less rapidly or shrinking. In other words,
our economic mix is shifting from industries and activities that are more
energy intensive to ones that are less so. A fi nal source of decarbo-
nization has been the shift in energy sources away from the most
