THE ECONOMIC ORIGINS OF CLIMATE CHANGE - The Climate Casino - page 21

40
30

Global CO 2 emissions
Trend (2.6% per year)

20

10
8
6

4
3

2

1
1900

1920 1940 1960 1980 2000

Figure 2. Global CO 2 emissions, 1900-2010.

The net effect of all these decisions around the world is shown in
Figure 2, which displays the long-term trend in global CO 2 emissions
over the period from 1900 to 2010 . 2 There have been periods of fast
growth and of slow growth, but on average, emissions grew at a rate
of 2.6 percent per year. This upward trend is the source of our worry.
These rising emissions are leading to rising CO 2 concentrations in the
atmosphere, which is what produces climate change.
I note here one geeky detail about the fi gure: The vertical scale on
the diagram, and on several others in this topic, is a ratio scale. This is a
diagram in which equal vertical distances have equal proportions; thus,
for example, the vertical distance from 200 to 400 is the same as that
from 400 to 800. Ratio scales are convenient because a straight line
(one with a constant slope) has a constant rate of growth or decline. If
you look at Figure 3, you see that a given percentage increase looks the
same no matter where it occurs on the chart.
It will be useful to give the global totals here. Global CO 2 emissions
have been growing because the global economy has been growing. The
world's population has expanded from around 2 billion in 1900 to over