Geoscience Reference
In-Depth Information
PROJECTIONS: BASIC PRINCIPLES
Understanding future climate change begins with a set of projections
for inputs into climate models. These are primarily the paths of emis-
sions of CO 2 and other GHGs. To keep the discussion manageable, I fo-
cus primarily on CO 2 because it is the most important of the GHGs, but
complete assessments include other gases as well. When looking at ac-
tual projections, I use CO 2 equivalent (CO 2 -e), which adds together the
contributions of all the GHGs and expresses them as the effect of an
equivalent amount of CO 2 .
How do statisticians and economists make projections? They begin
by estimating statistical relationships using the historical data along
with the underlying physical laws and economic relationships. With
these results, a demographer or economist can then make a statisti-
cally based projection of future trends. The advantage of the statistical
approach is that it can be reproduced and updated. That is, because
each of the steps can be performed using publicly available data and com-
puter software, the estimates can be checked or challenged by other
scientists.
As I discuss above, total CO 2 emissions are driven by three compo-
nents: population, GDP per capita, and the carbon intensity of GDP.
Mathematically, the CO 2 growth rate is equal to the sum of the growth
rates of each of the three components. Data for 2010 and projections for
2050 are shown for the United States and for the world in Table 1 . 7
These projections for 2050 assume that countries make no policies to
reduce emissions. The estimates come from the Yale DICE model, but
they do not differ markedly from those found in other studies.
Look fi rst at the top half for the United States. As Table 1 shows,
population is projected to grow at 0.6 percent per year, per capita out-
put to grow at 1.7 percent per year, and the CO 2 output ratio to decline
at 1.6 percent per year. On the basis of these assumptions, CO 2 emis-
sions would grow at 0.7 percent per year, increasing emissions by about
one-third by midcentury.
Similar calculations can be made for different regions. Most eco-
nomic models have modules for the different components. They might
 
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