Geoscience Reference
In-Depth Information
world. We need to know the consequences of rising GHG concentrations.
Because we cannot calculate all these complicated formulas in our heads,
we use computerized models to project the results of past and future
economic growth on emissions, climate, and thence upon human and
natural systems.
So how do economists and natural scientists project future climate
change? It is necessarily a two-step procedure. The fi rst step, explained
in this chapter, is to estimate the future emissions of CO
2
and other
important GHGs. The second step is to put those emissions estimates
into climate and other geophysical models to project future change in
CO
2
concentrations, temperature, and other important variables. This
second step is discussed in Chapter 4. I begin by discussing an impor-
tant element in modern natural and social sciences: the use of models.
A complete picture of future climate change requires projections of
the economy, energy use, CO
2
and other emissions, and different cli-
matic variables, along with the impacts in various sectors. A
projection
is
a conditional or “if-then” statement. It states, “If a given set of input
events take place, then we calculate that the following output events
will occur.” Economists often make this kind of projection, as in, “Given
current fi scal and monetary policies and the impact of the Euro crisis,
we expect real output to rise 2 percent next year.” Similarly, scientists
and economists use projections for future climate change. The main
inputs we need are variables like the path of annual CO
2
and other GHG
emissions. With these inputs and knowledge about the relevant physics,
chemistry, biology, and geography, climate scientists can calculate the
time paths for temperature and precipitation, sea level, sea ice, and
many other variables.
Because humans cannot calculate such projections in our heads,
they are all done with computerized models. What is a model? There
are different kinds of models, from model trains to architectural models
to scientifi c models. The basic idea is that a model is a simplifi ed picture
of a more complex reality. Economists represent the complex set of rela-
tionships that govern output, infl ation, and fi nancial markets using
“macroeconomic models.” These are mathematical and computerized
