Environmental Engineering Reference
In-Depth Information
because they have very similar
assumptions
about
emissions over the twenty-
rst century (about
parts
per million of CO
equivalent emissions (CO
e)) and
are also close to the BAU scenario. There is a consensus
among the
experts
that
any temperature
increase
beyond
ºC (
.
ºF) is quite dangerous. The AR
best
estimate would get that
.
ºC increase at about
ppm
CO
e.
The range in
Table
comes from many different
groups trying to predict what will happen to the global
climate as more and more greenhouse gases are added to
the atmosphere. Each has its own model. Although there
are many similarities among the models, there are some
differences. It is the differences that lead to the spread in
the prediction of the temperature increase that will
happen by the end of the century. We do not know
enough as yet to be sure which of the models is most
nearly correct, but the fact that even with their uncer-
tainties the models give results that are close to each
other is an indication that none is wildly wrong. The
new results give a slightly higher expected temperature
rise by the end of this century, and also decrease the
spread in the predictions of the many models. As time
goes on we will see what the real-world facts say about
the various models.
All of the models are quite complex and have adjustable
parameters that I call
.
These knobs can be turned
to increase or decrease the importance of some of the
processes that go into making the model, and each model
turns its knobs to get agreement with the past. I call this
tune-up process
“
knobs.
”
fixing up things so that you
agree with what you know has already happened. What
“
postdiction,
”
