Environmental Engineering Reference
In-Depth Information
called a
The starting point is the
world as it is, and the calculation sees how it changes (is
perturbed) when greenhouse gases are added to the
atmosphere. The oceans are there, their currents are
“
perturbation analysis.
”
flowing, the atmospheric winds blow, the ice is in place,
and the computer grinds away step by step to predict
the future as greenhouse gases accumulate.
There are also natural phenomena that occur randomly
from time to time and have to be put in explicitly. For
example, major volcanic eruptions, like that of Mount
Pinatubo in the Philippines in
, throw large amounts
of material into the upper atmosphere that affect the
albedo (the re
ection of incoming solar radiation back
out into space) directly and indirectly by affecting cloud
formation. This gives a cooling effect that lasts a few years
until the volcanic material falls out of the atmosphere.
Things like this cannot be predicted in advance. After an
event like Mount Pinatubo, the material ejected into the
sky has to be added explicitly and the models run again.
Fortunately for those doing the predictions, effects from
these kinds of events do not last for a long time and are
not really important for the long term, although they do
contribute to the seemingly random
fluctuations in the
planetary temperature.
A more important issue is predicting how human activ-
ities will change the amount of greenhouse gases put into
the atmosphere. Scenarios are created that predict how
energy use grows over time and what the mix of fuels will
be. From this the amount of greenhouse gas going into
the atmosphere for each scenario is derived. The IPCC
used six main scenarios in AR
, each with a few variations,
but only four in AR
. These scenarios go through the
