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two models, from NCAR and GFDL (respectively,
the National Center for Atmospheric Research
and the Geophysical Fluid Dynamics Laboratory)
included ocean coupling (i.e., could be classified
as AOGCMs). Others employed a 'slab' mixed
layer (top 50m or so) ocean and with various other
simplifications, such as no horizontal ocean heat
transport, prescribed horizontal ocean heat
transport, and fixed zonally averaged cloud cover.
Horizontal resolution (model grid cell size)
was coarse, typically in the order of 500km.
Simulations included climate responses to transi-
ent increases (1 percent per year) in CO
2
(with
the NCAR and GFDL models) and equilibrium
experiments for a doubling of CO
2
(in which
models were run until an equilibrium climate
state was reached). By the Second Assessment
Report (1995), typical horizontal resolution had
increased to about 250km, and ocean coupling
had been improved. Additional refinements
included treatment of the radiative effects of
anthropogenic sulfate aerosols and volcanic
eruptions. Eleven groups with 11 AOGCMs
participated. Simulations included transient
increases in CO
2
of 1 percent per year as well as
other greenhouse gas change scenarios. By the
Third Assessment Report (2001), horizontal
resolutions had been further increased, with more
robust treatment of the ocean (e.g., of overturn-
ing circulations) and land-surface interactions.
Nineteen AOGCMs participated. Models used for
the Fourth Assessment Report (2007) were even
more mature, with some including atmospheric
chemistry and interactive vegetation. A total of 23
AOGCMs were evaluated, representing the work
of 16 modeling groups from 11 countries.
An important feature of the third and fourth
Assessment Reports is that simulations with the
different models used a range of greenhouse
gas emission scenarios (contained in a Special
Report on Emission Scenarios, or SRES) based on
differing views of the global future. This was a
major advance over simply assuming a 1 percent
per year growth rate or a doubling of CO
2
. One
set of emissions scenarios (A1) assumes rapid
economic growth, global population peaking in
mid-century and then declining, and the intro-
duction of more efficient technologies. Three
variants are: A1F1, fossil fuel intensive; A1T, non-
fossil energy sources; and A1B, a balance across
all energy sources. Scenario A2 considers global
heterogeneity, increasing population, and
fragmented and slower technological change. A
second set includes B1 where population trends
are as in A1, but the global economy is service and
information based, with clean, resource-efficient
technologies. B2 envisages slower population
increase, intermediate levels of economic develop-
ment, and diverse, regionally oriented techno-
logical change. Of all these scenarios, A1B (often
termed 'Business as Usual', or BAU) has been the
most widely examined.
Figure 13.15
shows projected changes in the
concentrations of CO
2
, CH
4
and CFC-11 through
the twenty-first century, based on four of the
scenarios. Depending on the scenario, CO
2
concentrations are projected to rise to between
540 and 970ppm by 2100, corresponding to
increases of 90 and 250 percent above the pre-
industrial level. Methane concentration changes
will range between - 190ppb and + 1970ppb above
1998 levels by 2100. In 1995 it was estimated that
to stabilize the concentration of greenhouse
gases at 1990 levels would require the following
percentage reductions in emissions resulting from
human activities: CO
2
>60 percent; CH
4
15-20
percent; N
2
O 70-80 percent; CFCs 70-85 percent.
The 2001 IPCC report notes that to stabilize CO
2
concentrations at 450 (650)ppm would require
anthropogenic emissions to drop below 1990
levels within a few decades (about a century).
Given the strong growth in emissions since
2001, even stronger reductions would be needed
today.
The associated projected increases in anthro-
pogenic radiative forcing (relative to pre-
industrial conditions) corresponding to the
SRES cases of
Figure 13.15
are shown in
Figure
13.16
. The projected range is 4 to 9W m
-2
by 2100.
Aerosol impacts would reduce these numbers
