Model Parameters
Carbon Cycle Model
This button allows the user to examine non-climate-related uncertainties
in the carbon cycle model by changing the 1980s-mean value of net
land-use change CO2 emissions. The default value for this variable
is 1.1GtC (Gigatonnes of Carbon) per year, which is the IPCC Second Assessment
Report (SAR) best estimate. There is considerable uncertainty, however,
about the true value of these land-use change emissions. The IPCC SAR uncertainty
range is from 0.4 to 1.8GtC per year. Choosing a low value leads to higher
concentrations and vice versa. MAGICC runs the carbon cycle model three times,each
with a different Dn80s value: 1.8, 1.1 and 0.4. These settings,
and the associated CO2-fertilisation factor, determine the high
and low ends of the range for the MAGICC display CO2 Concentration
Graphs. It should be noted that the IPCC Third Assessment Report (TAR)
suggests slightly different values for the above parameters. The MAGICC carbon
cycle model, however, when climate feedbacks are included (the default case),
gives CO2 concentrations that agree well with the IPCC TAR values
(see Technical Report).
C-Cycle Climate Feedbacks
An important change from the SAR to the TAR was the inclusion and quantification
of the effects of climate feedbacks on the carbon cycle. The net effect
of these is a positive feedback, so their inclusion leads to higher concentrations
than would otherwise be obtained. There is considerable uncertainty in the
value of these feedbacks, and each of the three carbon cycle models used
in the IPCC TAR [the Joos et al. (Bern), Jain et al.(ISAM) and MAGICC models]
have different values. MAGICC uses feedback values that give a net climate
feedback that is somewhat lower than the values from the other two models.
These feedbacks can be turned on or off (default is on) so that users can
judge the importance of the MAGICC climate feedbacks.
Aerosol Forcing
There are four components to the aerosol forcing used by MAGICC. All four
components are subject to large uncertainties. The default values in MAGICC
are those used in the IPCC TAR, but the user can select, as alternatives,
high or low values for aerosol forcing.
Direct: this forcing is the direct, clear-sky effect of sulfate
aerosols formed from fossil fuel combustion, i.e., aerosols derived from SO2
emissions. MAGICC uses a default current (1990) forcing estimate of -0.4Wm-2.
Indirect: the indirect forcing effect of aerosols is more uncertain
than the direct effect. MAGICC uses a default value for the current indirect
forcing of -0.8Wm-2.
Biospheric: this forcing is due to aerosols emitted from biomass
burning, the sum of both organic and carbonaceous aerosol forcings. The default
for the current value of this forcing component is -0.2Wm-2. Forcing
is assumed to run parallel to gross land-use change emissions. (Note that,
in the emissions scenarios, it is net land-use change emissions that are
prescribed. Net emissions are gross emissions less regrowth. The MAGICC carbon
cycle model calculates gross land-use change emissions internally.)
FOC: this acronym stands for Fossil plus Organic Carbonaceous
aerosol forcing (often called 'soot'). The default for the current value
of this forcing component is 0.1Wm-2, with forcing scaled to run
parallel to SO2 emissions.
The separate components of aerosol forcing cannot be altered by the user.
Instead, as a guide to the importance of aerosol forcing uncertainties, the
user can choose high, default (mid) or low values of total aerosol forcing.
The default total forcing in 1990 is -1.3Wm-2. The low and high
values are -0.8Wm-2 and -1.8Wm-2.
Climate Sensitivity
The climate sensitivity defines the equilibrium response of global-mean surface
air temperature to a doubling of the CO2 concentration. The IPCC
SAR best-estimate of this parameter was 2.5°C, with a range from 1.5°
to 4.5°C. For the TAR projections a central value of 2.6°C was used
(the median for the seven AOGCMs whose results were used to calibrate MAGICC
-- see below). In default mode MAGICC runs each emissions scenario three
times through the climate model, each time with a different value for the
climate sensitivity: 1.5°, 2.6° and 4.5°C. These three runs
define the low and the high ends of the range of global temperature projections
(approximately the 90% confidence interval) displayed in the Temperature
Graphs.
It is possible to enter a user-defined value for the climate sensitivity.
Click on the relevant box and enter your own value. This choice will now
form part of your parameter selection for the User Model through which MAGICC
also runs the two emissions scenarios. The recommended range from which
to select the climate sensitivity is 0.5° to 5.5°C.
Thermohaline Circulation (THC)
An important determinant of temperature changes and oceanic thermal expansion
is how the thermohaline circulation changes. The default case corresponds
to a moderate slow-down of the THC as the globe warms, at a rate equal to
the median of THC change results for the seven AOGCMs whose results were
used to calibrate MAGICC. To illustrate the influence this slow-down has
on temperature and sea level, the user can select either the default (varying
THC) mode, or a constant THC.
Vertical Diffusivity (Kz)
Another important determinant of temperature changes and oceanic thermal
expansion is how the speed with which oceanic mixing transports heat from
the surface into the deeper ocean, parameterized in MAGICC by the vertical
diffusivity. The default case (2.3cm2/s) corresponds to the median
value for effective diffusivity for the seven AOGCMs whose results were
used to calibrate MAGICC. To quantify the influence this choice has on temperature
and sea level, the user can edit this value.
Ice Melt
Sea level rise is determined by oceanic thermal expansion, and ice melt
and other water balance terms. In the TAR, uncertainty ranges were estimated
for all non-expansion terms. The default in MAGICC uses all of the TAR central
estimates. The user can choose to use, instead, either the high or low extremes
(concatenating the two-sigma high and low limits for all non-expansion terms).
Model
In the TAR, global-mean temperature and sea level rise uncertainties were
determined by simulating the results of seven AOGCMs. MAGICC was first calibrated
to match the temperature and expansion outputs for these seven models forced
with 1%/yr compound CO2 changes (from the CMIP2 data base). MAGICC was
then run for each of the seven parameter sets for the full range of SRES
emissions scenarios (an exercise that would have been computationally impossible
to carry out using the original AOGCMs). The 'Model' button allows the user
to simulate global-mean temperature and sea level changes for any of the
seven models used in the TAR: GFDL, CSIRO, HadCM3, HadCM2, ECH4-OPYC, PCM,
or CSM. The correct MAGICC parameter values and non-expansion sea-level model
parameters to do this are hard-wired into the MAGICC code.