From EnviroWiki
Model for the Assessment of Greenhouse-gas Induced Climate Change
MAGICC is a set of coupled gas-cycle, climate and ice-melt models that allows one to determine the global-mean temperature and sea-level consequences of user-specified emissions scenarios. MAGICC is designed for two purposes:
- To compare the implications of a "policy" emissions scenario with a "reference" scenario, in order to see how effective a particular emissions control policy is relative to a base-line "no policy" case.
- To determine the sensitivity of results to model parameter changes. The latter may be done by comparing results from a given emissions scenario for a user-specified set of parameters with those for a default1 set of parameters.
MAGICC includes all the major greenhouse gases (except tropospheric ozone2) and the effects of fossil-fuel derived SO2 emissions through sulphate aerosol effects3, and accounts for the negative forcing effect of halocarbon-induced stratospheric ozone depletion. While the component models of MAGICC are conceptually simple, they nevertheless represent the state-of-the-art in their areas and simulate reliably the results of more complex and far more computationally-demanding models. On an 80486-based microcomputer a complete MAGICC run takes 10-20 seconds to complete.
The input emissions scenarios require values to be specified at 11 discrete dates between 1990 and 2100 (inclusive) for the following: fossil CO2, net land-use-change CO2, CH4, N2O, CO, NOx, VOCs, CFC11, CFC12, HCFC22, HFCl34a4 and SO2. The primary inputs to MAGICC are the user-selected policy and reference emissions scenarios. These are selected from an emissions scenario library (both scenarios may be the same if desired). The next step is to select either default or user-specified model parameters for the gas cycle, climate model and sea level (ice-melt) model parameters.
MAGICC executes four complete model runs over 1765-2100. To explain what these are and how they arise, consider the two initial question types. First, if a policy analyst wished to evaluate the climate effects of a particular emissions policy, he or she would choose to compare a Policy emissions scenario with a background Reference emissions scenario. For climate and sea level model parameters, it would be sufficient to use only the current best guess1 set of values by choosing the Default1 model parameter options. The two sets of results could be labeled PD and RD.
Alternatively, a scientist or educator may be interested in examining the sensitivity of the various models that comprise MAGICC to model parameter assumptions. In this case, he or she could choose the same emissions scenario for the policy and reference case, and then select a specific set of model parameters (User values) that differed from the default values. If R is used for the single emissions scenario, then the results could be labeled RU and RD, where RD is the same as in the first example. In general, one can combine these two types of application and produce four output data sets, PD, RD, PU and RU.
In addition to running these four cases, MAGICC estimates uncertainty ranges due to model parameter uncertainties relative to the Default model parameter set. To do this, each run calculates four sets of CO2 concentrations and four sets of CH4 concentrations (over 1990-2100) corresponding to low, default (best-guess), high, and user-specified model parameter sets. For gas concentrations, there are therefore two primary sets of model-generated data for each emissions scenario, corresponding to user-selected gas-cycle model parameters (in RU and PU) and default model parameters (in RD and PD). Both concentration data sets are used to force the climate model, which is an upwelling-diffusion energy-balance climate model. In each of the four climate model simulations, the climate and sea level models are run three times corresponding to low, mid1 or user, and high model parameters values. Table 1 give a summary of the runs.
Notes:
(1) L, M, H. U denote Low, Mid (i.e., best guess or default), High and User-selected model parameters
(2) In the Default cases, the M values are computed twice for CO2 and CH4 (i.e., the U mode in the User case is replaced by M)
(3) L, M and H for the temp/mean-sea-level models correspond to climate sensitivities of 1.5 deg. C, 2.5 deg. C and 4.5 deg. C, best-guess values for other upwelling-diffusion model parameters, and values for all ice-melt model parameters (bar the initial "small" glacier mass) that produce low, mid and high ice-melt (i.e. low climate sensitivity goes with low ice melt, etc.)
(4) For the User case, the best-guess climate and sea-level model parameter results are only produced if specifically selected by using the appropriate de bult options.
(5) Only those items in bold are shown in the graphical displays.
Input and output are displayed both graphically and in tabular form. Convenient input and output summary tables may be viewed on screen (and printed directly from the screen) for the user model-parameter cases, RU and PU (default model values will, of course, be shown if default model parameter selections are made initially). More detailed output for all four cases may be viewed or printed from DOS in the files MAGOUTRD.DAT, MAGOUTRU.DAT, MAGOUTPD.DAT and MAGOUTPU.DAT. Graphical displays are given for input emissions, and output concentrations, temperature, sea level, and radiative forcing (gas by gas). For CO2 and CH4 concentration, uncertainties are indicated by showing the low and high model-parameter cases. Uncertainties for temperature and sea level are illustrated similarly (for the default concentration case only).
