Reproduced with permission, from: Wigley, T.M.L. 1994. MAGICC (Model for the Assessment of Greenhouse-gas Induced Climate Change): User's Guide and Scientific Reference Manual. National Center for Atmospheric Research, Boulder, Colo.

MAGICC

(Model for the Assessment of Greenhouse-gas Induced Climate Change)

User's Guide and Scientific Reference Manual

T.M.L. Wigley
Climatic Research Unit
University of East Anglia
Norwich, UK
and
National Center for Atmospheric Research
P.O. Box 3000
Boulder, CO 80303, USA*

*mailing address
fax: (303) 497-2699                             October 1994
email: wigley@ncar.ucar.edu

CONTENTS

1.  Introduction

2.  A Teaching Example
     2.1 Background
     2.2 Selecting the Policy and Reference emissions scenarios
     2.3 Editing the gas cycle model parameters
     2.4 Editing the climate and sea-level model parameters
     2.5 Running MAGICC and viewing tabulated results
     2.6 Viewing the graphical output
           2.6.1 Gas Emissions
           2.6.2 Gas Concentrations
           2.6.3 Radiative Forcing
           2.6.4 Temperature and Sea Level Change
     2.7 Greenland Ice-melt

3.  Additional Operational Items
     3.1 Editing the emissions library
     3.2 Gas lifetimes and radiative forcing sensitivities
     3.3 Gas cycle model options
     3.4 Changing climate model parameters
     3.5 Stratospheric water vapour forcing

4.  Scientific Details
     4.1 Introduction
     4.2 Carbon Dioxide
     4.3 Methane
     4.4 Nitrous Oxide
     4.5 Halocarbons
     4.6 Sulphate Aerosols
     4.7 The Climate Model
     4.8 Ice-melt Models

5.  Acknowledgments

6.  References

1. INTRODUCTION

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.

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

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¹ Default, best guess and mid are used interchangeably.

²This is not included in the present version, but will be included in later versions.

³The effects of emissions from biomass burning will be included in a later version of MAGICC.

⁴These four halocarbons are used as proxies for a much larger set of gases, which are divided into four groups with the named gases as key members. For HFC134a, the emissions input required is equivalent HFC134a, the GWP-weighted sum of emissions for CFC14 (CF4), CFC116 (C2F6), HFC125, HFC143a and HFC152a. The calculated radiative forcing for equiv-HFC134a closely approximates the total forcing for this group of gases. For the other three halocarbons, individual gas concentrations are carried through the calculations, converted to radiative forcing, and then scaled up to account for other gases in the groups. While approximate, the errors involved in this scaling up process are very small (of order 0.01-0.02 W/m²) for all realistic emissions scenarios.

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.

Click for Table 1.

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.

¹ Default, best guess and mid are used interchangeably.

2. A TEACHING EXAMPLE

2.1 Background

To understand the overall operation of MAGICC, the reader is referred to the accompanying flow chart (Fig. 1). To put flesh on this skeleton, we consider a specific example. This is a rather complex example, chosen to illustrate a reasonable selection both of MAGICC's features and of the questions that may be addressed using MAGICC. As noted in the Introduction, two different types of question may be addressed: what is the effect of a particular emissions control policy; and how do the results vary if model parameter values are altered? Both will be considered simultaneously here.

The first question is: how much climate and sea level change might be averted by a specific emissions control strategy? To answer this we compare results for a reference and a policy emissions scenario.

For the reference emissions scenario we choose the 1992 IPCC scenario, IS92a (Leggett et al., 1992). This is IPCC's central "existing policies" scenario; as such, it is often used as a reference case. For the policy scenario, we choose IS92d. This is also an "existing policies" scenario (Leggett et al., 1992): however, it is based on different background assumptions for population growth, economic growth, etc. that lead to lower overall emissions. Nevertheless, it can serve well as an example of what might be achieved if one assumed that IS92a background assumptions applied and that specific policies were introduced to reduce emissions of all gases¹. The emissions for these scenarios are shown in Tables 2 and 3.

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¹ All, that is, except alternate (non-ozone-depleting) halocarbons, as characterized here by HFC134a. For CFC substitutes, IS92a and IS92d differ in the split between HCFC22 and HFC134a: IS92d assumes a phase out of HCFC22 and a greater usage of HFC134a than IS92a. This is more in accord with the Copenhagen amendment to the Montreal Protocol, which was agreed to after the IS92 scenarios were derived.

Click for Table 3.

Click for Figure 1.

• For CO2, we ask what effect the use of the most recent IPCC information on the carbon budget (Schimel et al., 1994) has on future CO2 concentrations and radiative forcing compared with using IPCC90 information (from Watson et al., 1990).

• For CH4, we investigate the effect of assuming a constant lifetime (of 11 years) compared with the range of uncertainty for the variable lifetime case.

• For N2O and CFC11, we compare results for IPCC94 and IPCC90 lifetimes.

• For SO2, we consider the effect of assuming a low value for the aerosol forcing.

• For the climate sensitivity we consider a value near the upper limit of the most likely range.

• For ice-melt, we look specifically at the contribution from Greenland using a high value for the mass-balance sensitivity.

Click for Table 4.

*Net land-use-change emissions averaged over the 1980s. This is a proxy for the strength of the CO2 fertilization effect--higher values require higher fertilization to balance the contemporary carbon budget. Further details are given in Wigley (1993) and in the text.

**See the following Technical Details section. The default model has a lifetime that varies with the concentrations of CH4, CO, NOx, and VOCs. Further details are given in Osborn and Wigley (1994).

2.2 Selecting the Policy and Reference emissions scenarios

The first step is to move into the MAGICC directory by typing CD\MAGICC and hitting the 'Enter' key. Typing MAGICC and 'Enter' will then display the first MAGICC screen. The choices then are: to enter the climate/sea level model part of MAGICC; to calculate Global Warming Potentials (GWPs); to view a help screen; to access the model documentation; or to quit. The GWP calculations in this version of MAGICC are out-of-date, and should not be used.

All choices are made through the CLIMATE MODELLING screen. The first task is to select and install a policy scenario and a reference scenario². These may be selected by clicking on the 'Change' buttons at the top of the screen (or keying 'A' or 'B'). Clicking on 'Change' for the policy scenario leads to the CHANGE POLICY SCENARIO screen. To select a new policy scenario, move the mouse arrow to anywhere on the appropriate line (IS92d in this case), and click. Clicking on Okay (or keying 'O') selects the chosen scenario and returns the user to the CLIMATE MODELLING screen. Clicking on Cancel ignores the selection and returns the user to the CLIMATE MODELLING screen retaining the original scenario selection. The Help button on this screen has not been activated and should be ignored. After returning to the CLIMATE MODELLING screen, repeat the above process for the reference scenario to install IS92a.

2.3 Editing the gas cycle model parameters

Back at the CLIMATE MODELLING screen, the next task is to set the appropriate gas-cycle model parameters (viz. lines 1,2,3 and 4 in Table 4). To do this, move the mouse and click on the User Defined button in the Gas Cycle Parameters panel (or type '2'). This brings up the GAS CYCLE MODEL PARAMETERS screen.

In the Carbon Cycle Model panel, we wish to select a user-defined value for the 1980s-mean value of net land-use-change emissions (Dn(1980s)) of 1.6 GtC/yr; i.e., the best-guess value recommended in the 1990 IPCC report (the current best guess value is 1.1 GtC/yr). First, click on the User button (or type 'S'). Then, using the move-right arrow on the keyboard, move the edit bar in the Dn(1980s) box to the right of the first decimal place, key 'Backspace' three times to delete the numbers to the left of the edit bar, and type in 1.6 (the box should now read 1.600).

Now move the mouse to the Methane Model panel and choose 'Constant' (or type 'O') on the CH4 lifetime row. Next, to select a fixed lifetime value, click on the edit button (or type #). This will bring up the GAS LIFETIMES AND DQ/DC screen. This screen allows one to change lifetimes and forcing sensitivities (dQ/dC) for a large selection of gases. For climate model calculations, however, only changes in CH4, N2O, CFC11, CFC12, HCFC22 and HFCl34a will be used--the other gases are included for GWP calculations. To change the methane lifetime, use the move-right arrow to move the edit bar to the right of the first decimal place, delete appropriately using the 'Backspace' key, and type in the new value. Clicking on Okay returns you to the GAS CYCLE MODEL PARAMETERS screen. You will see that the revised lifetime is now displayed below the 'Constant' button.

The next task is to alter the N2O and CFC11 lifetimes. To do this, move the mouse to the N2O and Halocarbons box and click on Edit (or type 'L'). This will cause the GAS LIFETIMES AND DQ/DC screen to be displayed again showing N2O details on the

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1 Hitting the 'Esc' key at any time will return the user to the previous menu. For most screens 'Enter' has the same effect.

2 An unused version of MAGICC will have no emissions scenarios installed. It is necessary to install a policy and an emissions scenario in order to run MAGICC.

Having completed the N2O and CFC11 lifetime changes and returned to the GAS CYCLE screen, the final task is to alter the aerosol forcing. Here, the user has only four choices, a best-guess ('medium') value, low and high values which are 0.5 and 1.5 times the medium value (following Wigley and Raper, 1992), and zero. Move the arrow to the SO2 forcing box and click on 'Low'.

2.4 Editing the climate and sea-level model parameters

The next step is to change the climate sensitivity, specified in MAGICC by the value of the equilibrium global-mean temperature change for a CO2 doubling ([[Delta]]T2x). To do this, return from the GAS CYCLE screen to the CLIMATE MODELLING screen by clicking on the Okay button. Next, move the mouse to and click on the User defined button in the Climate Model Parameters panel. This brings up the CLIMATE MODEL PARAMETERS screen, where the user can change the five main climate model parameters; viz. [[Delta]]T2x, the vertical diffusivity in the ocean (K), the mixed-layer depth (h), the upwelling rate (w) and the sinking-water to global-mean temperature change ratio [[Pi]]).

Editing climate model parameters is carried out as previously. The left and right arrow keys move the edit bar accordingly, 'Enter' moves the bar to the next box, and 'Backspace' deletes the figure to the left of the edit bar. New numbers appear to the left of the edit bar when typed on the keyboard. In the current example, only [[Delta]]T2x should be edited, from its default value of 2.5 deg. C to 4.0 deg. C. Now return to the CLIMATE MODELLING screen using the Okay button.