IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis

8.8.3 Earth System Models of Intermediate Complexity

Pictorially, EMICs can be defined in terms of the components of a three-dimensional vector (Claussen et al., 2002): the number of interacting components of the climate system explicitly represented in the model, the number of processes explicitly simulated and the detail of description. Some basic information on the EMICs used in Chapter 10 of this report is presented in Table 8.3. A comprehensive description of all EMICs in operation can be found in Claussen (2005). Actually, there is a broad range of EMICs, reflecting the differences in scope. In some EMICs, the number of processes and the detail of description are reduced to simulate feedbacks between as many components of the climate system as feasible. Others, with fewer interacting components, are utilised in long-term ensemble experiments to investigate specific aspects of climate variability. The gap between some of the most complicated EMICs and AOGCMs is not very large. In fact, this particular class of EMICs is derived from AOGCMs. On the other hand, EMICs and simple climate models differ much more. For instance, EMICs as well as AOGCMs realistically represent the large-scale geographical structures of the Earth, like the shape of continents and ocean basins, which is certainly not the case for simple climate models.

Table 8.3. Description of the EMICs used in Chapter 10. The naming convention for the models is as agreed by all modelling groups involved. An asterisk after a component or parametrization means that this component or parametrization was not activated in the experiments discussed in Chapter 10.

Name Atmospherea Oceanb Sea Icec Coupling/Flux Adjustmentsd Land Surfacee Biospheref Ice Sheetsg 
E1: BERN2.5CC (Plattner et al., 2001; Joos et al., 2001) EMBM, 1-D (φ), NCL, 7.5° x 15° (Schmittner and Stocker, 1999) FG with parametrized zonal pressure gradient, 2-D (φ, z), 3 basins, RL, ISO, MESO, 7.5°x15°, L14 (Wright and Stocker, 1992) 0-LT, 2-LIT (Wright and Stocker, 1993) PM, NH, NW (Stocker et al., 1992; Schmittner and Stocker, 1999) NST, NSM (Schmittner and Stocker, 1999) BO (Marchal et al., 1998), BT (Sitch et al., 2003; Gerber et al., 2003), BV (Sitch et al., 2003; Gerber et al., 2003)   
E2: C-GOLDSTEIN (Edwards and Marsh, 2005) EMBM, 2-D(φ,λ), NCL, 5° x 10° (Edwards and Marsh, 2005) FG, 3-D, RL, ISO, MESO, 5° x 10°, L8 (Edwards and Marsh, 2005) 0-LT, DOC, 2-LIT (Edwards and Marsh, 2005) GM, NH, RW (Edwards and Marsh, 2005) NST, NSM, RIV (Edwards and Marsh, 2005)     
E3: CLIMBER-2 (Petoukhov et al., 2000) SD, 3-D, CRAD, ICL, 10° x 51°, L10 (Petoukhov et al., 2000) FG with parametrized zonal pressure gradient, 2-D (φ, z), 3 basins, RL, 2.5°, L21 (Wright and Stocker, 1992) 0-LT, DOC, 2-LIT (Petoukhov et al., 2000) NM, NH, NW (Petoukhov et al., 2000) 1-LST, CSM, RIV (Petoukhov et al., 2000) BO (Brovkin et al., 2002), BT (Brovkin et al., 2002), BV (Brovkin et al., 2002) TM, 3-D, 0.75° x 1.5°, L20* (Calov et al., 2005) 
E4: CLIMBER-3a (Montoya et al., 2005) SD, 3-D, CRAD, ICL, 7.5° x 22.5°, L10 (Petoukhov et al., 2000) PE, 3-D, FS, ISO, MESO, TCS, DC*, 3.75° x 3.75°, L24 (Montoya et al., 2005) 2-LT, R, 2-LIT (Fichefet and Morales Maqueda, 1997) AM, NH, RW (Montoya et al., 2005) 1-LST, CSM, RIV (Petoukhov et al., 2000) BO* (Six and Maier-Reimer, 1996), BT* (Brovkin et al., 2002), BV* (Brovkin et al., 2002)   
E5: LOVECLIM (Driesschaert, 2005) QG, 3-D, LRAD, NCL, T21 (5.6° x 5.6°), L3 (Opsteegh et al., 1998) PE, 3-D, FS, ISO, MESO, TCS, DC, 3° x 3°, L30 (Goosse and Fichefet, 1999) 3-LT, R, 2-LIT (Fichefet and Morales Maqueda, 1997) NM, NH, RW (Driesschaert., 2005) 1-LST, BSM, RIV (Opsteegh et al., 1998) BO (Mouchet and François, 1996), BT (Brovkin et al., 2002), BV (Brovkin et al., 2002) TM, 3-D, 10 km x 10 km, L30 (Huybrechts, 2002) 
E6: MIT-IGSM2.3 (Sokolov et al., 2005) SD, 2-D (φ, z), CRAD, ICL, 4°, L11 (Sokolov and Stone, 1998), CHEM* (Mayer et al., 2000) PE, 3-D, FS, ISO, MESO, 4° x 4°, L15 (Marshall et al., 1997) 3-LT, 2-LIT (Winton, 2000) AM, GH, GW (Sokolov et al., 2005) 10-LST, CSM (Bonan et al., 2002) BO (Parekh et al., 2005), BT (Felzer et al., 2005), BV* (Felzer et al., 2005)    
E7: MOBIDIC (Crucifix et al., 2002) QG, 2-D (φ, z), CRAD, NCL, 5°, L2 (Gallée et al., 1991) PE with parametrized zonal pressure gradient, 2-D (φ, z), 3 basins, RL, DC, 5°, L15 (Hovine and Fichefet, 1994) 0-LT, PD, 2-LIT (Crucifix et al., 2002) NM, NH, NW (Crucifix et al., 2002) 1-LST, BSM (Gallée et al., 1991) BO* (Crucifix, 2005), BT* (Brovkin et al., 2002), BV (Brovkin et al., 2002) M, 1-D (φ), 0.5° (Crucifix and Berger, 2002) 
E8: UVIC (Weaver et al., 2001) DEMBM, 2-D (φ, λ), NCL, 1.8° x 3.6° (Weaver et al., 2001) PE, 3-D, RG, ISO, MESO, 1.8° x 3.6° (Weaver et al., 2001)  0-LT, R, 2-LIT (Weaver et al., 2001) AM, NH, NW (Weaver et al., 2001) 1-LST, CSM, RIV (Meissner et al., 2003) BO (Weaver et al., 2001), BT (Cox, 2001), BV (Cox, 2001) M, 2-D (φ, λ), 1.8° x 3.6°* (Weaver et al., 2001) 

Notes:

a EMBM = energy-moisture balance model; DEMBM = energy-moisture balance model including some dynamics; SD = statistical-dynamical model; QG = quasi-geostrophic model; 1-D (φ) = zonally and vertically averaged; 2-D(φ, λ) = vertically averaged; 2-D(φ, z) = zonally averaged; 3-D = three-dimensional; LRAD = linearized radiation scheme; CRAD = comprehensive radiation scheme; NCL = non-interactive cloudiness; ICL = interactive cloudiness; CHEM = chemistry module; horizontal and vertical resolutions: the horizontal resolution is expressed either as degrees latitude x longitude or as spectral truncation with a rough translation to degrees latitude x longitude; the vertical resolution is expressed as ‘Lm’, where m is the number of vertical levels.

b FG = frictional geostrophic model; PE = primitive equation model; 2-D (φ, z) = zonally averaged; 3-D = three-dimensional; RL = rigid lid; FS = free surface; ISO = isopycnal diffusion; MESO = parametrization of the effect of mesoscale eddies on tracer distribution; TCS = complex turbulence closure scheme; DC = parametrization of density-driven down-sloping currents; horizontal and vertical resolutions: the horizontal resolution is expressed as degrees latitude x longitude; the vertical resolution is expressed as ‘Lm’, where m is the number of vertical levels.

c n-LT = n-layer thermodynamic scheme; PD = prescribed drift; DOC = drift with oceanic currents; R = viscous-plastic or elastic-viscous-plastic rheology; 2-LIT = two-level ice thickness distribution (level ice and leads).

Notes (continued):

d PM = prescribed momentum flux; GM = global momentum flux adjustment; AM = momentum flux anomalies relative to the control run are computed and added to climatological data; NM = no momentum flux adjustment; GH = global heat flux adjustment; NH = no heat flux adjustment; GW = global freshwater flux adjustment; RW = regional freshwater flux adjustment; NW = no freshwater flux adjustment.

e NST = no explicit computation of soil temperature; n-LST = n-layer soil temperature scheme; NSM = no moisture storage in soil; BSM = bucket model for soil moisture; CSM = complex model for soil moisture; RIV = river routing scheme.

f BO = model of oceanic carbon dynamics; BT = model of terrestrial carbon dynamics; BV = dynamical vegetation model.

g TM = thermomechanical model; M = mechanical model (isothermal); 1-D (φ) = vertically averaged with east-west parabolic profile; 2-D (φ, λ) = vertically averaged; 3-D = three-dimensional; horizontal and vertical resolutions: the horizontal resolution is expressed either as degrees latitude x longitude or kilometres x kilometres; the vertical resolution is expressed as ‘Lm’, where m is the number of vertical levels.

Since the TAR, EMICs have intensively been used to study past and future climate changes (see Chapters 6, 9 and 10). Furthermore, a great deal of effort has been devoted to the evaluation of those models through coordinated intercomparisons.

Figure 8.17 compares the results from some of the EMICs utilised in Chapter 10 (see Table 8.3) with observation-based estimates and results of GCMs that took part in AMIP and CMIP1 (Gates et al., 1999; Lambert and Boer, 2001). The EMIC results refer to simulations in which climate is in equilibrium with an atmospheric CO2 concentration of 280 ppm. Figures 8.17a and 8.17b show that the simulated latitudinal distributions of the zonally averaged surface air temperature for boreal winter and boreal summer are in good agreement with observations, except at northern and southern high latitudes. Interestingly, the GCM results also exhibit a larger scatter in these regions, and they somewhat deviate from data there. Figures 8.17c and 8.17d indicate that EMICs satisfactorily reproduce the general structure of the observed zonally averaged precipitation. Here again, at most latitudes, the scatter in the EMIC results seems to be as large as the scatter in the GCM results, and both EMIC and GCM results agree with observational estimates. When these EMICs are allowed to adjust to a doubling of atmospheric CO2 concentration, they all simulate an increase in globally averaged annual mean surface temperature and precipitation that falls largely within the range of GCM results (Petoukhov et al., 2005).

Figure 8.17

Figure 8.17. Latitudinal distributions of the zonally averaged surface air temperature (a, b) and precipitation rate (c, d) for boreal winter (DJF) (a, c) and boreal summer (JJA) (b, d) as simulated at equilibrium by some of the EMICs used in Chapter 10 (see Table 8.3) for an atmospheric CO2 concentration of 280 ppm. In (a) and (b), observational data merged from Jennings (1975), Jones (1988), Schubert et al. (1992), da Silva et al. (1994) and Fiorino (1997) are shown by crosses. In (c) and (d), observation-based estimates from Jaeger (1976; crosses) and Xie and Arkin (1997; open circles) are shown. The vertical grey bars indicate the range of GCM results from AMIP and CMIP1 (see text). Note that the model versions used in this intercomparison have no interactive biosphere and ice sheet components. The MIT-UW model is an earlier version of MIT-IGSM2.3. Adapted from Petoukhov et al., 2005.

The responses of the North Atlantic MOC to increasing atmospheric CO2 concentration and idealised freshwater perturbations as simulated by EMICs have also been compared to those obtained by AOGCMs (Gregory et al., 2005; Petoukhov et al., 2005; Stouffer et al., 2006). These studies reveal no systematic difference in model behaviour, which gives added confidence to the use of EMICs.

In a further intercomparison, Rahmstorf et al. (2005) compared results from 11 EMICs in which the North Atlantic Ocean was subjected to a slowly varying change in freshwater input. All the models analysed show a characteristic hysteresis response of the North Atlantic MOC to freshwater forcing, which can be explained by Stommel’s (1961) salt advection feedback. The width of the hysteresis curve varies between 0.2 and 0.5 Sv in the models. Major differences are found in the location of the present-day climate on the hysteresis diagram. In seven of the models, the present-day climate for standard parameter choices is found in the bi-stable regime, while in the other four models, this climate is situated in the mono-stable regime. The proximity of the present-day climate to Stommel’s bifurcation point, beyond which NADW formation cannot be sustained, varies from less than 0.1 Sv to over 0.5 Sv.

A final example of EMIC intercomparison is discussed in Brovkin et al. (2006). Earth System Models of Intermediate Complexity that explicitly simulate the interactions between atmosphere, ocean and land surface were forced by a reconstruction of land cover changes during the last millennium. In response to historical deforestation of about 18 x 106 km2, all models exhibited a decrease in globally averaged annual mean surface temperature in the range of 0.13°C to 0.25°C, mainly due to the increase in land surface albedo. Further experiments with the models forced by the historical atmospheric CO2 trend reveal that, for the whole last millennium, the biogeophysical cooling due to land cover changes is less pronounced than the warming induced by the elevated atmospheric CO2 level (0.27°C–0.62°C). During the 19th century, the cooling effect of deforestation appears to counterbalance, albeit not completely, the warming effect of increasing CO2 concentration.