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

6.6.3 Comparisons of Millennial Simulations with Palaeodata

A range of increasingly complex climate models has been used to simulate NH temperatures over the last 500 to 1,000 years using both natural and anthropogenic forcings (Figure 6.13). These models include an energy balance formulation (Crowley et al., 2003, Gerber et al., 2003), two- and three-dimensional reduced complexity models (Bertrand et al., 2002b; Bauer et al., 2003), and three fully coupled AOGCMs (Ammann et al., 2003; Von Storch et al., 2004; Tett et al., 2007).

Comparison and evaluation of the output from palaeoclimate simulations is complicated by their use of different historical forcings, as well as by the way indirect evidence of the history of various forcings is translated into geographically and seasonally specific radiative inputs within the models. Some factors, such as orbital variations of the Earth in relation to the Sun, can be calculated accurately (e.g., Berger, 1977; Bradley et al., 2003b) and directly implemented in terms of latitudinal and seasonal changes in incoming shortwave radiation at the top of the atmosphere. For the last 2 kyr, although this forcing is incorporated in most models, its impact on climate can be neglected compared to the other forcings (Bertrand et al., 2002b).

Figure 6.13

Figure 6.13. Radiative forcings and simulated temperatures during the last 1.1 kyr. Global mean radiative forcing (W m–2) used to drive climate model simulations due to (a) volcanic activity, (b) solar irradiance variations and (c) all other forcings (which vary between models, but always include greenhouse gases, and, except for those with dotted lines after 1900, tropospheric sulphate aerosols). (d) Annual mean NH temperature (°C) simulated under the range of forcings shown in (a) to (c), compared with the concentration of overlapping NH temperature reconstructions (shown by grey shading, modified from Figure 6.10c to account for the 1500 to 1899 reference period used here). All forcings and temperatures are expressed as anomalies from their 1500 to 1899 means and then smoothed with a Gaussian-weighted filter to remove fluctuations on time scales less than 30 years; smoothed values are obtained up to both ends of each record by extending the records with the mean of the adjacent existing values. The individual series are identified in Table 6.2.

Table 6.2. Climate model simulations shown in Figure 6.13.

Series  Modela  Model type  Forcingsb  Reference  
GSZ2003  ECHO-G  GCM  SV -G - - - -  González-Rouco et al., 2003  
ORB2006  ECHO-G/MAGICC  GCM adj. using EBMc  SV -G -A -Z  Osborn et al., 2006  
TBC..2006  HadCM3  GCM  SVOG -ALZ  Tett et al., 2007  
AJS..2006  NCAR CSM  GCM  SV -G -A -Z  Mann et al., 2005b  
BLC..2002  MoBiDiC  EMIC  SV -G -AL -  Bertrand et al., 2002b  
CBK..2003  -  EBMc  SV -G -A - -  Crowley et al., 2003  
GRT..2005  ECBilt-CLIO  EMIC  SV -G -A - -  Goosse et al., 2005b  
GJB..2003  Bern CC  EBMc  SV -G -A -Z  Gerber et al., 2003  
B..03-14C  Climber2  EMIC (solar from 14C)  SV - -C -L -  Bauer et al., 2003  
B..03-10Be  Climber2  EMIC (solar from 10Be)  SV - -C -L -  Bauer et al., 2003  
GBZ..2006  ECHO-G  GCM  SV -G - - - -  González-Rouco et al., 2006 
SMC2006  ECHAM4/OPYC3  GCM  SV -G -A -Z  Stendel et al., 2006 


a Models: ECHO-G = ECHAM4 atmospheric GCM/HOPE-G ocean GCM, MAGICC = Model for the Assessment of Greenhouse-gas Induced Climate Change, HadCM3 = Hadley Centre Coupled Model 3; NCAR CSM = National Center for Atmospheric Research Climate System Model, MoBiDiC = Modèle Bidimensionnel du Climat , ECBilt-CLIO = ECBilt-Coupled Large-scale Ice Ocean, Bern CC = Bern Carbon Cycle-Climate Model, CLIMBER2 = Climate Biosphere Model 2, ECHAM4/OPYC3 = ECHAM4 atmospheric GCM/Ocean Isopycnal GCM 3.

b Forcings: S = solar, V = volcanic, O = orbital, G = well-mixed greenhouse gases, C = CO2 but not other greenhouse gases, A = tropospheric sulphate aerosol, L = land use change, Z=tropospheric and/or stratospheric ozone changes and/or halocarbons.

c EBM = Energy Balance Model.

Over recent millennia, the analysis of the gas bubbles in ice cores with high deposition rates provides good evidence of greenhouse gas changes at near-decadal resolution (Figure 6.4). Other factors, such as land use changes (Ramankutty and Foley, 1999) and the concentrations and distribution of tropospheric aerosols and ozone, are not as well known (Mickley et al., 2001). However, because of their magnitude, uncertainties in the history of solar irradiance and volcanic effects are more significant for the pre-industrial period.