TIER1 CCSM4 20th Century (1850-2005)

AR5 Case Specifications

(Taken directly from https://cmip.llnl.gov/cmip5/docs/Taylor_CMIP5_design.pdf)

Purposes and key diagnostics: 
3.2 Historical (mid-1800's - 2005) 
   a) Evaluate model performance against present climate and observed climate change. 
   b) Provides initial conditions for future scenario experiments 
   c) Enables detection and attribution studies - evaluation of human 
      impact on past climate (see expts. 7.1-7.3). 
   d) For models with full representation of the carbon cycle, the surface fluxes 
      of CO2 will be saved in order to calculate allowable emissions implied by 
      the prescribed changes in atmospheric CO2 and the uptake/release of CO2 
      by the oceans and terrestrial biosphere.  The separate effects on these 
      surface fluxes of climate change alone (i.e., the carbon-climate  feedback) 
      and CO2 concentration changes alone can be estimated by comparing  the 
      allowable emissions in expts. 3.2 and 4.2 with those found in expts. 5.4 
      and 5.5.

3.2-E  Historical ensemble 
   a) Better isolate the externally-forced response from total response (which is 
      of particular importance in so-called detection and attribution studies), 
      and obtain an estimate of the "unforced" variability as a residual. 
   b) Enables assessment of statistical significance of differences between 
      simulated and observed fields and between different simulations 
   c) Better determine evolving climatology and the statistics of rare events.

Further notes and issues that need to be considered include the following: 
  2) The simulations in Table 3 are referred to as prescribed "concentration" runs since 
     the well-mixed gases like CO2 will be prescribed, not calculated from emissions.  
     Other gases (e.g., ozone) might also be prescribed, but perhaps as a function of 
     altitude, latitude, longitude, and month of year (i.e., seasonally varying).  In some 
     models reactive species might be calculated with simple chemistry models, while 
     in others they might be prescribed.  The same is true of aerosol species.  
  3) Specified land-use changes will be supplied to the modeling groups for 20th and 
     21st century climates, but the implementation of these datasets and whether or not 
     to include dynamic vegetation is up to the individual modeling groups.  
  4) Care must be taken in accounting for volcanic eruptions that occurred prior to 
     1850 and also in the future because this can especially impact sea level changes, 
     which respond on multi-century time-scales.  If we completely neglect volcanoes 
     prior and after the historical period, then we shall exaggerate their effect on the 
     historical sea level record because during this period the average forcing will 
     become negative (relative to the pre-industrial control).  If we include a 
     background volcanic aerosol forcing in the pre-industrial control run, then the 
     same background aerosol should probably be included in the future runs, 
     otherwise there would be a slight exaggeration in the warming (and in sea level 
     increases) throughout the future runs, which would almost certainly be unrealistic.  
     However, imposing a background volcanic aerosol instantaneously in year 2006 
     of the "future" runs (see Table 1) would also be unrealistic because there were no 
     major volcanic eruptions in 2006.  It is recommended that either volcanic aerosols 
     should be omitted entirely from both the control and future runs, or, alternatively, 
     the same background aerosol should be prescribed in both runs.  
  5) It is recommended that some representation of the solar cycle be included in the 
     20th and 21st century simulations, though that is left up to the discretion of the 
     modeling groups.    
  9) For groups choosing to specify (rather than calculate) the time-varying and 
     evolving ozone concentrations, the most accurate option is to rely on a three 
     dimensional (latitude, altitude, time) monthly mean ozone time series based on 
     observations wherever available and based on model output for the period pre- 
     1970 and in the future (consistent with the chosen RCP). Two options will be 
     made available for use in CMIP5: 
     # Option 1: A merged observationally-based and model-based dataset.  
       i. For the well-observed period (1979-2006): An activity under the auspices 
          of SPARC will create a consensus observational stratospheric ozone 
          database. The monthly mean database will be zonal means (5zones) with 
          global coverage, extending from the tropopause to 70 km at high vertical 
          resolution (~1 km), and spanning the period 1979 to 2006 with no missing 
          values. A fixed monthly mean tropospheric ozone climatology, on the 
          same zonal and vertical grid, and representative of the period 1979 to 
          2006, will be appended to the transient stratospheric ozone fields to 
          provide a seamless database. While this approach can be expected to 
          provide the most accurate past stratospheric ozone forcing, fixed 
          tropospheric concentrations are of course unrealistic and clearly cannot 
          reproduce time-varying tropospheric ozone radiative forcing. 
      ii. For the "historical" period (1850-2006):  Regression coefficients will be 
          calculated for halocarbon effects (EESC) and/or linear trend and various 
          known natural forcings (volcanic aerosol, solar, ENSO, QBO). The 
          regression coefficients will be used to extrapolate that data back in time, 
          and form a stratospheric ozone time series backward to cover the entire 
          time period 1850-2006.  
     iii. For the future (2007 and beyond): A similar procedure could be used to 
          extrapolate into the future, and would capture changes due to halocarbons 
          which will be an important driver of future ozone behavior. However, 
          coupled chemistry climate model (CCM) simulations9 indicate that future 
          stratospheric ozone abundance is likely to be significantly affected by 
          climate change, and it is not yet possible to estimate this contribution 
          statistically from observations. Therefore, the SPARC CCMVal activity is 
          proposing to provide a stratospheric dataset for CMIP5 that extends the 
          observational database into the future, based on CCM simulations that 
          include the effects of climate change as well as halocarbon changes. 
     # Option 2: An entirely model-based dataset: A model-based vertically resolved, 
       monthly mean, full atmosphere ozone and tropospheric aerosol database from 
       1850 to 2150 from CCM simulations for the entire time period, past and 
       future, will be provided by AC&C activity 4. This has the advantage of being 
       a physically consistent model dataset throughout time and space and including 
       responses to all relevant forcings/composition changes such as methane and 
       nitrous oxide trends since the pre-industrial. However, the models that have 
       thus far expressed willingness to provide output to this activity are models that 
       in general emphasize the troposphere, placing therefore less emphasis and 
       computational resources on stratospheric physics and chemistry.