(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.