Control runs establish the basic climate of the model. Control runs are long integrations where the model input forcings (solar irradiance, sulfates, ozone, greenhouse gases) are held constant and are not allowed to evolve with time. Usually the input forcings are held fixed either at present day values (i.e., for year 2000 or 2000 Control Run) or a pre-industrial values (i.e., for 1870 or 1870 Control Run). Note that in this context, "fixed" can have two different meanings. The solar forcing values are held fixed a constant, non varying number. The sulfate, ozone and greenhouse gases values, however, are fixed to continually cycle over the same 12-month input dataset every year. The CCSM is then run for an extended period of model time, 100s of years, up to about 1000 years, until the system is close to equilibrium (i.e., with only minor drifts in deep ocean temperature, surface temperature, top-of-the-atmosphere fluxes, etc).
Future greenhouse gas emissions are the product of very complex dynamic systems, determined by driving forces such as demographic development, socio-economic development, and technological change. Their future evolution is highly uncertain. Scenarios are not specific predictions or forecasts of future climate. Rather, scenarios are plausible alternative futures. Each scenario is an example of what can happen under particular assumptions on use of fossil fuel and other human activities. Scenarios assist in climate modeling, help to examine potential climate change and explore vulnerabilities of humans and ecosystems under a changed climate. IPCC Special Report on Emissions Scenarios (SRES) ( Summary for policymakers or Full report) describes these scenarios.
Climate models are an imperfect representation of the earth’s climate system and climate modelers employ a technique called ensembling to capture the range of possible climate states. A climate model run ensemble consists of two or more climate model runs made with the exact same climate model, using the exact same boundary forcings, where the only difference between the runs is the initial conditions. An individual simulation within a climate model run ensemble is referred to as an ensemble member. The different initial conditions result in different simulations for each of the ensemble members due to the nonlinearity of the climate model system. Essentially, the earth’s climate can be considered to be a special ensemble that consists of only one member. Averaging over a multi-member ensemble of model climate runs gives a measure of the average model response to the forcings imposed on the model.
The CCSM IPCC simulations consist of 26 different runs with the CCSM3
• One 500-year control run
• A 5-member ensemble simulating the 1870-2000 historical period
• Four 5-member ensembles corresponding to the IPCC A2, A1B, B1 and constant-20th-Century-forcing future scenarios.
The control run defines the long-term climate of the simulated system. This simulated climate will be slightly different from the actual climate of the earth. Five 1870-2000 simulations were branched from five different points in the 1870 control run and four IPCC scenarios (i.e., A2, A1B, B1 and 20th-Century-constant-emissions) were run into the future starting from the end of each of the five different 1870-2000 historical simulations. The following figures step through this progression.
The 500-year control run can be seen in Figure 1.
In this control run, the model boundary (input) forcings are held constant at values for the year 1870 and the model is integrated forward in time for 500 years. During each year, the model is forced with the same year 1870 values. The natural variability of the simulated climate system will results in a non-constant, time-varying, surface temperature. After the initial adjustment period, the time-varying surface temperature of a balanced model will stay within a control climate envelope of the model denoted by the red and blue lines in Figure 1.
Figure 2 illustrates the manner in which the 5-member 1870-2000 historical ensemble (the five purple lines) was branched from the 1870 control run
The 5-member 1870-2000 historical ensemble is made up of five different runs that were initialized from different years in the 1870 control run and run forward in time to the year 2000. Unlike the 1870 control run, the five historical ensembles received time-evolving boundary forcings that correspond to the observed forcing values during the period from 1870 to 2000. In each case, the simulated climate’s response to the increasing greenhouse gasses in the atmosphere during this period is reflected in an increase of the surface temperatures. However, as the CCSM is a non-linear system, different state of the control run at the five different initial condition year results in 5 slightly different simulations of the 1870-2000 period.
The initial conditions for the five different 1870-2000 simulations correspond to year 360, 380, 400, 420 and 440 of the 1870 control run. The five
different years are differentiated by a letter a-e appended to the case name of the five 1870-2000 simulations. While one of the ensemble members may
most closely resemble the actual earth’s climate from 1870-2000, any of the five ensemble members is a possible climate state during this period.
Four IPCC scenarios were run from the year 2000 to 2100 to simulate a wide range of different future emission choices. These are denoted by the red, green, blue, and tan lines that correspond to the IPCC A2, A1B, A2 and constant 20th century forcing scenarios in the next figure.
For each of the four IPCC scenarios, one run was initialized from year 2000 of each of the five 1870-200 historical simulations as shown in Figure 3.
Again, the five different year-2000 states results in five different realizations of each of the four IPCC scenarios. An ensemble of the IPCC future runs is made up of the five different runs of the same IPCC scenario. In Figure 3, the five members of the A1B ensemble are called out by the five green A1B boxes.
The different forcings of the four IPCC scenarios are reflected in different future model surface temperature responses. The constant 20th century forcing shows the least increase in future surface temperature, the B1 and A1B scenarios displays moderate increases and the A2 scenario results in the largest response. After the year 2100, The B1 and A1B scenarios were extended another 100 years with the forcings fixed at year 2100 values.