20th Century Constant Emissions model run has forcing constituents that are held at year 2000 values. In this scenario the climate system is committed to a certain amount of change based upon the current concentrations of the various constituents in the atmosphere. For more information visit IPCC special report on Emissions Scenarios. You can read Summary for policymakers or Full report.

20th Century Constant Emissions run set is represented by the five ensemble members. 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. For more information about ensemble members see Help.