The Madden-Julian oscillation (MJO), as represented by the Max Planck Institute for Meteorology Earth System Model (MPI-ESM), is analyzed for the first time over time periods ranging from decades to more than a millennium. Particular attention is paid to the behavior of the MJO index as calculated from the leading pair of empirical orthogonal functions (EOFs) derived from a multivariate EOF analysis. The analysis of 1000 year simulations with the MPI-ESM and its predecessor reveals significant interannual (2–6 years) to interdecadal (10–20 years) internal variability of the MJO but relatively little evidence of significant variability at longer timescales in unforced runs. A 1200 year experiment forced by the best estimates of solar variability, volcanism, and changing atmospheric composition indicates that the MJO simulated in the twentieth century is very similar to the MJO simulated since AD 800. The analysis of sensitivity experiments shows the influence of different external forcings: solar variability may contribute to MJO variability on 11 and 22 year periods, but this is difficult to separate from internal variability; and there is a hint of enhanced decadal variability associated with volcanic forcing. Land use change and changes associated with anthropogenic forcing over the twentieth century have no detectable effect on the simulated MJO. An increase of the CO2 concentrations by 1% per year starting in AD 1850 leads to an increase in the MJO strength in the twenty-first century, as does the warming associated with an abrupt quadrupling of the atmospheric CO2 concentration, suggesting that the MJO may intensify with warming.