A new model of the global circuit has been constructed that treats more realistically than previous models the external influences due to varying charged particle fluxes from space, the internal variability due to the transport of radon by atmospheric dynamics, and the varying aerosol populations. An approximate treatment of the effects of explosive volcanic eruptions, that introduce SO2 and H2O into the stratosphere, involves the creation of ultrafine aerosol particles in the downward branch of the Brewer-Dobson circulation from sulfuric acid aerosol particles vaporized in the upward branch. A large increase in stratospheric column resistance occurs at high latitudes, but for much of the time this column resistance is reduced by stratospheric ionizing radiation due to relativistic electron precipitation. Here we present results for the variability of the global circuit due to the variations of the aerosol populations in the stratosphere and troposphere, the variability of the cosmic ray flux, and the geographical and seasonal variability of the near-surface radioactivity. We discuss inferred changes in the circuit parameters associated with climate change, on interannual to Milankovich timescales.