Twentieth century secular decrease in the atmospheric potential gradient

[1] Current flowing in the global atmospheric electrical circuit (AEC) substantially decreased during the twentieth century. Fair-weather potential gradient (PG) observations in Scotland and Shetland show a previously unreported annual decline from 1920 to 1980, when the measurements ceased. A 25% reduction in PG occurred in Scotland 1920-50, with the maximum decline during the winter months. This is quantitatively explained by a decrease in cosmic rays (CR) increasing the thunderstorm-electrosphere coupling resistance, reducing the ionospheric potential V-I. Independent measurements of V-I also suggest a reduction of 27% from 1920-50. The secular decrease will influence fair weather atmospheric electrical parameters, including ion concentrations and aerosol electrification. Between 1920-50, the PG showed a negative correlation with global temperature, despite the positive correlation found recently between surface temperature and V-I. The 1980s stabilisation in V-I may arise from compensation of the continuing CR-induced decline by increases in global temperature and convective electrification.