Stanhill, G., Institute of Soil Water and Environmental Sciences, ARO Volcani Center, POB6, Bet Dagan 50250, Israel
Cohen, S., Institute of Soil Water and Environmental Sciences, ARO Volcani Center, POB6, Bet Dagan 50250, Israel

Annual values of sunshine duration (SS) measured in Japan between 1890 and 2002 were used as a proxy for global irradiance (Eg↓) to study trends and changes in solar forcing at the Earth's surface. Proxy relationships established for the two SS recorders used in the JMA network both yielded estimates of mean annual values of Eg↓ with RMS < 6%. A first order integrated moving average model (ARIMA) adequately described the time course of SS and Eg↓, which indicated a small, irregular but significant annual increase in solar forcing during the 20th century averaging 0.08 W m-2 or 2.3 hours of Jordan SS recorder sunshine, equivalent to 0.5% per decade. The rate of increase was four times the average in the first four and last three decades of the century reaching a maximum after 1980. The negative effect of the five major volcanic eruptions on Eg↓ was shown to yield a significant linear negative forcing of -41 W m-2 per unit AOD (stratospheric aerosol optical depth). The degree of negative solar forcing was related to latitude: between 25° and 44°N each degree shift to the North was associated with an annual increase in Eg↓ averaging 0.02%. The time course of changes in solar radiation in Japan during the 20th century resembled that measured in air temperature; correlations between annual values of Eg↓ and those in the air temperature of the Northern Hemisphere were very highly significant (P < 0.001) both for the concurrent and preceding year. © 2008, Meteorological Society of Japan.