Solar radiation changes in Japan during the 20th century: Evidence from sunshine duration measurements

Annual values of sunshine duration (SS) measured in Japan between 1890 and 2002 were used as a proxy for global irradiance (E-g down arrow) 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 E-g down arrow with RMS < 6%. A first order integrated moving average model (ARIMA) adequately described the time course of SS and E-g down arrow, which indicated a small, irregular but significant annual increase in solar forcing during the 20(th) 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 E-g down arrow 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 degrees and 44 degrees N each degree shift to the North was associated with an annual increase in E-g down arrow averaging 0.02%. The time course of changes in solar radiation in Japan during the 20(th) century resembled that measured in air temperature; correlations between annual values of E-g down arrow and those in the air temperature of the Northern Hemisphere were very highly significant (P < 0.001) both for the concurrent and preceding year.