Long-term trends in solar irradiance in Ireland and their potential effects on gross primary productivity

Long-term trends in total solar irradiance (Rs), diffuse irradiance (Rd) and pan evaporation (Epan), from various sites in Ireland, were examined over the last 20–50 years. This information was used to estimate the impact of changes in Rs and Rd on gross primary productivity (GPP) of forest, arable and grassland ecosystems. Analysis of the data indicated a significant (P < 0.0001) reduction in Rs of 19% (27.7 MJ m−2 year−1) and Rd by 16% (12.2 MJ m−2 year−1) from 1955 to 1984. However, we show Rs has remained unchanged, or even increased at some sites since the mid-1980s, whilst Rd has continued to decrease. Decreases in measured and calculated (Preistley–Taylor function) Epan between 1955 and 1984 were consistent with the observed changes in Rs. Long-term changes in Rs were associated with variations in atmospheric optical properties, as evident from a change in the slope of the relationship between the diffuse fraction (Rd:Rs) and the clearness index (Rs:Ro). At Dublin Airport an increase in Rs and Rs:Ro since the 1980s was consistent with a decrease in black smoke and sulphate concentrations. We show that combined alterations in Rs and Rd, over the last 50 years, had only a small impact on annual GPP of temperate terrestrial ecosystems (a decrease of 0.5 to 1.7 t C ha−1 decade−1). These reductions in GPP were larger in arable and grassland systems, where productivity is light-limited and particularly sensitive to changes in Rd:Rs.