New evidence on the dimming/brightening phenomenon and decreasing diurnal temperature range in Iran (1961–2009)



For a better understanding of multidecadal climate change, as well as for the production of solar power, there is a growing need for knowledge of the trends in incident sunlight at the Earth's surface, but a lack of a long-term sunlight time series dictates that a proxy measure is needed. In this study, variations of sunshine duration and diurnal temperature range (DTR) are used as proxies for surface solar radiation. Annual and seasonal composites of both variables from 29 stations are analyzed from 1961 through 2009 across the different types of climates of Iran. The annual sunshine duration mean time series shows a decrease from the early 1960s to the late 1970s, in line with the widespread dimming of surface solar radiation observed during this period. By the early 1980s, there is an increase in sunshine through the end of the 20th century, aligning with a well-known and well-documented brightening period. In addition, a renewed dimming is observed during the 2000s, with a sharp drop in 2009. A linear trend estimated over the 1961–2009 period was not found to be statistically significant. However, the annual DTR time series shows a widespread and statistically significant decrease since the 1960s, although the series ends without relevant variations after the 1990s. An agreement in the interannual variability of sunshine and DTR is observed except for the summer season. On decadal time scales, only the spring DTR series shows a partial agreement with sunshine series. Nevertheless, the recent leveling off in the DTR series supports a transition in the radiative regime.