Long-term changes in insolation and temperatures at different altitudes

Over the past few years, ground- and space-based atmospheric measurements have revealed a large inter-decadal variability in the amount of radiation reaching the Earth's surface, also known as global dimming and brightening. However, the underlying physical causes of these changes remain unexplained. Clouds and aerosols, or their interactions, could both be responsible for the insolation changes, which in turn may impact the radiative balance of the planet. Here, making use of the special topology and clean environment of the Canary Islands, we compare trends in sunshine duration and temperature series, as a function of altitude. The temperature dataset is constituted by a series of mean, minimum and maximum temperatures, and daily temperature ranges. We find that the insolation and temperature trends are identical at sea level and at more than 2 km height, but the changes in diurnal temperature range are not, suggesting a possible urban heat effect at the sea level location, as well as a possible different influence of clouds and/or aerosols at different altitudes. We also find that during the summer, especially at the high altitude site, there is a clear correspondence between daytime insolation and nighttime cloud-free atmospheric extinction measurements. This suggests that atmospheric aerosol concentrations are the major contributor to the variations in the flux of solar radiation reaching the ground at high altitude sites over the Canary Islands.