Less sulphates, cleaner air , more sunshine hitting the ground.

An observed decline of surface shortwave radiation (SSR) in Europe discovered from about 1950s until about the 1980s and many parts of the world is attributed to increasing emissions of anthropogenic aerosols (dimming phase). The followed increase of SSR in some regions (brightening phase) is a consequence of the clean air business in Europe.

The simulations with detailed treatment of aerosols and their interaction with clouds are needed for understanding the regional SSR trends. The NASA GISS ModelE2 is used in this study. It is based on transient simulations with natural and anthropogenic forcings.

We compare two simulations with transient aerosol emissions with the focus on aerosol effects on clouds. For the annual mean SSR, the dimming trends range between -4.4 W/m² over the Mediterranean region and -1.7 W/m² over the middle Europe. Brightening trends range from 5.9 W/m² over France to 2.4 W/m² over Russian European region, also for the annual average SSR.

Both dimming and brightening trends are stronger for summer with larger variability in SSR. The temperature trends are more consistent for summer, e.g. decreasing for dimming and increasing for brightening.

The cooling trends lead to increase in cloud cover for summer dimming, and the warming trend during the summer brightening is accompanied by cloud decreases. The increasing low clouds during the dimming accompanied by the upward tendency in the water cloud optical depth, e.g. an enhancement of negative cloud radiative forcing due to an increase in both cloud fractions and cloud optical depths during dimming.

Dimming periods are characterized by the positive trends in sulfate concentrations. Rapid reduction of sulfate aerosols have started in the last 20-25 years that allowed more solar radiation reach the Earth surface for the brightening periods over Europe.

This is so called direct sulfate aerosol effect on climate, more sulfates during dimming cause more reflection of sun light back to space and less solar flux reaching the ground. Cleaner air from sulfates during brightening allow more solar radiation at the surface and stronger warming. Increase of aerosols during dimming produced the enhancement of the reflective effect of aerosols on clouds while reduction of aerosols during brightening leads to decrease of aerosol effect on clouds.

Less sulphates

More sunshine