What was done
The authors "present observational evidence of a strong decline in aerosol optical depth over mainland Europe during the last two decades of rapid warming" -- when air temperatures rose by about 1°C after 1980 -- via analyses of "aerosol optical depth measurements from six specific locations and surface irradiance measurements from a large number of radiation sites in Northern Germany and Switzerland."

What was learned
Ruckstuhl et al. report that "the measurements show a decline in aerosol concentration of up to 60%, which has led to a statistically significant increase of solar irradiance under cloud-free skies since the 1980s." The value of the direct aerosol effect of this radiative forcing is approximately 0.84 Wm^-2; and when combined with the concomitant cloud-induced radiative forcing of about 0.16 Wm^-2, it leads to a total radiative surface climate forcing over mainland Europe of about 1 Wm^-2. This forcing, in the words of the thirteen German and Swiss researchers, "has most probably strongly contributed to the recent rapid warming in Europe."

What it means
Cleaning up significantly polluted skies can provide an even greater impetus for climate warming than does the carbon dioxide that is concurrently emitted to them, as has apparently been the case over mainland Europe for the past quarter century. Hence, it is totally unfair to ascribe the high temperatures the continent has experienced in recent years to the concomitant ongoing rise in the air's CO2 content. The lion's share of the warming has likely been produced by the removal from the atmosphere of true air pollutants.