[1] The rapid temperature increase of 1°C over mainland Europe since 1980 is considerably larger than the temperature rise expected from anthropogenic greenhouse gas increases. Here we present aerosol optical depth measurements from six specific locations and surface irradiance measurements from a large number of radiation sites in Northern Germany and Switzerland. The measurements show a decline in aerosol concentration of up to 60%, which have led to a statistically significant increase of solar irradiance under cloud-free skies since the 1980s. The measurements confirm solar brightening and show that the direct aerosol effect had an approximately five times larger impact on climate forcing than the indirect aerosol and other cloud effects. The overall aerosol and cloud induced surface climate forcing is $+1 W m À2 dec À1 and has most probably strongly contributed to the recent rapid warming in Europe. Citation: Ruckstuhl, C., et al.

Abstract. Small changes in the radiation budget at the earth's surface can lead to large climatological responses when persistent over time. With the increasing debate on anthropogenic influences on climatic processes during the 1980s the need for accurate radiometric measurements with higher temporal resolution was identified, and it was determined that the existing measurement networks did not have the resolution or accuracy required to meet this need. In 1988 the WMO therefore proposed the establishment of a new international Baseline Surface Radiation Network (BSRN), which should collect and centrally archive high-quality ground-based radiation measurements in 1 min resolution. BSRN began its work in 1992 with 9 stations; currently (status 2018-01-01), the network comprises 59 stations (delivering data to the archive) and 9 candidates (stations recently accepted into the network with data forthcoming to the archive) distributed over all continents and oceanic environments. The BSRN database is the World Radiation Monitoring Center (WRMC). It is hosted at the Alfred Wegener Institute (AWI) in Bremerhaven, Germany, and now offers more than 10 300 months of data from the years 1992 to 2017. All data are available at https://doi.org/10.1594/PANGAEA.880000 free of charge.

[1] Mainland Europe's temperature rise of about 1°C since the 1980s is considerably larger than expected from anthropogenic greenhouse warming. Here we analyse shortwave and longwave surface forcings measured in Switzerland and Northern Germany and relate them to humidity-and temperature increases through the radiation-and energy budget. Shortwave climate forcing from direct aerosol effects is found to be much larger than indirect aerosol cloud forcing, and the total shortwave forcing, that is related to the observed 60% aerosol decline, is two to three times larger than the longwave forcing from rising anthropogenic greenhouse gases. Almost tree quarters of all the shortwave and longwave forcing energy goes into the turbulent fluxes, which increases atmospheric humidity and hence the longwave forcing by water vapour feedback. With anthropogenic aerosols now reaching low and stable values in Europe, solar forcing will subside and future temperature will mainly rise due to anthropogenic greenhouse gas warming. Citation: Philipona, R., K. Behrens, and C. Ruckstuhl

developed by PMOD/WRC took part in these intercomparisons. The internal consistency of the IRIS radiometers and the agreement with the ACP were within ±1 W m −2 , providing traceability of atmospheric longwave irradiance to the international system of units with unprecedented accuracy. Measurements performed during the two field campaigns and over the past 4 years have shown that the World Infrared Standard Group (WISG) of pyrgeometers is underestimating clear-sky atmospheric longwave irradiance by 2 to 6 W m −2 , depending on the amount of integrated water vapor (IWV). This behavior is an instrument-dependent feature and requires an individual sensitivity calibration of each pyrgeometer with respect to an absolute reference such as IRIS or ACP. For IWV larger than 10 mm, an average sensitivity correction of +6.5% should be applied to the WISG in order to be consistent with the longwave reference represented by the ACP and IRIS radiometers. A concerted effort at international level will need to be implemented in order to correct measurements of atmospheric downwelling longwave irradiance traceable to the WISG.