Trends in the erythemal radiant exposure from re-evaluated measurements (1976–2023) with biometers in Belsk, Poland, and their sources from corresponding ozone, aerosol, and cloud observations

Abstract. The world's longest homogenised series of measured erythemal solar irradiance comes from biometers operating in Belsk, Poland (51.84° N, 20.79° E), in 1976–2023. Linear trends in erythemal radiant exposure (ERE) are calculated for the first (1976–1999) and second (2000–2023) halves of the observation period. A statistically significant increasing trend of 6.9 % per decade was found for annual ERE in the first period. In the second period, only the trend in seasonal (June–August) ERE was statistically significant (3.1 % per decade). The method proposed here to reveal the sources of the ERE trends involves the construction of separate and combined forcings from clear-sky (total column ozone and aerosol optical depth) and cloud proxies (sunshine duration, clearness index). The superposition of these proxy effects over 1985–1999 was the source of the positive trend in annual ERE for the first half of the observations. Before 1985, clear-sky and cloud effects had cancelled each other. The maximum ERE growth rate of 19.4 % per decade over 10 years was found in 1984–1993, with overlapping forcing effects of decreasing ozone and cloudiness responsible. Clear-sky and cloud effects stabilised after 1996 and 2005, respectively. The cloud effect has begun to force a positive trend in the annual ERE again since 2015 due to increasing cloud transparency and/or the disappearance of cloud cover. Comparisons of the performance of linear and non-linear versions of the ERE models show that interaction effects between clear-sky and cloud proxies can be neglected in trend analyses.