Placing limits on long-term variations in quiet-Sun irradiance and their contribution to total solar irradiance and solar radiative forcing of climate

Abstract

Recent reconstructions of Total Solar Irradiance (TSI) postulate that quiet Sun variations could give significant changes to solar power input to Earth’s climate (radiative climate forcings of 0.7-1.1Wm-2 over 1700-2019) arising from changes in quiet-Sun magnetic fields that have not, as yet, been observed. Reconstructions without such changes yield solar forcings that are smaller by a factor of more than 10. We study the quiet-Sun TSI since 1995 for three reasons: (1) this interval shows rapid decay in average solar activity following the grand solar maximum in 1985 (such that activity in 2019 was broadly equivalent to that in 1900); (2) there is improved consensus between TSI observations; and (3) it contains the first modelling of TSI that is independent of the observations. Our analysis shows the most likely upward drift in quiet-Sun radiative forcing since 1700 is between +0.07 and 0.13Wm-2. Hence we cannot yet discriminate between the quiet-Sun TSI being enhanced or reduced during the Maunder and Dalton sunspot minima, although there is a growing consensus from the combinations of models and observations that it was slightly enhanced. We present reconstructions that add quiet-Sun TSI and its uncertainty to models that reconstruct the effects of sunspots and faculae