Dimming of the 17th Century Sun

TL;DR

This paper suggests that non-linear effects in small network faculae could cause greater solar dimming during prolonged minima, potentially explaining the Little Ice Age cooling.

Contribution

It introduces a non-linear relationship between facular area and TSI contribution, highlighting the importance of small network faculae at low activity levels.

Findings

Increased TSI contribution per unit area from small network faculae by a factor of 2-4.

Non-linear relation can produce significant solar dimming during minima.

Mechanism consistent with observed 11-year cycle during Maunder Minimum.

Abstract

Reconstructions of total solar irradiance (TSI) rely mainly on linear relations between TSI variation and indices of facular area. When these are extrapolated to the prolonged 15th - 17th century Sp\"orer and Maunder solar activity minima, the estimated solar dimming is insufficient to explain the mid- millennial climate cooling of the Little Ice Age. We draw attention here to evidence that the relation departs from linearity at the lowest activity levels. Imaging photometry and radiometry indicate an increased TSI contribution per unit area from small network faculae by a factor of 2-4 compared to larger faculae in and around active regions. Even partial removal of this more TSI - effective network at prolonged minima could enable climatically significant solar dimming, yet be consistent with the weakened but persistent 11- yr cycle observed in Be 10 during the Maunder Minimum. The mechanism we suggest would not alter previous findings that increased solar radiative forcing is insufficient to account for 20th century global warming.