How Dim Can the Sun Be?

Author: Philip R. Goode
Big Bear Solar Observatory

The Earth’s climate depends on the Sun’s radiative output and the Earth’s reflectance. Variations in the Sun’s radiative output have been carefully measured from space for more than twenty years. The Sun’s irradiance is about 0.1% greater at activity maximum. It is often argued that over the Holocene the Sun has been appreciably dimmer (upwards of 0.5%) than it is now at activity minimum. Is this possible? Recent information from the helioseismic detectors aboard SOHO have shed light on the physical origin of the evolution revealing that the Sun is probably cooler when its radiative output is greatest. The results also imply a sharp lower limit on the variation of the solar output. This limit implies that if the Sun caused the net sunlight reaching Earth to be lower in the recent past, the effect is indirect rather than direct. Combined with data from SOHO (later SDO), SORCE will provide essential data to advance our understanding of the connections between the Sun’s short timescale and historical timescale variations.