  The variable Sun is the most likely candidate for natural forcing of past
climate change on time scales of 50 to 1000 years. Evidence for this
understanding is that the terrestrial climate correlates positively with solar
activity. During the past 10,000 years, the Sun has experienced substantial
variations in activity and there have been numerous attempts to reconstruct
solar irradiance. While there is general agreement on how solar forcing varied
during the last several hundred years --- all reconstructions are proportional
to the solar activity --- there is scientific controversy on the magnitude of
solar forcing. We present a reconstruction of the Total and Spectral Solar
Irradiance covering 130 nm--10 $\mu$m from 1610 to the present with annual
resolution and for the Holocene with 22-year resolution. We assume that the
minimum state of the quiet Sun in time corresponds to the observed quietest
area on the present Sun. Then we use available long-term proxies of the solar
activity, which are $^{10}$Be isotope concentrations in ice cores and 22-year
smoothed neutron monitor data, to interpolate between the present quiet Sun and
the minimum state of the quiet Sun. This determines the long-term trend in the
solar variability which is then superposed with the 11-year activity cycle
calculated from the sunspot number. The time-dependent solar spectral
irradiance from about 7000 BC to the present is then derived using a
state-of-the-art radiation code. We derive a total and spectral solar
irradiance that was substantially lower during the Maunder minimum than
observed today. The difference is remarkably larger than other estimations
published in the recent literature. The magnitude of the solar UV variability,
which indirectly affects climate is also found to exceed previous estimates. We
discuss in details the assumptions which leaded us to this conclusion.
