Is There a Secular Variation of the Total Solar Irradiance?

This most basic question about TSI has competing answers, based on the same time series of total irradiance measurements that began in the late 1970s with Nimbus-7 (Hickey et al, 1980). The essential problem is the "ACRIM gap," when the overlapping TSI data was sparse and ill-characterized for sensor degradation. Willson and Mordvinov (2003) consider only the TSI measurements from space, shown below.


They assert that a secular increase in total irradiance is required to link the observations at the sunspot minima of the 1980s and 1990s.

Fröhlich and Lean (1998) come to the opposite conclusion using detailed proxy records to bridge the "ACRIM GAP" smoothly:
   

This controversy has enormous significance for human society and for our understanding of the Sun. Secular variation cannot be proven or ruled out by present observations. Our proposed flight will take place near the next solar minimum, ideal for a sensitive search for sources of TSI variation.

Hickey, P., Stoer, L. L., Jacobowitz, H., Pellegrino, P., Maschhoff, R. H., House, F., and Haar, T. HJ., Initial Solar Irradiance Determinations ftom Nimbus 7 Cavity Radiometer Measurements, Science, 208, 281 (1980).

Wilson, R. C., Mordivov, A. V., Secular tatal solar irradiance trend during solar cycles 21-23, Gaophys. Rev. Lett., 30, 1199 (2003).

Frölich, C., and Lean, J., The Sun's total irradiance: cycles and trends in the past two decades and associated climate change uncertainities, Geophys. Res. Lett., 25, 4377 (1998).