Differences among total solar irradiance (TSI) estimates are most pronounced during the so-called “ACRIM Gap” of 1989–1991, when available satellite observations disagree in trend and no observations exist from satellites with on-board calibration. Different approaches to bias-correcting noisy satellite observations lead to discrepancies of up to 0.7 W/m2 in the change in TSI during the Gap. Using a Bayesian hierarchical model for Total Solar Irradiance (BTSI), we jointly infer TSI during the ACRIM Gap from satellite observations and proxies of solar activity. In addition, BTSI yields estimates of noise and drift in satellite observations and calibration for proxy records. We find that TSI across the ACRIM Gap changes by only 0.01 W/m2, with a 95% confidence interval of [-0.07, 0.09] W/m2. Our results are consistent with the PMOD CPMDF and Community Consensus TSI reconstructions and inconsistent with the 0.7 W/m2 trend reported in the ACRIM composite reconstruction. Constraints on the trend across the ACRIM Gap are primarily obtained through constraints on the drift in the Nimbus-7 satellite that are afforded by overlapping satellite and proxy observations.

Spectral analyses of the Greenland Ice Sheet Project 2 (GISP2) δ18O record has been interpreted to show a 1/(1470 yr) spectral peak that is highly statistically significant (p<0.01) relative to a null model, H0, consisting of an auto-regressive order one process. In this study H0 is generalized using an auto-regressive moving-average process that is removed from the GISP2 δ18O time series, yielding a spectral estimate that is approximately level. A rule of thumb is proposed for evaluating the adequacy of H0 involving comparing the expected and observed variance of the logarithm of a leveled spectral estimate. After suitably leveling GISP2 δ18O and accounting for multiple hypothesis testing, multitaper spectral estimation indicates that the 1/(1470 yr) peak is insignificant (p>0.05). The proposed techniques for fitting H0 should be generally applicable to evaluating peaks in other geophysical spectral estimates.