A Bayesian Model for Inferring Total Solar Irradiance From Proxies and Direct Observations: Application to the ACRIM Gap

Abstract

Differences among total solar irradiance (TSI) estimates are most pronounced during the so‐called “ACRIM Gap” of 1989–1991, when available satellite‐based observations disagree in trend and no observations exist from satellites with on‐board calibration. Different approaches to bias‐correcting noisy satellite‐based 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 TSI (BTSI), we jointly infer TSI during the ACRIM Gap from satellite‐based observations and proxies of solar activity. In addition, BTSI yields estimates of noise and drift in satellite‐based 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.