Re-evaluating solar irradiance reconstructions: No evidence of large secular trends

Constraining the long-term variations in total solar irradiance (TSI) is important for assessing the Sun's influence on Earth's climate. -2 $), while two models suggest substantially larger changes of 2.1--5.9 W,m$^ -2 . One of these models is the semi-empirical Code for the High spectral ResolutiOn recoNstructiOn of Solar irradiance (CHRONOS), while the other is an independent empirical reconstruction. Although the two models differ substantially in architecture, both use cosmogenic isotope records combined with neutron monitor data to describe the long-term irradiance variability. However, cross-calibrating modulation potential reconstructions from cosmogenic isotopes against neutron monitor data remains highly uncertain. We reassess the origin and magnitude of the secular trends inferred by the two current TSI reconstructions exhibiting the largest secular variability. We update both reconstructions using recent heliospheric modulation potential and open solar flux reconstructions based on geomagnetic data and neutron monitor measurements, which provide a more reliable connection between cosmogenic isotope and neutron monitor records. We further apply a more robust smoothing methodology for the long-term series. These allow the secular component of the reconstructions to be extended consistently to the satellite era and for comparison of the resulting TSI reconstructions with direct TSI measurements. We find that both original reconstructions substantially overestimate the secular variability in irradiance. -2 $ between the 1700 and 1986 minima, while the second reconstruction returns about 0.2--0.25 W m$^ -2 . Our analysis indicates that the previously inferred large secular trends arose primarily from improper linking of cosmogenic isotope and neutron monitor records as well as issues in the adopted smoothing approach. -2 , consistent with the majority of current irradiance models.