Sub-Milankovitch solar forcing of past climates: Mid and late Holocene perspectives

A comparison was performed of solar activity and terrestrial temperature records, both derived from tree rings (i.e., without dating uncertainties), with identification of detailed and highly quantified time-and timescale-dependent characteristics of solar forcing on climate through the current inter glacial in the context of oceanic variability. The tree-ring-derived temperature record from high latitudes of Europe (Lapland) exhibits persistent annual-to-millennial-scale variations, with multidecadal to multicentennial periodicities reminiscent of the Sun's periodicities. At millennial scales, cool temperatures coincided with large-scale glacial maxima. Moreover, millennial and bimillennial modes of climate variability were correlative with variations in sunspot numbers on similar scales, with near-century and near-zero lags, respectively. Although they were subtle in amplitude, the sub-Milankovitch-scale changes in the reception of the Sun's energy could thus suffice to noticeably modulate interglacial climate variations. The relative significance of timescale-dependent, Sun-climate linkages has likely varied during the mid and late Holocene times, respectively. Thus, the warmer and cooler paleotemperatures during the Medieval Climate Anomaly and Little Ice Age were better explained by solar variations on a millennial rather than bimillennial scale. The observed variations may have occurred in association with internal climate amplification (likely, thermohaline circulation and El Nino-Southern Oscillation activity). The near-centennial delay in climate in responding to sunspots indicates that the Sun's in-fluence on climate arising from the current episode of high sunspot numbers may not yet have manifested itself fully in climate trends. If neglected in climate models, this lag could cause an underestimation of twenty-first-century warming trends.