Solar history and paleohydrology during the last two millennia

Fluvial paleohydrology provides a link between solar variability and terrestrial climate during the last two millennia by revealing progressive changes in the degree of seasonality which reflect latitudinal shifts in climatic belts. A latitudinally diachronous phase of channel deposition which prevailed in AD 300–1850 in Eurasia between 45° and 25°N indicates more equable stream discharges prompted by a temporary southward displacement of the Atlantic depression zone. A similar effect in central Mexico, between 18° and 28°N, points to changes in the contribution of winter and autumn rains to the annual total which may stem from latitudinal shifts in the high‐pressure cells. The radiocarbon evidence suggests that the displacements coincided with an increase in solar activity that reached a maximum in about AD 600. The proposed association is consistent with evidence from 1948–1966 for increased frontal activity in the North Atlantic during sunspot maxima, although it is not yet clear whether the process was mediated by UV absorption in the lower stratosphere or by the flux of MeV‐GeV particles.