Identifying climate variables that interchange with volcanic eruptions as cooling forces during the Common Era’s ice ages

Volcanism is known to be an instigating factor for the Late Antique Little Ice Age (LALIA, 536–660) and the Little Ice Age (LIA, 1250–1850), but little is known about when the effect of volcanism ends, and which other mechanisms prolong a cold period that includes the ice-ages’ cold periods, but also continued periods with persistent cooling. Here we show, with a high-resolution lead-lag method, where the stratospheric aerosol optical depth (SAOD) generated by volcanic emissions ceases to precede the Northern Hemisphere summer temperature (NHST). We find that five climate mechanisms cool the Northern Hemisphere (percentage time in parentheses): SAOD (51 %), total solar irradiance (TSI, 2 %), the North Atlantic oscillation (NAO, 11 %), the interdecadal Pacific oscillation (IPO, 28 %) and CO2 (16 %). The last four variables overlap, and altogether the five climate variables cover 89 % of the cold period that includes LALIA and LIA. In contrast, we find an increase in atmospheric CO2 over a brief period just after large volcanic eruptions. During the cold period, the five variables lead NHST, are in a cooling mode, and have sufficient strength to cool the Northern Hemisphere.