Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea‐ice/ocean feedbacks

Abstract

Northern Hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes. However, the causes of superposed century‐scale cold summer anomalies, of which the Little Ice Age (LIA) is the most extreme, remain debated, largely because the natural forcings are either weak or, in the case of volcanism, short lived. Here we present precisely dated records of ice‐cap growth from Arctic Canada and Iceland showing that LIA summer cold and ice growth began abruptly between 1275 and 1300 AD, followed by a substantial intensification 1430–1455 AD. Intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium. A transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times, and that cold summers can be maintained by sea‐ice/ocean feedbacks long after volcanic aerosols are removed. Our results suggest that the onset of the LIA can be linked to an unusual 50‐year‐long episode with four large sulfur‐rich explosive eruptions, each with global sulfate loading >60 Tg. The persistence of cold summers is best explained by consequent sea‐ice/ocean feedbacks during a hemispheric summer insolation minimum; large changes in solar irradiance are not required.

An abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type episodes

“…These results contrast with previous studies in which similar abrupt events were attributed to the volcanic forcing alone in climate simulations of the past millennium (Miller et al 2012;Schleussner and Feulner 2013;Schleussner et al 2015). The sensitivity experiments used here, however, still include changes in the other imposed external forcings (see Sect.…”

“…The NAO reconstruction of Trouet et al 2009 was nevertheless contested in a later study Lehner et al (2012), and Ortega et al (2015) presented an alternative NAO reconstruction in which no phase shift was found between the MCA and LIA. Further uncertainty arises from the fact that the previous studies attributed these North Atlantic climate anomalies during the LIA to either internal climate variability (Drijfhout et al 2013) or external forcing, including a series of strong, decadally-paced volcanic eruptions (e.g., Miller et al 2012;Schleussner and Feulner 2013) and changes in the incoming solar radiation during the thirteenth century (Lehner et al 2013). …”

“…A reduction in the meridional heat transported by the AMOC was invoked in earlier studies to explain anomalous cold conditions in the North Atlantic reconstructed during the LIA (e.g., Lund et al 2006;Miller et al 2012;Lehner et al 2013). This hypothesis, however, found no support in a recent reconstruction of the AMOC strength (Rahmstorf et al 2015).…”

“…Sensitivity experiments without volcanic forcing can nonetheless produce similar abrupt events; a necessary causal link between the volcanic cluster and the SPG shift can therefore be excluded. Instead, preconditioning by internal variability explains 2014; Ran et al 2011;Miettinen et al 2012Miettinen et al , 2015MoffaSánchez et al 2014a) and salinity in the North Atlantic (e.g., Jensen et al 2004;Moffa-Sánchez et al 2014a), summer temperatures of northeastern North America (Gennaretti et al 2014), ice cap growth of the Arctic Canada or off Iceland and East Greenland (Massé et al 2008;Miller et al 2012;Miettinen et al 2015), and of the inflow of Atlantic waters into the Nordic Seas (Dylmer et al 2013). Yet attribution and the physical mechanism responsible for such an abrupt SPG weakening remain unclear, as well as the connection between this event and the other climate changes reconstructed in the North Atlantic during the LIA.…”

Assessing reconstruction techniques of the Atlantic Ocean circulation variability during the last millennium

“…The pseudoreconstructed FCT, however, underestimates/overestimates and reconstructions possible (e.g., Jansen et al 2007;Fernández-Donado et al 2013;Stocker et al 2014). Climate simulations have also been used to explore the importance of external forcing and internal climate variability to explain the most relevant climate excursions of the last millennium, such as the Medieval Climate Anomaly (MCA, 950-1250) and the Little Ice Age (LIA, 1450-1850) (e.g., Miller et al 2012;Pages 2k Consortium 2013).…”