What triggered the Little Ice Age?

The Little Ice Age (LIA) was one of the coldest periods of the postglacial period in the Northern Hemisphere. Although there is increasing evidence that this time interval was associated with weakening of the subpolar gyre (SPG), the sequence of events that led to its weakened state has yet to be explained. Here, we use a recently reconstructed record of Atlantic Multidecadal Variability (AMV) to provide insights into the sequence of events that led to the LIA. We show that the LIA was preceded by an intrusion of warm Atlantic water into the Nordic Seas in the late 1300s. The intrusion was a consequence of persistent atmospheric blocking over the North Atlantic, linked to unusually high solar forcing in times of lower volcanic activity. The warmer water led to the breakup of sea ice and calving of tidewater glaciers. Weakening of the blocking anomaly in the late 1300s allowed the large volume of ice that had accumulated to be exported into the North Atlantic, contributing to the weakening of the Atlantic Meridional Overturning Circulation (AMOC).

The modern spatial fingerprints involving fast AMOC changes are captured by many highly resolved records from around the Atlantic during the transition from the late 1300s to the early 1400s. Paleoclimatic evidence from the Tropics suggest a more northerly Intertropical Convergence Zone (ITCZ) in the late 1300s followed by a rapid southward shift of the ITCZ in the early 1400s, which is consistent with model simulations of the climatic response in the Tropics to a slowdown in AMOC. While this Atlantic intrusion into the Nordic Seas triggered the main phase of the LIA, the cooling condition was maintained by higher volcanic activity in the ensuing decades that was coincident with lower solar irradiance.