Was there a volcanic-induced long-lasting cooling over the Northern Hemisphere in the mid-6th–7th century?

Abstract

The climate of the Northern Hemisphere (NH) in the mid-6th century was one of the coldest during the last 2 millennia based on multiple paleo-proxies. While the onset of this cold period can be clearly connected to the volcanic eruptions in 536 and 540 Common Era (CE), the duration, extent, and magnitude of the cold period are uncertain. Proxy data are sparse for the first millennium, which compounds the uncertainties of the reconstructions. To better understand the mechanisms of the prolonged cooling, we analyze new transient simulations over the Common Era and enhance the representation of mid-6th to 7th century climate by additional ensemble simulations covering 520–680 CE. We use the Max Planck Institute Earth System Model to apply the external forcing as recommended in the Paleoclimate Modelling Intercomparison Project phase 4. After the four large eruptions in 536, 540, 574, and 626 CE, a significant mean surface climate response in the NH lasting up to 20 years is simulated. The 2 m air temperature shows a cooling over the Arctic in winter, corresponding to the increase in Arctic sea ice, mainly in the Labrador Sea and to the east of Greenland. The increase in sea-ice extent relates to a decrease in the northward ocean heat transport into the Arctic within the first 2 years after the eruptions and to an increase in the Atlantic meridional overturning circulation, which peaks 10 years after the eruptions. A decrease in the global ocean heat content is simulated after the eruptions that does not recover during the simulation period. These ocean–sea-ice interactions sustain the surface cooling, as the cooling lasts longer than is expected solely from the direct effects of the volcanic forcing, and are thus responsible for the multi-decadal surface cooling. In boreal summer, the main cooling occurs over the continents at midlatitudes. A dipole pattern develops with high sea level pressure and a decrease in both precipitation and evaporation poleward of 40∘ N. In addition, more pronounced cooling over land compared to ocean leads to an enhanced land–sea contrast. While our model ensemble simulations show a similar ∼20-year summer cooling over NH land after the eruptions as tree ring reconstructions, a volcanic-induced century-long cooling, as reconstructed from tree ring data, does not occur in our simulations.