The end of the last glacial is marked by abrupt climatic shifts, which gave way to our current warm climate, the Holocene, 11 700 years ago. According to previous geologic evidence temperatures were extremely cold in Europe 12 500–11 700 years ago, during a period termed the Younger Dryas. The severe cold climate during the period has been attributed to increased input of meltwater from the shrinking continental ice sheets to the Atlantic Ocean.
A new study by the University of Helsinki and international collaborators shows that summer temperatures might have been up to ten degrees higher than previously thought. The reason for the surprising result is found from the North Atlantic.
"The glacial meltwaters changed the density-driven Atlantic Meridional Overturning Circulation (AMOC), causing very cold ocean temperatures. Blocking, quasi-stationary high-pressure systems formed on the continent, cutting Europe off the cold Atlantic westerlies in the summer. Summers were warm but short," describes researcher Minna Väliranta from the University of Helsinki.
Atmospheric blocking pattern resembles today’s heat waves
The results are highly relevant to climate change science, because of the worrisome analogy of the mechanism to that of the heat waves of our millennium.
Blocking high-pressure systems cause heat waves on the continent, a phenomenon linked to ocean circulation. For example, scientists have recently reported that meltwaters from the Greenland ice sheet slow down the Gulf Stream.
"Climate warming is particularly pronounced at high latitudes. It has significant and broad implications," says Väliranta.
The current study combines high-resolution climate simulations with climate reconstructions. The reconstructions are based on macroscopic remains and fossil pollen of aquatic plants that have preserved in European lake sediments since the Younger Dryas.
Frederik Schenk, Minna Väliranta, Francesco Muschitiello, Lev Tarasov, Maija Heikkilä, Svante Björck, Jenny Brandefelt, Arne V. Johansson, Jens-Ove Näslund, Barbara Wohlfarth. Warm summers during the Younger Dryas cold reversal. Nature Communications, 2018; 9 (1) DOI:10.1038/s41467-018-04071-5