Barbara Wohlfarth and Frederik Schenk during field work.
Barbara Wohlfarth and Frederik Schenk during field work. The ancient lake site of Hässeldala in southern Sweden is one of the sites used in their study to reconstruct summer temperatures in the past from lake sediment cores. Photo: Laura Parducci

Record-breaking cold ocean temperatures across the central North Atlantic in recent years suggest that the northward oceanic heat transport through the Atlantic Meridional Overturning Circulation (AMOC) has reached a long-term minimum and might be the weakest for at least 1,000 years. Most climate models project that the already observed slowdown will continue under global warming scenarios although a complete collapse appears unlikely – at least based on current models.

However, the climate history tells that we do not need to wait for a complete collapse to get a drastic climate response. A new study published in Nature Communications investigates how the strong cooling of the North Atlantic Ocean in response to a partial collapse of the AMOC impacted the European climate around 12 000 years ago. The period, called the Younger Dryas (YD), is the latest and one of the most extreme rapid cooling events that occurred during a phase of rapid warming at the climate transition from the late-glacial to our current warm climate, the Holocene.

Although the last major climate shock in response to a partial collapse of the AMOC 12 000 years ago took place under quite different conditions, new climate model simulations emphasize a potentially worrisome analogy to today’s situation. First, the AMOC collapse takes place during a time of rapid warming. Second, while winters become extremely cold, our study shows that European summers might get even warmer. While warm summers do not sound bad, the mechanism leading to this additional warming is responsible for several of the worst heat waves and droughts in Europe.

The simulation demonstrates that the mechanism behind this “cold-ocean-warm-summer” feedback is related to so-called atmospheric blocking. This blocking consists of high pressure systems which become quasi-stationary in their position for at least five days or even several weeks. These weather patterns are known to lead to extreme warming and drought in summer or extreme cold waves during winter. These high pressure systems cut Europe off from the warming westerlies in winter or the cooling westerlies in summer and cause the most extreme heat or cold waves. The simulation suggests now that a very cold North Atlantic leads to intensified atmospheric blocking 12 000 years ago and warm summers. The latter is confirmed by new geological evidence in the same study.

“If the slowdown of the Atlantic overturning proceeds as observed and projected by most climate models for the future, atmospheric blocking might increase in intensity and/or frequency and could lead to even stronger heat waves than we would expect from the gradual warming trend in response to greenhouse gases.” says Frederik Schenk, lead author of the study and researcher at Bolin Centre for Climate Research, Stockholm University.

The study was conducted by an international group of researchers and received funding from the Swedish Nuclear Fuel and Waste Management Company (SKB).

Article in Nature Communications: Schenk et al. (2018) Warm summers during the Younger Dryas cold reversal