The Younger Dryas was a period (12,900–11,600 BP) towards the end of the last glaciation when glaciers re-advanced in Scotland and the tundra plants, including the eponymous Dryas, replaced the Bølling-Allerød forests in Denmark. These changes indicate the Younger Dryas was a cold interval in Europe, an interpretation challenged by a new paper by Schenk et al in Nature Communications.
Schenk et al base their reinterpretation on climate model output supported by plant macrofossil and pollen which are used as indicator species to infer July temperatures. Their model simulates autumns, winters and springs that were colder during the Younger Dryas than the Bølling-Allerød, and summers that were short but warmer. The paper suggests, based on the model output, various mechanisms that could give summer warming in an otherwise cold period, especially atmospheric blocking preventing winds from the cold ocean blowing far into Europe.
I’m not in a position to evaluate their model, so I’m going to have a closer look at their proxy data and compare it with other proxy data.
Since the replacement of forest by tundra or steppe during the Younger Dryas is well known, it is a little surprising to see palaeobotanical evidence being used to suggest warm Younger Dryas summers. Schenk et al base their temperature reconstruction on “plant species that indicate local presence focusing primarily on specific plant species, such as aquatics and riparian species “, rather than the full assemblage. I cannot find any discussion in the text as to why the aquatics should indicate warmth while the forests decline, but of course summer temperature is not the only factor driving vegetation – drought also plays an important role.
The glacial readvance is probably one of the best pieces of evidence for cool summers in the Younger Dryas. Within Europe, Younger Dryas glacial re-advances have been reported from, for example, the Iberian Peninsula, South Wales, North Wales, Northern England, Northern Ireland, Scotland, Norway, Iceland, the Alps, Poland, Romania, Macedonia. For glaciers to re-advance, summer temperatures need to be low. Alternatively winter precipitation could to be very high, but it is generally thought that Europe was arid in the Younger Dryas because of the extensive sea ice in the North Atlantic (some of the evidence for aridity is dependent on estimates of summer temperature, but some is independent). Glacial re-advances near the Atlantic coast could be consistent with Schenk et al, as the coast might not be protected by atmospheric blocking, but glacial advances in central Europe are not. Schenk et al do not consider the glacial evidence.
The one possible way to reconcile Schenk et al with the glaciological evidence is to argue that the dating of the glacial re-advances to the Younger Dryas is suspect. Bromley et al (2014) make this argument for Scotland, but at other sites the evidence is unambiguous, for example at Kråkenes where the well dated lake core contains sediment derived from a cirque glacier in the Younger Dryas.
Schenk et al consider the numerous chironomid-based reconstructions of summer temperature which are almost unanimous in reconstructing colder conditions in the Younger Dryas than the preceding Bølling-Allerød but dismiss this evidence. They argue that “chironomid assemblages have been shown to incorporate a number of environmental signals (e.g., catchment vegetation, nutrient supply, lake status and depth, seasonality) apart from the ambient summer air temperature“. This is correct, but the aquatic indicators used by Schenk et al will be sensitive to many of the same factors.
Specifically, Schenk et al argue that changes in seasonality – cold springs with late-lying snow and short summers affect the chironomid-based reconstructions. Why these seasonality shifts would not likewise affect the aquatic plant indicators is not explained.
The indicator values are calculated as from the temperature at the range limit of each species in Finland. Range limits are inherently uncertain and as the reconstruction is based on the indicator value of the least cold-tolerant species in the assemblage, the reconstructions will also be very uncertain. I do wonder how the range limits would change if a larger geographic area was considered – a question that could easily be tested with GBIF data.
I am not convinced by this paper.