Swedish scientist's century-old peat bog data proves eerily accurate on climate shifts

Peatlands provide some of the most continuous and informative archives for reconstructing Holocene environmental and climatic changes. Their potential was recognised in the late 19th century, but systematic investigations began in the early 20th century, notably through the work of Swedish scientists. Among these, Erik Granlund made a particularly influential contribution by identifying a series of “recurrence surfaces” (rekurrensytor, RY) in southern Swedish peat bogs. These stratigraphic boundaries, marking transitions from highly humified to less humified peat, were interpreted as evidence of abrupt shifts from relatively dry and warm phases to wetter and cooler conditions. Based on extensive peatland surveys, Granlund proposed several such shifts during the middle and Late-Holocene. Two of his most distinct recurrence surfaces, RY V (~4250 cal a BP) and RY III (~2550 cal a BP), correspond closely to the globally recognised 4.2 and 2.8 ka climatic events, making his early identification of the 4.2 ka event particularly remarkable. The youngest surfaces, RY II (~1550 cal a BP) and RY I (~750 cal a BP), were less frequently recorded but appear to coincide with climatic fluctuations documented elsewhere in northwestern Europe. The uncertain RY IV (~3150 cal a BP), mainly observed in southwestern Sweden, may represent a local or regionally restricted event. This paper revisits Granlund’s concept of recurrence surfaces, evaluates their significance in light of modern paleoenvironmental approaches, and considers whether aspects of his interpretations may have been influenced by confirmation bias.