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Cold extremes affect ecosystems and societies at high latitudes, but their frequency and intensity remain poorly understood. Here, we use incompletely lignified tree rings, blue rings (BRs), as highly selective markers of ephemeral summer cooling in northern Scandinavia. A 300-year record from the northern treeline reveals that BRs were common during the 18th and 19th centuries but vanished entirely after 1902, offering biological evidence for exceptional pace of Arctic warming during the 20th century. Juniperus shrubs recorded only the most severe cold extremes (mean z-score −1.86 versus −1.36 in Pinus trees). Reflecting this difference in sensitivity, BRs were found more often in trees (24 years) than in shrubs (14 years). Unlike traditional tree-ring proxies, BRs provide finer temporal resolution and likely reflect intra-seasonal cooling events. Integrating BRs with historical records demonstrates their power to link cold extremes with societal impacts, exemplified by the 1867 major famine in Northern Europe. These findings position BRs as a distinctive tool for resolving climate extremes–society nexus during pre-instrumental period.