Summer temperatures in southern Siberia not seen since the Holocene Thermal Maximum

The thermal history of continental Eurasia and regional responses to anthropogenic warming remains poorly understood prior to instrumental records. Like the faster-than-global warming of the Arctic1, southern Siberia has also warmed rapidly, which leads to increasing recurrence of heat waves and drought, escalates wildfires2, and increases the vulnerability of regional permafrost against thaw3. Due mainly to the lack of long, well dated and quantitative climate records from vast Siberia, the anthropogenically-driven regional warming cannot easily be placed in a longer-term perspective. Clumped isotope analyses on biogenic carbonates can provide quantitative estimates (TΔ47) of the (water) temperature during the formation of these carbonates.Here we use clumped isotope thermometry on lacustrine carbonates (bivalves, gastropods) from shallow (7 m long, 14C-dated sediment core4. The oldest sample is from the Last Glacial Maximum (∼25 ka BP), while two modern samples were collected in 2023. Shells were manually cleaned and homogenised using an agate mortar and between four and 28 replicates (each weighing 120 to 350 mg) were analysed using a clumped-isotope-dedicated NU Perspective IRMS.As Lake Ochaul is ice-covered between October and April, and the biogenic carbonate from the analysed molluscs forms preferentially in the warm summer season, the obtained TΔ47 values represent warm season water temperatures. The reconstructed temperatures range from +2.5±6°C to 21±4°C, with the highest values well in the range of modern water and air temperatures observed during July and August.Our TΔ47 results indicate that until the mid 19th century, warm season water temperature of Lake Ochaul closely followed summer insolation. Warm season water temperatures were highest during the Holocene Thermal Maximum5 and declined in response to lower summer insolation during the later Holocene. Interestingly, all core top and modern samples show TΔ47 values significantly higher than expected if insolation was the sole forcing on temperature. This is consistent with lake water temperatures today, which reach values characteristic of the Holocene Thermal Maximum. Consequently, as modern water temperatures deviate so strikingly from the insolation trend, the results of this study indicate that anthropogenic warming now drives regional temperature dynamics in southern Siberia. References1 Hantemirov et al. (2022) Current Siberian heating is unprecedented during the past seven millennia. Nat. Comms. 13:49682 Huang et al. (2024) Escalating wildfires in Siberia driven by climate feedbacks under a warming Arctic in the 21st century. AGU Advances 5, e2023AV0011513 Vaks et al. (2025) Arctic speleothems reveal nearly permafrost-free Northern Hemisphere in the late Miocene. Nat. Comms. 16:54834 Kobe et al. (2022) Not herbs and forbs alone: pollen‐based evidence for the presence of boreal trees and shrubs in Cis‐Baikal (Eastern Siberia) derived from the Last Glacial Maximum sediment of Lake Ochaul. JQS 37, 868–8835 Tarasov et al. (2025) Environmental and cultural transformations in the Lake Baikal Region reflect hemispheric-scale changes in temperature and atmospheric circulation over the past 8800 years. GPC 256:105157