Arctic Siberia summers were up to 10°C warmer than today during the Last Interglacial, study finds

Interglacials are, as the name suggests, warm periods between planetary glaciations when the expanse of ice on Earth shrinks. Currently, we are in an 11,000 year-long interglacial period known as the Holocene. Prior to this, the Last Interglacial occurred between 115,000 and 130,000 years ago.

During this time, Earth experienced summers that were almost completely ice-free and there was significant vegetation growth in polar regions, changing the ecosystems for life to flourish. Scientists can look to this Last Interglacial as a potential analog for future global warming.

Indeed, new research, currently under review for publication in the Climate of the Past journal, has turned to the geological record of the Arctic to understand how terrestrial environments responded to the warmer world. Here, warming was amplified compared to the rest of the northern hemisphere due to ice albedo feedbacks, whereby solar insolation melted ice sheets, reducing the amount of radiation reflected back out to space and causing further warming, creating a positive feedback loop.

Dr. Lutz Schirrmeister, of the Helmholtz Center for Polar and Marine Research in Germany, and colleagues have turned to particular landscapes generated in areas experiencing permafrost, where the ground has remained frozen for at least two years.

Thermokarst topography is unique to such regions, characterized by hollows and hummocks that form when ice-rich permafrost thaws and the surface slumps due to a lack of ice in the pore spaces between sediments. Nowadays, these depressions also fill with water, producing thermokarst lakes.

Dr. Schirrmeister and the team investigated coastal sections along the Dmitry Laptev Strait, Siberia, via sediment cores drilled during fieldwork between 1999 and 2014, which preserve alternating layers of peaty plant matter with clays and silts. These distinctive layers represent the changing landscape through time between shallower boggy terrain where plants could grow, to deeper lake deposits. Today, the study area is a mixture of drier tundra with substantial plant growth, grasses and wetlands underlain by 400–600m of permafrost.

From these cores, the scientists used a combination of sediment analysis with fossil remains of plants (pollen, leaves and stems), insects (beetles and midges), crustaceans (ostracods) and animals (water fleas and mollusks) to reconstruct the paleoenvironment.

Combined with modeling, this data highlights that steppe or tundra-steppe (grassland and low-growing shrubs) environments prevailed in the area at the beginning of the Last Interglacial, but that birch and larch forests proliferated during the middle of the event, with the treeline being 270km north of its current position during the peak.

The researchers ultimately identified up to 10°C more summer warming in northern Siberia during the Last Interglacial compared to summers today, with fossilized plant material suggesting that mean temperatures of the warmest month could have reached 15°C, while fossil beetles indicate the coldest temperature may have been -38°C. Today, the respective mean temperatures are approximately 3°C and -34°C.

Having said this, in June 2020, the town of Verkhoyansk in Russia measured the highest temperature ever recorded above the Arctic Circle at 38°C, while the lowest temperature recorded is -69°C in Greenland. While these were anomalous, the continued changing climate highlights the need to look to the past to inform the future, when such conditions could become more common.

Dr. Schirrmeister notes that while the Last Interglacial warming mostly impacted summer temperatures, future climate change is expected to more broadly impact winter months due to anthropogenic activity. Nevertheless, ice sheet retreat, loss of sea ice and melting permafrost are all observed in the Arctic today, highlighting the importance of continued research into the sensitivity of Earth to rising temperatures during the Last Interglacial.

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