Ancient Greenland sediment reveals: moderate but prolonged warmth melted major ice 416,000 years ago

A University at Buffalo- led study of long- forgotten sediment from Camp Century (a Cold War U.S. military base in northwest Greenland) shows that summers there were about 4.7°C (8.5°F) warmer than today during an ice- free period roughly 416,000 years ago.

The sediment was recovered in the 1960s from nearly a mile (about 1.6 km) beneath the ice sheet during drilling at Camp Century, which was tied to the secret Project Iceworm effort to base nuclear missiles under the ice.

The samples were brought to the University at Buffalo, later moved to Denmark, and largely forgotten until a freezer cleanout rediscovered them around 2018. Earlier work had already shown the site was ice- free about 416,000 years ago (during Marine Isotope Stage 11), when the sediment was last exposed to sunlight; this required the Greenland Ice Sheet to retreat more than 60 miles (about 97 km) inland from its present edge in that region.

This new analysis (published September 28, 2026, in Climate of the Past) is the first quantitative reconstruction of land- based summer temperatures in Greenland for that ancient warm period.

Researchers examined fatty molecules (brGDGTs) produced by bacteria that were preserved in the sediment. The types and proportions of these molecules vary with temperature; comparing them to modern Arctic lake samples allowed the temperature estimate.

Key implications include:

• Moderately warmer summers (similar in magnitude to some Holocene peaks roughly 5,000- 10,000 years ago) coincided with substantial ice- sheet retreat.
• Duration of warming appears critical: the ~416,000- year- ago warmth may have lasted tens of thousands of years, allowing more time for melting, whereas shorter Holocene warmth left Camp Century ice- covered.
• Researchers caution against viewing this as reassuring for the present. Current human- caused warming is more intense, and projections indicate Arctic temperatures could rise 3.3– 10°C by 2100 (depending on emissions), with warmth potentially lasting centuries to millennia. Both magnitude and duration matter for ice- sheet response.

Lead author John Michael Aguilar (UB RENEW Institute postdoc) and corresponding author Elizabeth Thomas (UB Earth Sciences) note that the work improves understanding of how the ice sheet responds to sustained warming.

The analysis required accounting for elevation changes as the ice sheet advanced and retreated.

The research was supported by the National Science Foundation.

This builds on prior Camp Century sediment studies that documented past tundra vegetation and ice- free conditions, underscoring the Greenland Ice Sheet’s sensitivity to prolonged warmth.

Could rise 3.3– 10°C by 2100?

The figure comes primarily from the IPCC Sixth Assessment Report (AR6, 2021), Working Group I, Chapter 4.

Exact IPCC AR6 statement

The Arctic is projected to warm much more than the global average (Arctic amplification). For annual-average near-surface air temperature in the Arctic (roughly 67.7°N- 90°N) by 2081- 2100 relative to 1995- 2014:

• ~3 °C under the low- emissions scenario SSP1- 2.6
• ~10 °C under the medium- to- high scenario SSP3- 7.0
• ~12 °C under the very high- emissions scenario SSP5- 8.5

More detailed multi-model ranges (5- 95%) from the same chapter include values such as:

Scenario	Median / central	Range (approx.)
SSP1-2.6	3.3 °C	0.4–7.5 °C
SSP2-4.5	5.4 °C	2.8–10.0 °C
SSP3-7.0	7.7 °C	4.5–13.4 °C
SSP5-8.5	10.0 °C	6.2–15.2 °C

The University at Buffalo press release’s “3.3 to 10 °C” range is therefore a reasonable summary of the low- to- high emissions projections reported by the IPCC.

SSP5- 8.5 is still referenced in older studies and some risk assessments, but it is no longer treated as a plausible “business- as- usual” pathway and is being phased out of the core scenario set for the next IPCC assessment cycle.

SSP5- 8.5 remains in the scientific literature and is still occasionally used for sensitivity analysis, but the climate research community and the scenario developers preparing for IPCC AR7 no longer regard it as a credible central or likely high- end pathway. Newer scenario sets have a lower and narrower high-end range.

Marine Isotope Stage 11 (MIS 11) was a long interglacial period in Earth’s recent geologic past, roughly spanning 424,000 to 374,000 years ago.

What Marine Isotope Stages are

Marine Isotope Stages are numbered intervals defined primarily from the oxygen isotope ratios (δ¹⁸O) in fossil foraminifera shells in deep- sea sediment cores.

These ratios mainly reflect global ice volume and deep- ocean temperature. Odd- numbered stages are interglacials (warmer, lower ice volume); even-numbered stages are glacials (colder, higher ice volume).

The current interglacial is MIS 1 (the Holocene).

Key characteristics of MIS 11

• Duration: One of the longest interglacials of the past 500,000–800,000 years. The main warm phase (often called MIS 11c) lasted roughly 25,000– 30,000 years or more in many records, substantially longer than the Holocene so far (~11,700 years).
• Orbital context: It occurred during a period of low orbital eccentricity, which reduced the amplitude of precessional cycles. This produced insolation patterns more similar to those of the Holocene than any other recent interglacial, making MIS 11 a frequently cited analogue for the present interglacial (though CO₂ levels and other factors differ).
• Temperature: Globally, peak warmth was broadly comparable to or only modestly warmer than the Holocene in many regions. High- latitude and Arctic records often show more pronounced warmth. In the Camp Century study, northwestern Greenland summers were ~4.7 °C warmer than 20th- century conditions, similar to peak Holocene warmth there.
• Sea level: Estimates of peak global mean sea level range from about 6- 13 m above present (some older interpretations suggested higher). This implies significant reduction of the Greenland Ice Sheet and possibly parts of the Antarctic ice sheets.
• Ice sheets: Evidence (including the Camp Century sediments dated to ~416,000 years ago) shows substantial retreat of the Greenland Ice Sheet. Modeling and proxy data indicate the prolonged duration of moderate warmth was critical for allowing major ice loss, even though peak temperatures were not extreme.

Why it matters for modern climate

MIS 11 demonstrates that sustained moderate warmth lasting many thousands of years can drive major ice-sheet retreat and multi- meter sea- level rise. The Camp Century paper and related studies emphasize that both the magnitude and the duration of summer warmth are important for Greenland Ice Sheet response. Modern human- caused warming (? Mea Culpa?) is occurring faster and is already more intense in many metrics; if elevated temperatures persist for centuries to millennia (as projected under higher- emissions scenarios), the ice- sheet response could be substantial.

In short, MIS 11 is a valuable natural experiment showing how the Greenland Ice Sheet behaves under prolonged interglacial conditions broadly comparable in temperature (but longer in duration) to parts of the Holocene.

Holocene- like summer climate during Marine Isotope Stage 11 in northwestern Greenland

Using lipid biomarkers (branched GDGTs for temperature and plant- wax hydrogen isotopes) preserved in the Camp Century subglacial sediments, the authors reconstructed conditions ~416,000 ± 38,000 years ago (during Marine Isotope Stage 11, or MIS 11):

• Summer air temperature (months above freezing) was 4.7 ± 3.2 °C warmer than 20th- century conditions at the site.
• These temperatures were similar to (or only slightly warmer than) peak Holocene warmth in Greenland, a time when the Camp Century site itself remained ice- covered.
• Atmospheric water- vapor isotopes were ²² ± 18 ‰ enriched in ²H, indicating a greater contribution of locally evaporated moisture (consistent with reduced ice cover and expanded vegetation).
• Conditions were cooler than the Last Interglacial (MIS 5e).

An older, lower sediment unit (likely Pliocene or early Pleistocene) recorded broadly similar temperatures.

Interpretation for ice- sheet sensitivity

Despite temperatures comparable to the Holocene, the Greenland Ice Sheet retreated far enough for the Camp Century site (currently under ~1.4 km of ice, >100 km from the present margin) to become ice- free. The authors attribute this to the prolonged duration of MIS 11 warmth (tens of thousands of years), which allowed more time for the ice sheet to respond, consistent with evidence from southern Greenland.

Key takeaway stated in the paper:

“Holocene- like MIS 11 conditions suggest that sustained moderate warmth can drive major Greenland Ice Sheet loss. … Therefore, efforts to reduce both the magnitude and duration of summer warmth in the coming centuries will be important to curbing ice- sheet retreat.”

This directly supports the implications discussed earlier: moderate but long-lasting summer warming is enough to produce substantial ice- sheet retreat, and modern human- driven warming is both more intense and projected to persist for centuries to millennia depending on emissions pathways.

The full paper (open- access PDF available on the page) includes detailed methods, calibrations, elevation and lapse- rate corrections, and comparisons with other Greenland paleoclimate records.

Journal: Climate of the Past

DOI: 10.5194/cp-22-1757-2026, 22, 1757–1779

Published: 28 September 2026

Provided: University at Buffalo

Authors: John Michael N. Aguilar,
Elizabeth K. Thomas,
Diana S. Aga,
Paul R. Bierman,
Jason P. Briner,
Isla S. Castañeda,
and Andrew J. Christ

Abstract

Determining the climatic conditions under which the Greenland Ice Sheet (GrIS) was smaller than present is important to quantify GrIS sensitivity to climate change. We use biomarkers in sediment collected beneath the GrIS at Camp Century, northwestern Greenland, to reconstruct summer temperature and atmospheric circulation 416 000 ± 38 000 years ago, at some point during Marine Isotope Stage 11 (MIS 11). We find that northwestern Greenland summer climate during MIS 11 was similar to the middle Holocene but different than the 20th century. Air temperature was 4.7 ± 3.2 °C warmer, and atmospheric water vapor isotope values were 22 ± 18 ‰ ²H-enriched, indicating a greater contribution of locally evapotranspired moisture. These conditions are similar to or slightly warmer than during peak Holocene warmth on Greenland, when the Camp Century site remained ice-covered, and cooler than the Last Interglacial (LIG). Biomarkers from lower in the section likely represent an earlier ice-free interval, potentially during the Pliocene or early Pleistocene, and record climatic conditions similar to MIS 11. These data add to the sparse available climate data for the early and middle Pleistocene on Greenland, and suggest interglacial periods had similar temperature throughout the Pleistocene. We interpret the reduced GrIS extent in northwestern Greenland during MIS11 compared to the Holocene, despite similar temperature, to indicate ice-sheet response to prolonged warmth, as reconstructed in southern Greenland. Therefore, efforts to reduce both the magnitude and duration of summer warmth in the coming centuries will be important to curbing ice-sheet retreat.