Fact check: What were CO2 levels during the last ice age?

1. Why the headline number (~180 ppm) dominates the conversation — and who produced it

Ice cores from Antarctica provide the primary direct evidence for glacial atmospheric CO2; analyses of trapped air yield continuous records across multiple glacial cycles. The Vostok core and later EPICA Dome C and Dronning Maud Land cores measured glacial concentrations around 180 ppm, a value repeatedly reported in synthesis and targeted studies and highlighted in both older and recent literature ^{[3] [1]}. EPICA Dome C extended the longer-term context and identified even lower minima during specific Marine Isotope Stages, while a 2010 plant-response study emphasized how these low CO2 levels were among the lowest encountered during the evolution of land plants, underlining the biological significance of the glacial low-CO2 state ^{[4] [1]}. Recent work continues to use ~180 ppm as a benchmark when testing climate model performance against paleoclimate constraints ^[2].

2. The outliers and why figures like 172 ppm appear in the literature

High-resolution ice-core work identified intervals with CO2 below 180 ppm; the EPICA Dome C study reports minima near 172 ppm during Marine Isotope Stage 16 and documents periods of CO2 below 180 ppm for millennial spans. These lower figures do not contradict the broader ~180 ppm headline; they reflect natural variability across different glacial stages and the improved temporal resolution of some cores ^{[4] [5]}. The existence of pulses or prolonged intervals with CO2 slightly lower than the LGM average points to the complex interplay of ocean solubility, biological carbon storage, and ice-sheet configurations that modulated atmospheric CO2 through glacial cycles ^[4].

3. How scientists measure and why uncertainty remains non‑zero

The core method is direct measurement of ancient air bubbles trapped in Antarctic ice, a technique with high fidelity but known caveats: gas age–ice age differences (bubbles enclose air after snowfall compacts), smoothing of rapid changes in lower-accumulation regions, and local thermal and deformation effects. These methodological factors mean reported values (e.g., 180 ppm) are robust within analytical uncertainties but can shift slightly as new cores, higher-resolution sampling, or refined gas-age models emerge ^{[3] [6]}. Studies published across decades show convergent results despite methodological evolution, which strengthens confidence in the broad conclusion that glacial CO2 was well below preindustrial levels ^{[2] [7]}.

4. What the glacial CO2 story means compared with the modern era and why context matters

Ice-core records show that preindustrial and modern CO2 concentrations are markedly higher than during glacial times; analyses explicitly state modern burdens are unprecedented over at least the last several hundred thousand years, contrasting with glacial minima near 180 ppm ^[3]. This comparison matters for climate sensitivity and ecosystem response: shifts of 100+ ppm are associated with large global temperature and ice-sheet changes, so the glacial-to-interglacial CO2 swing provides a real-world case for greenhouse-gas forcing. Recent model-data comparison studies use the LGM CO2 benchmark to constrain sensitivity and feedbacks in climate models, reinforcing the use of paleodata as an empirical test for projections ^[2].

5. Multiple perspectives and potential agendas in public discussion

Scientific sources consistently report low glacial CO2; disagreements are over details of timing, magnitude of short-term minima, and interpretation of drivers. Some public arguments selectively cite specific minima (e.g., 172 ppm) to imply broader instability in paleodata, while others emphasize the robust convergence of independent cores around ~180 ppm to stress continuity. Researchers publishing long records and model comparisons emphasize consilience across multiple cores and methods, whereas skeptics sometimes highlight analytical caveats to downplay the modern CO2 rise; readers should note these rhetorical uses and weigh the full suite of ice-core evidence and model tests ^{[4] [2] [3]}.