Researchers have analysed a 3-million-year-old ice core from Allan Hills in Antarctica, measuring atmospheric CO2 at 250 parts per million and methane at 507 parts per billion during the late Pliocene. This epoch featured global temperatures about 1°C warmer than today and sea levels up to 25 metres higher. The levels are lower than previous indirect estimates.
Bubbles trapped in ancient ice from Allan Hills provided the first direct measurements of Pliocene atmospheric gases. Julia Marks-Peterson of Oregon State University and her team extracted this 'blue' ice, which is older ice exposed near the surface due to wind erosion of younger layers. Their analysis revealed CO2 levels of 250 ppm and methane at 507 ppb around 3 million years ago. In the following cooling period, CO2 saw a small decrease while methane showed no notable change. These figures contrast with indirect estimates placing CO2 at around 400 ppm, similar to today's levels, and current methane near 2000 ppb. Marks-Peterson noted, “We definitely were a bit surprised.” She added, “Maybe the Earth system is even more sensitive to changes in CO2 than we have understood. That’s a little bit of a scary thought and something that I would say that our record can’t answer yet.” Cristian Proistosescu at the University of Illinois Urbana-Champaign, not involved in the study, commented, “Ultimately, any new data that suggests Pliocene CO2 levels were lower than previously expected means future climate change might be worse than previously expected.” Tim Naish at Victoria University of Wellington urged caution: “It’s way too early to throw the baby out with the bathwater,” calling for records from warmer Pliocene periods. Thomas Chalk from the European Center for Research and Education in Environmental Geosciences affirmed the low CO2 values but highlighted potential distortion in ancient ice, questioning whether they represent a cold snap, warm interval, or average. The team plans to compare results with the Beyond EPICA continuous ice core exceeding 1 million years. The study appears in Nature (DOI: 10.1038/s41586-025-10032-y).