Ancient ice records reveal stable greenhouse gas levels over 3 million years
New shallow Allan Hills blue-ice cores extend greenhouse-gas records to about 3.1–0.5 million years ago. They show mean CH4 changing little over the period, with CO2 falling by ~20 ppm from 2.9 to 1.2 Ma and then staying stable within ±10 ppm through the mid‑Pleistocene Transition; respiration-corrected samples from 2.8–3.1 Ma yield CO2 of 250 ± 10 ppm, indistinguishable from the early Pleistocene. These results suggest long‑term stability of atmospheric CO2 and CH4 and demonstrate that ice-core gas measurements can be extended into the late Pliocene, providing snapshots of climate during a era of global cooling.
