Understanding the evolution of Arctic polar climate from the protracted
warmth of the middle Pliocene into the earliest glacial cycles in the
Northern Hemisphere has been hindered by the lack of continuous, highly
resolved Arctic time series. Evidence from Lake El’gygytgyn, in
northeast (NE) Arctic Russia, shows that 3.6 to 3.4 million years ago,
summer temperatures were ~8°C warmer than today, when the partial
pressure of CO₂ was ~400 parts per million. Multiproxy
evidence suggests extreme warmth and polar amplification during the
middle Pliocene, sudden stepped cooling events during the
Pliocene-Pleistocene transition, and warmer than present Arctic summers
until ~2.2 million years ago, after the onset of Northern Hemispheric
glaciation. Our data are consistent with sea-level records and other
proxies indicating that Arctic cooling was insufficient to support
large-scale ice sheets until the early Pleistocene.