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The Pliocene epoch, which lasted from 5.3 million to 2.6 million years ago, was a consequential time in Earth's history. The Isthmus of Panama formed, connecting North and South America, and the accumulation of ice at the poles reshaped the world's biogeography.

Notably, the Pliocene was also the last time Earth's atmospheric carbon dioxide (CO₂) concentrations approached or exceeded 400 parts per million before the 21st century. (Atmospheric CO₂ averaged 425.40 parts per million in December 2024 at Hawaii's Mauna Loa Observatory.)

The patterns and changes in the climate system during the Pliocene could presage how the planet will respond to near-future climate change. For instance, paleoclimatology tells us that smaller ice sheets in the Pliocene caused sea levels to swell more than 20 meters higher than today. Is such a rise in store for the rest of the century?

To clarify how the ancient climate responded to elevated CO₂ levels, Jessica Tierney and colleagues reconstructed early (5.0–4.5 million years ago) and middle (3.5–3.0 million years ago) Pliocene climate conditions using a technique called paleoclimate data assimilation.

The approach blends geologic observations of past sea surface temperatures—gleaned from molecular lipids and microfossils preserved in ancient marine sediments, which change their chemical properties with water temperature—w¾±³Ù³ó climate models to estimate past climate states. They called their global reconstruction "plioDA." The research is in the journal AGU Advances.

The plioDA reconstruction shows that the time period was warmer than previously thought—4.1℃ warmer than preindustrial conditions, on average. Today's climate, in comparison, is about 1.5℃ warmer than preindustrial conditions.

The model also calls previous assumptions into question: Past work has proposed that the ocean was forming deep water (a cold, salty layer of the ocean beneath the surface and transitional thermocline) during the Pliocene. But the surface salinity values shown in plioDA render deepwater formation during the Pliocene unlikely.

The research provides an updated view of the climate during the Pliocene, which is considered an analog to today's climate conditions. In addition, the study updates estimates of global temperature changes, the sensitivity of the climate to CO₂ concentrations, and the state of the ocean in the bygone epoch.

This story is republished courtesy of Eos, hosted by the American Geophysical Union. Read the original story .