A critical role of ocean–sea ice interactions in the pronounced warmth during the Miocene Climatic Optimum

When Earth Was Surprisingly Warm

About 17 million years ago, our planet slipped into a natural heatwave called the Miocene Climatic Optimum. Global temperatures rose far above today’s levels, and the polar regions warmed especially strongly. Yet even our best climate models have struggled to reproduce just how warm it got, particularly near the poles. This study asks a simple but powerful question: was the key to that ancient warmth not just greenhouse gases in the air, but also what was happening in the oceans and sea ice at high latitudes?

A Past Climate That Challenges Our Models

Geological evidence suggests that during the Miocene Climatic Optimum, global surface temperatures were roughly 8–10 °C warmer than in preindustrial times, with polar oceans sometimes more than 10 °C warmer than today. Carbon dioxide levels were likely two to three times higher than before the Industrial Revolution, which helps explain the warmth but not the surprisingly small temperature difference between the equator and the poles. Many previous modeling studies could warm the planet overall, but they consistently failed to heat the high latitudes enough. This mismatch raised doubts about how well we understand both past and future warm climates.

Putting Two Virtual Earths to the Test

The authors used two sophisticated Earth system models—NorESM1-F and IPSL-CM5A2—and gave them the same Miocene geography, vegetation, ice sheets and two plausible carbon dioxide levels. That setup allowed a fair comparison of how each model handled the same ancient world. Both virtual Earths warmed considerably, by about 4–8 °C on average, in line with many reconstructions. But they diverged sharply in the Arctic. NorESM produced extreme polar warming, with Arctic surface temperatures more than 20 °C above preindustrial values and an almost ice-free Arctic Ocean. IPSL, by contrast, showed much more modest polar warming and seasonal sea ice that still covered large areas in winter. When the model results were compared with fossil and chemical temperature clues from rocks and sediments, NorESM matched the unusually warm high-latitude oceans better than IPSL, though it ran too hot on some land areas.

How Ocean Currents and Sea Ice Tip the Balance

To understand why the models behaved so differently, the researchers dissected the energy budget and circulation patterns. In both worlds, extra greenhouse gases trapped more heat and melting ice brightened less of the planet’s surface, allowing it to absorb more sunlight. Clouds also added some extra warming in the far north. But the crucial difference lay in the oceans and sea ice. In NorESM, a strong overturning circulation in the Atlantic pumped large amounts of warm, salty water toward the Arctic and mixed it downward, while deep waters from the south weakened. This vigorous circulation, combined with a wide open seaway between the Atlantic and Arctic, flooded the polar ocean with heat and salt. The saltier water was harder to freeze, and strong mixing continually brought warmth to the surface, preventing sea ice from reforming. With sea ice almost gone year-round, the dark ocean soaked up even more solar energy, further boosting polar temperatures. IPSL, by contrast, simulated a weaker overturning circulation, reduced northward heat transport, and persistent winter sea ice that helped keep the Arctic cooler.

Checking the Atmosphere’s Role

The team also tested whether differences in the atmosphere alone could explain the contrasting results. Both models showed weakened tropical-to-polar air circulation, a pattern similar to what is expected in future warming scenarios. When the researchers forced an atmosphere-only version of NorESM with sea surface temperatures and sea ice patterns borrowed from the IPSL runs, the resulting warming pattern looked much more like IPSL than NorESM. This experiment showed that, on large scales, the atmospheres of the two models behave similarly. The real leverage came from how each model’s ocean and sea ice systems responded—particularly how much heat the ocean carried north and how easily the Arctic could lose its ice cover.

Lessons for the Future from an Ancient Ocean

In simple terms, this work argues that the Miocene Climatic Optimum may have been a fundamentally different kind of polar climate—one in which strong ocean currents, salty surface waters, and greatly reduced sea ice worked together with greenhouse gases to supercharge high-latitude warming. NorESM’s version of that world, with a powerful Atlantic circulation and nearly ice-free Arctic, fits the available evidence better than a cooler, ice-rich Arctic, even if it may push the sea-ice loss to an extreme. The study highlights that getting past and future warm climates right is not just about setting the correct carbon dioxide level. It also requires capturing how oceans and sea ice interact, and how sensitive these systems are to changes in geography and forcing. Better comparisons among many models and more geological clues about ancient sea ice and deep ocean circulation will be essential to pin down how our own warming oceans might reshape polar climate in the centuries ahead.

Citation: Tan, N., Fluteau, F., Zhang, Z. et al. A critical role of ocean–sea ice interactions in the pronounced warmth during the Miocene Climatic Optimum. Commun Earth Environ 7, 326 (2026). https://doi.org/10.1038/s43247-026-03324-2

Keywords: Miocene Climatic Optimum, ocean circulation, sea ice, polar amplification, Atlantic overturning