Ocean circulation changes rather than global warming drove temperature trends across the Miocene Climatic Optimum.

Early-to-Middle Miocene [23 to 13.8 million years ago (Ma)] climates were ∼6° to 9°C warmer than today. Around 16.9 Ma, benthic foraminiferal oxygen isotope (δ18O) records suggest pronounced warming, known as the Miocene Climatic Optimum (MCO) onset. However, explaining this putative global warming with carbon dioxide (CO2) forcing faces challenges in terms of timing and forcing magnitude. Our new biomarker-based ocean temperature records, combined with published data, reveal no substantial mean global upper ocean warming across the MCO. Instead, temperature changes differ considerably between sites, with some regions showing cooling. Climate model sensitivity tests indicate that these patterns are best explained by ocean circulation changes, likely due to gateway changes and Antarctic ice sheet variations. This suggests that the MCO signature is better attributed to ocean circulation changes rather than a CO2-driven global warming event. The δ18O signal therefore represents warmer deepwater sourcing and/or reduced Antarctic glacial volume, resolving the longstanding challenge of explaining MCO warming without a pronounced CO2 rise.