Climate proxies indicate coupling between changes in atmospheric pCO2, global temperatures, and ice volume over much of the Cenozoic. Evidence has been presented for decoupling of these factors in the Miocene, though the cause of the apparent decoupling was uncertain. Here, we revisit Deep Sea Drilling Program (DSDP) Site 608 (24–9 Ma) in the North Atlantic Ocean, to provide improved constraints on sea-surface temperatures (SSTs) using the TEX86 and proxies, and use these to recalculate atmospheric pCO2. From the Oligocene/Miocene boundary to the middle Miocene Climatic Optimum (MCO, ca. 23.03 to ca. 14.5 Ma), SSTs at Site 608 were upward of 30 °C, nearly 15 °C warmer than modern. During the Middle Miocene Climatic Transition (MMCT), ca. 14.5 to ca. 12.5 Ma), SSTs cooled by ∼6 °C. Lower SSTs persisted until the end of our record at 9 Ma. Our organic proxy derived SST estimates are considerably higher than those previously calculated from planktonic foraminiferal oxygen isotope data, leading to reassessed alkenone pCO2 estimates ∼65 to ∼175 ppm higher than previously calculated, with other assumptions held constant. A pCO2 decrease from an average of ∼430 ppm in MCO to ∼305 ppm after the MMCT, in step with the ∼6 °C SST cooling, demonstrates coupling of climate and the carbon cycle, as well as a highly sensitive climate system.