Mid-Pliocene warmth: Stronger greenhouse and stronger conveyor

Three million years ago, prior to the onset of northern hemisphere glaciation, global mean temperatures may have been as much as 3.5 degrees C warmer than at present. We present evidence, based on the carbon isotopic composition of marine organic matter, that atmospheric CO2 levels at this time were on average only about 35% higher than the preindustrial value of 280 ppm. We also present carbon isotopic evidence for stronger thermohaline circulation in the Atlantic Ocean during the warmest intervals and propose that the North Atlantic ''conveyor belt'' may act as a positive feedback to global warming by enhancing sea ice retreat and decreasing high latitude albedo. Based on our results, it seems unlikely that the mid Pliocene warm period was a doubled CO2 world.