Warming in a warm world: Orbital CO2 forcing variations in the warm Pliocene

The Mid Pliocene warm period ( 3.26 to 3.00 million years ago, Ma) is the last time in Earth's history with CO₂ levels above 400ppm, and both sea level and temperature are thought to be significantly higher than today. Understanding the links between the heat trapping gas CO₂ and climate is important for divining our near future, even with rapidly implemented CO₂ mitigation strategies. We present orbitally resolved ( 3000 years) boron isotope CO₂ data from the Caribbean Sea over the interval from M2 to KM5, a period of warming with similar orbital characteristics to today. Using the high power of the boron isotope proxy to reconstruct relative radiative forcing change from CO₂ we show that cycles, though apparent in the record, show a change in forcing from CO₂ of no more than 2.6 W/m² , which is similar if not larger than the late Pleistocene (2 W/m²). This difference is in drastic contrast with global δ¹⁸O record that shows an amplified glacial-interglacial signal in the late Pleistocene (2‰) compared to the Pliocene (0.5‰). However, the M2 glaciation (3.3 Ma) is associated with no substantially reduced CO₂ forcing relatively to previous and subsequent Pliocene 'glacials'. These data show the importance of ice sheets as a climate regulator, both as a primary contributor to the Earth's radiative forcing balance but also in the regulation of CO₂.