Constraints on past CO2 and climate sensitivity from global temperature and sea level reconstructions across the Plio-Pleistocene

In contrast to previous approaches, new reconstructions of changes in global mean surface temperature and global mean sea level (GMSL) include large variability in GMSL throughout the Pleistocene. Here we assess these reconstructions from an energy balance perspective by using them to force a global carbon cycle model in different scenarios that capture the spread in existing CO2 reconstructions. Results suggests that a high CO2 scenario (300–450 ppm before 2 million years ago) is most consistent with the new climate reconstructions. When radiative forcing from the high CO2 scenario and land ice albedo derived from the new GMSL reconstruction are corrected for other forcings and feedbacks, but ignoring any state-dependency, these results for the past 4 million years suggest an equilibrium climate sensitivity of 1.8–1.9 K which is just below the 5–95% confidence range of recent estimates using other approaches (2.1–4.0 K).