Global and regional sea-surface temperature changes over the Marine Isotopic Stage 9e and Termination IV

Abstract. The Marine Isotope Stage (MIS) 9, occurring approximately from 300 to 335 ka, represents an important period for studying the dynamics of Earth's climate. Interest in studying this interglacial period stems from the fact that is associated with the highest atmospheric CO₂ concentrations over the last 800 ka (excluding anthropogenic CO₂ emissions). Numerous reconstructions of the sea surface temperatures (SST) are available over this time interval, but it is challenging to assess the regional and global patterns of climate variability and to infer temporal sequences of changes from numerous marine sediment records located in different parts of the world and whose chronologies originate from different dating strategies. In this study, we present the first spatio-temporal SST synthesis over the interval 300 to 350 ka, covering this interglacial period and its preceding deglaciation (Termination IV, ~335 to ~350 ka). We include 98 high-resolution SST reconstructions and we establish a common temporal framework between the selected marine records, based on the latest reference ice core chronology (AICC2023). We also homogenize the proxy-calibration strategy by applying a single method for each proxy. Chronological and calibration uncertainties are quantified using Bayesian and Monte Carlo procedures. Finally, through a Monte Carlo approach, we generate global and regional SST stacks relative to Pre-Industrial Era over Termination IV and MIS 9.

We highlight significant differences in terms of temporal variability, amplitude, and timing of changes in the SST records across the globe across the studied time interval. While the patterns of SST changes are homogeneous at basin-scale, heterogeneous interglacial SST peaks are observed across ocean basins. The interglacial surface temperature peaks in extra-tropic basins are similar or warmer than the pre-industrial period (PI), while intra-tropic areas appears to be colder relative to PI during glacial optimum. In addition, the timing in interglacial surface temperature peaks differ across the different regions. These regional temperature variations suggest that atmospheric and oceanic dynamics played a greater role than global radiative forcing in shaping the MIS 9 climate. The heterogeneous timing of changes across the different regions contribute to a smoothed global response in terms of both timing and amplitude. Consequently, we find that at a global scale MIS 9e SST was as warm as the pre-industrial period (~ -0.2°C ± 0.3 °C). Converted into surface air temperatures (~ -0.4°C ± 0.6 °C), this estimate agrees within the uncertainty range with previous studies based on a smaller number of records with lower temporal resolution. We also compare our results on MIS 9 and Termination IV with published SST syntheses from more recent interglacial periods (MIS 5e and Holocene) and deglacial periods (Termination I and II). We find that the global deglacial surface air warming during Termination IV is similar in amplitude (~5.3 °C) to that observed during Terminations I and II. Finally, a comparison of deglacial warming rates for these three terminations to the warming trend of the last 60 years emphasizes that the rapidity of modern climate change is unprecedented within the context of these past deglaciations.