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

Abstract

The Marine Isotope Stage (MIS) 9e, occurring approximately from 335 to 320 ka, represents an important period for studying the dynamics of Earth's climate. Interest in studying this interglacial period stems from the fact that it is associated with the highest atmospheric CO2 concentrations over the last 800 ka (excluding anthropogenic CO2 emissions). Numerous reconstructions of sea surface temperatures (SST) cover this time interval, yet synthesizing them into consistent regional- and global-scale climate signals is challenging because they are scattered across the globe and based on heterogeneous chronological frameworks. In this study, we present the first spatio-temporal SST synthesis over the interval 350 to 300 ka, covering this interglacial period and its preceding deglaciation (Termination IV, ĝ1/4 350 to ĝ1/4 335 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-scale 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 over 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 to or warmer than the pre-industrial period (PI), while intra-tropic areas appear to be colder relative to PI during interglacial 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 9e climate. The heterogeneous timing of changes across the different regions contribute to a smoothed global-scale response in terms of both timing and amplitude. Consequently, we find that at a global scale MIS 9e SST was as warm as the PI (ĝ1/4 -0.1 ± 0.2 °C). Converted into surface air temperatures (ĝ1/4 -0.3 ± 0.3 °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 9e and Termination IV with published SST syntheses covering 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 (ĝ1/4 5.7 °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.