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During the Mid-Pleistocene Transition (MPT), ca. 1200–800 000 years ago (ka), the Earth's glacial cycles changed from 41 to 100 kyr periodicity. The emergence of this longer ice age periodicity was accompanied by higher global ice volume in glacial periods and lower global ice volume in interglacial periods. Since there is no known change in external orbital forcing across the MPT, it is generally agreed that the cause of this transition is internal to the Earth system. Resolving the climate, carbon cycle, and cryosphere processes responsible for the MPT remains a major challenge in Earth and palaeoclimate science. To address this challenge, the international ice core community has prioritised recovery of an ice core record spanning the MPT interval. Here we present results from a simple generalised least-squares (GLS) model that predicts atmospheric CO2 out to 1.8 Myr. Our prediction utilises existing records of atmospheric carbon dioxide (CO2) from Antarctic ice cores spanning the past 800 kyr along with the existing LR04 benthic δ18Ocalcite stack (Lisiecki and Raymo, 2005; hereafter "benthic δ18O stack") from marine sediment cores. Our predictions assume that the relationship between CO2 and benthic δ18O over the past 800 000 years can be extended over the last 1.5 million years. The implicit null hypothesis is that there has been no fundamental change in feedbacks between atmospheric CO2 and the climate parameters represented by benthic δ18O , global ice volume, and ocean temperature. We test the GLS-model-predicted CO2 concentrations against observed blue ice CO2 concentrations, δ11B -based CO2 reconstructions from marine sediment cores, and δ13C of leaf-wax-based CO2 reconstructions (Higgins et al., 2015; Yan et al., 2019; Yamamoto et al., 2022). We show that there is no clear evidence from the existing blue ice or proxy CO2 data to reject our predictions or our associated null hypothesis. A definitive test and/or rejection of the null hypothesis may be provided following recovery and analysis of continuous oldest ice core records from Antarctica, which are still several years away. The record presented here should provide a useful comparison for the oldest ice core records and an opportunity for further constraints on the processes involved in the MPT. [ABSTRACT FROM AUTHOR]

Antarctica is the coldest, driest, highest, and windiest continent, and it is home to 70 research stations from 29 countries. Antarctica is a remote and harsh place, without many weather stations or people. This means scientists must come up with creative ways to learn about the continent's past climate. Scientists use sediment that builds up on the ocean floor to explore what Earth was like more than a million years ago; and they use satellites to learn about today's climate. But how can we know what happened in the time between? This is where ice can help! When snow falls in Antarctica, it builds up in layers that get squeezed into ice. These layers hold traces of the past climate. The ice in Antarctica is the oldest anywhere in the world. Hundreds of thousands of years ago until now. These studies help us to better understand the past climate and to predict the future climate. [ABSTRACT FROM AUTHOR]

During the Mid-Pleistocene Transition (MPT), ca. 1250–800 kya, the Earth's glacial cycles changed from 41 ky to 100 ky periodicity. The emergence of this longer ice-age periodicity was accompanied by higher global ice volume in glacial periods and lower global ice volume in interglacial periods. Since there is no known change in external orbital forcing across the MPT, it is generally agreed that the cause of this transition is internal to the earth system. Resolving the climate–carbon cycle–cryosphere dynamics processes responsible for the MPT remains a major challenge in ice core and climate science. To address this challenge, the international ice core community has prioritized recovery of an ice core record spanning the MPT interval. The results from such 'oldest ice' projects are still several years away. Our objective here it to make an advanced prediction of atmospheric CO2 out to 1.5 my. Our prediction utilizes existing records of atmospheric carbon dioxide (CO2) from Antarctic ice cores spanning the past 800 ky along with the existing benthic water stable isotope (ẟ18O) record from marine sediment cores. Our predictions assume that the relationship between CO2 and benthic ẟ18O over the past 800 thousand years can be extended over the last one and a half million years. The implied null hypothesis is that there has been no fundamental change in the global climate–carbon cycle–cryosphere feedback systems across the MPT. We find that our predicted CO2 record is significantly lower during glacial intervals than the existing blue-ice and boron isotope-based estimates of CO2 that pre-date the continuous 800 ky CO2 record. Our predicted glacial CO2 concentrations are ~9 ppm below glacial CO2 concentrations observed in blue ice data at ca. 1 mya and ~19 ppm below glacial CO2 concentrations reconstructed from boron isotopic data over ca ~1.1–1.25 mya. These results support rejection of our null hypothesis and provide quantitative evidence of a fundamental shift in the global climate–carbon cycle–cryosphere feedback systems across the MPT. However, the definitive test of the various theories explaining the MPT will be comparison of our predicted records with the forthcoming oldest ice core records. [ABSTRACT FROM AUTHOR]