Predicting trends in atmospheric CO2 across the Mid-Pleistocene Transition using existing climate archives.

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 (&#948¹⁸O) record from marine sediment cores. Our predictions assume that the relationship between CO₂ and benthic &#948¹⁸O 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]