Abstract
The Pliocene–Pleistocene Transition (PPT), from around 3.2 to 2.5 million years ago (Ma), represented a major shift in the climate system and was characterized by a gradual cooling trend and the appearance of large continental ice sheets over northern Eurasia and North America. Paleo evidence indicates that the PPT was accompanied and possibly caused by a decrease in atmospheric CO2, but the temporal resolution of CO2 reconstructions is low for this period of time and uncertainties remain large. Therefore, instead of applying existent CO2 reconstructions we solved an ‘inverse’ problem by finding a schematic CO2 concentration scenario that allows us to simulate the temporal evolution of key climate characteristics in agreement with paleoclimate records. To this end, we performed an ensemble of transient simulations with an Earth system model of intermediate complexity from which we derived a best guess transient CO2 scenario for the interval from 3.2 to 2.4 Ma that gives the best fit between the simulated and reconstructed benthic δ18O and global sea surface temperature evolution. Our data-constrained CO2 scenarios are consistent with recent CO2 reconstructions and suggest a gradual CO2 decline from 375–425 to 275–300 ppm, between 3.2 and 2.4 Ma. In addition to a gradual decline, the best fit to paleoclimate data requires the existence of pronounced CO2 variability coherent with the 41-kyr (1 kyr = 1000 years) obliquity cycle. In our simulations the long-term CO2 decline is accompanied by a relatively abrupt intensification of Northern Hemisphere glaciation at around 2.7 Ma. This is the result of a threshold behaviour of the ice sheets response to gradual CO2 decrease and orbital forcing. The simulated Northern Hemisphere ice sheets during the early Pleistocene glacial cycles reach a maximum volume equivalent to a sea level drop of about 40 m. Both ice volume and benthic δ18O are dominated by 41-kyr cyclicity. Our simulations suggest that before 2.7 Ma Greenland was ice free during summer insolation maxima and only partly ice covered during periods of minimum summer insolation. A fully glaciated Greenland comparable to its present-day ice volume is modelled only during glacial maxima after 2.7 Ma and more continuously after 2.5 Ma.
Highlights
The PlioceneePleistocene transition (PPT) from around 3.2e3.0 to 2.5 Ma before present represented a major change in climate dynamics characterized by increased climate variability and the intensification of Northern Hemisphere glaciation at around 2.7 Ma (Maslin et al, 1998; Mudelsee and Raymo, 2005)
2015) and varied the temperature anomaly introduced to account for changes in the simulated Greenland ice sheet area as described in Section 2.1 in the range 1e3 C. 3) In a third step we explore the effect of the transient Greenland glaciations on climate and the rest of the NH ice sheets by prescribing the ice fractions of the GrIS modelled in step 2 in transient CLIMBER-2 experiments
In this paper we presented a novel model-based approach to reconstructing plausible CO2 scenarios that accompanied the PlioceneePleistocene transition
Summary
The PlioceneePleistocene transition (PPT) from around 3.2e3.0 to 2.5 Ma before present represented a major change in climate dynamics characterized by increased climate variability and the intensification of Northern Hemisphere glaciation (iNHG) at around 2.7 Ma (Maslin et al, 1998; Mudelsee and Raymo, 2005). Estimates of CO2 concentrations during the late Pliocene remain very uncertain and typically range between 350 and 450 ppm (Raymo et al, 1996; Pagani et al, 2010; Seki et al, 2010). It is believed that higher CO2 concentrations, different orography, vegetation cover and ice-sheet area contributed to the warmer Pliocene climate (Lunt et al, 2012; Willeit et al, 2013). According to various geological evidence, the peak sea level during the MPWP was 10e30 m higher than present Even the state of the Greenland ice sheet during the MPWP remains highly uncertain (Dolan et al, 2015)
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