Amazing stuff! Climate changes all the time for millions of years!
We still know very little about our oceans!
"Global mean sea level (GMSL) and climate are inextricably intertwined [???], a point of great relevance in our warming world. Clark et al. present a reconstruction of GMSL changes over the past 4.5 million years that accounts for temperature-driven changes in the oxygen isotopic compositions of the main ice sheets, as well as changes in ice volume and ocean temperature. This approach provides important insights about ice sheet volumes and variability and the forces that drive conditions over time."
From the abstract:
"Structured Abstract
INTRODUCTION
The oxygen isotopic composition (18O/16O) of benthic foraminifera shells (expressed as δ18Ob) has long provided a simple but powerful record of the long-term evolution of combined changes in deep ocean temperature (expressed as δ18OT) and global mean sea level, with the latter recorded by changes in the δ18O of seawater (expressed as δ18Osw) due to ice sheet growth and decay. More recently, a widely used strategy to isolate δ18Osw by regressing the δ18Ob record against times of known sea level reinforced the longstanding inference from the δ18Ob record that sea level change during the past 4.5 million years (Myr) experienced two transitions towards lower sea level lowstands.
The first transition [3 to 2.5 million years ago (Ma)] represents increasingly larger Northern Hemisphere (NH) ice sheets that fluctuated with a 41-thousand-year (kyr) periodicity.
The second middle Pleistocene transition (MPT) (1.2 to 0.62 Ma) is characterized by even larger fluctuations of NH ice sheets, which changed from a 41-kyr periodicity to a dominant 100-kyr periodicity.
In both cases, the transitions occurred in the absence of any changes in the orbital forcing by the Sun, suggesting that the cause(s) of these transitions was internal to the climate system.
RATIONALE
Despite general agreement that each of the two transitions represented an increase in the size of NH ice sheet fluctuations, the regression-based method to reconstruct sea level remains uncertain because by default, it reproduces the δ18Ob variability. Indeed, multiple lines of evidence—including from terrestrial, marine, and geophysical records that constrain ice sheet extent, as well as from δ18Osw which is derived by subtracting known δ18OT from the δ18Ob record—suggest that fluctuations of large NH ice sheets have occurred throughout the past 2.5 Myr, with the MPT thus recording a change in periodicity but not in ice sheet size. If correct, this scenario poses new challenges in understanding the 41-kyr periodicity of large ice sheets in a warmer world as well as the change in ice sheet variability during the MPT.
RESULTS
We converted a new δ18Osw record to sea level by applying a mass-balance approach that accounts for time-varying temperature and ice volume effects on the δ18O of ice sheets.
We find that the first transition in the δ18Ob record represents an increase in NH ice sheet fluctuations to a size that then reoccurred throughout the remaining 2.5 Myr whereas the second transition represents a decrease in mean ocean temperature accompanied by an increase in its variability.
Our climate model results show that the height of large pre-MPT ice sheets allowed them to be in surface mass balance under warmer-than-present global temperatures.
Growth and decay of these large ice sheets was paced by 41-kyr obliquity forcing, with deglaciation occurring once ice sheets exceeded a certain size and became unstable. We propose that changes in the Southern Ocean carbon cycle during the MPT modulated the response of global temperature and ice sheets to 41-kyr obliquity forcing, resulting in the emergence of the ~100-kyr signal.
CONCLUSION
Our work creates a new paradigm for the development of the Pleistocene ice ages, in particular the idea that large changes in sea level occurred throughout the Pleistocene rather than temperature varying with ice volume as the Earth cooled. In doing so, our findings modify as well as add several fundamental challenges to our understanding of ice sheet–climate interactions.
Underlying each of these challenges is the fact that the dominant orbital-scale sea level variability and its changes over the past 3 Myr are not the ones that would be predicted solely by the associated orbital forcing, suggesting internal feedbacks of the climate system that we propose are largely driven by changes in the Southern Ocean carbon cycle and the effects of these changes on CO2 and global temperature. High-resolution ice core CO2 records that extend beyond 0.8 Ma are needed to test our hypotheses."
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