the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Polar amplification of orbital-scale climate variability in the early Eocene greenhouse world
Abstract. Climate variability is typically amplified towards polar regions. The underlying causes, notably albedo and humidity changes, are challenging to accurately quantify with observations or models, hampering projections of future polar amplification. Polar amplification reconstructions from the ice-free early Eocene (~56–48 million years ago) can exclude ice albedo effects, but the required tropical temperature records for resolving timescales shorter than multi-million years are lacking. Here, we reconstruct early Eocene tropical sea surface temperature variability by presenting an up to ~4 kyr-resolution biomarker-based temperature record from Ocean Drilling Program Site 959, located in the tropical Atlantic Ocean. This record shows warming across multiple orbitally paced carbon cycle perturbations, coeval with high-latitude-derived deep-ocean bottom waters, showing that these events represent transient global warming events (hyperthermals). This implies that orbital forcing caused global temperature variability through carbon cycle feedbacks. Importantly, deep-ocean temperature variability was amplified by a factor 1.7–2.3 compared to the tropical surface ocean, corroborating available long-term estimates. This implies that fast atmospheric feedback processes controlled meridional temperature gradients on multi-million year, as well as orbital timescales during the early Eocene.
Our combined records have several other implications. First, our amplification factor is somewhat larger than the same metric in fully-coupled simulations of the early Eocene (1.1–1.3), suggesting that models slightly underestimate the non-ice related — notably hydrological — feedbacks that cause polar amplification of climate change. Second, even outside the hyperthermals, we find synchronous eccentricity-forced temperature variability in the tropics and deep ocean that represent global mean sea surface temperature variability of up to 0.7 °C, and requires significant variability in atmospheric pCO2. We hypothesize that the responsible carbon cycle feedbacks that are independent of ice, snow and frost-related processes might play an important role in Phanerozoic orbital-scale climate variability throughout geological time, including Pleistocene glacial-interglacial climate variability.
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Status: closed
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RC1: 'Comment on cp-2023-70', Anonymous Referee #1, 14 Nov 2023
- AC1: 'Reply on RC1', Chris Fokkema, 20 Mar 2024
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RC2: 'Comment on cp-2023-70', Anonymous Referee #2, 01 Mar 2024
Fokkema et al. reported orbitally-resolved SSTs and associated data from a tropical Atlantic site. They first confirmed that the Eocene “hyperthermal” events are present in the tropical ocean. Then, these TEX86-based SSTs were used together with benthic foram d18O to constrain Polar Amplification in the ice-free greenhouse world. They identified a persistent PA factor that appears to be robust across different archives, proxy calibrations, and timescales. They concluded that feedback other than ice-albedo, probably involving the carbon cycle, was in play. This is a solid study with a large number of measurements used to provide robust paleoenvironmental reconstructions, while considering potential caveats (such as upwelling at the studied site and different calibrations of TEX86). These results were used to strengthen the series of studies conducted recently by the authors and other folks to constrain PA over different periods of the Cenozoic. I actually reviewed an earlier version of this manuscript for a different journal and don’t have many remaining comments. I suggest publication after minor revisions.
The only thing I can think of is to use this opportunity to expand a little bit more on the discussion of feedback mechanisms operating on orbital timescales in the early Cenozoic. This should be feasible given the expertise of the climate dynamics in the author’s team. What could be important but currently missing in the model? Also, any thought experiment on the potential drivers of these carbon cycle changes on orbital cycles (I know we still haven’t really figured out the late Pleistocene G-IG CO2 changes, but what are the potential ways to balance the carbon budget of the Eocene? What factors could be more important in the greenhouse world than the Pleistocene)?
Citation: https://doi.org/10.5194/cp-2023-70-RC2 - AC2: 'Reply on RC2', Chris Fokkema, 20 Mar 2024
Status: closed
-
RC1: 'Comment on cp-2023-70', Anonymous Referee #1, 14 Nov 2023
- AC1: 'Reply on RC1', Chris Fokkema, 20 Mar 2024
-
RC2: 'Comment on cp-2023-70', Anonymous Referee #2, 01 Mar 2024
Fokkema et al. reported orbitally-resolved SSTs and associated data from a tropical Atlantic site. They first confirmed that the Eocene “hyperthermal” events are present in the tropical ocean. Then, these TEX86-based SSTs were used together with benthic foram d18O to constrain Polar Amplification in the ice-free greenhouse world. They identified a persistent PA factor that appears to be robust across different archives, proxy calibrations, and timescales. They concluded that feedback other than ice-albedo, probably involving the carbon cycle, was in play. This is a solid study with a large number of measurements used to provide robust paleoenvironmental reconstructions, while considering potential caveats (such as upwelling at the studied site and different calibrations of TEX86). These results were used to strengthen the series of studies conducted recently by the authors and other folks to constrain PA over different periods of the Cenozoic. I actually reviewed an earlier version of this manuscript for a different journal and don’t have many remaining comments. I suggest publication after minor revisions.
The only thing I can think of is to use this opportunity to expand a little bit more on the discussion of feedback mechanisms operating on orbital timescales in the early Cenozoic. This should be feasible given the expertise of the climate dynamics in the author’s team. What could be important but currently missing in the model? Also, any thought experiment on the potential drivers of these carbon cycle changes on orbital cycles (I know we still haven’t really figured out the late Pleistocene G-IG CO2 changes, but what are the potential ways to balance the carbon budget of the Eocene? What factors could be more important in the greenhouse world than the Pleistocene)?
Citation: https://doi.org/10.5194/cp-2023-70-RC2 - AC2: 'Reply on RC2', Chris Fokkema, 20 Mar 2024
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