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  • 1
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    The effect of obliquity-driven changes on paleoclimate sensitivity during the late Pleistocene (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 45 (13). pp. 6661-6671.
    Details
    Publication Date: 2021-02-08
    Description: We reanalyze existing paleodata of global mean surface temperature ΔTg and radiative forcing ΔR of CO2 and land ice albedo for the last 800,000 years to show that a state‐dependency in paleoclimate sensitivity S, as previously suggested, is only found if ΔTg is based on reconstructions, and not when ΔTg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land‐ice sheet growth and sea level fall) the multi‐millennial component of reconstructed ΔTg diverges from CO2, while in simulations both variables vary more synchronously, suggesting that the differences during these times are due to relatively low rates of simulated land ice growth and associated cooling. To produce a reconstruction‐based extrapolation of S for the future we exclude intervals with strong ΔTg‐CO2 divergence and find that S is less state‐dependent, or even constant (state‐independent), yielding a mean equilibrium warming of 2–4 K for a doubling of CO2.
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  • 2
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    Late Pleistocene Carbon Cycle Revisited by Considering Solid Earth Processes (2020)
    AGU (American Geophysical Union) | Wiley
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    Publication Date: 2023-02-08
    Description: The importance of volcanic CO2 release, continental weathering, and coral reef growth on the global carbon cycle has been highlighted by several different studies. Based on these independent approaches, we here revisit the last 800 kyr with the box model BICYCLE, which has been extended to be able to address these solid Earth contributions to the carbon cycle. We show that the volcanic outgassing of CO2 as a function of sea level change from mid‐ocean ridges and hot spot island volcanoes cannot be the generic process that leads during phases of falling obliquity to a sea level‐CO2 decoupling as has been suggested before. The combined contribution from continental and marine volcanism, if both lagging sea level change by 4 kyr, might have added up to 13 ppm to the glacial/interglacial CO2 rise. Coral reef growth as suggested by an independent model is during glacial terminations about an order of magnitude too high to be reconciled with meaningful carbon cycle dynamics. Global riverine input of bicarbonate caused by silicate and carbonate weathering is suggested to have been stable over Termination I. However, if weathering fluxes are changed by up to 50% in sensitivity experiments, the corresponding bicarbonate input might contribute less than 20 ppm to the deglacial atmospheric CO2 rise. The overall agreement of results with the new process‐based sediment module and the previously applied time‐delayed response function to mimic carbonate compensation gives confidence in the results obtained in previous applications of the BICYCLE model without solid Earth processes.
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  • 3
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    Toward Reconciling Radiocarbon Production Rates With Carbon Cycle Changes of the Last 55,000 Years (2022)
    AGU (American Geophysical Union) | Wiley
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    Publication Date: 2024-02-07
    Description: Since it is currently not understood how changes in 14C production rate (Q), and in the carbon cycle, can be combined to explain the reconstructed atmospheric Δ14C record, we discuss possible reasons for this knowledge gap. Reviewing the literature, we exclude that changes in the content of atoms in the atmosphere, which produce cosmogenic 14C after being hit by galactic cosmic rays, might be responsible for parts of the observed differences. When combining Q with carbon cycle changes, one needs to understand the changes in the atmospheric 14C inventory, which are partially counterintuitive. For example, during the Last Glacial Maximum, Δ14C was ∼400‰ higher compared with preindustrial times, but the 14C inventory was 10% smaller. Some pronounced changes in atmospheric Δ14C do not correspond to any significant changes in the atmospheric 14C inventory, since CO2 was changing simultaneously. Using two conceptually different models (BICYCLE-SE and LSG-OGCM), we derive hypothetical Qs by forcing the models with identical atmospheric CO2 and Δ14C data. Results are compared with the most recent data-based estimates of Q derived from cosmogenic isotopes. Millennial-scale climate change connected to the bipolar seesaw is missing in the applied models, which might explain some, but probably not all, of the apparent model-data disagreement in Q. Furthermore, Q based on either data from marine sediments or ice cores contains offsets, suggesting an interpretation deficit in the current data-based approaches.
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  • 4
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    Role of the Deglacial Buildup of the Great Barrier Reef for the Global Carbon Cycle (2022)
    AGU (American Geophysical Union) | Wiley
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    Publication Date: 2024-02-07
    Description: The carbon isotope 13C is commonly used to attribute the last deglacial atmospheric CO2 rise to various processes. Here we show that the growth of the world's largest reef system, the Great Barrier Reef (GBR), is marked by a pronounced decrease in δ13C in absolutely dated fossil coral skeletons between 12.8 and 11.7 ka, which coincides with a prominent minimum in atmospheric δ13CO2 and the Younger Dryas. The event follows the flooding of a large shelf platform and initiation of an extensive barrier reef system at 13 ka. Carbon cycle simulations show the coral δ13C decrease was mainly caused by the combination of isotopic fractionation during reef carbonate production and the decomposition of organic land carbon on the newly flooded shallow-water platform. The impacts of these processes on atmospheric CO2 and δ13CO2, however, are marginal. Thus, the GBR was not contributing to the last deglacial δ13CO2 minimum at ∼12.4 ka.
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