An Early Cenozoic perspective on Greenhouse warming and carbon cycle dynamics

Department of Earth and Planetary Sciences, University of California at Santa Cruz, Santa Cruz, California 95060, USA.
Nature (Impact Factor: 41.46). 02/2008; 451(7176):279-83. DOI: 10.1038/nature06588
Source: PubMed


Past episodes of greenhouse warming provide insight into the coupling of climate and the carbon cycle and thus may help to predict the consequences of unabated carbon emissions in the future.

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    • "From the beginning of the Pleistocene (2.5 Myr ago) onwards, the long-term averages of both records nearly coincide . They show a similarly weakly declining trend over the Mid-Pleistocene Transition (1.5 to 0.7 Myr ago), when power in the δ 18 O spectrum shifts from 41 kyr to 100 kyr (Lisiecki and Raymo, 2005; Zachos et al., 2008; Bintanja and Van de Wal, 2008 ). Conversely , the higher variability in our simulation continues longer, lasting until the end of the Mid-Pleistocene Transition (0.8 Myr ago). "
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    ABSTRACT: During the past five million yrs, benthic δ18O records indicate a large range of climates, from warmer than today during the Pliocene Warm Period to considerably colder during glacials. Antarctic ice cores have revealed Pleistocene glacial–interglacial CO2 variability of 60–100 ppm, while sea level fluctuations of typically 125 m are documented by proxy data. However, in the pre-ice core period, CO2 and sea level proxy data are scarce and there is disagreement between different proxies and different records of the same proxy. This hampers comprehensive understanding of the long-term relations between CO2, sea level and climate. Here, we drive a coupled climate–ice sheet model over the past five million years, inversely forced by a stacked benthic δ18O record. We obtain continuous simulations of benthic δ18O, sea level and CO2 that are mutually consistent. Our model shows CO2 concentrations of 300 to 470 ppm during the Early Pliocene. Furthermore, we simulate strong CO2 variability during the Pliocene and Early Pleistocene. These features are broadly supported by existing and new δ11B-based proxy CO2 data, but less by alkenone-based records. The simulated concentrations and variations therein are larger than expected from global mean temperature changes. Our findings thus suggest a smaller Earth System Sensitivity than previously thought. This is explained by a more restricted role of land ice variability in the Pliocene. The largest uncertainty in our simulation arises from the mass balance formulation of East Antarctica, which governs the variability in sea level, but only modestly affects the modeled CO2 concentrations.
    Full-text · Article · Apr 2016 · Earth and Planetary Science Letters
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    • "The Paleocene–Eocene Thermal Maximum (PETM, ∼56 Ma) is the most well-studied example of a hyperthermal event and perhaps our best geologic analog for future climate change. Hyperthermals like the PETM are identified in the sedimentary record by three primary lines of evidence: 1) a negative carbon isotopic (δ 13 C) excursion (CIE) (McInerney and Wing, 2011; Zachos et al., 2008), 2) dissolution of marine carbonates (CaCO 3 ) (Zachos et al., 2005), and 3) a transient increase in temperature indicated by paleotemperature proxies (Dunkley Jones et al., 2013; Zachos et al., 2008). Together, these lines of evidence are consistent with the PETM being driven by massive release of isotopically records from shallow marine and terrestrial settings on the basis of carbon isotope stratigraphy (e.g. "
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    ABSTRACT: As an episode of rapid global warming associated with the release of massive quantities of carbon to the atmosphere and oceans, the Paleocene–Eocene Thermal Maximum (PETM, ∼56 Ma) is considered a potential analog for modern anthropogenic carbon emissions. However, the prevailing order of magnitude uncertainty in the rate of carbon release during the PETM precludes any straightforward comparison between the paleo-record and the modern. Similar barriers exist to the interpretation of many other carbon isotope excursions in the geological record. Here we use the Earth system model cGENIE to quantify the consequences of differing carbon emissions rates on the isotopic record of different carbon reservoirs. We explore the consequences of a range of emissions scenarios – from durations of carbon input of years to millennia and constant versus pulsed emissions rates, and trace how the isotopic signal is imprinted on the different carbon reservoirs. From this, we identify a characteristic relationship between the difference in carbon isotope excursion sizes between atmospheric CO2 and dissolved inorganic carbon (DIC) and the duration of carbon emissions. To the extent that available isotopic data spanning the PETM constrain the size of the marine and atmospheric carbon isotopic excursions, applying this empirical relationship suggests the duration of the component of carbon emissions that dominates the isotopic signal could be less than 3000 yr. However, utilizing the ratio of excursion size in the atmosphere to ocean as a metric to constrain duration of carbon emissions highlights the necessity to strengthen estimates for these two measurements across the PETM. Our general interpretive framework could be equally applied in assessing rates of carbon emissions for other geological events.
    Full-text · Article · Feb 2016 · Earth and Planetary Science Letters
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    • "By 1.8 Ma, hominins had expanded their range beyond the tropics of Africa and into more variable environments of Asia where they encountered new extremes of temperature and types of vegetation. There was substantial environmental instability in the Pliocene and Pleistocene (Zachos et al., 2008), as temperatures and rainfall fluctuated dramatically, especially after about 700,000 years ago. "

    Full-text · Article · Jan 2016
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