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Between about 55.5 and 52 million years ago, Earth experienced a series of sudden and extreme global warming events (hyperthermals) superimposed on a long-term warming trend (1). The first and [...]

Environmental conditions during the past 24 million years are thought to have been favourable for enhanced rates of atmospheric carbon dioxide drawdown by silicate chemical weathering (1-7). Proxy records indicate, however, that the Earth's atmospheric carbon dioxide concentrations did not fall below about 200-250 parts per million during this period (8). The stabilization of atmospheric carbon dioxide concentrations near this minimum value suggests that strong negative feedback mechanisms inhibited further drawdown of atmospheric carbon dioxide by high rates of global silicate rock weathering. Here we investigate one possible negative feedback mechanism, occurring under relatively low carbon dioxide concentrations and in warm climates, that is related to terrestrial plant productivity and its role in the decomposition of silicate minerals9-11. We use simulations of terrestrial and geochemical carbon cycles and available experimental evidence to show that vegetation activity in upland regions of active orogens was severely limited by near-starvation of carbon dioxide in combination with global warmth over this period. These conditions diminished biotic-driven silicate rock weathering and thereby attenuated an important long-term carbon dioxide sink. Although our modelling results are semi-quantitative and do not capture the full range of biogeochemical feedbacks that could influence the climate, our analysis indicates that the dynamic equilibrium between plants, climate and the geosphere probably buffered the minimum atmospheric carbon dioxide concentrations over the past 24 million years., Alkenone-based proxy records8 indicate that atmospheric carbon dioxide (C[O.sub.2]) concentrations ranged between 1,000 and 1,500 parts per million (p.p.m.) from about 45 to 34 million years (Myr) ago (the middle [...]

The Early Jurassic (early Toarcian, ca. 183 Ma) carbon cycle perturbation is characterized by an ~-5%0 [[delta.sup.13]C excursion in the exogenic carbon reservoirs, a 1000 ppm rise in atmospheric C[O.sub.2], and a 6-7 [degrees]C warming. Two proposed explanations for this presumed global carbon cycle perturbation are the liberation of massive amounts of isotopically light C[H.sub.4] from (1) Gondwanan coals by heating during the intrusive eruption of the Karoo-Ferrar large igneous province (LIP) or (2) the thermal dissociation of gas hydrates. Carbon cycle modeling indicates that the release of C[H.sub.4] from Gondwanan coals synchronous with the eruption of the Karoo-Ferrar LIP fails to reproduce the magnitude or timing of the C[O.sub.2] and [[delta.sup.13]C excursions. However, sensitivity analyses constrained by a marine cyclostratigraphically dated [[delta.sup.13]C record indicate that both features of geologic record can be explained with the huge input of ~15,340-24,750 Gt C over ~220 k.y., a result possibly pointing to the involvement of hydrothermal vent complexes in the Karoo Basin. The simulated release of >6000 Gt C from gas hydrates also reproduces aspects of the early Toarcian rock record, but the large mass involved raises fundamental questions about its formation, storage, and release. Keywords: atmospheric C[O.sub.2], carbon cycle, [[delta.sup.13]C, gas hydrates, Gondwanan coals.

Author(s): Gabriel J. Bowen (corresponding author) [1, 3]; David J. Beerling [2]; Paul L. Koch [1]; James C. Zachos [1]; Thomas Quattlebaum [1] An abrupt climate warming of 5 to [...]

The species-specific inverse relation between atmospheric [CO.sub.2] concentration and stomatal frequency for many woody angiosperm species is being used increasingly with fossil leaves to reconstruct past atmospheric C[O.sup.2] levels. To extend our limited knowledge of the responsiveness of conifer needles to C[O.sub.2] fluctuations, the stomatal frequency response of four native North American conifer species (Tsuga heterophylla, Picea glauca, Picea mariana, and Larix laricina) to a range of historical C[O.sub.2] mixing ratios (290 to 370 ppmV) was analyzed. Because of the specific mode of leaf development and the subsequent stomatal patterning in conifer needles, the stomatal index of these species was not affected by C[O.sub.2]. In contrast, a new measure of stomatal frequency, based on the number of stomata per millimeter of needle length, decreased significantly with increasing C[O.sub.2]. For Tsuga heterophylla, the stomatal frequency response to C[O.sub.2] changes in the last century is validated through assessment of the influence of other biological and environmental variables. Because of their sensitive response to C[O.sub.2], combined with a high preservation capacity, fossil needles of Tsuga heterophylla, Picea glauca, P. mariana, and Larix laricina have great potential for detecting and quantifying past atmospheric C[O.sub.2] fluctuations. Key words: C[O.sub.2]; conifers; leaf morphology; North America; quantification methods; stomata; stomatal density.

Paleoceanographic studies indicate that isotopic curves observed in the Mesozoic's Early Toarcian and Early Aptian eras suggest significant, episodic methane releases which generated warm conditions similar to the present-day greenhouse effect. Implications for global carbon research are discussed.

The benefits of implementing negative emission technologies for a century (years 2070-2170) on the global warming response to cumulative carbon emissions until year 2420 are assessed with a comprehensive set of intermediate complexity Earth system model integrations. Model integrations include 82 different model realisations covering a wide range of plausible climate states. The global warming response is assessed in terms of two key climate metrics: the effective transient climate response to cumulative CO2 emissions (eTCRE), measuring the surface warming response to cumulative carbon emissions and associated non-CO2 (RCP4.5) forcing, and the zero emissions commitment (ZEC), measuring the extent of any continued warming after net zero is reached. The TCRE is approximated from eTCRE by removing the contributions of non-CO2 forcing as 2.15 °C EgC-1 (with a 10-90 % range of 1.6 to 2.8 °C EgC-1). During the net positive emission phases, the eTCRE decreases from 2.62 (1.90 to 3.65) to 2.30 (1.73 to 3.23) °C EgC-1 due to a weakening in the increase in radiative forcing with an increase in atmospheric carbon, which is partly opposed by an increasing fraction of the radiative forcing warming the surface as the ocean stratifies. During the negative and zero emission phases, a progressive reduction of the eTCRE to 2.0 (1.4 to 2.8) °C EgC-1 is driven by the reducing airborne fraction as CO2 is drawn down by the ocean. The model uncertainty in the slopes of warming versus cumulative CO2 emissions varies from being controlled by the radiative feedback parameter during positive emissions to also being affected by ocean circulation and carbon-cycle parameters during zero or net-negative emissions. There is hysteresis in atmospheric CO[sub 2] and surface warming, where atmospheric CO2 and surface temperature are higher after peak emissions compared with before peak emissions. The continued warming after emissions cease defining the ZEC for the model mean without carbon capture is -0.01 °C at 25 years and decreases in time to -0.15 °C at 90 years after emissions cease. However, there is a spread in the ensemble with a temperature overshoot occurring in 50 % of the ensemble members at year 25. The ZEC only becomes negative in all ensemble members if modest carbon capture is included. Hence, incorporating negative emissions enhances the ability to meet climate targets and avoid risk of continued warming after net zero is reached. [ABSTRACT FROM AUTHOR]