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Author: Schellnhuber, H.J. × Subject: carbon cycle ×

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Now showing 1 - 3 of 3
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    Strong time dependence of ocean acidification mitigation by atmospheric carbon dioxide removal
    ([London] : Nature Publishing Group UK, 2019) Hofmann, M.; Mathesius, S.; Kriegler, E.; van Vuuren, D.P.; Schellnhuber, H.J.
    In Paris in 2015, the global community agreed to limit global warming to well below 2 ∘C, aiming at even 1.5 ∘C. It is still uncertain whether these targets are sufficient to preserve marine ecosystems and prevent a severe alteration of marine biogeochemical cycles. Here, we show that stringent mitigation strategies consistent with the 1.5 ∘C scenario could, indeed, provoke a critical difference for the ocean’s carbon cycle and calcium carbonate saturation states. Favorable conditions for calcifying organisms like tropical corals and polar pteropods, both of major importance for large ecosystems, can only be maintained if CO2 emissions fall rapidly between 2025 and 2050, potentially requiring an early deployment of CO2 removal techniques in addition to drastic emissions reduction. Furthermore, this outcome can only be achieved if the terrestrial biosphere remains a carbon sink during the entire 21st century.
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    Decomposing the effects of ocean warming on chlorophyll a concentrations into physically and biologically driven contributions
    (Bristol : IOP Publishing, 2013) Olonscheck, D.; Hofmann, M.; Worm, B.; Schellnhuber, H.J.
    Recently compiled observational data suggest a substantial decline in the global median chlorophyll a concentration over the 20th century, a trend that appears to be linked to ocean warming. Several modelling studies have considered changes in the ocean's physical structure as a possible cause, while experimental work supports a biological mechanism, namely an observed increase in zooplankton grazing rate that outpaces phytoplankton production at higher temperatures. Here, we present transient simulations derived from a coupled ocean general circulation and carbon cycle model forced by atmospheric fields under unabated anthropogenic global warming (IPCC SRES A1FI scenario). The simulations account for both physical and biological mechanisms, and can reproduce about one quarter of the observed chlorophyll a decline during the 20th century, when using realistically parameterized temperature sensitivity of zooplankton metabolism (Q10 between 2 and 4) and phytoplankton growth (Q10 ~ 1.9). Therefore, we have employed and re-calibrated the standard ecosystem model which assumes a lower temperature sensitivity of zooplankton grazing (Q10 = 1.1049) by re-scaling phytoplankton growth rates and zooplankton grazing rates. Our model projects a global chlorophyll a decline of >50% by the end of the 21st century. While phytoplankton abundance and chlorophyll a experience pronounced negative effects, primary production and zooplankton concentrations are less sensitive to ocean warming. Although changes in physical structure play an important role, much of the simulated change in chlorophyll a and productivity is related to the uneven temperature sensitivity of the marine ecosystem.
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    Reduction of biosphere life span as a consequence of geodynamics
    (Abingdon : Taylor and Francis Ltd., 2000) Franck, S.; Block, A.; Von Bloh, W.; Bounama, C.; Schellnhuber, H.J.; Svirezhev, Y.
    The long-term co-evolution of the geosphere-biosphere complex from the Proterozoic up to 1.5 billion years into the planet's future is investigated using a conceptual earth system model including the basic geodynamic processes. The model focusses on the global carbon cycles as mediated by life and driven by increasing solar luminosity and plate tectonics. The main CO2 sink, the weathering of silicates, is calculated as a function of biologic activity, global run-off and continental growth. The main CO2 source, tectonic processes dominated by sea-floor spreading, is determined using a novel semi-empirical scheme. Thus, a geodynamic extension of previous geostatic approaches can be achieved. As a major result of extensive numerical investigations, the 'terrestrial life corridor', i.e., the biogeophysical domain supporting a photosynthesis-based ecosphere in the planetary past and in the future, can be identified. Our findings imply, in particular, that the remaining life-span of the biosphere is considerably shorter (by a few hundred million years) than the value computed with geostatic models by other groups. The 'habitable-zone concept' is also revisited, revealing the band of orbital distances from the sun warranting earth-like conditions. It turns out that this habitable zone collapses completely in some 1.4 billion years from now as a consequence of geodynamics.