| 1. |
- Armstrong McKay, David I., et al.
(författare)
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Reduced carbon cycle resilience across the Palaeocene-Eocene Thermal Maximum
- 2018
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 14:10, s. 1515-1527
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Tidskriftsartikel (refereegranskat)abstract
- Several past episodes of rapid carbon cycle and climate change are hypothesised to be the result of the Earth system reaching a tipping point beyond which an abrupt transition to a new state occurs. At the Palaeocene-Eocene Thermal Maximum (PETM) at similar to 56 Ma and at subsequent hyperthermal events, hypothesised tipping points involve the abrupt transfer of carbon from surface reservoirs to the atmosphere. Theory suggests that tipping points in complex dynamical systems should be preceded by critical slowing down of their dynamics, including increasing temporal auto-correlation and variability. However, reliably detecting these indicators in palaeorecords is challenging, with issues of data quality, false positives, and parameter selection potentially affecting reliability. Here we show that in a sufficiently long, high-resolution palaeorecord there is consistent evidence of destabilisation of the carbon cycle in the similar to 1.5 Myr prior to the PETM, elevated carbon cycle and climate instability following both the PETM and Eocene Thermal Maximum 2 (ETM2), and different drivers of carbon cycle dynamics preceding the PETM and ETM2 events. Our results indicate a loss of resilience (weakened stabilising negative feedbacks and greater sensitivity to small shocks) in the carbon cycle before the PETM and in the carbon-climate system following it. This pre-PETM carbon cycle destabilisation may reflect gradual forcing by the contemporaneous North Atlantic Volcanic Province eruptions, with volcanism-driven warming potentially weakening the organic carbon burial feedback. Our results are consistent with but cannot prove the existence of a tipping point for abrupt carbon release, e.g. from methane hydrate or terrestrial organic carbon reservoirs, where as we find no support for a tipping point in deep ocean temperature.
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| 2. |
- Boysen, Lena R., et al.
(författare)
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The limits to global-warming mitigation by terrestrial carbon removal
- 2017
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Ingår i: Earth's Future. - 2328-4277. ; 5:5, s. 463-474
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Tidskriftsartikel (refereegranskat)abstract
- Massive near-term greenhouse gas emissions reduction is a precondition for staying well below 2 degrees C global warming as envisaged by the Paris Agreement. Furthermore, extensive terrestrial carbon dioxide removal (tCDR) through managed biomass growth and subsequent carbon capture and storage is required to avoid temperature overshoot in most pertinent scenarios. Here, we address two major issues: First, we calculate the extent of tCDR required to repair delayed or insufficient emissions reduction policies unable to prevent global mean temperature rise of 2.5 degrees C or even 4.5 degrees C above pre-industrial level. Our results show that those tCDR measures are unable to counteract business-as-usual emissions without eliminating virtually all natural ecosystems. Even if considerable (Representative Concentration Pathway 4.5 [RCP4.5]) emissions reductions are assumed, tCDR with 50% storage efficiency requires >1.1 Gha of the most productive agricultural areas or the elimination of > 50% of natural forests. In addition, > 100 MtN/yr fertilizers would be needed to remove the roughly 320 GtC foreseen in these scenarios. Such interventions would severely compromise food production and/or biosphere functioning. Second, we reanalyze the requirements for achieving the 160-190 GtC tCDR that would complement strong mitigation action (RCP2.6) in order to avoid 2 degrees C overshoot anytime. We find that a combination of high irrigation water input and/or more efficient conversion to stored carbon is necessary. In the face of severe trade-offs with society and the biosphere, we conclude that large-scale tCDR is not a viable alternative to aggressive emissions reduction. However, we argue that tCDR might serve as a valuable supporting actor for strong mitigation if sustainable schemes are established immediately. Plain Language Summary In 2015, parties agreed to limit global warming to well below 2 degrees C above pre-industrial levels. However, this requires not only massive near-term greenhouse gas emissions reductions but also the application of negative emission techniques that extract already emitted carbon dioxide from the atmosphere. Specifically, this could refer to the establishment of extensive plantations of fast-growing tree and grass species in combination with biomass conversion to carbon-saving products. Although such deployment is seen as promising, its carbon sequestration potentials and possible side-effects still remain to be studied in depth. In this study, we analyzed two feasibility aspects of such a negative emissions approach using biomass plantations and carbon utilization pathways. First, we show that biomass plantations with subsequent carbon immobilization are likely unable to repair insufficient emission reduction policies without compromising food production and biosphere functioning due to its space-consuming properties. Second, the requirements for a strong mitigation scenario staying below the 2 degrees C target would require a combination of high irrigation water input and development of highly effective carbon process chains. Although we find that this strategy of sequestering carbon is not a viable alternative to aggressive emission reductions, it could still support mitigation efforts if sustainably managed.
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| 3. |
- Lenton, Timothy M., et al.
(författare)
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Earliest land plants created modern levels of atmospheric oxygen
- 2016
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 113:35, s. 9704-9709
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Tidskriftsartikel (refereegranskat)abstract
- The progressive oxygenation of the Earth's atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O-2) first approached modern levels (similar to 21%) remains unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435-392 Ma, and the appearance of fossil charcoal indicates O-2 > 15-17% by 420-400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from similar to 470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burialthe net long-term source of O-2. We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved similar to 30% of today's global terrestrial net primary productivity by similar to 445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (similar to 2,000) than marine biomass (similar to 100), such that a given weathering flux of phosphorus could support more organic carbon burial. Furthermore, recent experiments suggest that early plants selectively increased the flux of phosphorus (relative to alkalinity) weathered from rocks. Combining these effects in a model of long-term biogeochemical cycling, we reproduce a sustained +2% increase in the carbonate carbon isotope (delta C-13) record by similar to 445 Ma, and predict a corresponding rise in O-2 to present levels by 420-400 Ma, consistent with geochemical data. This oxygen rise represents a permanent shift in regulatory regime to one where fire-mediated negative feedbacks stabilize high O-2 levels.
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| 4. |
- Steffen, Will, et al.
(författare)
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Trajectories of the Earth System in the Anthropocene
- 2018
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 115:33, s. 8252-8259
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Tidskriftsartikel (refereegranskat)abstract
- We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a Hothouse Earth pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System-biosphere, climate, and societies-and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values.
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