| 1. |
- Chang, Kuang Yu, et al.
(författare)
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Observational constraints reduce model spread but not uncertainty in global wetland methane emission estimates
- 2023
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Ingår i: Global Change Biology. - 1354-1013. ; 29:15, s. 4298-4312
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Tidskriftsartikel (refereegranskat)abstract
- The recent rise in atmospheric methane (CH4) concentrations accelerates climate change and offsets mitigation efforts. Although wetlands are the largest natural CH4 source, estimates of global wetland CH4 emissions vary widely among approaches taken by bottom-up (BU) process-based biogeochemical models and top-down (TD) atmospheric inversion methods. Here, we integrate in situ measurements, multi-model ensembles, and a machine learning upscaling product into the International Land Model Benchmarking system to examine the relationship between wetland CH4 emission estimates and model performance. We find that using better-performing models identified by observational constraints reduces the spread of wetland CH4 emission estimates by 62% and 39% for BU- and TD-based approaches, respectively. However, global BU and TD CH4 emission estimate discrepancies increased by about 15% (from 31 to 36 TgCH4 year−1) when the top 20% models were used, although we consider this result moderately uncertain given the unevenly distributed global observations. Our analyses demonstrate that model performance ranking is subject to benchmark selection due to large inter-site variability, highlighting the importance of expanding coverage of benchmark sites to diverse environmental conditions. We encourage future development of wetland CH4 models to move beyond static benchmarking and focus on evaluating site-specific and ecosystem-specific variabilities inferred from observations.
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| 2. |
- Dinauer, Ashley, et al.
(författare)
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Mysteriously high I14C of the glacial atmosphere : Influence of 14C production and carbon cycle changes
- 2020
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 16:4, s. 1159-1185
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Tidskriftsartikel (refereegranskat)abstract
- Despite intense focus on the span classCombining double low lineinline-formulag1/4190/span drop in atmospheric span classCombining double low lineinline-formulaI14C/span during Heinrich Stadial 1 at span classCombining double low lineinline-formulag1/417.4/span-14.6 span classCombining double low lineinline-formulaka/span, the specific mechanisms responsible for the apparent span classCombining double low lineinline-formulaI14C/span excess in the glacial atmosphere have received considerably less attention. The computationally efficient Bern3D Earth system model of intermediate complexity, designed for long-term climate simulations, allows us to address a very fundamental but still elusive question concerning the atmospheric span classCombining double low lineinline-formulaI14C/span record: how can we explain the persistence of relatively high span classCombining double low lineinline-formulaI14C/span values during the millennia after the Laschamp event? Large uncertainties in the pre-Holocene span classCombining double low lineinline-formula14C/span production rate, as well as in the older portion of the span classCombining double low lineinline-formulaI14C/span record, complicate our qualitative and quantitative interpretation of the glacial span classCombining double low lineinline-formulaI14C/span elevation. Here we begin with sensitivity experiments that investigate the controls on atmospheric span classCombining double low lineinline-formulaI14C/span in idealized settings. We show that the interaction with the ocean sediments may be much more important to the simulation of span classCombining double low lineinline-formulaI14C/span than had been previously thought. In order to provide a bounded estimate of glacial span classCombining double low lineinline-formulaI14C/span change, the Bern3D model was integrated with five available estimates of the span classCombining double low lineinline-formula14C/span production rate as well as reconstructed and hypothetical paleoclimate forcing. Model results demonstrate that none of the available reconstructions of past changes in span classCombining double low lineinline-formula14C/span production can reproduce the elevated span classCombining double low lineinline-formulaI14C/span levels during the last glacial. In order to increase atmospheric span classCombining double low lineinline-formulaI14C/span to glacial levels, a drastic reduction of air-sea exchange efficiency in the polar regions must be assumed, though discrepancies remain for the portion of the record younger than span classCombining double low lineinline-formulag1/433/span span classCombining double low lineinline-formulaka/span. We end with an illustration of how the span classCombining double low lineinline-formula14C/span production rate would have had to evolve to be consistent with the span classCombining double low lineinline-formulaI14C/span record by combining an atmospheric radiocarbon budget with the Bern3D model. The overall conclusion is that the remaining discrepancies with respect to glacial span classCombining double low lineinline-formulaI14C/span may be linked to an underestimation of span classCombining double low lineinline-formula14C/span production and/or a biased-high reconstruction of span classCombining double low lineinline-formulaI14C/span over the time period of interest. Alternatively, we appear to still be missing an important carbon cycle process for atmospheric span classCombining double low lineinline-formulaI14C/span.
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| 3. |
- Fischer, Hubertus, et al.
(författare)
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Palaeoclimate constraints on the impact of 2 °C anthropogenic warming and beyond
- 2018
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Ingår i: Nature Geoscience. - : Springer Science and Business Media LLC. - 1752-0894 .- 1752-0908. ; 11:7, s. 474-485
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Tidskriftsartikel (refereegranskat)abstract
- Over the past 3.5 million years, there have been several intervals when climate conditions were warmer than during the pre-industrial Holocene. Although past intervals of warming were forced differently than future anthropogenic change, such periods can provide insights into potential future climate impacts and ecosystem feedbacks, especially over centennial-to-millennial timescales that are often not covered by climate model simulations. Our observation-based synthesis of the understanding of past intervals with temperatures within the range of projected future warming suggests that there is a low risk of runaway greenhouse gas feedbacks for global warming of no more than 2 °C. However, substantial regional environmental impacts can occur. A global average warming of 1–2 °C with strong polar amplification has, in the past, been accompanied by significant shifts in climate zones and the spatial distribution of land and ocean ecosystems. Sustained warming at this level has also led to substantial reductions of the Greenland and Antarctic ice sheets, with sea-level increases of at least several metres on millennial timescales. Comparison of palaeo observations with climate model results suggests that, due to the lack of certain feedback processes, model-based climate projections may underestimate long-term warming in response to future radiative forcing by as much as a factor of two, and thus may also underestimate centennial-to-millennial-scale sea-level rise.
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| 4. |
- Heinze, Christoph, et al.
(författare)
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The quiet crossing of ocean tipping points
- 2021
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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. ; 118:9
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Tidskriftsartikel (refereegranskat)abstract
- Anthropogenic climate change profoundly alters the ocean's environmental conditions, which, in turn, impact marine ecosystems. Some of these changes are happening fast and may be difficult to reverse. The identification and monitoring of such changes, which also includes tipping points, is an ongoing and emerging research effort. Prevention of negative impacts requires mitigation efforts based on feasible research-based pathways. Climate-induced tipping points are traditionally associated with singular catastrophic events (relative to natural variations) of dramatic negative impact. High-probability high-impact ocean tipping points due to warming, ocean acidification, and deoxygenation may be more fragmented both regionally and in time but add up to global dimensions. These tipping points in combination with gradual changes need to be addressed as seriously as singular catastrophic events in order to prevent the cumulative and often compounding negative societal and Earth system impacts.
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| 5. |
- Jungclaus, Johann H., et al.
(författare)
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The PMIP4 contribution to CMIP6 - Part 3 : The last millennium, scientific objective, and experimental design for the PMIP4 past1000 simulations
- 2017
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Ingår i: Geoscientific Model Development. - : Copernicus GmbH. - 1991-959X .- 1991-9603. ; 10:11, s. 4005-4033
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Tidskriftsartikel (refereegranskat)abstract
- The pre-industrial millennium is among the periods selected by the Paleoclimate Model Intercomparison Project (PMIP) for experiments contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and the fourth phase of the PMIP (PMIP4). The past1000 transient simulations serve to investigate the response to (mainly) natural forcing under background conditions not too different from today, and to discriminate between forced and internally generated variability on interannual to centennial timescales. This paper describes the motivation and the experimental set-ups for the PMIP4-CMIP6 past1000 simulations, and discusses the forcing agents orbital, solar, volcanic, and land use/land cover changes, and variations in greenhouse gas concentrations. The past1000 simulations covering the pre-industrial millennium from 850 Common Era (CE) to 1849 CE have to be complemented by historical simulations (1850 to 2014 CE) following the CMIP6 protocol. The external forcings for the past1000 experiments have been adapted to provide a seamless transition across these time periods. Protocols for the past1000 simulations have been divided into three tiers. A default forcing data set has been defined for the Tier 1 (the CMIP6 past1000) experiment. However, the PMIP community has maintained the flexibility to conduct coordinated sensitivity experiments to explore uncertainty in forcing reconstructions as well as parameter uncertainty in dedicated Tier 2 simulations. Additional experiments (Tier 3) are defined to foster collaborative model experiments focusing on the early instrumental period and to extend the temporal range and the scope of the simulations. This paper outlines current and future research foci and common analyses for collaborative work between the PMIP and the observational communities (reconstructions, instrumental data).
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| 6. |
- Lin, Shangrong, et al.
(författare)
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Underestimated Interannual Variability of Terrestrial Vegetation Production by Terrestrial Ecosystem Models
- 2023
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Ingår i: Global Biogeochemical Cycles. - 0886-6236. ; 37:4
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Tidskriftsartikel (refereegranskat)abstract
- Vegetation gross primary production (GPP) is the largest terrestrial carbon flux and plays an important role in regulating the carbon sink. Current terrestrial ecosystem models (TEMs) are indispensable tools for evaluating and predicting GPP. However, to which degree the TEMs can capture the interannual variability (IAV) of GPP remains unclear. With large data sets of remote sensing, in situ observations, and predictions of TEMs at a global scale, this study found that the current TEMs substantially underestimate the GPP IAV in comparison to observations at global flux towers. Our results also showed the larger underestimations of IAV in GPP at nonforest ecosystem types than forest types, especially in arid and semiarid grassland and shrubland. One cause of the underestimation is that the IAV in GPP predicted by models is strongly dependent on canopy structure, that is, leaf area index (LAI), and the models underestimate the changes of canopy physiology responding to climate change. On the other hand, the simulated interannual variations of LAI are much less than the observed. Our results highlight the importance of improving TEMs by precisely characterizing the contribution of canopy physiological changes on the IAV in GPP and of clarifying the reason for the underestimated IAV in LAI. With these efforts, it may be possible to accurately predict the IAV in GPP and the stability of the global carbon sink in the context of global climate change.
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| 7. |
- Lindgren, Amelie, 1987-
(författare)
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Northern Permafrost Region Soil Carbon Dynamics since the Last Glacial Maximum : a terrestrial component in the glacial to interglacial carbon cycle
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- At the Last Glacial Maximum (LGM), after ~100,000 years of relatively cold temperatures and progressively lower atmospheric carbon dioxide (CO2) concentrations, CO2 levels reached ~180 ppm, which is less than half of what we see today in a much warmer world (~400 ppm). Although much of this increase since the LGM is due to human-induced emissions, about 100 ppm of this increase can be attributed to natural variations seen over glacial to interglacial cycles. The sources for this natural CO2 rise remain unclear despite considerable efforts to constrain its origin. This thesis attempts to describe and quantify the role of soil carbon in this context, with emphasis on the permafrost hypothesis, which states that a shift from glacial to interglacial conditions released permafrost soil carbon to the atmosphere during the deglaciation. We present empirical estimates of the change in the Northern permafrost area between the LGM and present, and the associated soil carbon stock changes. We also partition these soil carbon stock changes at millennial intervals to capture not only the size but the timing of change. We find that the soil carbon stocks north of the Tropics decreased after the LGM to reach a minimum around 10,000 years ago, after which stocks increased to more than compensate for past losses. This may present part of a solution to untangle the marine and atmospheric 13C record, where the marine records suggest that the terrestrial carbon stock has grown since the LGM, while the atmospheric record also indicates terrestrial losses. To estimate the mineral soil carbon stocks, we have relied on vegetation reconstructions. Some of these reconstructions were created with a novel data-driven machine learning approach. This method may facilitate robust vegetation reconstruction when evidence of past conditions is readily available. Results in this thesis highlight the importance of permafrost, loess deposits and peatlands when considering the soil carbon cycle over long time scales.
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| 8. |
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| 9. |
- Muscheler, Raimund, et al.
(författare)
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Solar activity during the last 1000 yr inferred from radionuclide records
- 2006
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Ingår i: Quaternary Science Reviews. - 0277-3791. ; 26, s. 82-97
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Tidskriftsartikel (refereegranskat)abstract
- Identification of the causes of past climate change requires detailed knowledge of one of the most important natural factors—solar forcing. Prior to the period of direct solar observations, radionuclide abundances in natural archives provide the best-known proxies for changes in solar activity. Here we present two independent reconstructions of changes in solar activity during the last 1000 yr, which are inferred from 10Be and 14C records. We analyse the tree-ring 14C data (SHCal, IntCal04 from 1000 to 1510 AD and annual data from 1511 to 1950 AD) and four 10Be records from Greenland ice cores (Camp Century, GRIP, Milcent and Dye3) together with two 10Be records from Antarctic ice cores (Dome Concordia and South Pole). In general, the 10Be and 14C records exhibit good agreement that allows us to obtain reliable estimates of past solar magnetic modulation of the radionuclide production rates. Differences between 10Be records from Antarctica and Greenland indicate that climatic changes have influenced the deposition of 10Be during some periods of the last 1000 yr. The radionuclide-based reconstructions of past changes in solar activity do not always agree with the sunspot record, which indicates that the coupling between those proxies is not as close as has been sometimes assumed. The tree-ring 14C record and 10Be from Antarctica indicate that recent solar activity is high but not exceptional with respect to the last 1000 yr.
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| 10. |
- Muscheler, Raimund, et al.
(författare)
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Solar activity during the last 1000 yr inferred from radionuclide records
- 2007
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 26:1-2, s. 82-97
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Tidskriftsartikel (refereegranskat)abstract
- Identification of the causes of past climate change requires detailed knowledge of one of the most important natural factors-solar forcing. Prior to the period of direct solar observations, radionuclide abundances in natural archives provide the best-known proxies for changes in solar activity. Here we present two independent reconstructions of changes in solar activity during the last 1000 yr, which are inferred from Be-10 and C-14 records. We analyse the tree-ring C-14 data (SHCal, IntCa104 from 1000 to 1510 AD and annual data from 1511 to 1950 AD) and four Be-10 records from Greenland ice cores (Camp Century, GRIP, Milcent and Dye3) together with two Be-10 records from Antarctic ice cores (Dome Concordia and South Pole). In general, the Be-10 and C-14 records exhibit good agreement that allows us to obtain reliable estimates of past solar magnetic modulation of the radionuclide production rates. Differences between Be-10 records from Antarctica and Greenland indicate that climatic changes have influenced the deposition of Be-10 during some periods of the last 1000 yr. The radionuclide-based reconstructions of past changes in solar activity do not always agree with the sunspot record, which indicates that the coupling between those proxies is not as close as has been sometimes assumed. The tree-ring C-14 record and Be-10 from Antarctica indicate that recent solar activity is high but not exceptional with respect to the last 1000 yr. (c) 2006 Elsevier Ltd. All rights reserved.
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