Abstract. Fighting land degradation of semi-arid and climate-sensitive grasslands are among the most urgent tasks of current eco-political agenda. Northern China and Mongolia are particularly prone to surface transformations caused by heavily increased livestock numbers during the 20th century. Extensive overgrazing and resource exploitation amplify regional climate change effects and trigger intensified surface transformation, which forces policy-driven interventions to prevent desertification. In the past, the region has been subject to major shifts in environmental and socio-cultural parameters, what makes it difficult to measure the extent of the regional anthropogenic impact and global climate change. This article analyses historical written sources, palaeoenvironmental data, and Normalized Difference Vegetation Index (NDVI) temporal series from the Moderate Resolution Imaging Spectroradiometer (MODIS) to compare landcover change during the Little Ice Age (LIA) and the reference period 2000–2018. Results show that decreasing precipitation and temperature records led to increased land degradation during the late 17th century. However, modern landcover data shows enhanced expansion of bare lands contrasting an increase in precipitation (Ptotal) and maximum temperature (Tmax). Vegetation response during the early growing season (March–May) and the late grazing season (September) does not relate to Ptotal and Tmax and generally low NDVI values indicate no major grassland recovery over the past 20 years.
{"title":"Monitoring landcover change and desertification processes in northern China and Mongolia using historical written sources and vegetation indices","authors":"Michael Kempf","doi":"10.5194/CP-2021-5","DOIUrl":"https://doi.org/10.5194/CP-2021-5","url":null,"abstract":"Abstract. Fighting land degradation of semi-arid and climate-sensitive grasslands are among the most urgent tasks of current eco-political agenda. Northern China and Mongolia are particularly prone to surface transformations caused by heavily increased livestock numbers during the 20th century. Extensive overgrazing and resource exploitation amplify regional climate change effects and trigger intensified surface transformation, which forces policy-driven interventions to prevent desertification. In the past, the region has been subject to major shifts in environmental and socio-cultural parameters, what makes it difficult to measure the extent of the regional anthropogenic impact and global climate change. This article analyses historical written sources, palaeoenvironmental data, and Normalized Difference Vegetation Index (NDVI) temporal series from the Moderate Resolution Imaging Spectroradiometer (MODIS) to compare landcover change during the Little Ice Age (LIA) and the reference period 2000–2018. Results show that decreasing precipitation and temperature records led to increased land degradation during the late 17th century. However, modern landcover data shows enhanced expansion of bare lands contrasting an increase in precipitation (Ptotal) and maximum temperature (Tmax). Vegetation response during the early growing season (March–May) and the late grazing season (September) does not relate to Ptotal and Tmax and generally low NDVI values indicate no major grassland recovery over the past 20 years.\u0000","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"37 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2021-02-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"121500278","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
C. Timmreck, M. Toohey, D. Zanchettin, S. Brönnimann, Elin Lundstadt, R. Wilson
Abstract. The 1809 eruption is one of the most recent unidentified volcanic eruptions with a global climate impact. Even though the eruption ranks as the 3rd largest since 1500 with an eruption magnitude estimated to be two times that of the 1991 eruption of Pinatubo, not much is known of it from historic sources. Based on a compilation of instrumental and reconstructed temperature time series, we show here that tropical temperatures show a significant drop in response to the ~1809 eruption, similar to that produced by the Mt. Tambora eruption in 1815, while the response of Northern Hemisphere (NH) boreal summer temperature is spatially heterogeneous. We test the sensitivity of the climate response simulated by the MPI Earth system model to a range of volcanic forcing estimates constructed using estimated volcanic stratospheric sulfur injections (VSSI) and uncertainties from ice core records. Three of the forcing reconstructions represent a tropical eruption with approximately symmetric hemispheric aerosol spread but different forcing magnitudes, while a fourth reflects a hemispherically asymmetric scenario without volcanic forcing in the NH extratropics. Observed and reconstructed post-volcanic surface NH summer temperature anomalies lie within the range of all the scenario simulations. Therefore, assuming the model climate sensitivity is correct, the VSSI estimate is accurate within the uncertainty bounds. Comparison of observed and simulated tropical temperature anomalies suggests that the most likely VSSI for the 1809 eruption would be somewhere between 12–19 Tg of sulfur. Model results show that NH large-scale climate modes are sensitive to both volcanic forcing strength and its spatial structure. While spatial correlations between the N-TREND NH temperature reconstruction and the model simulations are weak in terms of the ensemble mean model results, individual model simulations show good correlation over North America and Europe, suggesting the spatial heterogeneity of the 1810 cooling could be due to internal climate variability.
{"title":"The unidentified volcanic eruption of 1809: why it remains a\u0000climatic cold case","authors":"C. Timmreck, M. Toohey, D. Zanchettin, S. Brönnimann, Elin Lundstadt, R. Wilson","doi":"10.5194/CP-2021-4","DOIUrl":"https://doi.org/10.5194/CP-2021-4","url":null,"abstract":"Abstract. The 1809 eruption is one of the most recent unidentified volcanic eruptions with a global climate impact. Even though the eruption ranks as the 3rd largest since 1500 with an eruption magnitude estimated to be two times that of the 1991 eruption of Pinatubo, not much is known of it from historic sources. Based on a compilation of instrumental and reconstructed temperature time series, we show here that tropical temperatures show a significant drop in response to the ~1809 eruption, similar to that produced by the Mt. Tambora eruption in 1815, while the response of Northern Hemisphere (NH) boreal summer temperature is spatially heterogeneous. We test the sensitivity of the climate response simulated by the MPI Earth system model to a range of volcanic forcing estimates constructed using estimated volcanic stratospheric sulfur injections (VSSI) and uncertainties from ice core records. Three of the forcing reconstructions represent a tropical eruption with approximately symmetric hemispheric aerosol spread but different forcing magnitudes, while a fourth reflects a hemispherically asymmetric scenario without volcanic forcing in the NH extratropics. Observed and reconstructed post-volcanic surface NH summer temperature anomalies lie within the range of all the scenario simulations. Therefore, assuming the model climate sensitivity is correct, the VSSI estimate is accurate within the uncertainty bounds. Comparison of observed and simulated tropical temperature anomalies suggests that the most likely VSSI for the 1809 eruption would be somewhere between 12–19 Tg of sulfur. Model results show that NH large-scale climate modes are sensitive to both volcanic forcing strength and its spatial structure. While spatial correlations between the N-TREND NH temperature reconstruction and the model simulations are weak in terms of the ensemble mean model results, individual model simulations show good correlation over North America and Europe, suggesting the spatial heterogeneity of the 1810 cooling could be due to internal climate variability.","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"33 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2021-01-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130202335","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Alexandre Devers, J. Vidal, C. Lauvernet, O. Vannier
Abstract. Surface observations are usually too few and far between to properly assess multidecadal variations at the local scale and characterize historical local extreme events at the same time. A data assimilation scheme has been recently presented to assimilate daily observations of temperature and precipitation into downscaled reconstructions from a global extended reanalysis through an Ensemble Kalman fitting approach and derive high-resolution fields. Recent studies also showed that assimilating observations at high temporal resolution does not guarantee correct multidecadal variations. The current paper thus proposes (1) to apply this scheme over France and over the 1871–2012 period based on the SCOPE Climate reconstructions background dataset and all available daily historical surface observations of temperature and precipitation, (2) to develop an assimilation scheme at the yearly time scale and to apply it over the same period and lastly, (3) to derive the FYRE Climate reanalysis, a 25-member ensemble hybrid dataset resulting from the daily and yearly assimilation schemes, spanning the whole 1871–2012 period at a daily and 8-km resolution over France. Assimilating daily observations only allows reconstructing accurately daily characteristics, but fails in reproducing robust multidecadal variations when compared to independent datasets. Combining the daily and yearly assimilation schemes, FYRE Climate clearly performs better than the SCOPE Climate background in terms of bias, error, and correlation, but also better than the Safran reference surface reanalysis over France available from 1958 onward only. FYRE Climate also succeeds in reconstructing both local extreme events and multidecadal variability. It is made freely available from http://doi.org/10.5281/zenodo.4005573 (precipitation) and http://doi.org/10.5281/zenodo.4006472 (temperature).
{"title":"FYRE Climate: A high-resolution reanalysis of daily precipitation\u0000and temperature in France from 1871 to 2012","authors":"Alexandre Devers, J. Vidal, C. Lauvernet, O. Vannier","doi":"10.5194/CP-2020-156","DOIUrl":"https://doi.org/10.5194/CP-2020-156","url":null,"abstract":"Abstract. Surface observations are usually too few and far between to properly assess multidecadal variations at the local scale and characterize historical local extreme events at the same time. A data assimilation scheme has been recently presented to assimilate daily observations of temperature and precipitation into downscaled reconstructions from a global extended reanalysis through an Ensemble Kalman fitting approach and derive high-resolution fields. Recent studies also showed that assimilating observations at high temporal resolution does not guarantee correct multidecadal variations. The current paper thus proposes (1) to apply this scheme over France and over the 1871–2012 period based on the SCOPE Climate reconstructions background dataset and all available daily historical surface observations of temperature and precipitation, (2) to develop an assimilation scheme at the yearly time scale and to apply it over the same period and lastly, (3) to derive the FYRE Climate reanalysis, a 25-member ensemble hybrid dataset resulting from the daily and yearly assimilation schemes, spanning the whole 1871–2012 period at a daily and 8-km resolution over France. Assimilating daily observations only allows reconstructing accurately daily characteristics, but fails in reproducing robust multidecadal variations when compared to independent datasets. Combining the daily and yearly assimilation schemes, FYRE Climate clearly performs better than the SCOPE Climate background in terms of bias, error, and correlation, but also better than the Safran reference surface reanalysis over France available from 1958 onward only. FYRE Climate also succeeds in reconstructing both local extreme events and multidecadal variability. It is made freely available from http://doi.org/10.5281/zenodo.4005573 (precipitation) and http://doi.org/10.5281/zenodo.4006472 (temperature).\u0000","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"584 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2021-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132358433","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Gabriella M. Weiss, J. Lattaud, M. V. D. van der Meer, T. Eglinton
Abstract. The Baltic Sea experienced a number of marine transgressions and regressions throughout the Holocene. These fluctuations in sea level coupled with substantial regional ice melt led to isostatic adjustment and periodic isolation from the North Sea. Here, we determine the distributions and isotopic signatures of organic compounds preserved in a sediment record spanning the last ~ 11 ka in order to reconstruct environmental change under these dynamic conditions. Carbon and hydrogen isotope ratios of short-, mid-, and long-chain n-alkanes along with long-chain diol and glycerol dialkyl glycerol tetraether abundances were analyzed from Arkona Basin sediments sampled from the western Baltic Sea. In the earliest part of the record (10–8.2 ka), hydrogen isotope values of higher plant-derived n-alkanes revealed a change in dominant water source from an ice melt-derived to a precipitation-dominated hydrological regime. Following this shift in water source, carbon isotope values of n-alkanes suggest diversification of vegetation. Shifts in hydrology and vegetation did not coincide with established phase boundaries, but instead occurred mid-phase or spanned phase transitions, highlighting the fact that proxies may record changes on different time scales and suggesting that climate in the region was dynamic throughout the Holocene.
{"title":"Co-evolution of terrestrial and aquatic ecosystem structure with\u0000hydrological change in the Holocene Baltic Sea","authors":"Gabriella M. Weiss, J. Lattaud, M. V. D. van der Meer, T. Eglinton","doi":"10.5194/CP-2020-163","DOIUrl":"https://doi.org/10.5194/CP-2020-163","url":null,"abstract":"Abstract. The Baltic Sea experienced a number of marine transgressions and regressions throughout the Holocene. These fluctuations in sea level coupled with substantial regional ice melt led to isostatic adjustment and periodic isolation from the North Sea. Here, we determine the distributions and isotopic signatures of organic compounds preserved in a sediment record spanning the last ~ 11 ka in order to reconstruct environmental change under these dynamic conditions. Carbon and hydrogen isotope ratios of short-, mid-, and long-chain n-alkanes along with long-chain diol and glycerol dialkyl glycerol tetraether abundances were analyzed from Arkona Basin sediments sampled from the western Baltic Sea. In the earliest part of the record (10–8.2 ka), hydrogen isotope values of higher plant-derived n-alkanes revealed a change in dominant water source from an ice melt-derived to a precipitation-dominated hydrological regime. Following this shift in water source, carbon isotope values of n-alkanes suggest diversification of vegetation. Shifts in hydrology and vegetation did not coincide with established phase boundaries, but instead occurred mid-phase or spanned phase transitions, highlighting the fact that proxies may record changes on different time scales and suggesting that climate in the region was dynamic throughout the Holocene.","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"132 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2021-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128440108","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Liangjun Zhu, Shuguang Liu, Haifeng Zhu, D. Cooper, D. Yuan, Yu Zhu, Zongshan Li, Yuandong Zhang, Hanxue Liang, Xu Zhang, Wenqi Song, Xiaochun Wang
Abstract. The unstable sensitivity of growth-climate relationships greatly restricts tree-ring-based paleoclimate reconstructions, especially in areas with frequent divergence problems, such as the temperate zone in northeast China. Here, we propose an original tree-species mixing method to overcome this obstacle and improve the stability and reliability of reconstruction models. We take the tree-ring based growing-season minimum temperature reconstruction for the northern Changbai Mountains in northeast China as an example to illustrate the method. Compared with previous temperature reconstruction models, our reconstruction model is more stable and reliable and explains up to 68 % of the variance. It is also highly consistent with historical records and tree-ring-based temperature reconstructions from the nearby Xiaoxing'an Mountains and from across the Northern Hemisphere. Our reconstruction uses two different tree species and is more accurate than temperature reconstructions developed from a single species. Over the past 259 years (AD 1757–2015), five significant cold periods and five warm periods were identified. The reconstruction indicates rapid warming since the 1980s, which is consistent with other instrumental and reconstructed records. We also found the Atlantic Multidecadal Oscillation plays a crucial role in driving the growing-season minimum temperature in the northern Changbai Mountains.
{"title":"Supplementary material to \"Overcoming model instability in tree-ring-based temperature \u0000reconstructions using a multi-species method: A case study from the \u0000Changbai Mountains, northeastern China\"","authors":"Liangjun Zhu, Shuguang Liu, Haifeng Zhu, D. Cooper, D. Yuan, Yu Zhu, Zongshan Li, Yuandong Zhang, Hanxue Liang, Xu Zhang, Wenqi Song, Xiaochun Wang","doi":"10.5194/CP-2021-2","DOIUrl":"https://doi.org/10.5194/CP-2021-2","url":null,"abstract":"Abstract. The unstable sensitivity of growth-climate relationships greatly restricts tree-ring-based paleoclimate reconstructions, especially in areas with frequent divergence problems, such as the temperate zone in northeast China. Here, we propose an original tree-species mixing method to overcome this obstacle and improve the stability and reliability of reconstruction models. We take the tree-ring based growing-season minimum temperature reconstruction for the northern Changbai Mountains in northeast China as an example to illustrate the method. Compared with previous temperature reconstruction models, our reconstruction model is more stable and reliable and explains up to 68 % of the variance. It is also highly consistent with historical records and tree-ring-based temperature reconstructions from the nearby Xiaoxing'an Mountains and from across the Northern Hemisphere. Our reconstruction uses two different tree species and is more accurate than temperature reconstructions developed from a single species. Over the past 259 years (AD 1757–2015), five significant cold periods and five warm periods were identified. The reconstruction indicates rapid warming since the 1980s, which is consistent with other instrumental and reconstructed records. We also found the Atlantic Multidecadal Oscillation plays a crucial role in driving the growing-season minimum temperature in the northern Changbai Mountains.\u0000","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"26 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2021-01-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114732985","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-12-23DOI: 10.5194/cp-2020-160-supplement
J. Lohmann, A. Svensson
Abstract. A significant influence of major volcanic eruptions on regime shifts and long-term climate variability has been suggested previously. But a statistical assessment of this has been hampered by inaccurate synchronization of large volcanic eruptions to changes in past climate. Here, this is achieved by combining a new record of bipolar volcanism from Greenland and Antarctic ice cores with records of abrupt climate change derived from the same ice cores. We show that at > 99 % confidence bipolar volcanic eruptions occurred more frequently than expected by chance just before the onset of Dansgaard-Oeschger events, the most prominent large-scale abrupt climate changes of the last glacial period. Out of 20 climate change events in the 12–60 ka period, 5 (7) occur within 20 (50) years after a bipolar eruption. Thus, such large eruptions may act as short-term triggers for large-scale abrupt climate change, and may explain part of the variability of Dansgaard-Oeschger cycles.
{"title":"On the Role of Volcanism in Dansgaard-Oeschger Cycles","authors":"J. Lohmann, A. Svensson","doi":"10.5194/cp-2020-160-supplement","DOIUrl":"https://doi.org/10.5194/cp-2020-160-supplement","url":null,"abstract":"Abstract. A significant influence of major volcanic eruptions on regime shifts and long-term climate variability has been suggested previously. But a statistical assessment of this has been hampered by inaccurate synchronization of large volcanic eruptions to changes in past climate. Here, this is achieved by combining a new record of bipolar volcanism from Greenland and Antarctic ice cores with records of abrupt climate change derived from the same ice cores. We show that at > 99 % confidence bipolar volcanic eruptions occurred more frequently than expected by chance just before the onset of Dansgaard-Oeschger events, the most prominent large-scale abrupt climate changes of the last glacial period. Out of 20 climate change events in the 12–60 ka period, 5 (7) occur within 20 (50) years after a bipolar eruption. Thus, such large eruptions may act as short-term triggers for large-scale abrupt climate change, and may explain part of the variability of Dansgaard-Oeschger cycles.\u0000","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"46 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123631745","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract. Sea-ice cover over the Southern Ocean responds to and impacts Southern Ocean dynamics and, thus, mid to high latitude climate in the Southern Hemisphere. In addition, sea-ice cover can significantly modulate the carbon exchange between the atmosphere and the ocean. As climate models are the only tool available to project future climate changes, it is important to assess their performance in simulating past changes. The Last Glacial Maximum (LGM, ∼21,000 years ago) represents an interesting target as it is a relatively well documented period with climatic conditions and a carbon cycle very different from pre-industrial conditions. Here, we study the changes in seasonal Antarctic sea-ice cover as simulated in numerical PMIP3 and LOVECLIM simulations of the LGM, and their relationship with windstress and ocean temperature. Simulations and paleo-proxy records suggest a fairly well constrained glacial winter sea-ice edge at 51.5° S (1 sigma range: 50°–55.5° S). Simulated glacial summer sea-ice cover however differs widely between models, ranging from almost no sea ice to a sea-ice edge reaching 55.5° S. The austral summer multi-model mean sea-ice edge lies at ∼60.5° S (1 sigma range: 57.5°–70.5° S). Given the lack of strong constraints on the summer sea-ice edge based on sea-ice proxy records, we extend our model-data comparison to summer sea-surface temperature. Our analysis suggests that the multi-model mean summer sea ice provides a reasonable, albeit upper end, estimate of the austral summer sea-ice edge allowing us to conclude that the multi-model mean of austral summer and winter sea-ice cover seem to provide good estimates of LGM conditions. Using these best estimates, we find that there was a larger sea-ice seasonality during the LGM compared to the present day.
{"title":"Evaluating seasonal sea-ice cover over the Southern Ocean from the Last Glacial Maximum","authors":"Ryan A. Green, L. Menviel, K. Meissner, X. Crosta","doi":"10.5194/cp-2020-155","DOIUrl":"https://doi.org/10.5194/cp-2020-155","url":null,"abstract":"Abstract. Sea-ice cover over the Southern Ocean responds to and impacts Southern Ocean dynamics and, thus, mid to high latitude climate in the Southern Hemisphere. In addition, sea-ice cover can significantly modulate the carbon exchange between the atmosphere and the ocean. As climate models are the only tool available to project future climate changes, it is important to assess their performance in simulating past changes. The Last Glacial Maximum (LGM, ∼21,000 years ago) represents an interesting target as it is a relatively well documented period with climatic conditions and a carbon cycle very different from pre-industrial conditions. Here, we study the changes in seasonal Antarctic sea-ice cover as simulated in numerical PMIP3 and LOVECLIM simulations of the LGM, and their relationship with windstress and ocean temperature. Simulations and paleo-proxy records suggest a fairly well constrained glacial winter sea-ice edge at 51.5° S (1 sigma range: 50°–55.5° S). Simulated glacial summer sea-ice cover however differs widely between models, ranging from almost no sea ice to a sea-ice edge reaching 55.5° S. The austral summer multi-model mean sea-ice edge lies at ∼60.5° S (1 sigma range: 57.5°–70.5° S). Given the lack of strong constraints on the summer sea-ice edge based on sea-ice proxy records, we extend our model-data comparison to summer sea-surface temperature. Our analysis suggests that the multi-model mean summer sea ice provides a reasonable, albeit upper end, estimate of the austral summer sea-ice edge allowing us to conclude that the multi-model mean of austral summer and winter sea-ice cover seem to provide good estimates of LGM conditions. Using these best estimates, we find that there was a larger sea-ice seasonality during the LGM compared to the present day.\u0000","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"31 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123234744","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
T. Leutert, S. Modestou, S. Bernasconi, A. N. Meckler
Abstract. The middle Miocene climate transition (MMCT, ~14.5–13.0 Ma) was associated with a significant expansion of Antarctic ice, but the mechanisms triggering the event remain enigmatic. We present a new clumped isotope (∆47) bottom water temperature (BWT) record from 16.0 Ma to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean, and compare it to existing BWT records. We show that BWTs in the Southern Ocean were ~8–10 °C during the middle Miocene greenhouse, and thus considerably warmer than today. Nonetheless, bottom water δ18O (calculated from foraminiferal δ18O and ∆47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset demonstrates that BWTs at Site 747 decreased by ~3–5 °C across the MMCT. This cooling preceded the stepped main increase in global ice volume, and appears to have been followed by a transient bottom water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Additional processes and feedbacks required for substantial ice growth may have contributed to the observed decoupling of Southern Ocean BWT and global ice volume.
{"title":"Southern Ocean bottom water cooling and ice sheet expansion\u0000during the middle Miocene climate transition","authors":"T. Leutert, S. Modestou, S. Bernasconi, A. N. Meckler","doi":"10.5194/cp-2020-157","DOIUrl":"https://doi.org/10.5194/cp-2020-157","url":null,"abstract":"Abstract. The middle Miocene climate transition (MMCT, ~14.5–13.0 Ma) was associated with a significant expansion of Antarctic ice, but the mechanisms triggering the event remain enigmatic. We present a new clumped isotope (∆47) bottom water temperature (BWT) record from 16.0 Ma to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean, and compare it to existing BWT records. We show that BWTs in the Southern Ocean were ~8–10 °C during the middle Miocene greenhouse, and thus considerably warmer than today. Nonetheless, bottom water δ18O (calculated from foraminiferal δ18O and ∆47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset demonstrates that BWTs at Site 747 decreased by ~3–5 °C across the MMCT. This cooling preceded the stepped main increase in global ice volume, and appears to have been followed by a transient bottom water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Additional processes and feedbacks required for substantial ice growth may have contributed to the observed decoupling of Southern Ocean BWT and global ice volume.\u0000","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"17 8","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"120909997","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Venugopal, N. Bertler, Rebecca L. Pyne, H. Kjær, V. Winton, P. Mayewski, G. Cortese
Abstract. Nitrate (NO3−), an abundant aerosol in polar snow, is a complex environmental proxy to interpret owing to the variety of its sources and its susceptibility to post-depositional processes. During the last glacial period, when the dust level in the Antarctic atmosphere was higher than today by a factor up to ~25, mineral dust appears to have a stabilizing effect on the NO3− concentration. However, the exact mechanism remains unclear. Here, we present new and highly resolved records of NO3− and non-sea salt calcium (nssCa2+, a proxy for mineral dust) from the Roosevelt Island Climate Evolution (RICE) ice core for the period 26–40 kilo years Before Present (ka BP). This interval includes seven millennial-scale Antarctic Isotope Maxima (AIM) events, against the background of a glacial climate state. We observe a significant correlation between NO3− and nssCa2+ over this period and especially during AIM events. We put our observation into a spatial context by comparing the records to existing data from east Antarctic cores of EPICA Dome C (EDC), Vostok and central Dome Fuji. The data suggest that nssCa2+ is contributing to the effective scavenging of NO3− from the atmosphere through the formation of Ca(NO3)2. The geographic pattern implies that the process of Ca(NO3)2 formation occurs during the long-distance transport of mineral dust from the mid-latitude source regions by Southern Hemisphere Westerly Winds (SHWW) and most likely over the Southern Ocean. Since NO3− is dust-bound and the level of dust mobilized through AIM events is mainly regulated by the latitudinal position of SHWW, we suggest that NO3− may also have the potential to provide insights into paleo-westerly wind pattern during the events.
摘要硝态氮(NO3−)是极地雪中丰富的气溶胶，由于其来源的多样性和对沉积后过程的敏感性，是一个复杂的环境代用物。在末次冰期，当南极大气中的粉尘水平比今天高约25倍时，矿物粉尘似乎对NO3−浓度具有稳定作用。然而，确切的机制尚不清楚。在这里，我们获得了来自罗斯福岛气候演化(RICE)冰芯的26 - 40k年前(ka BP)的NO3−和非海盐钙(nssCa2+，矿物粉尘的代用物)的新的高分辨率记录。这一间隔包括在冰川气候状态背景下的7个千年尺度的南极同位素极大期(AIM)事件。我们观察到NO3−和nssCa2+在这一时期，特别是在AIM事件期间具有显著的相关性。通过与EPICA Dome C (EDC)、Vostok和中央Dome Fuji的南极东部岩心的现有数据进行比较，我们将观测结果置于空间背景下。这些数据表明，nssCa2+通过形成Ca(NO3)2，有助于有效清除大气中的NO3−。地理格局表明，Ca(NO3)2的形成过程发生在中纬度源区矿物粉尘被南半球西风(SHWW)长距离输送的过程中，且极有可能发生在南大洋上空。由于NO3−是沙尘束缚的，而通过AIM事件动员的沙尘水平主要受SHWW纬度位置的调节，我们认为NO3−可能也有可能在事件期间提供对古西风模式的见解。
{"title":"Mineral Dust Influence on the Glacial Nitrate Record from the RICE\u0000Ice Core, West Antarctica and Environmental Implications","authors":"A. Venugopal, N. Bertler, Rebecca L. Pyne, H. Kjær, V. Winton, P. Mayewski, G. Cortese","doi":"10.5194/cp-2020-151","DOIUrl":"https://doi.org/10.5194/cp-2020-151","url":null,"abstract":"Abstract. Nitrate (NO3−), an abundant aerosol in polar snow, is a complex environmental proxy to interpret owing to the variety of its sources and its susceptibility to post-depositional processes. During the last glacial period, when the dust level in the Antarctic atmosphere was higher than today by a factor up to ~25, mineral dust appears to have a stabilizing effect on the NO3− concentration. However, the exact mechanism remains unclear. Here, we present new and highly resolved records of NO3− and non-sea salt calcium (nssCa2+, a proxy for mineral dust) from the Roosevelt Island Climate Evolution (RICE) ice core for the period 26–40 kilo years Before Present (ka BP). This interval includes seven millennial-scale Antarctic Isotope Maxima (AIM) events, against the background of a glacial climate state. We observe a significant correlation between NO3− and nssCa2+ over this period and especially during AIM events. We put our observation into a spatial context by comparing the records to existing data from east Antarctic cores of EPICA Dome C (EDC), Vostok and central Dome Fuji. The data suggest that nssCa2+ is contributing to the effective scavenging of NO3− from the atmosphere through the formation of Ca(NO3)2. The geographic pattern implies that the process of Ca(NO3)2 formation occurs during the long-distance transport of mineral dust from the mid-latitude source regions by Southern Hemisphere Westerly Winds (SHWW) and most likely over the Southern Ocean. Since NO3− is dust-bound and the level of dust mobilized through AIM events is mainly regulated by the latitudinal position of SHWW, we suggest that NO3− may also have the potential to provide insights into paleo-westerly wind pattern during the events.","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"20 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124947389","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
N. Yeung, L. Menviel, K. Meissner, A. Taschetto, T. Ziehn, M. Chamberlain
Abstract. Due to different orbital configurations, high northern latitude boreal summer insolation was higher during the Last Interglacial period (LIG; 129–116 thousand years before present, ka) than during the preindustrial period (PI), while high southern latitude austral summer insolation was lower. The climatic response to these changes is studied here with focus on the southern hemispheric monsoons, by performing an equilibrium experiment of the LIG at 127 ka with the Australian Earth System Model, ACCESS-ESM1.5, as part of the Paleoclimate Model Intercomparison Project 4 (PMIP4). In our simulation, mean surface air temperature increases by 6.5 °C over land during boreal summer between 40° N and 60° N in the LIG compared to PI, leading to a northward shift of the Inter-Tropical Convergence Zone (ITCZ) and a strengthening of the North African and Indian monsoons. Despite 0.4 °C cooler conditions in austral summer in the Southern Hemisphere (0–90° S), annual mean air temperatures are 1.2 °C higher at southern mid-to-high latitudes (40° S–80° S). These differences in temperature are coincident with a large-scale reorganisation of the atmospheric circulation. The ITCZ shifts southward in the Atlantic and Indian sectors during the LIG austral summer compared to PI, leading to increased precipitation over the southern tropical oceans. However, the decline in Southern Hemisphere insolation during austral summer induces a significant cooling over land, which in turn weakens the land-sea temperature contrast, leading to an overall reduction (−20 %) in monsoonal precipitation over the Southern Hemisphere's continental regions. The intensity and areal extent of the Australian, South American and South African monsoons are consistently reduced. This is associated with greater pressure and subsidence over land due to a strengthening of the southern hemispheric Hadley cell during austral summer.
{"title":"Weak Southern Hemispheric monsoons during the Last Interglacial\u0000period","authors":"N. Yeung, L. Menviel, K. Meissner, A. Taschetto, T. Ziehn, M. Chamberlain","doi":"10.5194/cp-2020-149","DOIUrl":"https://doi.org/10.5194/cp-2020-149","url":null,"abstract":"Abstract. Due to different orbital configurations, high northern latitude boreal summer insolation was higher during the Last Interglacial period (LIG; 129–116 thousand years before present, ka) than during the preindustrial period (PI), while high southern latitude austral summer insolation was lower. The climatic response to these changes is studied here with focus on the southern hemispheric monsoons, by performing an equilibrium experiment of the LIG at 127 ka with the Australian Earth System Model, ACCESS-ESM1.5, as part of the Paleoclimate Model Intercomparison Project 4 (PMIP4). In our simulation, mean surface air temperature increases by 6.5 °C over land during boreal summer between 40° N and 60° N in the LIG compared to PI, leading to a northward shift of the Inter-Tropical Convergence Zone (ITCZ) and a strengthening of the North African and Indian monsoons. Despite 0.4 °C cooler conditions in austral summer in the Southern Hemisphere (0–90° S), annual mean air temperatures are 1.2 °C higher at southern mid-to-high latitudes (40° S–80° S). These differences in temperature are coincident with a large-scale reorganisation of the atmospheric circulation. The ITCZ shifts southward in the Atlantic and Indian sectors during the LIG austral summer compared to PI, leading to increased precipitation over the southern tropical oceans. However, the decline in Southern Hemisphere insolation during austral summer induces a significant cooling over land, which in turn weakens the land-sea temperature contrast, leading to an overall reduction (−20 %) in monsoonal precipitation over the Southern Hemisphere's continental regions. The intensity and areal extent of the Australian, South American and South African monsoons are consistently reduced. This is associated with greater pressure and subsidence over land due to a strengthening of the southern hemispheric Hadley cell during austral summer.","PeriodicalId":263057,"journal":{"name":"Climate of The Past Discussions","volume":"45 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125042784","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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