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  • 1
    Publication Date: 2019-09-24
    Description: Limited research has focussed on historical droughts during the pre-instrumental weather-recording period in semi-arid to arid human-inhabited environments. Here we describe the unique nature of droughts over semi-arid central Namibia (southern Africa) between 1850 and 1920. More particularly, our intention is to establish temporal shifts of influence and impact that historical droughts had on society and the environment during this period. This is achived through scrutinizing documentary records sourced from a variety of archives and libraries. The primary source of information comes from misssonary diaries, letters and reports. These missionaries were based at a variety of stations across the central Namibian region and thus collectively provide insight to sub-regional (or site specific) differences in hydro-meteorological conditions, and drought impacts and responses. Earliest instrumental rainfall records (1891–1913) from several missionary stations or settlements are used to quantify hydro-meteorological conditions and compare with documentary sources. The work demonstrates strong-sub-regional contrasts in drought conditions during some given drought events and the dire implications of failed rain seasons, the consequences of which lasted many months to several years. The paper advocates that human experience and associated reporting of drought events depends strongly on social, environmental, spatial and societal developmental situations and perspectives. To this end, the reported experiences, impacts and responses to drought over this 70 year period portray both common and changeable attributes through time.
    Print ISSN: 1814-9340
    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 2
    Publication Date: 2019-09-24
    Description: Droughts pose a climatic hazard that can have a profound impacts on past societies. Using documentary sources, this paper studies the occurrence and impacts of spring-summer droughts in pre-industrial England from 1200 to 1700. The types of records, source availability and changes in record keeping over time are described, and an overview of droughts in those 500 years is provided. The focus lies on a structural survey over the drought impacts most relevant to human livelihood. This includes the agricultural and pastoral sectors of agrarian production, health, the fire risk to settlements and the drop in water levels or dwindling of water supplies. Whereas due the specific characteristics of wheat cultivation in medieval and early modern England, the grain production was comparatively resilient to drought, livestock farming was under threat when rainfall fell noticeably below average. The most important problem in warm and dry summers, however, was the risk to health. Partly steeply raised mortality levels were associated with these conditions during the study period, because malaria, gastrointestinal disease and plague showed an affinity to heat and drought. Adaptation strategies to reduce the stress posed by summer droughts are included in the study.
    Print ISSN: 1814-9340
    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 3
    Publication Date: 2019-09-19
    Description: Droughts derive from a deficit of precipitation and belong to the most dangerous natural hazards for human societies. Documentary data of the pre-modern and early modern times contain direct and indirect information on precipitation that allow the production of reconstructions with the methods of historical climatology. For this study, two drought indices have been created on the basis of documentary data produced in Bern, Switzerland (DIB) and in Rouen, France (DIR) for the period from 1315 to 1715. These two indices have been compared to a third supra-regional drought index for Switzerland (SDI), Germany, France, the Netherlands, and Belgium synthesised from precipitation reconstruction based on historical climatology. The results of the study show that the documentary data from Bern mainly contain summer droughts, whereas the data from Rouen rather allow the reconstruction of spring droughts. The comparison of the three indices shows that the DIB and the DIR most probably do not contain all actual drought events, but they also detect droughts that do not appear in the SDI. This fact suggests that more documentary data from single places, such as historical city archives, should be examined in the future and added to larger reconstructions in order to obtain more complete drought reconstructions.
    Print ISSN: 1814-9340
    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 4
    Publication Date: 2019-09-19
    Description: In this study, the dead carbon fraction (DCF) variations in stalagmite M1-5 from Socotra Island in the western Arabian Sea were investigated through a new set of high-precision U-series and radiocarbon (14C) dates. The data reveal an extreme case of very high and also climate dependent DCF values. For M1-5 an average DCF of 56.2 ± 3.4 % is observed between 27 and 18 kyr BP. Such high DCF values indicate a high influence of aged soil organic matter (SOM) and nearly completely closed system carbonate dissolution conditions. Towards the end of the last glacial period decreasing Mg/Ca ratios suggest an increase in precipitation which caused a marked change in the soil carbon cycling as indicated by sharply decreasing DCF. This is in contrast to the relation of soil infiltration and reservoir age observed in stalagmites from temperate zones. For Socotra Island, which is influenced by the East African–Indian monsoon, we propose that more humid conditions and enhanced net-infiltration after the LGM led to denser vegetation and thus lowered the DCF by increased 14CO2 input into the soil zone. The onset of the Younger Dryas (YD) is represented in the record by the end of DCF decrease with a sudden change to much higher and extremely variable reservoir ages. Our study highlights the dramatic variability of soil carbon cycling processes and vegetation feedback on Socotra Island manifested in stalagmite reservoir ages on both long-term trends and sub-centennial timescales, thus providing evidence for climate influence on stalagmite radiocarbon. This is of particular importance for studies focussing on 14C calibration and atmospheric reconstruction through stalagmites which relies on largely climate independent soil carbon cycling above the cave.
    Print ISSN: 1814-9340
    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 5
    Publication Date: 2019-09-16
    Description: Imposing freshwater flux (FWF) variations in the North Atlantic is an effective method to cause reorganizations of the Atlantic Meridional Overturning Circulation (AMOC) in climate models. Through this approach, models have been able to reproduce the abrupt climate changes of the last glacial period. Such exercises have been useful for gaining insight into a wealth of processes regarding the widespread climatic consequences of AMOC variations. However, an issue that has passed unnoticed is the fact that the timing of the FWF applied in these studies is inconsistent with reconstructions. Here we focus on the deglaciation to show that imposing a FWF that is derived from the sea-level record results in a simulated AMOC evolution in a poor fit with the data, revealing an inconsistency between the generally accepted FWF mechanism and the resulting climatic impacts. Based on these negative results, we propose that the trigger of deglacial abrupt climate changes is not yet fully identified and that mechanisms other than FWF forcing should be explored more than ever.
    Print ISSN: 1814-9340
    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 6
    Publication Date: 2019-09-13
    Description: We investigate the changes in terrestrial natural methane emissions between the Last Glacial Maximum (LGM) and preindustrial (PI) by performing time-slice experiments with a methane-enabled version of MPI-ESM, the Max Planck Institute for Meteorology Earth System Model. We consider all natural sources of methane except for emissions from wild animals and geological sources, i.e. emissions from wetlands, fires, and termites. Changes are dominated by changes in tropical wetland emissions, with mid-to-high latitude wetlands playing a secondary role, and all other natural sources being of minor importance. The emissions are determined by the interplay of vegetation productivity, a function of CO2 and temperature, source area size, affected by sea level and ice sheet extent, and the state of the West African Monsoon, with increased emissions from north Africa during strong monsoon phases. We show that it is possible to explain the difference in atmospheric methane between LGM and PI purely by changes in emissions. As emissions more than double between LGM and PI, changes in the atmospheric lifetime of CH4, as proposed in other studies, are not required.
    Print ISSN: 1814-9340
    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 7
    Publication Date: 2019-09-12
    Description: The present article deals with the reconstruction of drought time series in Germany since 1500. The reconstructions are based on written records from the historical climate and environmental database tambora.org, early and official instrument data as well as precipitation and temperature indices. From the historical descriptions of the weather climatic processes and their effects and consequences for the environment and society, action paths and drought categories are derived. Furthermore, a historical precipitation index (HPI) is calculated and correlated with the SPI index. These are correlated and quantified in a rating scheme with modern rainfall indices and recent drought categories. Finally, a Historical Drought Index (HDI) and a Historical Wet Index (HWI) derived from the hygric indices are presented. On this basis, the long-term development of dryness and drought and significant accumulations and extremes in Germany since 1500 are discussed.
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    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 8
    Publication Date: 2019-09-09
    Description: The climate of the Southern Hemisphere (SH) is strongly influenced by variations in the El Niño-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Due to the temporally very limited instrumental records in most parts of the SH, very little is known about the relationship between these two key modes of variability and its stability over time. Here, we use proxy-based reconstructions and climate model simulations to quantify changes in tropical-extratropical SH teleconnections as represented by the correlation between the ENSO and SAM indices. Reconstructions indicate mostly negative correlations back to around 1400 CE confirming the pattern seen in the instrumental record over the last few decades. An ensemble of last millennium simulations of the model CESM1 confirms this pattern with very stable ensemble mean correlations around −0.3. Individual forced simulations, the pre-industrial control run and the proxy-based reconstructions indicate intermittent periods of positive correlations and particularly strong negative correlations. The fluctuations of the ENSO-SAM correlations are not significantly related to solar nor volcanic forcing in both proxy and model data, indicating that they are driven by internal variability in the climate system. Pseudoproxy experiments indicate that the currently available proxy records are able to reproduce the tropical-extratropical teleconnection patterns back to around 1600 CE. We analyse the spatial temperature and sea level pressure patterns during periods of positive and particularly strong negative teleconnections in the CESM model. Results indicate no consistent pattern during periods where the ENSO-SAM teleconnection changes its sign. However, periods of very strong negative SH teleconnections are associated with negative temperature anomalies across large fractions of the extra-tropical Pacific and a strengthening of the Aleutian Low.
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    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 9
    Publication Date: 2019-09-06
    Description: Global and regional climate changed dramatically with the expansion of the Antarctic Ice sheet at the Eocene-Oligocene Transition (EOT). These large-scale changes are generally linked to declining atmospheric pCO2 levels and/or changes in Southern Ocean gateways such as the Drake Passage around this time. To better understand the Southern Hemisphere regional climatic changes and the impact of glaciation on the Earth’s oceans and atmosphere at the EOT, we compiled a database of sea and land surface temperature reconstructions from a range of proxy records and compared this with a series of fully-coupled climate model simulations. Regional patterns in the proxy records of temperature show that cooling across the EOT was less at high latitudes and greater at mid-latitudes. Climate model simulations have some issues in capturing the zonal mean latitudinal temperature profiles shown by the proxy data, but certain simulations do show moderate-good performance at recreating the temperature patterns shown in the data. When taking into account the absolute temperature before and after the EOT, as well as the change in temperature across it, simulations with a closed Drake Passage before and after the EOT or with an opening of the Drake Passage across the EOT perform poorly, whereas simulations with a drop in atmospheric pCO2 in combination with ice growth generally perform better. This provides further support to previous research that changes in atmospheric pCO2 are more likely to have been the driver of the EOT climatic changes, as opposed to opening of the Drake Passage.
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    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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  • 10
    Publication Date: 2019-09-06
    Description: Volcanic eruptions are important drivers of climate variability on both seasonal and multi-decadal time scales as a result of atmosphere-ocean coupling. While the direct response after equatorial eruptions emerges as the positive phase of the North Atlantic Oscillation in the first two years after an eruption, less is known about high latitude northern hemisphere eruptions. In this study we assess the difference between equatorial and high latitude volcanic eruptions through the reconstructed atmospheric circulation and stable water isotope records of Greenland ice cores for the last millennia (1241–1979 CE), where the coupling mechanism behind the long-term response is addressed. The atmospheric circulation is studied through the four main modes of climate variability in the North Atlantic, the Atlanti Ridge (AtR), Scandinavian Blocking (ScB) and the positive and negative phase of the North Atlantic Oscillation (NAO+/NAO−). We report a difference in the atmospheric circulation response after equatorial eruptions compared to the response after high latitude eruptions, where NAO+ and AtR seem to be more associated with equatorial eruptions while NAO- and ScB seems to follow high latitude eruptions. This response is present during the first five years and then again in years 8–12 after both equatorial and high latitude eruptions. Such a prolonged response is evidence of an ocean-atmosphere coupling that is initiated through different mechanisms, where we suspect sea ice to play a key role.
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    Electronic ISSN: 1814-9359
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union (EGU).
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