Stable water isotopes in the global water cycle: Atmospheric model simulations and application to proxy data

Stable isotopes of water recorded in polar ice cores are used to reconstruct past temperatures and the fractionation during phase changes make them a useful tracer of the hydrological cycle. This study focuses on the global and regional variations in the distribution of water isotopes with changes in the climate. Sensitivity experiments and time-slice simulations for the Last Glacial Maximum (LGM), Heinrich Stadial-1 and mid-Holocene climates were carried out both to understand the boundary conditions that exert the maximum influences on the isotopic composition of precipitation, and to reproduce the isotopic distribution of precipitation during these time periods. The numerical climate model, the National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM3.0 fitted with an oxygen-isotope module (Iso- CAM), is used to carry out the experiments. The first part of this study focuses on understanding the distribution of oxygen isotopes in precipitation during the LGM and to associate the anomalies from the control climate with the influence of different boundary condition constraints. Results from a pre-industrial control simulation are compared against experiments in which the influence of individual boundary conditions (greenhouse gases, ice-sheet albedo and topography, sea-surface temperature (SST), and orbital parameters) were changed each at a time to the LGM values to assess their individual impact. The results show that the SST and ice-sheet topography changes during the LGM are responsible for most of the modeled variations in the climate and hence the 18Oprecip distribution. In this study a detailed analysis of the seasonal and annual variations of 18Oprecip for the control and a combined LGM simulation is carried out. tion, the spatial and temporal slopes between the 18Oprecip and surface temperature are calculated for the combined LGM and control simulations over Greenland and Antarctica, which are compared with the reconstructions from the ice-cores and those simulated with other isotope models. Secondly, four different time slice experiments - preindustrial, mid-Holocene, LGM, and Heinrich Stadial-1 - were carried out to analyze the water isotope distribution over the African continent during these time periods. The local and non-local climate influences on the hydrogen isotope composition of precipitation ( Dprecip) during these different climates are investigated. The study highlights the strong impact of convection and rainout on the Dprecip over the tropics, along with the changes in large-scale circulation. In addition, model results for Dprecip for these time periods are compared with Dwax data obtained from the stable hydrogen isotope composition of plant leaf-wax n-alkanes, and show a qualitative agreement between the proxy and the model data. In a third part of the thesis, the present-day distribution of the isotopes in precipitation and water vapor were compared with the observations. The measurements of isotopes in water vapor have the advantage over the isotopes in precipitation that the observations are available around the year and also over arid regions where the precipitation events are very few. The study highlights the robustness of the results as well as some of the drawbacks of the model due to deficiencies in reproducing the hydrology over the land and because of the simplistic cloud isotope scheme

Global Tipping Points 2023 Report: Ch1.4 – Tipping points in ocean and atmosphere circulations.

This chapter assesses scientific evidence for tipping points across circulations in the ocean and atmosphere. The warming of oceans, modified wind patterns and increasing freshwater influx from melting ice hold the potential to disrupt established circulation patterns. We find evidence for tipping points in the Atlantic Meridional Overturning Circulation (AMOC), the North Atlantic Subpolar Gyre (SPG), and the Antarctic Overturning Circulation, which may collapse under warmer and ‘fresher’ (i.e. less salty) conditions. A slowdown or collapse of these oceanic circulations would have far-reaching consequences for the rest of the climate system, such as shifts in the monsoons. There is evidence that this has happened in the past, having led to vastly different states of the Sahara following abrupt changes in the West African monsoon, which we also classify as a tipping system. Evidence about tipping of the monsoons over South America and Asia is limited, however large-scale deforestation or air pollution are considered as potential sources of destabilisation. Although theoretically possible, there is little indication for tipping points in tropical clouds or mid-latitude atmospheric circulations. Similarly, tipping towards a more extreme or persistent El Niño Southern Oscillation (ENSO) state is not sufficiently supported by models and observations. While the thresholds for many of these systems are uncertain, tipping could be devastating for many millions of people. Stabilising climate (along with minimising other pressures, like aerosol pollution and ecosystem degradation) is critical for reducing the likelihood of reaching tipping points in the ocean-atmosphere system

Safe and just Earth system boundaries

The stability and resilience of the Earth system and human well-being are inseparably linked 1-3, yet their interdependencies are generally under-recognized; consequently, they are often treated independently 4,5. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice) 4. The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future

Safe and just Earth system boundaries

The stability and resilience of the Earth system and human well-being are inseparably linked1-3, yet their interdependencies are generally under-recognized; consequently, they are often treated independently4,5. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice)4. The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future

Stabilen Wasserisotopen im globalen Wasserkreislauf : Atmosphärische Modellsimulationen und Anwendung auf Proxy-Daten

Stable isotopes of water recorded in polar ice cores are used to reconstruct past temperatures and the fractionation during phase changes make them a useful tracer of the hydrological cycle. This study focuses on the global and regional variations in the distribution of water isotopes with changes in the climate. Sensitivity experiments and time-slice simulations for the Last Glacial Maximum (LGM), Heinrich Stadial-1 and mid-Holocene climates were carried out both to understand the boundary conditions that exert the maximum influences on the isotopic composition of precipitation, and to reproduce the isotopic distribution of precipitation during these time periods. The numerical climate model, the National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM3.0 fitted with an oxygen-isotope module (Iso- CAM), is used to carry out the experiments. The first part of this study focuses on understanding the distribution of oxygen isotopes in precipitation during the LGM and to associate the anomalies from the control climate with the influence of different boundary condition constraints. Results from a pre-industrial control simulation are compared against experiments in which the influence of individual boundary conditions (greenhouse gases, ice-sheet albedo and topography, sea-surface temperature (SST), and orbital parameters) were changed each at a time to the LGM values to assess their individual impact. The results show that the SST and ice-sheet topography changes during the LGM are responsible for most of the modeled variations in the climate and hence the 18Oprecip distribution. In this study a detailed analysis of the seasonal and annual variations of 18Oprecip for the control and a combined LGM simulation is carried out. tion, the spatial and temporal slopes between the 18Oprecip and surface temperature are calculated for the combined LGM and control simulations over Greenland and Antarctica, which are compared with the reconstructions from the ice-cores and those simulated with other isotope models. Secondly, four different time slice experiments - preindustrial, mid-Holocene, LGM, and Heinrich Stadial-1 - were carried out to analyze the water isotope distribution over the African continent during these time periods. The local and non-local climate influences on the hydrogen isotope composition of precipitation ( Dprecip) during these different climates are investigated. The study highlights the strong impact of convection and rainout on the Dprecip over the tropics, along with the changes in large-scale circulation. In addition, model results for Dprecip for these time periods are compared with Dwax data obtained from the stable hydrogen isotope composition of plant leaf-wax n-alkanes, and show a qualitative agreement between the proxy and the model data. In a third part of the thesis, the present-day distribution of the isotopes in precipitation and water vapor were compared with the observations. The measurements of isotopes in water vapor have the advantage over the isotopes in precipitation that the observations are available around the year and also over arid regions where the precipitation events are very few. The study highlights the robustness of the results as well as some of the drawbacks of the model due to deficiencies in reproducing the hydrology over the land and because of the simplistic cloud isotope scheme

Sources of water vapor and their effects on water isotopes in precipitation in the Indian monsoon region: a model-based assessment

Abstract Climate records of ratios of stable water isotopes of oxygen (δ18O) are used to reconstruct the past Indian monsoon precipitation. Identifying the sources of water vapor is important in understanding the role of monsoonal circulation in the δ18O values, to aid in monsoon reconstructions. Here, using an isotope-enabled Earth system model, we estimate the contributions of oceanic and terrestrial water vapor sources to two major precipitation seasons in India—the Southwest monsoon and the Northeast monsoon, and their effects on the δ18O in precipitation (δ18Op). We find that the two monsoon seasons have different dominant sources of water vapor because of the reversal in atmospheric circulation. While Indian Ocean regions, Arabian Sea, and recycling are the major sources of the Southwest monsoon precipitation, North Pacific Ocean and recycling are two crucial sources of Northeast monsoon precipitation. The δ18Op of the Southwest monsoon precipitation is determined by contributions from the Indian Ocean sources and recycling. Despite reduced precipitation, more negative δ18Op values are simulated in the Northeast monsoon season due to larger negative δ18Op contributions from the North Pacific. Our results imply that changes in atmospheric circulation and water vapor sources in past climates can influence climate reconstructions using δ18O

Potential roles of CO2 fertilization, nitrogen deposition, climate change, and land use and land cover change on the global terrestrial carbon uptake in the twenty-first century

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Modeling the sensitivity of precipitation oxygen isotopes to the Last Glacial Maximum boundary conditions: A study using Community Atmosphere Model (CAM3.0)

To understand the validity of d18O proxy records as indicators of past temperature change, a series of experiments was conducted using an atmospheric general circulation model fitted with water isotope tracers (Community Atmosphere Model version 3.0, IsoCAM). A pre-industrial simulation was performed as the control experiment, as well as a simulation with all the boundary conditions set to Last Glacial Maximum (LGM) values. Results from the pre-industrial and LGM simulations were compared to experiments in which the influence of individual boundary conditions (greenhouse gases, ice sheet albedo and topography, sea surface temperature (SST), and orbital parameters) were changed each at a time to assess their individual impact. The experiments were designed in order to analyze the spatial variations of the oxygen isotopic composition of precipitation (d18Oprecip) in response to individual climate factors. The change in topography (due to the change in land ice cover) played a significant role in reducing the surface temperature and d18Oprecip over North America. Exposed shelf areas and the ice sheet albedo reduced the Northern Hemisphere surface temperature and d18Oprecip further. A global mean cooling of 4.1 °C was simulated with combined LGM boundary conditions compared to the control simulation, which was in agreement with previous experiments using the fully coupled Community Climate System Model (CCSM3). Large reductions in d18Oprecip over the LGM ice sheets were strongly linked to the temperature decrease over them. The SST and ice sheet topography changes were responsible for most of the changes in the climate and hence the d18Oprecip distribution among the simulations

A review of the major drivers of the terrestrial carbon uptake: model-based assessments, consensus, and uncertainties

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