Diurnal temperature range (DTR) is a useful index of climatic change in addition to mean temperat... more Diurnal temperature range (DTR) is a useful index of climatic change in addition to mean temperature changes. Observational records indicate that DTR has decreased over the last 50 years due to differential changes in minimum and maximum temperatures. However, modelled changes in DTR in previous climate model simulations of this period are smaller than those observed, primarily due to an overestimate of changes in maximum temperatures. This present study examines DTR trends using the latest generation of global climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5), and utilises the novel CMIP5 detection and attribution experimental design of variously forced historical simulations (natural only, greenhouse gas only and all anthropogenic and natural forcings). Comparison of observed and modelled changes in DTR over the period of 1951-2005 again reveals that global DTR trends are lower in model simulations than observed across the 27 member multi-model ensemble analysed here. Modelled DTR trends are similar for both experiments incorporating all forcings and for the historical experiment with greenhouse gases only, while no DTR trend is discernible in the naturally-forced historical experiment. The persistent underestimate of DTR changes in this latest multi-model evaluation appears to be related to ubiquitous model deficiencies in cloud cover and land surface processes that impact the accurate simulation of regional minimum or maximum temperatures changes observed during this period. Different model processes are likely responsible for subdued simulated DTR trends over the various analysed regions.
[1] Anthropogenic contributions to the record hot 2013 Australian summer are investigated using a... more [1] Anthropogenic contributions to the record hot 2013 Australian summer are investigated using a suite of climate model experiments. This was the hottest Australian summer in the observational record. Australian area-average summer temperatures for simulations with natural forcings only were compared to simulations with anthropogenic and natural forcings for the period 1976–2005 and the RCP8.5 high emission simulation (2006–2020) from nine CMIP5 models. Using fraction of attributable risk to compare the likelihood of extreme Australian summer temperatures between the experiments, it was very likely (>90% confidence) there was at least a 2.5 times increase in the odds of extreme heat due to human influences using simulations to 2005, and a five-fold increase in this risk using simulations for 2006–2020. The human contribution to the increased odds of Australian summer extremes like 2013 was substantial, while natural climate variations alone, including El Niño Southern Oscillation, are unlikely to explain the record temperature.
Deuterium excess (d) is interpreted in conventional paleoclimate reconstructions as a tracer of o... more Deuterium excess (d) is interpreted in conventional paleoclimate reconstructions as a tracer of oceanic source region conditions, such as temperature, where precipitation originates. Previous studies have adopted coisotopic approaches (using both d18O and d) to estimate past changes in both site and oceanic source temperatures for ice core sites using empirical relationships derived from conceptual distillation models, particularly Mixed
Cloud Isotopic Models (MCIMs). However, the relationship between d and oceanic surface conditions remains unclear in past contexts. We investigate this climate-isotope relationship for sites in Greenland and Antarctica using multiple simulations of the water
isotope-enabled Goddard Institute for Space Studies ModelE-R general circulation model and apply a novel suite of model vapor source distribution (VSD) tracers to assess d as a proxy for source temperature variability under a range of climatic conditions. Simulated average source temperatures determined by the VSDs are compared to synthetic source temperature estimates calculated using MCIM equations linking d to source region
conditions. We show that although deuterium excess is generally a faithful tracer of source temperatures as estimated by the MCIM approach, large discrepancies in the isotope-climate relationship occur around Greenland during the Last Glacial Maximum simulation, when precipitation seasonality and moisture source regions were notably different from the present. This identified sensitivity in d as a source temperature proxy suggests that quantitative climate reconstructions from deuterium excess should be treated with caution for some sites when boundary conditions are significantly different from the present day. Also, the exclusion of the influence of humidity and other evaporative source changes inMCIMregressions may be a limitation of quantifying source temperature fluctuations from deuterium excess in some instances.
Both North and East Africa experienced more humid conditions during the early and mid-Holocene ep... more Both North and East Africa experienced more humid conditions during the early and mid-Holocene epoch (11,000-5,000 yr BP; 11-5 ka) relative to today. The North African Humid Period has been a major focus of paleoclimatic study, and is now understood to represent a response of the hydrological cycle to the increase in boreal summer insolation and associated ocean, atmosphere and land surface feedbacks. Meanwhile, the mechanisms responsible for the coeval East African Humid Period are poorly understood. Here, we use results from isotope-enabled coupled climate modeling experiments to investigate the cause of the East African Humid Period. The modeling results are interpreted alongside proxy records of both water balance and the isotopic composition of rainfall. Our simulations show that the orbitally-induced increase in dry season precipitation and the subsequent reduction in precipitation seasonality can explain the East African Humid Period, and this scenario agrees well with regional lake level and pollen paleoclimate data. Changes in zonal moisture flux from both the Atlantic and Indian Ocean account for the simulated increase in precipitation from June through November. Isotopic paleoclimate data and simulated changes in moisture source demonstrate that the western East African Rift Valley in particular experienced more humid conditions due to the influx of Atlantic moisture and enhanced convergence along the Congo Air Boundary. Our study demonstrates that zonal changes in moisture advection are an important determinant of climate variability in the East African region.
Little is known about the possible teleconnections between abrupt climatic changes originating in... more Little is known about the possible teleconnections between abrupt climatic changes originating in the North Atlantic and precipitation dynamics in the Australian–Indonesian summer monsoon (AISM) domain. We examine the climatic impacts of Heinrich stadial 3 (HS3) and Greenland interstadials 4 and 3 (GIS4/3) on AISM-associated precipitation through a high resolution analysis of stable isotope (δ18O, δ13C) and trace element (Mg/Ca, P/Ca) ratios in a stalagmite from Liang Luar cave, Flores, Indonesia. Sixteen high precision 230Th dates indicate that stalagmite LR07-E1 grew rapidly (~0.3–1.0 mm/yr) in two phases between ~31.5– 30.1 ka and ~27.8–25.6 ka, separated by a ~2.3 kyr unconformity. Temporally consistent abrupt responses occur in the Flores record during HS3 and GIS4, which are coherent with changes in stalagmite δ18O records from China and Brazil. The response of low-latitude precipitation to HS3 cooling and GIS4 warming, as demonstrated by the widely separated sites, comprises three distinct simplified phases: (1) a strong southward migration of the ITCZ during HS3 is associated with a decrease in rainfall at Liang Luar cave and in China, while wetter conditions are reconstructed from Brazil, (2) represents the peak of HS3 impacts and an extended hiatus begins in the Flores record and (3) where suggested dry conditions at Liang Luar throughout GIS4 form part of a coherent north–south anti-phasing in precipitation changes. The reconstructed changes are also broadly consistent with NASA GISS ModelE-R simulations of a Heinrich-like freshwater perturbation in the North Atlantic basin, which produces a southward shift in the ITCZ. The relationship between the palaeoclimate records indicates that atmospheric teleconnections rapidly propagate and synchronise climate change across the hemispheres during periods of abrupt climate change. Our findings augment recent proposals that large-scale atmospheric re-organisations during stadials and interstadials play a key role in driving changes in atmospheric CO2 concentration, air temperature and global climate change.
Water isotope records such as speleothems provide extensive evidence of past tropical hydrologica... more Water isotope records such as speleothems provide extensive evidence of past tropical hydrological changes. During Heinrich events, isotopic changes in monsoon regions have been interpreted as implying a widespread drying through the Northern Hemisphere tropics and an antiphased precipitation response in the south. Here, we examine the sources of this variability using a water isotope-enabled general circulation model, Goddard Institute for Space Studies ModelE. We incorporate a new suite of vapour source distribution tracers to help constrain the impact of precipitation source region changes on the isotopic composition of precipitation and to identify nonlocal amount effects. We simulate a collapse of the North Atlantic meridional overturning circulation with a large freshwater input to the region as an idealised analogue to iceberg discharge during Heinrich events. An increase in monsoon intensity, defined by vertical wind shear, is modelled over the South American domain, with small decreases simulated over Asia. Simulated isotopic anomalies agree well with proxy climate records, with lighter isotopic values simulated over South America and enriched values across East Asia. For this particular abrupt climate event, we identify which climatic change is most likely linked to water isotope change – changes in local precipitation amount, monsoon intensity, water vapour source distributions or precipitation seasonality. We categorise individual sites according to the climate variability that water isotope changes are most closely associated with, and find that the dominant isotopic controls are not consistent across the tropics – simple local explanations, in particular, fall short of explaining water isotope variability at all sites. Instead, the best interpretations appear to be site specific and often regional in scale.
Recent drilling eventually reached to the base of the sediments of Lynch's Crater on the Atherton... more Recent drilling eventually reached to the base of the sediments of Lynch's Crater on the Atherton Tableland within the humid tropics of north-eastern Australia. This paper incorporates results from the sequence extension into a complete record from the site, much of which has only been presented previously in summary form. A more certain chronology for the record is provided by recent radiocarbon dating on the topmost sediments and the application of time series analysis to the whole sequence. This suggests that the vegetation and climate are forced essentially by northern hemisphere insolation and ice volume, probably operating through sea level and sea surface temperature changes. The extended record is considered to cover the last 230,000 years and the pattern of complex rainforest expansion during wetter interglacial periods and its replacement by drier rainforest and sclerophyll vegetation during drier glacials is maintained. Superimposed on this cyclicity are trends resulting from both the evolution of the lake basin as well as external influences that include climate and, within the last 45,000 years, people.
Diurnal temperature range (DTR) is a useful index of climatic change in addition to mean temperat... more Diurnal temperature range (DTR) is a useful index of climatic change in addition to mean temperature changes. Observational records indicate that DTR has decreased over the last 50 years due to differential changes in minimum and maximum temperatures. However, modelled changes in DTR in previous climate model simulations of this period are smaller than those observed, primarily due to an overestimate of changes in maximum temperatures. This present study examines DTR trends using the latest generation of global climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5), and utilises the novel CMIP5 detection and attribution experimental design of variously forced historical simulations (natural only, greenhouse gas only and all anthropogenic and natural forcings). Comparison of observed and modelled changes in DTR over the period of 1951-2005 again reveals that global DTR trends are lower in model simulations than observed across the 27 member multi-model ensemble analysed here. Modelled DTR trends are similar for both experiments incorporating all forcings and for the historical experiment with greenhouse gases only, while no DTR trend is discernible in the naturally-forced historical experiment. The persistent underestimate of DTR changes in this latest multi-model evaluation appears to be related to ubiquitous model deficiencies in cloud cover and land surface processes that impact the accurate simulation of regional minimum or maximum temperatures changes observed during this period. Different model processes are likely responsible for subdued simulated DTR trends over the various analysed regions.
[1] Anthropogenic contributions to the record hot 2013 Australian summer are investigated using a... more [1] Anthropogenic contributions to the record hot 2013 Australian summer are investigated using a suite of climate model experiments. This was the hottest Australian summer in the observational record. Australian area-average summer temperatures for simulations with natural forcings only were compared to simulations with anthropogenic and natural forcings for the period 1976–2005 and the RCP8.5 high emission simulation (2006–2020) from nine CMIP5 models. Using fraction of attributable risk to compare the likelihood of extreme Australian summer temperatures between the experiments, it was very likely (>90% confidence) there was at least a 2.5 times increase in the odds of extreme heat due to human influences using simulations to 2005, and a five-fold increase in this risk using simulations for 2006–2020. The human contribution to the increased odds of Australian summer extremes like 2013 was substantial, while natural climate variations alone, including El Niño Southern Oscillation, are unlikely to explain the record temperature.
Deuterium excess (d) is interpreted in conventional paleoclimate reconstructions as a tracer of o... more Deuterium excess (d) is interpreted in conventional paleoclimate reconstructions as a tracer of oceanic source region conditions, such as temperature, where precipitation originates. Previous studies have adopted coisotopic approaches (using both d18O and d) to estimate past changes in both site and oceanic source temperatures for ice core sites using empirical relationships derived from conceptual distillation models, particularly Mixed
Cloud Isotopic Models (MCIMs). However, the relationship between d and oceanic surface conditions remains unclear in past contexts. We investigate this climate-isotope relationship for sites in Greenland and Antarctica using multiple simulations of the water
isotope-enabled Goddard Institute for Space Studies ModelE-R general circulation model and apply a novel suite of model vapor source distribution (VSD) tracers to assess d as a proxy for source temperature variability under a range of climatic conditions. Simulated average source temperatures determined by the VSDs are compared to synthetic source temperature estimates calculated using MCIM equations linking d to source region
conditions. We show that although deuterium excess is generally a faithful tracer of source temperatures as estimated by the MCIM approach, large discrepancies in the isotope-climate relationship occur around Greenland during the Last Glacial Maximum simulation, when precipitation seasonality and moisture source regions were notably different from the present. This identified sensitivity in d as a source temperature proxy suggests that quantitative climate reconstructions from deuterium excess should be treated with caution for some sites when boundary conditions are significantly different from the present day. Also, the exclusion of the influence of humidity and other evaporative source changes inMCIMregressions may be a limitation of quantifying source temperature fluctuations from deuterium excess in some instances.
Both North and East Africa experienced more humid conditions during the early and mid-Holocene ep... more Both North and East Africa experienced more humid conditions during the early and mid-Holocene epoch (11,000-5,000 yr BP; 11-5 ka) relative to today. The North African Humid Period has been a major focus of paleoclimatic study, and is now understood to represent a response of the hydrological cycle to the increase in boreal summer insolation and associated ocean, atmosphere and land surface feedbacks. Meanwhile, the mechanisms responsible for the coeval East African Humid Period are poorly understood. Here, we use results from isotope-enabled coupled climate modeling experiments to investigate the cause of the East African Humid Period. The modeling results are interpreted alongside proxy records of both water balance and the isotopic composition of rainfall. Our simulations show that the orbitally-induced increase in dry season precipitation and the subsequent reduction in precipitation seasonality can explain the East African Humid Period, and this scenario agrees well with regional lake level and pollen paleoclimate data. Changes in zonal moisture flux from both the Atlantic and Indian Ocean account for the simulated increase in precipitation from June through November. Isotopic paleoclimate data and simulated changes in moisture source demonstrate that the western East African Rift Valley in particular experienced more humid conditions due to the influx of Atlantic moisture and enhanced convergence along the Congo Air Boundary. Our study demonstrates that zonal changes in moisture advection are an important determinant of climate variability in the East African region.
Little is known about the possible teleconnections between abrupt climatic changes originating in... more Little is known about the possible teleconnections between abrupt climatic changes originating in the North Atlantic and precipitation dynamics in the Australian–Indonesian summer monsoon (AISM) domain. We examine the climatic impacts of Heinrich stadial 3 (HS3) and Greenland interstadials 4 and 3 (GIS4/3) on AISM-associated precipitation through a high resolution analysis of stable isotope (δ18O, δ13C) and trace element (Mg/Ca, P/Ca) ratios in a stalagmite from Liang Luar cave, Flores, Indonesia. Sixteen high precision 230Th dates indicate that stalagmite LR07-E1 grew rapidly (~0.3–1.0 mm/yr) in two phases between ~31.5– 30.1 ka and ~27.8–25.6 ka, separated by a ~2.3 kyr unconformity. Temporally consistent abrupt responses occur in the Flores record during HS3 and GIS4, which are coherent with changes in stalagmite δ18O records from China and Brazil. The response of low-latitude precipitation to HS3 cooling and GIS4 warming, as demonstrated by the widely separated sites, comprises three distinct simplified phases: (1) a strong southward migration of the ITCZ during HS3 is associated with a decrease in rainfall at Liang Luar cave and in China, while wetter conditions are reconstructed from Brazil, (2) represents the peak of HS3 impacts and an extended hiatus begins in the Flores record and (3) where suggested dry conditions at Liang Luar throughout GIS4 form part of a coherent north–south anti-phasing in precipitation changes. The reconstructed changes are also broadly consistent with NASA GISS ModelE-R simulations of a Heinrich-like freshwater perturbation in the North Atlantic basin, which produces a southward shift in the ITCZ. The relationship between the palaeoclimate records indicates that atmospheric teleconnections rapidly propagate and synchronise climate change across the hemispheres during periods of abrupt climate change. Our findings augment recent proposals that large-scale atmospheric re-organisations during stadials and interstadials play a key role in driving changes in atmospheric CO2 concentration, air temperature and global climate change.
Water isotope records such as speleothems provide extensive evidence of past tropical hydrologica... more Water isotope records such as speleothems provide extensive evidence of past tropical hydrological changes. During Heinrich events, isotopic changes in monsoon regions have been interpreted as implying a widespread drying through the Northern Hemisphere tropics and an antiphased precipitation response in the south. Here, we examine the sources of this variability using a water isotope-enabled general circulation model, Goddard Institute for Space Studies ModelE. We incorporate a new suite of vapour source distribution tracers to help constrain the impact of precipitation source region changes on the isotopic composition of precipitation and to identify nonlocal amount effects. We simulate a collapse of the North Atlantic meridional overturning circulation with a large freshwater input to the region as an idealised analogue to iceberg discharge during Heinrich events. An increase in monsoon intensity, defined by vertical wind shear, is modelled over the South American domain, with small decreases simulated over Asia. Simulated isotopic anomalies agree well with proxy climate records, with lighter isotopic values simulated over South America and enriched values across East Asia. For this particular abrupt climate event, we identify which climatic change is most likely linked to water isotope change – changes in local precipitation amount, monsoon intensity, water vapour source distributions or precipitation seasonality. We categorise individual sites according to the climate variability that water isotope changes are most closely associated with, and find that the dominant isotopic controls are not consistent across the tropics – simple local explanations, in particular, fall short of explaining water isotope variability at all sites. Instead, the best interpretations appear to be site specific and often regional in scale.
Recent drilling eventually reached to the base of the sediments of Lynch's Crater on the Atherton... more Recent drilling eventually reached to the base of the sediments of Lynch's Crater on the Atherton Tableland within the humid tropics of north-eastern Australia. This paper incorporates results from the sequence extension into a complete record from the site, much of which has only been presented previously in summary form. A more certain chronology for the record is provided by recent radiocarbon dating on the topmost sediments and the application of time series analysis to the whole sequence. This suggests that the vegetation and climate are forced essentially by northern hemisphere insolation and ice volume, probably operating through sea level and sea surface temperature changes. The extended record is considered to cover the last 230,000 years and the pattern of complex rainforest expansion during wetter interglacial periods and its replacement by drier rainforest and sclerophyll vegetation during drier glacials is maintained. Superimposed on this cyclicity are trends resulting from both the evolution of the lake basin as well as external influences that include climate and, within the last 45,000 years, people.
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Papers by Sophie Lewis
Cloud Isotopic Models (MCIMs). However, the relationship between d and oceanic surface conditions remains unclear in past contexts. We investigate this climate-isotope relationship for sites in Greenland and Antarctica using multiple simulations of the water
isotope-enabled Goddard Institute for Space Studies ModelE-R general circulation model and apply a novel suite of model vapor source distribution (VSD) tracers to assess d as a proxy for source temperature variability under a range of climatic conditions. Simulated average source temperatures determined by the VSDs are compared to synthetic source temperature estimates calculated using MCIM equations linking d to source region
conditions. We show that although deuterium excess is generally a faithful tracer of source temperatures as estimated by the MCIM approach, large discrepancies in the isotope-climate relationship occur around Greenland during the Last Glacial Maximum simulation, when precipitation seasonality and moisture source regions were notably different from the present. This identified sensitivity in d as a source temperature proxy suggests that quantitative climate reconstructions from deuterium excess should be treated with caution for some sites when boundary conditions are significantly different from the present day. Also, the exclusion of the influence of humidity and other evaporative source changes inMCIMregressions may be a limitation of quantifying source temperature fluctuations from deuterium excess in some instances.
interpretations appear to be site specific and often regional in
scale.
tropics of north-eastern Australia. This paper incorporates results from the sequence extension into a complete record from the site, much of which has only been presented previously in summary form. A more certain chronology for the record is provided by recent radiocarbon dating on the topmost sediments and the application of time series analysis to the whole sequence. This suggests that the vegetation and climate are forced essentially by northern hemisphere insolation and ice volume, probably operating through sea level and sea surface temperature changes. The extended record is considered to cover the last 230,000 years and the pattern of complex rainforest expansion during wetter interglacial periods and its replacement by drier rainforest and sclerophyll vegetation
during drier glacials is maintained. Superimposed on this cyclicity are trends resulting from both the evolution of the lake basin as well as external influences that include climate and, within the last 45,000 years, people.
Cloud Isotopic Models (MCIMs). However, the relationship between d and oceanic surface conditions remains unclear in past contexts. We investigate this climate-isotope relationship for sites in Greenland and Antarctica using multiple simulations of the water
isotope-enabled Goddard Institute for Space Studies ModelE-R general circulation model and apply a novel suite of model vapor source distribution (VSD) tracers to assess d as a proxy for source temperature variability under a range of climatic conditions. Simulated average source temperatures determined by the VSDs are compared to synthetic source temperature estimates calculated using MCIM equations linking d to source region
conditions. We show that although deuterium excess is generally a faithful tracer of source temperatures as estimated by the MCIM approach, large discrepancies in the isotope-climate relationship occur around Greenland during the Last Glacial Maximum simulation, when precipitation seasonality and moisture source regions were notably different from the present. This identified sensitivity in d as a source temperature proxy suggests that quantitative climate reconstructions from deuterium excess should be treated with caution for some sites when boundary conditions are significantly different from the present day. Also, the exclusion of the influence of humidity and other evaporative source changes inMCIMregressions may be a limitation of quantifying source temperature fluctuations from deuterium excess in some instances.
interpretations appear to be site specific and often regional in
scale.
tropics of north-eastern Australia. This paper incorporates results from the sequence extension into a complete record from the site, much of which has only been presented previously in summary form. A more certain chronology for the record is provided by recent radiocarbon dating on the topmost sediments and the application of time series analysis to the whole sequence. This suggests that the vegetation and climate are forced essentially by northern hemisphere insolation and ice volume, probably operating through sea level and sea surface temperature changes. The extended record is considered to cover the last 230,000 years and the pattern of complex rainforest expansion during wetter interglacial periods and its replacement by drier rainforest and sclerophyll vegetation
during drier glacials is maintained. Superimposed on this cyclicity are trends resulting from both the evolution of the lake basin as well as external influences that include climate and, within the last 45,000 years, people.