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Papers by Bette Otto-bliesner
Biogeosciences
interactions regulate N availability for plant growth and for emissions of nitrous oxide (N 2 O) ... more interactions regulate N availability for plant growth and for emissions of nitrous oxide (N 2 O) and the uptake of carbon dioxide. Future projections of these terrestrial greenhouse gas fluxes are strikingly divergent, leading to major uncertainties in projected global warming. Here we analyse the large increase in terrestrial N 2 O emissions over the past 21 000 years as reconstructed from ice-core isotopic data and presented in part 1 of this study. Remarkably, the increase occurred in two steps, each realized over decades and within a maximum of 2 centuries, at the onsets of the major deglacial Northern Hemisphere warming events. The data suggest a highly dynamic and responsive global N cycle. The increase may be explained by an increase in the flux of reactive N entering and leaving ecosystems or by an increase in N 2 O yield per unit N converted. We applied the LPX-Bern dynamic global vegetation model in deglacial simulations forced with Earth system model climate data to investigate N 2 O emission patterns, mechanisms, and C-N coupling. The N 2 O emission changes are mainly attributed to changes in temperature and precipitation and the loss of land due to sea-level rise. LPX-Bern simulates a deglacial increase in N 2 O emissions but underestimates the reconstructed increase by 47 %. Assuming timeindependent N sources in the model to mimic progressive N limitation of plant growth results in a decrease in N 2 O emissions in contrast to the reconstruction. Our results appear consistent with suggestions of (a) biological controls on ecosystem N acquisition and (b) flexibility in the coupling of the C and N cycles during periods of rapid environmental change. A dominant uncertainty in the explanation of the reconstructed N 2 O emissions is the poorly known N 2 O yield per N lost through gaseous pathways and its sensitivity to soil conditions. The deglacial N 2 O record provides a constraint for future studies.
PAGES news
The impact of ice sheet size and geometry on both climate and the ice sheet itself is investigate... more The impact of ice sheet size and geometry on both climate and the ice sheet itself is investigated with general circulation models. This information is used to simulate the observed change of ice sheet and sea level throughout the ice age cycle.
The Paleontological Society Special Publications
Mechanisms for maintaining wann temperatures at high latitudes and in continental interiors durin... more Mechanisms for maintaining wann temperatures at high latitudes and in continental interiors during the Cretaceous are incompletely understood. Previous model simulations implicate paleogeography, increased atmospheric CO2, and increased ocean heat transport as mechanisms of high-latitude wannth. However, published model simulations that incorporate these factors cannot reproduce the summer and winter wannth indicated by temperature proxies such as fossil plants. This could indicate model deficiencies or the lack of at least one important factor in model simulations.
Earth Systems and Environment
In modern environmental and climate science it is necessary to assimilate observational datasets ... more In modern environmental and climate science it is necessary to assimilate observational datasets collected over decades with outputs from numerical models, to enable a full understanding of natural systems and their sensitivities. During the twentieth and twenty-first centuries, numerical modelling became central to many areas of science from the Bohr model of the atom to the Lorenz model of the atmosphere. In modern science, a great deal of time and effort is devoted to developing, evaluating, comparing and modifying numerical models that help us synthesise our understanding of complex natural systems.
Biogeosciences Discussions
Land ecosystems currently take up a quarter of the human-caused carbon dioxide emissions. Future ... more Land ecosystems currently take up a quarter of the human-caused carbon dioxide emissions. Future projections of this carbon sink are strikingly divergent, leading to major uncertainties in projected global warming. This situation partly 15 reflects our insufficient understanding of carbon-nitrogen (C-N) interactions and particularly of the controls on biological N fixation (BNF). It is difficult to infer ecosystem responses for century time scales, relevant for global warming, from the comparatively short instrumental records and laboratory or field experiments. Here we analyse terrestrial emissions of nitrous oxide (N2O) over the past 21,000 years as reconstructed from ice-core isotopic data and presented in part I of this study.
Geophysical Research Letters
Proceedings of the National Academy of Sciences of the United States of America, Feb 21, 2017
The response of the El Niño/Southern Oscillation (ENSO) to tropical volcanic eruptions has import... more The response of the El Niño/Southern Oscillation (ENSO) to tropical volcanic eruptions has important worldwide implications, but remains poorly constrained. Paleoclimate records suggest an "El Niño-like" warming 1 year following major eruptions [Adams JB, Mann ME, Ammann CM (2003) Nature 426:274-278] and "La Niña-like" cooling within the eruption year [Li J, et al. (2013) Nat Clim Chang 3:822-826]. However, climate models currently cannot capture all these responses. Many eruption characteristics are poorly constrained, which may contribute to uncertainties in model solutions-for example, the season of eruption occurrence is often unknown and assigned arbitrarily. Here we isolate the effect of eruption season using experiments with the Community Earth System Model (CESM), varying the starting month of two large tropical eruptions. The eruption-year atmospheric circulation response is strongly seasonally dependent, with effects on European winter warming, the Inte...
Geophysical Research Letters, 2016
Geoscientific Model Development Discussions, 2016
Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenari... more Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high (> 800 ppmv) atmospheric CO<sub>2</sub> concentrations. Although a post-hoc intercomparison of Eocene (~50 million years ago, Ma) climate model simulations and geological data has been carried out previously, models of past high-CO<sub>2</sub> periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the latest Paleocene and the early Eocene. Together these form the first phase of DeepMIP – the deeptime model intercomparison project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design consists of three core paleo simulations and a set of optional sensitivity studies. The experimental design specifies and provides guidance on boundary conditions associate...
Journal of Climate, 2016
The hydroclimate response to volcanic eruptions depends both on volcanically induced changes to t... more The hydroclimate response to volcanic eruptions depends both on volcanically induced changes to the hydrologic cycle and on teleconnections with the El Niño–Southern Oscillation (ENSO), complicating the interpretation of offsets between proxy reconstructions and model output. Here, these effects are separated, using the Community Earth System Model Last Millennium Ensemble (CESM-LME), by examination of ensemble realizations with distinct posteruption ENSO responses. Hydroclimate anomalies in monsoon Asia and the western United States resemble the El Niño teleconnection pattern after “Tropical” and “Northern” eruptions, even when ENSO-neutral conditions are present. This pattern results from Northern Hemisphere (NH) surface cooling, which shifts the intertropical convergence zone equatorward, intensifies the NH subtropical jet, and suppresses the Southeast Asian monsoon. El Niño events following an eruption can then intensify the ENSO-neutral hydroclimate signature, and El Niño proba...
Nature communications, Jan 21, 2015
The ongoing retreat of glaciers globally is one of the clearest manifestations of recent global w... more The ongoing retreat of glaciers globally is one of the clearest manifestations of recent global warming associated with rising greenhouse gas concentrations. By comparison, the importance of greenhouse gases in driving glacier retreat during the most recent deglaciation, the last major interval of global warming, is unclear due to uncertainties in the timing of retreat around the world. Here we use recently improved cosmogenic-nuclide production-rate calibrations to recalculate the ages of 1,116 glacial boulders from 195 moraines that provide broad coverage of retreat in mid-to-low-latitude regions. This revised history, in conjunction with transient climate model simulations, suggests that while several regional-scale forcings, including insolation, ice sheets and ocean circulation, modulated glacier responses regionally, they are unable to account for global-scale retreat, which is most likely related to increasing greenhouse gas concentrations.
Journal of Climate, 2015
An unforced simulation of the Community Climate System Model, version 4 (CCSM4), is found to have... more An unforced simulation of the Community Climate System Model, version 4 (CCSM4), is found to have Greenland warming and cooling events that resemble Dansgaard–Oeschger cycles in pattern and magnitude. With the caveat that only three transitions were available to be analyzed, it is found that the transitions are triggered by stochastic atmospheric forcing. The atmospheric anomalies change the strength of the subpolar gyre, leading to a change in Labrador Sea sea ice concentration and meridional heat transport. The changed climate state is maintained over centuries through the feedback between sea ice and sea level pressure in the North Atlantic. Indications that the initial atmospheric pressure anomalies are preceded by precipitation anomalies in the western Pacific warm pool are discussed. The full evolution of the anomalous climate state depends crucially on the climatic background state.
Developments in Paleoenvironmental Research, 2004
... 91191 Gif sur Yvette cedex France CHRIS D. HEWITT (chris.hewitt@metoffice.com) Hadley Centre ... more ... 91191 Gif sur Yvette cedex France CHRIS D. HEWITT (chris.hewitt@metoffice.com) Hadley Centre for Climate Prediction and Research Met Office FitzRoy Road, Exceter Devon, EX1 3BP UK AKIO KITOH (kitoh@mri-jma.go ...
Chinese Science Bulletin, 2014
Biogeosciences
interactions regulate N availability for plant growth and for emissions of nitrous oxide (N 2 O) ... more interactions regulate N availability for plant growth and for emissions of nitrous oxide (N 2 O) and the uptake of carbon dioxide. Future projections of these terrestrial greenhouse gas fluxes are strikingly divergent, leading to major uncertainties in projected global warming. Here we analyse the large increase in terrestrial N 2 O emissions over the past 21 000 years as reconstructed from ice-core isotopic data and presented in part 1 of this study. Remarkably, the increase occurred in two steps, each realized over decades and within a maximum of 2 centuries, at the onsets of the major deglacial Northern Hemisphere warming events. The data suggest a highly dynamic and responsive global N cycle. The increase may be explained by an increase in the flux of reactive N entering and leaving ecosystems or by an increase in N 2 O yield per unit N converted. We applied the LPX-Bern dynamic global vegetation model in deglacial simulations forced with Earth system model climate data to investigate N 2 O emission patterns, mechanisms, and C-N coupling. The N 2 O emission changes are mainly attributed to changes in temperature and precipitation and the loss of land due to sea-level rise. LPX-Bern simulates a deglacial increase in N 2 O emissions but underestimates the reconstructed increase by 47 %. Assuming timeindependent N sources in the model to mimic progressive N limitation of plant growth results in a decrease in N 2 O emissions in contrast to the reconstruction. Our results appear consistent with suggestions of (a) biological controls on ecosystem N acquisition and (b) flexibility in the coupling of the C and N cycles during periods of rapid environmental change. A dominant uncertainty in the explanation of the reconstructed N 2 O emissions is the poorly known N 2 O yield per N lost through gaseous pathways and its sensitivity to soil conditions. The deglacial N 2 O record provides a constraint for future studies.
PAGES news
The impact of ice sheet size and geometry on both climate and the ice sheet itself is investigate... more The impact of ice sheet size and geometry on both climate and the ice sheet itself is investigated with general circulation models. This information is used to simulate the observed change of ice sheet and sea level throughout the ice age cycle.
The Paleontological Society Special Publications
Mechanisms for maintaining wann temperatures at high latitudes and in continental interiors durin... more Mechanisms for maintaining wann temperatures at high latitudes and in continental interiors during the Cretaceous are incompletely understood. Previous model simulations implicate paleogeography, increased atmospheric CO2, and increased ocean heat transport as mechanisms of high-latitude wannth. However, published model simulations that incorporate these factors cannot reproduce the summer and winter wannth indicated by temperature proxies such as fossil plants. This could indicate model deficiencies or the lack of at least one important factor in model simulations.
Earth Systems and Environment
In modern environmental and climate science it is necessary to assimilate observational datasets ... more In modern environmental and climate science it is necessary to assimilate observational datasets collected over decades with outputs from numerical models, to enable a full understanding of natural systems and their sensitivities. During the twentieth and twenty-first centuries, numerical modelling became central to many areas of science from the Bohr model of the atom to the Lorenz model of the atmosphere. In modern science, a great deal of time and effort is devoted to developing, evaluating, comparing and modifying numerical models that help us synthesise our understanding of complex natural systems.
Biogeosciences Discussions
Land ecosystems currently take up a quarter of the human-caused carbon dioxide emissions. Future ... more Land ecosystems currently take up a quarter of the human-caused carbon dioxide emissions. Future projections of this carbon sink are strikingly divergent, leading to major uncertainties in projected global warming. This situation partly 15 reflects our insufficient understanding of carbon-nitrogen (C-N) interactions and particularly of the controls on biological N fixation (BNF). It is difficult to infer ecosystem responses for century time scales, relevant for global warming, from the comparatively short instrumental records and laboratory or field experiments. Here we analyse terrestrial emissions of nitrous oxide (N2O) over the past 21,000 years as reconstructed from ice-core isotopic data and presented in part I of this study.
Geophysical Research Letters
Proceedings of the National Academy of Sciences of the United States of America, Feb 21, 2017
The response of the El Niño/Southern Oscillation (ENSO) to tropical volcanic eruptions has import... more The response of the El Niño/Southern Oscillation (ENSO) to tropical volcanic eruptions has important worldwide implications, but remains poorly constrained. Paleoclimate records suggest an "El Niño-like" warming 1 year following major eruptions [Adams JB, Mann ME, Ammann CM (2003) Nature 426:274-278] and "La Niña-like" cooling within the eruption year [Li J, et al. (2013) Nat Clim Chang 3:822-826]. However, climate models currently cannot capture all these responses. Many eruption characteristics are poorly constrained, which may contribute to uncertainties in model solutions-for example, the season of eruption occurrence is often unknown and assigned arbitrarily. Here we isolate the effect of eruption season using experiments with the Community Earth System Model (CESM), varying the starting month of two large tropical eruptions. The eruption-year atmospheric circulation response is strongly seasonally dependent, with effects on European winter warming, the Inte...
Geophysical Research Letters, 2016
Geoscientific Model Development Discussions, 2016
Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenari... more Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high (> 800 ppmv) atmospheric CO<sub>2</sub> concentrations. Although a post-hoc intercomparison of Eocene (~50 million years ago, Ma) climate model simulations and geological data has been carried out previously, models of past high-CO<sub>2</sub> periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the latest Paleocene and the early Eocene. Together these form the first phase of DeepMIP – the deeptime model intercomparison project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design consists of three core paleo simulations and a set of optional sensitivity studies. The experimental design specifies and provides guidance on boundary conditions associate...
Journal of Climate, 2016
The hydroclimate response to volcanic eruptions depends both on volcanically induced changes to t... more The hydroclimate response to volcanic eruptions depends both on volcanically induced changes to the hydrologic cycle and on teleconnections with the El Niño–Southern Oscillation (ENSO), complicating the interpretation of offsets between proxy reconstructions and model output. Here, these effects are separated, using the Community Earth System Model Last Millennium Ensemble (CESM-LME), by examination of ensemble realizations with distinct posteruption ENSO responses. Hydroclimate anomalies in monsoon Asia and the western United States resemble the El Niño teleconnection pattern after “Tropical” and “Northern” eruptions, even when ENSO-neutral conditions are present. This pattern results from Northern Hemisphere (NH) surface cooling, which shifts the intertropical convergence zone equatorward, intensifies the NH subtropical jet, and suppresses the Southeast Asian monsoon. El Niño events following an eruption can then intensify the ENSO-neutral hydroclimate signature, and El Niño proba...
Nature communications, Jan 21, 2015
The ongoing retreat of glaciers globally is one of the clearest manifestations of recent global w... more The ongoing retreat of glaciers globally is one of the clearest manifestations of recent global warming associated with rising greenhouse gas concentrations. By comparison, the importance of greenhouse gases in driving glacier retreat during the most recent deglaciation, the last major interval of global warming, is unclear due to uncertainties in the timing of retreat around the world. Here we use recently improved cosmogenic-nuclide production-rate calibrations to recalculate the ages of 1,116 glacial boulders from 195 moraines that provide broad coverage of retreat in mid-to-low-latitude regions. This revised history, in conjunction with transient climate model simulations, suggests that while several regional-scale forcings, including insolation, ice sheets and ocean circulation, modulated glacier responses regionally, they are unable to account for global-scale retreat, which is most likely related to increasing greenhouse gas concentrations.
Journal of Climate, 2015
An unforced simulation of the Community Climate System Model, version 4 (CCSM4), is found to have... more An unforced simulation of the Community Climate System Model, version 4 (CCSM4), is found to have Greenland warming and cooling events that resemble Dansgaard–Oeschger cycles in pattern and magnitude. With the caveat that only three transitions were available to be analyzed, it is found that the transitions are triggered by stochastic atmospheric forcing. The atmospheric anomalies change the strength of the subpolar gyre, leading to a change in Labrador Sea sea ice concentration and meridional heat transport. The changed climate state is maintained over centuries through the feedback between sea ice and sea level pressure in the North Atlantic. Indications that the initial atmospheric pressure anomalies are preceded by precipitation anomalies in the western Pacific warm pool are discussed. The full evolution of the anomalous climate state depends crucially on the climatic background state.
Developments in Paleoenvironmental Research, 2004
... 91191 Gif sur Yvette cedex France CHRIS D. HEWITT (chris.hewitt@metoffice.com) Hadley Centre ... more ... 91191 Gif sur Yvette cedex France CHRIS D. HEWITT (chris.hewitt@metoffice.com) Hadley Centre for Climate Prediction and Research Met Office FitzRoy Road, Exceter Devon, EX1 3BP UK AKIO KITOH (kitoh@mri-jma.go ...
Chinese Science Bulletin, 2014