Irene Malmierca-Vallet

Global Mean Ocean Temperature (GMOT) changes are thought to be both an important consequence and a possible cause of Dansgaard–Oeschger (DO) events, a series of abrupt climate changes during the last glacial. Despite extensive reconstructions of GMOT from ice cores for other periods, no reliable reconstructions exist for Marine Isotope Stage 3, when DO events were most common. We investigate model simulations showing unforced DO‐like oscillations alongside existing proxy reconstructions of Southern Ocean and Antarctic temperatures, which we show to be strongly linked to GMOT in the models though with consistently larger changes in Antarctic temperature than GMOT. We find that the models reproduce DO events following non‐Heinrich stadials well, but that the GMOT changes they show across these events are comparable to the measurement error for ice core GMOT reconstructions and so would be difficult to detect. The simulations we use, which are not forced by Heinrich meltwater, do not capture the larger Southern Ocean and Antarctic warming associated with DO events that follow Heinrich stadials, making extrapolating our results to these events challenging. Additionally, there is a statistically robust disagreement between the proxies and models regarding the relative warming in Antarctica and the Southern Ocean, which adds further uncertainty to our estimates of GMOT warming. Nonetheless, we find that the GMOT changes across these larger DO events in MIS3 would be detectable. Given the importance of GMOT for understanding DO events, these should therefore be priority targets for future efforts to reconstruct GMOT from ice cores.

Given that the Arctic could be ice-free in summer within the next ten to 20 years, accurately predicting low-ice states is of crucial importance. Paleo-evidence shows that the strong orbitally-induced high latitude insolation anomaly at 127 000 years ago (127 ky), of around +70 W m ⁻² in the Arctic during spring-summer, led to warm conditions and an Arctic that was occasionally or often ice-free during summer. Building on two Coupled Model Intercomparison Projects (CMIPs): the Sea-Ice Model Intercomparison Project and the Paleoclimate Modelling Intercomparison Project, we propose an Assessment Fast Track experiment, abrupt-127k , focusing on this seasonally ice-free, or near ice-free, Arctic at 127 ky. The abrupt-127k experiment is initialised from a piControl simulation and abruptly imposes observed values for the insolation distribution and greenhouse gas forcing at 127 ky. It provides a new opportunity to evaluate models used to compute climate projections, both against paleo-evidence and each other, during a known low Arctic sea ice state. As CMIP models are not usually tuned to paleo observations, abrupt-127k represents a true “out-of-sample” test. The abrupt-127k experiment has four key scientific objectives, to: ascertain the simulated Arctic sea ice state, including the presence and characteristics of last-ice areas; evaluate the simulated climates using Arctic paleo-evidence; characterise the central Arctic surface energy budget; and analyse the ice budget including ice melt, growth, and transport. We show that a large Arctic ice response will manifest within the first 30 years of the simulation, thus a single 100-year long run is sufficient for these objectives. Modelling groups are requested to follow standard CMIP output protocol for analysis, including the use of standard “fixed-length” output. Given abrupt-127k is similar in setup to abrupt-2xCO2 and abrupt-4xCO2 CMIP7 experiments, combined analysis of these abrupt-experiments will facilitate understanding of the impacts of instantaneous radiative forcing in the Arctic.

Extreme weather represents one of the most significant consequences of a warming climate. Improving constraints on how such events may manifest in the future is therefore a key priority, particularly for hazards that lead to severe societal, ecological, and financial impacts, such as heatwaves, extreme rainfall, droughts and their compounding effects.Past interglacial periods provide physically realised instances of warm-climate states that can be used to contextualise ongoing anthropogenic warming and to inform future changes. Each interglacial is characterised by a distinct combination of orbital forcing and greenhouse gas concentrations, ice-sheet configuration, and background climate. Comparing these periods allows the partial isolation of the roles played by different climate drivers and large-scale circulation patterns in shaping the frequency, intensity, variability and spatial distribution of extreme events.Here, we compare extreme weather characteristics across four interglacial periods: the mid-Holocene (6 ka), the Last Interglacial (127 ka), Marine Isotope Stage 11 (408 ka), and Marine Isotope Stage 31 (1072 ka), alongside a pre-industrial control. The analysis is based on preliminary equilibrium time-slice simulations conducted using the HadGEM3-GC5.0 coupled climate model, which also enables an initial assessment of model performance across a range of interglacial climates. We demonstrate how distinct warm-climate conditions have affected polar and global extreme events in the past and discuss the mechanisms underpinning these changes and their relevance for future climates.

The shape and extent of the Greenland Ice Sheet (GIS) during the Holocene remain a matter of considerable debate, with existing studies proposing a wide range of reconstructions. In this study, we aim to combine stable water isotopic information from ice cores with outputs from isotope-enabled climate models to investigate this problem. Directly exploring the space of possible ice sheet geometries through numerical simulations is computationally prohibitive. To address this challenge, we plan to develop a Gaussian process emulator that will serve as a statistical surrogate for the full climate model. The emulator will be trained on the results of a limited number of carefully designed simulations and will be used to enable fast, probabilistic predictions of model outputs at untried inputs. The inputs will consist of GIS morphologies, parameterized using a dimension-reduction technique adapted to the spherical geometry of the ice sheet. Using predictions from the emulator, we will explore the range of ice sheet morphologies that are compatible with available ice-core isotope measurements and other complementary observational data, including those collected during recent KANG-GLAC expeditions, with the goal of ultimately reducing uncertainty in reconstructions of Holocene GIS morphology.

Given that the Arctic could be ice-free in summer within the next ten to twenty years, accurately predicting low-ice states is of crucial importance. Paleo-evidence shows that the strong orbitally-induced high latitude insolation anomaly at 127,000 years ago (127 ky), of around +70 W m⁻² in the Arctic during spring-summer, led to warm conditions and an Arctic that was occasionally or often ice-free during summer. Building on two Coupled Model Intercomparison Projects (CMIPs): the Sea-Ice Model Intercomparison Project and the Paleoclimate Modelling Intercomparison Project, we propose an Assessment Fast Track experiment, abrupt-127k, focusing on this seasonally ice-free, or near ice-free, Arctic at 127 ky. The abrupt-127k experiment is initialised from a piControl simulation and abruptly imposes observed values for the insolation distribution and greenhouse gas forcing at 127 ky. It provides a new opportunity to evaluate models used to compute climate projections, both against paleo-evidence and each other, during a known low Arctic sea ice state. As CMIP models are not usually tuned to paleo observations, abrupt-127k represents a true ‘out-of-sample’ test. The abrupt-127k experiment has four key scientific objectives, to: ascertain the simulated Arctic sea ice state, including the presence and characteristics of last-ice areas; evaluate the simulated climates using Arctic paleo-evidence; characterise the central Arctic surface energy budget; and analyse the ice budget including ice melt, growth, and transport. We show that a large Arctic ice response will manifest within the first 30 years of the simulation, thus a single 100-year long run is sufficient for these objectives. Modelling groups are requested to follow standard CMIP output protocol for analysis, including the use of standard ‘fixed-length’ output. Given Fast Track abrupt- 127k is similar in setup to abrupt-2xCO2 and abrupt-4xCO2 experiments, combined analysis of these abrupt-experiments will facilitate understanding of the impacts of instantaneous radiative forcing in the Arctic.

Given that the Arctic could be ice-free in summer within the next ten to twenty years, accurately predicting low-ice states is of crucial importance. Paleo-evidence shows that the strong orbitally-induced high latitude insolation anomaly at 127,000 years ago (127 ky), of around +70 W m⁻² in the Arctic during spring-summer, led to warm conditions and an Arctic that was occasionally or often ice-free during summer. Building on two Coupled Model Intercomparison Projects (CMIPs): the Sea-Ice Model Intercomparison Project and the Paleoclimate Modelling Intercomparison Project, we propose an Assessment Fast Track experiment, abrupt-127k, focusing on this seasonally ice-free, or near ice-free, Arctic at 127 ky. The abrupt-127k experiment is initialised from a piControl simulation and abruptly imposes observed values for the insolation distribution and greenhouse gas forcing at 127 ky. It provides a new opportunity to evaluate models used to compute climate projections, both against paleo-evidence and each other, during a known low Arctic sea ice state. As CMIP models are not usually tuned to paleo observations, abrupt-127k represents a true ‘out-of-sample’ test. The abrupt-127k experiment has four key scientific objectives, to: ascertain the simulated Arctic sea ice state, including the presence and characteristics of last-ice areas; evaluate the simulated climates using Arctic paleo-evidence; characterise the central Arctic surface energy budget; and analyse the ice budget including ice melt, growth, and transport. We show that a large Arctic ice response will manifest within the first 30 years of the simulation, thus a single 100-year long run is sufficient for these objectives. Modelling groups are requested to follow standard CMIP output protocol for analysis, including the use of standard ‘fixed-length’ output. Given Fast Track abrupt- 127k is similar in setup to abrupt-2xCO2 and abrupt-4xCO2 experiments, combined analysis of these abrupt-experiments will facilitate understanding of the impacts of instantaneous radiative forcing in the Arctic.

Given that the Arctic could be ice-free in summer within the next ten to twenty years, accurately predicting low-ice states is of crucial importance. Paleo-evidence shows that the strong orbitally-induced high latitude insolation anomaly at 127,000 years ago (127 ky), of around +70 W m⁻² in the Arctic during spring-summer, led to warm conditions and an Arctic that was occasionally or often ice-free during summer. Building on two Coupled Model Intercomparison Projects (CMIPs): the Sea-Ice Model Intercomparison Project and the Paleoclimate Modelling Intercomparison Project, we propose an Assessment Fast Track experiment, abrupt-127k, focusing on this seasonally ice-free, or near ice-free, Arctic at 127 ky. The abrupt-127k experiment is initialised from a piControl simulation and abruptly imposes observed values for the insolation distribution and greenhouse gas forcing at 127 ky. It provides a new opportunity to evaluate models used to compute climate projections, both against paleo-evidence and each other, during a known low Arctic sea ice state. As CMIP models are not usually tuned to paleo observations, abrupt-127k represents a true ‘out-of-sample’ test. The abrupt-127k experiment has four key scientific objectives, to: ascertain the simulated Arctic sea ice state, including the presence and characteristics of last-ice areas; evaluate the simulated climates using Arctic paleo-evidence; characterise the central Arctic surface energy budget; and analyse the ice budget including ice melt, growth, and transport. We show that a large Arctic ice response will manifest within the first 30 years of the simulation, thus a single 100-year long run is sufficient for these objectives. Modelling groups are requested to follow standard CMIP output protocol for analysis, including the use of standard ‘fixed-length’ output. Given Fast Track abrupt- 127k is similar in setup to abrupt-2xCO2 and abrupt-4xCO2 experiments, combined analysis of these abrupt-experiments will facilitate understanding of the impacts of instantaneous radiative forcing in the Arctic.

Several general circulation models have now demonstrated the ability to simulate spontaneous millennial‐scale oscillations that resemble Dansgaard–Oeschger (DO) events. It is often unclear how representative of DO events these simulations are, particularly outside of the polar regions. To test this, we directly compare simulated changes from two isotope‐enabled models to a compilation of 111 speleothem records from 67 caves across the low‐ and mid‐latitudes. We find that both models successfully reproduce the observed pattern of changes in Europe and the Mediterranean, Asia, and Central America. However, they perform less well for Western North America, South America, and Oceania, and the simulated changes are also generally too small in their magnitude. Where the models do reproduce the observed changes, we find evidence that the isotopic variability is influenced by both local and remote drivers, but the remote drivers appear more important.