Donovan Patrick Dennis

We describe a new Earth system model (ESM) experiment protocol, as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP) project. We propose this as a protocol for the Coupled Model Intercomparison Project 7 (CMIP7). The protocol requires ESMs to run in CO ₂ -emission mode, with atmospheric CO ₂ a predicted variable. Forcing for the protocol consists solely of a constant emission of CO ₂ , based on each model's transient climate response to cumulative emissions of carbon dioxide (TCRE) value, to give a common global mean surface warming rate of 2 °C per century. This positive emission (ramp-up) experiment is started from the pre-industrial state of a given model. When the ramp-up run first exceeds a specified level of global warming (2 and 4 °C) relative to the model's pre-industrial global mean surface air temperature (GMSAT), CO ₂ emissions are set to zero and the positive emission run is branched into a zero-emission run. The zero-emission runs continue for 300 years. 50 years into each zero-emission run, CO ₂ emissions are set to the negative of the positive emission rate and the model run until GMSAT cools below the original pre-industrial value. Additionally, when the negative emission run started from the global warming level (GWL)=4 °C first drops below GWL=2 °C, a zero-emission run is branched off this, completing the set of experiments. Using this protocol, we are able to control the rate of global warming, and potentially also the rate of cooling, across participating models. TIPMIP experiments will support a range of analyses, including; an assessment of abrupt/rapid Earth system change under net zero CO ₂ emissions at a range of global warming levels, the long-term Earth system response to net zero CO ₂ emissions at these warming levels, the response to net negative CO ₂ emissions and the efficacy of negative emissions to drive cooling, and the reversibility of Earth system change under a pathway of positive (warming), zero, and negative (cooling) CO ₂ emissions.

This paper describes the experimental protocol for a set of coordinated simulations involving oceanic surface freshwater flux perturbations, conducted as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP). These simulations constitute the first phase of the TIPMIP-OCEAN domain. We propose this protocol for inclusion in the Coupled Model Intercomparison Project Phase 7 (CMIP7), although it can also be implemented within CMIP6+ or other types of coupled or ocean standalone models. This initial phase focuses primarily on the dynamics of the North Atlantic Ocean, particularly the Atlantic Meridional Overturning Circulation (AMOC). The different experiments are designed to (i) evaluate the impacts of a potential major AMOC weakening under a 2 °C global warming scenario, (ii) assess the sensitivity of the AMOC to combined global warming and freshwater forcing, (iii) investigate the potential recovery of the AMOC following the reversal of forcings, and (iv) compare past AMOC variations with available climate observations and reconstructions. Four categories of experiments are included. Experiment group A examines the effect of freshwater release around Greenland under ramp-up, stabilization, and ramp-down scenarios in both CO₂ emissions and freshwater input. Experiment group B complements this idealized set by using historical climate simulations and projections for 1850–2100, incorporating realistic estimates of Greenland Ice Sheet melt based on observations for the historical period and ice-sheet model projections for the future. Experiment group C extends the existing North Atlantic Hosing Model Intercomparison Project (NAHosMIP) by applying large freshwater perturbations to both control and 2 °C-warming simulations to assess how global warming influences AMOC reversibility. Finally, experiment group D imposes freshwater inputs, consistent with those inferred for the 8.2 kyr before present event, under pre-industrial conditions, in order to constrain model sensitivity to freshwater forcing using paleoclimate reconstructions. Together, these coordinated experiments will allow systematic evaluation of how different climate models respond to identical freshwater perturbations—an essential step toward better understanding the wide inter-model spread in North Atlantic dynamics and projected future AMOC changes.

This paper describes the experimental protocol for a set of coordinated simulations involving oceanic surface freshwater flux perturbations, conducted as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP). These simulations constitute the first phase of the TIPMIP-OCEAN domain. We propose this protocol for inclusion in the Coupled Model Intercomparison Project Phase 7 (CMIP7), although it can also be implemented within CMIP6+ or other types of coupled or ocean standalone models. This initial phase focuses primarily on the dynamics of the North Atlantic Ocean, particularly the Atlantic Meridional Overturning Circulation (AMOC). The different experiments are designed to (i) evaluate the impacts of a potential major AMOC weakening under a 2 °C global warming scenario, (ii) assess the sensitivity of the AMOC to combined global warming and freshwater forcing, (iii) investigate the potential recovery of the AMOC following the reversal of forcings, and (iv) compare past AMOC variations with available climate observations and reconstructions. Four categories of experiments are included. Experiment group A examines the effect of freshwater release around Greenland under ramp-up, stabilization, and ramp-down scenarios in both CO₂ emissions and freshwater input. Experiment group B complements this idealized set by using historical climate simulations and projections for 1850–2100, incorporating realistic estimates of Greenland Ice Sheet melt based on observations for the historical period and ice-sheet model projections for the future. Experiment group C extends the existing North Atlantic Hosing Model Intercomparison Project (NAHosMIP) by applying large freshwater perturbations to both control and 2 °C-warming simulations to assess how global warming influences AMOC reversibility. Finally, experiment group D imposes freshwater inputs, consistent with those inferred for the 8.2 kyr before present event, under pre-industrial conditions, in order to constrain model sensitivity to freshwater forcing using paleoclimate reconstructions. Together, these coordinated experiments will allow systematic evaluation of how different climate models respond to identical freshwater perturbations—an essential step toward better understanding the wide inter-model spread in North Atlantic dynamics and projected future AMOC changes.

This paper describes the experimental protocol for a set of coordinated simulations involving oceanic surface freshwater flux perturbations, conducted as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP). These simulations constitute the first phase of the TIPMIP-OCEAN domain. We propose this protocol for inclusion in the Coupled Model Intercomparison Project Phase 7 (CMIP7), although it can also be implemented within CMIP6+ or other types of coupled or ocean standalone models. This initial phase focuses primarily on the dynamics of the North Atlantic Ocean, particularly the Atlantic Meridional Overturning Circulation (AMOC). The different experiments are designed to (i) evaluate the impacts of a potential major AMOC weakening under a 2 °C global warming scenario, (ii) assess the sensitivity of the AMOC to combined global warming and freshwater forcing, (iii) investigate the potential recovery of the AMOC following the reversal of forcings, and (iv) compare past AMOC variations with available climate observations and reconstructions. Four categories of experiments are included. Experiment group A examines the effect of freshwater release around Greenland under ramp-up, stabilization, and ramp-down scenarios in both CO₂ emissions and freshwater input. Experiment group B complements this idealized set by using historical climate simulations and projections for 1850–2100, incorporating realistic estimates of Greenland Ice Sheet melt based on observations for the historical period and ice-sheet model projections for the future. Experiment group C extends the existing North Atlantic Hosing Model Intercomparison Project (NAHosMIP) by applying large freshwater perturbations to both control and 2 °C-warming simulations to assess how global warming influences AMOC reversibility. Finally, experiment group D imposes freshwater inputs, consistent with those inferred for the 8.2 kyr before present event, under pre-industrial conditions, in order to constrain model sensitivity to freshwater forcing using paleoclimate reconstructions. Together, these coordinated experiments will allow systematic evaluation of how different climate models respond to identical freshwater perturbations—an essential step toward better understanding the wide inter-model spread in North Atlantic dynamics and projected future AMOC changes.

Within the Tipping Point Modeling Intercomparison Project (TIPMIP; www.tipmip.org), we outline a coordinated experimental protocol designed for standalone land surface models (LSMs) as well as coupled Earth System Models (ESMs). The protocol targets key questions on permafrost feedbacks, thresholds, timescales, abrupt versus gradual, and irreversible changes, and interactions with other tipping elements, aiming to quantify plausible landscape transformations, associated greenhouse gas emissions, and their impacts on global energy, carbon, and hydrological cycles, as well as large scale circulation. Building on TIPMIP-ESM, the TIPMIP-permafrost Tier 1 experiments, defined as the core, priority experiments required for MIP participation, aim to use LSMs to quantify the extent to which permafrost area, ground ice volume, and soil carbon stocks exhibit path dependent behavior and reversibility when subjected to idealized warming, stabilization, and subsequent cooling phases. This includes evaluating whether the permafrost returns to its initial state as temperatures decline, or the extent to which certain changes, such as area loss, carbon redistribution, hydrological reorganization, or ground subsidence, persist despite cooling and thus remain effectively irreversible over time scales spanning a century to a several hundred years. In addition, during the stabilization phase we will examine whether permafrost degradation continues even after warming ceases. This involves assessing time lagged thaw response, identifying critical thresholds that may trigger rapid acceleration of degradation, and determining whether internal processes, such as shifts in soil moisture, ground subsidence and thermokarst lake formation, or carbon redistribution, amplify change through self reinforcing feedbacks. The emergence of such behaviors would indicate nonlinear system dynamics and a heightened susceptibility to tipping point transitions.Additional sensitivity experiments (Tier 2), aimed at understanding and quantifying specific processes that could induce permafrost tipping, will apply idealized forcing. The focus is on altering hydrologic conditions and modifying surface properties (e.g., vegetation, albedo, thermokarst lake distribution) to explore how these factors influence the onset and dynamics of tipping behavior.Tier 3 experiments, also sensitivity oriented, investigate the coupling between permafrost dynamics and other climate tipping elements using ESMs, for example, assessing the consequences of an AMOC collapse for high latitude permafrost stability, as well as the broader Earth system impacts of abrupt permafrost tipping.

Recent studies show that critical thresholds, tipping points, likely exist for the Antarctic Ice Sheet, and potentially individual drainage basins within Antarctica. Surpassing these critical thresholds for key glacier drainage basins can have significant impacts on the long-term sea-level contribution of the ice sheet, which may be irreversible. The marine ice sheet instability (MISI) is considered to be one of the key feedback mechanisms capable of triggering tipping dynamics in ice sheets, wherein grounding line retreat past a stable position along the bed can trigger further rapid and extensive retreat due to the internal dynamics of ice flow. Model simulations that exhibit this instability tend to project much higher degrees of sea-level rise than those where MISI is not initiated. As such, it is important to understand what factors determine if MISI takes effect and the timing of its onset. The sliding velocity of a glacier is a fundamental variable in calculating overall ice flow, and how the sliding velocity evolves in time is a critical factor in determining if MISI is triggered or not during model simulations projecting future glacier dynamics. While it is well understood that the sliding velocity is highly dependent upon the basal environment, much remains unknown regarding its specific characteristics including where sediment accumulates and how much is present, how meltwater flows through the basal environment, and how to best represent basal friction felt by the glacier. Uncertainties associated with both the physical processes governing ice sheet responses to climate warming, and the parameter choices associated with those physical processes make the analysis of potential ice sheet tipping points particularly difficult. Here, we aim to better understand the influence that the basal environment has on the tipping behavior of the Antarctic Ice Sheet. We use the Parallel Ice Sheet Model (PISM) to identify critical temperature thresholds that will lead to irreversible ice sheet mass loss and quantify the associated long-term sea-level commitment from Antarctica. Climate forcings will be dictated according to the TIPMIP protocols, following stylized warming scenarios and exploring the ice sheet equilibrium response to constant climates at different global warming levels. We analyze model results for differences in tipping behavior obtained by using various different representations of hydrology in the basal environment and ranging values for associated parameters. Here, we present the initial results of these experiments and discuss the relative importance of subglacial hydrology in determining if and when critical tipping points are exceeded for the Antarctic Ice Sheet.

Understanding the potential future evolution of the Greenland Ice Sheet (GrIS) is of critical importance for anticipating the consequences of global climate change. GrIS melt contributes between 0.5-0.8 mm per year to total global sea level rise and the total ice volume has the potential to raise sea levels by 7 meters. Furthermore, freshwater delivery to the North Atlantic has important implications for the stability of the density-driven Atlantic Meridional Overturning Circulation. A critical challenge in anticipating GrIS sea level rise contribution and North Atlantic freshwater delivery arises from the so-called inertia of the ice sheet, wherein present-day and near-term future warming may trigger ice loss that unfolds on century to millennial timescales. The GrIS is considered one of the Earth system’s tipping elements, components of the Earth system wherein crossing a critical global warming threshold leads to large-scale and nonlinear change in system state. Though it is subject to a number of self-amplifying (and -dampening) feedbacks, the susceptibility of the GrIS to tipping is not well-constrained, with particular uncertainties arising over long (1-10 kyr) timescales, given different global warming rates, and under potential scenarios of “stabilised” or landing climate (i.e., the Paris-agreed 1.5 C). The Tipping Points Modelling Intercomparison Project (TIPMIP) seeks to investigate the likelihood, impacts, and risk of crossing Earth system tipping points in so-called global tipping elements—components which, if tipped, have widespread consequences for the whole Earth system. Here we present initial explorations of the response of the GrIS to the transient, idealised warming scenarios following the TIPMIP-ESM and TIPMIP-ICESHEET domain protocols, with standalone, offline experiments undertaken using the Parallel Ice Sheet Model (PISM). These idealised warming scenarios have been designed to explore both the warming-induced triggers of tipping dynamics as well as their feedbacks over century to millennia timescales.

The terrestrial biosphere plays a crucial role in regulating biogeochemical cycles, storing carbon, and providing essential ecosystem services, yet the land carbon sink remains a large source of uncertainty in future climate projections. Under the combined pressures of climate change, land use, pests, and fire, major biomes such as the Amazon and boreal forests face widespread degradation and may exhibit tipping behavior. The Tipping Points Modelling Intercomparison Project (TIPMIP) brings together the modelling community to develop unified simulation protocols for Earth System Models and standalone models, including Dynamic Global Vegetation Models. In this context, TIPMIP-BIO aims at running idealized experiments of climate overshoot and stabilizations scenarios, in order to assess climatic and deforestation thresholds that could trigger nonlinear biosphere responses. The framework provides opportunities to examine key processes and feedbacks, including CO₂ fertilization and physiological responses, and to explore emerging questions such as plant trait variability and the impact of compound climate events.

Due to insufficient emission reductions in recent years, it is increasingly likely that global warming will exceed the 1.5 °C temperature limit in the late 2020s or 2030s. As a result, several Earth system tipping elements could, at least temporarily, have their tipping points surpassed, posing risks of large-scale and profound structural change. Tipping does not always occur immediately upon crossing such a critical threshold. If the length of time the driver is beyond the critical level is short enough, tipping could still be avoided for some slow-responding elements of the climate system. An improved understanding is therefore needed of whether tipping remains avoidable, for which systems, and under what conditions. Here, we review how minimising the magnitude and duration of any temperature overshoot beyond 1.5 °C could decrease tipping risks. Tipping elements with fast response times, such as warm-water coral reefs, are especially vulnerable to overshoot. In contrast, those with slow response times, such as polar ice sheets, may be less sensitive to temporary overshoot. Potential interactions between tipping elements and additional human pressures, such as deforestation in the Amazon or pollution and overfishing of coral reef habitats, may further lower tipping points, narrowing the range of overshoot trajectories that can still avoid it. The vulnerability of many tipping elements, even under shorter overshoot conditions, underscores that global warming must peak below 2 °C above pre-industrial levels, return to below 1.5 °C as quickly as possible (i.e. within this century), and to around 1 °C thereafter to limit tipping point risks.

We describe a new Earth system model (ESM) experiment protocol, as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP) project. We propose this as a protocol for the Coupled Model Intercomparison Project 7 (CMIP7). The protocol requires ESMs to run in CO₂-emission mode, with atmospheric CO₂ a predicted variable. Forcing for the protocol consists solely of a constant emission of CO₂, based on each model’s transient climate response to cumulative emissions of carbon dioxide (TCRE) value, to give a common global mean warming rate of 2 °C per century. This positive emission experiment is started from the pre-industrial state of a given model. When the ramp-up run first exceeds a specified level of global warming (2 °C and 4 °C) relative to the model’s pre-industrial global mean surface air temperature (GMSAT), CO₂ emissions are set to zero and the positive emission run is branched into a zero-emission run. The zero-emission runs continue for 300 years. At 50 years into each zero-emission run, CO₂ emissions are set to the negative of the positive emission rate and the model run until GMSAT cools below the original pre-industrial value. Additionally, when the negative emission run started from global warming level (GWL) = 4 °C first drops below GWL = 2 °C, a zero-emission run is branched off this, completing the set of experiments. Using this protocol, we are able to control the rate of global warming and cooling across participating models. TIPMIP experiments will support a range of analyses, including; an assessment of abrupt/rapid Earth system change, the long-term response to zero CO₂ emissions, the response to negative CO₂ emissions, the efficacy of negative emissions in driving cooling, and the reversibility of Earth system change under a pathway of positive (warming), zero (stabilization), and negative (cooling) CO₂ emissions.