CL1 | "Towards a temperature-homogenous planet?" Subtitle : "Are general circulation models able to simulate past warm and high GHG values climates? And do we have good proxies to reconstruct them especially at high latitudes?"
"Towards a temperature-homogenous planet?" Subtitle : "Are general circulation models able to simulate past warm and high GHG values climates? And do we have good proxies to reconstruct them especially at high latitudes?"
Convener: Gilles Ramstein | Co-conveners: Ayako Abe-Ouchi, Paul Valdes, Anna Ruth Halberstadt
| Mon, 24 Apr, 16:15–18:00 (CEST)
Room F1
Mon, 16:15
Present day climate is characterized by the presence of two ice sheets, one in each hemisphere, which is rare in the climate history of the Earth. This feature is strongly associated with the fact that during quaternary, the amount of GHG, especially atmospheric CO2 content in the atmosphere, was low compared to cenozoic's one. At the scale of centuries, the warming of high latitudes will be pivotal for humankind, but are we really able to diagnose such climate changes?
The warm climates of Cenozoic and Mesozoic offer the unique opportunity to investigate the climate behaviour in a rich GHG world. For many decades, a large number of scientists, from climate modeling groups to data reconstruction communities, have addressed several issues concerning the comparison of temperature simulations and proxies reconstructions for many warm periods at mid and high latitudes, but also between the surface and the bottom of paleo ocean. These efforts pointed out a large mismatch for mid and high latitude surface temperatures. Models largely under estimating temperature reconstructions derived from many terrestrial and oceanic proxies. A consequence was that the thermal gradient form equator to pole, which was very low by the reconstructions, has remained over estimated by the models, with a weak polar amplification. These issues have been exacerbated by Model Intercomparison Projects (MIP), which clearly pointed out that these mismatches were shared by most models (Pliomip for mid Pliocene and Deepmip for Eocene), and therefore, this mismatch can be considered as a robust feature.
This long-lasting paradox is associated with our ability to simulate earth's climate with a very low equator to pole temperature gradient compared to quaternary glacial inter-glacial cycles has been arising since the 1970s'. Therefore, it could be interesting to revisit this issue together with modelers of past warm climates and data people and discuss the plausible causes of this mismatch: lack of processes (cloud physics, GHG atmospheric content, aerosols...), but also uncertainties on data reconstructions. Moreover, this issue has important consequences for our ability to correctly understand and simulate the future climates, especially at high latitudes, and the interactions with ice sheets at scales of decades to centuries.

The ongoing climate change is already prominent. Its evolution during this century is the major topic tackled by the IPCC, even though some projections are investigated up to 2300. The different scenarios used by the IPCC, from SSP1-1.9 to SSP5-8.5 depict pCO2 emissions that may lead to a drastic increase, reaching 125 Gt/year at the end of the century. Such high values correspond to warm climates of the Cenozoic. It is therefore pivotal to know how the climate models currently used for projections are able to describe this warm and Pco2 rich climates. 

In a first step, thanks to mips (model intercomparison projects) similar to those used for projections (Cmip), we will depict the robust features and weaknesses of modelling mid-Pliocene, mid-Miocene, and Eocene climates when comparing model results and data reconstructions. For all these climates, there is a large mismatch between model simulation and reconstruction from different proxies at mid and high latitudes. Most of the models largely underestimate the temperature increase for these latitudes. This long-lasting paradox is also puzzling for future long-term projections. 

In a second step, we will investigate the uncertainties, limitations, but also the important advantages due to the possibility of comparing model results to real data. 

Last but not least, in a final step, we will investigate the possible explanations to partly solve this paradox, pointing out how these studies may help to better constrain long-term future climates.



16h15-17h: Part I.  How current MIPs deal with comparison with temperature reconstructions: a review from Cenozoic warm climates Mid/late Pliocene, Mid Miocene Climate Optimum and deeper warm climates

·       16h45-17h: Alan Haywood: Pliocene climate and the high latitudes: a data/model perspective 


·        16h30-16h45: Natalie Burls: Simulating Miocene warmth: insights from an opportunistic MultiModel ensemble (MioMIP1) and efforts towards a coordinated intercomparison (MioMIP2)


·       16h15-16h30: Dan Lunt: DeepMIP-Eocene: A window to a super-warm world, 50 million years ago, through an model-model-proxy-proxy intercomparison approach






17h-17h30: Part II. Forcing factors and validation


·       17h-17h10: James Rae: Cenozoic CO2: from the deep ocean to the atmosphere


·       17h10-17h20: Erin McClymont: Pliocene climate variability on glacial-interglacial timescales (PlioVAR): lessons learned from multi-proxy reconstructions of seasurface temperatures and data-model comparisons



·       17h20-17h30: Aisling Dolan: Efforts towards reconstructing ice sheets during the Pliocene



17h30-18h???Part III: Some hints to better understand this long-standing paradox


·       17h30-17h40: Ran Feng: Revisiting the low-gradient problem with weather-resolving atmosphere-ocean coupled simulations

·       17h40-17h50: Gerrit Lohmann: Effects of CO2 and Ocean Mixing on Miocene and Pliocene Temperature Gradients



·       17h50-18h:  Peter Hopcroft: Potential role of methane and other non-CO2 trace gases in past warm climates


Orals: Mon, 24 Apr | Room F1

Chairpersons: Gilles Ramstein, Ayako Abe-Ouchi, Paul Valdes
Part I. How current MIPs deal with comparison with temperature reconstructions: a review from Cenozoic warm climates Mid/late Pliocene, Mid Miocene Climate Optimum and deeper warm climates
On-site presentation
Alan Haywood

It is well established that during intervals of the Pliocene epoch climatic conditions were both warmer and wetter than the pre-industrial era. Since the first global compilations of geological proxy data and climate modelling studies, it has been known that the pattern of surface temperature change during the Pliocene was not spatially uniform, with both geological data and models showing an amplification of surface temperature change at higher latitudes. This is a trait which the Pliocene shares with other warm(er) climate states in Earth history. Over the last two decades our appreciation of the character of climate and environmental change in the high latitudes has evolved significantly with (a) the availability of new and multi-proxy reconstructions, and (b) through the application of different climate, vegetation and ice sheet models, methodologies and intercomparison projects (PlioMIP1 and 2 and PLISMIP). We have become increasingly aware of the complex interaction of different sources of uncertainty (in proxies, models, model boundary conditions and forcings) when assessing the degree to which climate models are able to reproduce the magnitude of climate change indicated by geological data. It is clear that broad and simple assumptions cannot be made regarding the efficacy of either proxy reconstructions or climate model simulations for the Pliocene high latitudes, and that the picture of Pliocene climate at the higher latitudes, and how well models simulate it, is a nuanced one.

Whilst modelling studies have tended to agree in demonstrating the primacy of greenhouse gas forcing on Pliocene warming as a global average, it is increasingly apparent how important other factors such as palaeogeography and ice-sheet reconstructions can be in determining the local and regional pattern of climate change in the high latitudes. Yet, at present many of these aspects remain poorly constrained.

Energy balance analyses have demonstrated the importance of clear sky albedo, cloud albedo and heat transports in determining the degree of warming at the high latitudes in climate models. This has helped to inspire new climate modelling studies using perturbed physics in order to explore model uncertainty. However, our focus has largely been on the assessment of the ability of climate models to simulate mean annual temperature change, rather than the seasonal expression of temperature change. Recent work has demonstrated that a large ensemble of climate models is generally able to simulate warm season temperatures in the high latitudes of the Northern Hemisphere, with the apparent discrepancy in mean annual surface temperatures being driven largely by climate models underestimating the magnitude of warming during the cold season. If this is true, it would place a useful additional constraint on how Pliocene climate simulations need to evolve in order to match proxy reconstructions more closely. 

How to cite: Haywood, A.: Pliocene climate and the high latitudes: a data/model perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8969,, 2023.

Virtual presentation
Natalie Burls

The Miocene epoch, spanning 23.03-5.33Ma, was a dynamic climate of sustained, polar-amplified warmth that peaked during the Miocene Climatic Optimum (16.75-14.5Ma). Miocene atmospheric CO2 concentrations are typically reconstructed between 300-600 ppmv with estimates as high as 800-1100 ppmv during the MCO. With surface temperature reconstructions pointing to substantial midlatitude and polar warmth, it is unclear what processes maintained the much weaker-than-modern equator-to-pole temperature gradient. Emanating from community discussions at MioMeet (hosted by the Bolin Centre for Climate Research in 2019), Burls et al. (2021) synthesize several Miocene climate modeling efforts, together with available terrestrial and ocean surface temperature reconstructions. The range of model-data agreement was evaluated, highlighting robust mechanisms operating across the Miocene modelling efforts, as well as the regions where the differences across models (coming from a combination of model differences in imposed non-CO2 Miocene boundary conditions and model feedback strengths) result in a large spread in warming responses. This MioMIP1 effort was an ensemble of opportunity: the models, boundary conditions, and reference datasets were state-of-the-art, but also inhomogeneous and not ideal for a formal intermodel comparison effort. Building on MioMIP1, the Miocene community is currently drafting the experimental design for a coordinated set of MioMIP2 simulations wherein participating modelling groups will use common boundary conditions. This talk will review the take-home findings from MioMIP1 and the status of the community’s MioMIP2 effort.

How to cite: Burls, N.: Simulating Miocene warmth: insights from an opportunistic Multi-Model ensemble (MioMIP1) and efforts towards a coordinated intercomparison (MioMIP2), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6492,, 2023.

On-site presentation
Dan Lunt and the The DeepMIP Team

DeepMIP-Eocene is a community project dedicated to improving our understanding of the super-warm Eocene climate, ~50 million years ago. The objectives of the DeepMIP group include fostering closer links between the palaeoclimate modelling and data communities; designing and carrying out paleoclimate model simulations; creating and synthesising datasets to enable meaningful model-data comparisons; and analysing the results with the aims of evaluating the models, understanding the reasons behind the model-model and model-data differences, and, where possible, providing suggestions for model improvements.

DeepMIP-Eocene is nearing completion of its first phase.  In this talk, I will present the key results to emerge from this first phase.  This includes the large-scale modelled features of the Eocene climate (including the causes of polar amplification), model-data comparisons (including an assessment of whether models have improved over time), climate and Earth system sensitivity (derived from both proxies and models), ocean circulation (including an assessment of likely regions of deep water formation),  sea ice and Arctic climate,  and African and Australian hydroclimate.

I will finish with an outlook to the next phase of DeepMIP-Eocene, including new aspects of the experimental design and novel analyses.

How to cite: Lunt, D. and the The DeepMIP Team: DeepMIP-Eocene: A window to a super-warm world, 50 million years ago, through an model-model-proxy-proxy intercomparison approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9627,, 2023.

Part II. Forcing factors and validation
On-site presentation
James Rae and the OldCO2NewArchives Collaborators

The Cenozoic is a time of climatic extremes: abrupt events and state changes pepper the transition from Hothouse warmth to the Pleistocene Icehouse, and these evolving climate regimes are accompanied by major changes in ocean chemistry and biota.  CO2 is thought to play a critical role in environmental change throughout this era, but despite recent progress, there is still much to learn on the Cenozoic evolution of the ocean-atmosphere CO2 system.  To address this, we present new reconstructions of ocean pH and atmospheric CO2 spanning the late Cretaceous to the Pleistocene, based on the boron isotope composition of benthic and planktic foraminifera.  These are accompanied by improved constraints on the secular evolution of seawater chemistry, which are critical for accurate and precise determination of ocean pH and atmospheric CO2 from boron isotopes.  Using the most reliable data and updated calculation routines, we find close coupling between the CO2 system of the deep ocean, the atmosphere, and climate over the last 66 million years.  Our data also highlight intervals of dynamic changes in the carbon cycle, such as the transition into the Early Eocene Climatic Optimum, where we suggest a novel link between changes in ocean circulation, redox, and the sulphur and carbon cycles.  Overall, our data demonstrate the persistence of CO2’s control on the climate system across varying boundary conditions, and the influence of both the long-term carbon cycle and shorter-term ocean biogeochemical cycling on Earth’s climate.

How to cite: Rae, J. and the OldCO2NewArchives Collaborators: Cenozoic CO2: from the deep ocean to the atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9880,, 2023.

On-site presentation
Erin McClymont, Sze Ling Hi, Heather Ford, Julia Tindall, and Alan Haywood and the PlioVAR Working Group

The Pliocene epoch (~2.6-5.3 million years ago) offers an opportunity to study a climate state in long-term equilibrium with current or predicted near-future atmospheric CO2 concentrations. Compared to today, the late Pliocene was characterised by a globally warmer climate, with reduced continental ice volume and reduced ocean/atmosphere circulation intensity. Towards the end of the Pliocene, there was a marked increase in glaciation in the northern hemisphere and atmospheric CO2 concentrations declined.

The Past Global Changes (PAGES) PlioVAR working group co-ordinated a synthesis of marine data to characterise spatial and temporal variability of Pliocene climate, underpinned by high quality data sets and robust stratigraphies. Here we present some of the main findings of this synthesis effort, including new assessments of sea surface temperatures (SSTs) during the KM5c interglacial (~3.2 million years ago) and Pliocene-Pleistocene intensification of northern hemisphere glaciation. We outline our approaches to integrating multi-proxy reconstructions of sea-surface temperatures from a globally distributed suite of marine sediment cores, which included a review and assessment of the impacts of SST calibration choice and interpretation. We show that an improved relationship between proxy data and climate models could be generated by focussing on a single interglacial. Differences between proxies, and between proxies and models, tended to be associated with surface ocean fronts or currents, although seasonality may also be important. The transition towards enhanced northern hemisphere glaciation at the end of the Pliocene had asynchronous trends and patterns in SST as well as benthic stable isotope records. We consider how these results might inform our understanding of past climate forcings and feedbacks during both warm intervals of the past and the development of larger ice sheets in the northern hemisphere. Additional proxy data is required from high-latitude regions of both hemispheres, to assess polar amplification and meridional temperature gradients. Co-ordinated multiproxy SST analyses will also significantly enhance our understanding and interpretation of the signals they record, and provide additional detail for comprehensive data-model comparisons.

How to cite: McClymont, E., Hi, S. L., Ford, H., Tindall, J., and Haywood, A. and the PlioVAR Working Group: Pliocene climate variability on glacial-interglacial timescales (PlioVAR): lessons learned from multi-proxy reconstructions of sea-surface temperatures and data-model comparisons, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14588,, 2023.

On-site presentation
Aisling Dolan, Daniel Hill, Alan Haywood, and Yvonne Smith

Understanding Polar climate and the stability of high-latitude ice sheets is incredibly important in light of predicted future climate change and the polar amplification of warming.  The mid-Pliocene warm period (3.3 to 3.0 Ma) has been a highly investigated time in Earth history and it is well established that during this time period temperatures were warmer and CO2 levels were elevated compared to that of the pre-industrial era.  Changes in the polar cryosphere are a key driver of Pliocene climate change and reduced equator-to-pole temperature gradients. During the mid-Pliocene, estimates of sea level change between 5m and 25m have been reconstructed based on geological evidence. Best estimates of maximum sea level rise are around 20m, suggesting a significant contribution to sea level from both the Greenland and Antarctic Ice sheets is likely at certain intervals within the mid-Pliocene. Despite the uncertainties, Pliocene sea level has become a key target for ice sheet models trying to simulate ice sheet melt and a test-bed for ice loss physics.

Alongside a series of modelling efforts to understand the broader Pliocene climate (PlioMIP1 and PlioMIP2), the Pliocene ice sheet modelling Intercomparison project (PLISMIP) was formed to investigate the dependency of ice sheet reconstructions on the specific climate or ice sheet model employed.   We detail investigations of ice sheet model parameterisations and initial conditions, climate model boundary conditions and the climate model used, and show the extent to which these impact our predictions of ice sheet configuration during the Pliocene. 

We consider the implications of having to prescribe an ice sheet configuration in large model intercomparison projects such as PlioMIP and how the results from PLISMIP and more recent independent ice sheet modelling work has influenced the experimental design in PlioMIP3 to include different ice sheet scenarios over Antarctica.  We also highlight key areas where there is the potential to use geological proxy data or employ enhanced modelling techniques to constrain our estimates of ice in a warmer world.  

How to cite: Dolan, A., Hill, D., Haywood, A., and Smith, Y.: Efforts towards reconstructing ice sheets during the Pliocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8995,, 2023.

Part III: Some hints to better understand this long-standing paradox
Virtual presentation
Ran Feng

The reduction in the tropical to mid-latitude sea surface temperature gradient, as shown by proxy records across the Pacific, is a common feature of past warm intervals but remains difficult for climate models to replicate. This model-data discrepancy (termed “the low-gradient problem”) is potentially tied to the parameterizations of cloud and moist convection in the models, which remain highly uncertain and largely limit the confidence of predicted future climate change. Here, focusing on the mid-Pliocene (4 - 3 Ma) interval, for which climate forcing conditions are relatively well constrained and global SSTs are well sampled, two sets of atmosphere-dynamical ocean coupled simulations with (at 25 km) and without (at 100 km) weather-resolving atmospheric resolution using Community Earth System Model version 1.3 are compared to identify whether better resolved cloud and moist convection can ameliorate the low-gradient problem. Preliminary results show more extensive mid-Pliocene warming in the North Pacific relative to the preindustrial simulated in the high-resolution simulations compared to the low-resolution simulations, whereas tropical Pacific warming is similar in both sets of simulations. The approximate partial radiative perturbation method and heat transport decomposition will be implemented to quantify differences in the shortwave cloud effects and responses of atmospheric dry static and latent energy and ocean heat transport between the high- and low-resolution simulations. The contributions from better resolved clouds and moist convection to the amplified North Pacific warming in the high-resolution simulations will be further quantified.

How to cite: Feng, R.: Revisiting the low-gradient problem with weather-resolving atmosphere-ocean coupled simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16511,, 2023.

On-site presentation
Gerrit Lohmann, Gregor Knorr, Akil Hossain, and Christian Stepanek

During the Cenozoic Era hothouse climate transformed to a state that allowed establishment of extensive ice-sheets. The transformation towards an overall cooler climate encompassed periods of relatively steady change of global temperatures which were interrupted by short-term aberrations of relatively rapid cooling or warming. Various drivers have been found to contribute to this complex process of climate cooling - among these drawdown of carbon dioxide, reorganized ocean circulation related to ocean gateway evolution, varying amplitude and geographic location of deep water upwelling and formation processes, and internal feedbacks related to changes in environmental and land surface conditions in particular at high latitudes and on the continents.

The fact that carbon dioxide as the most important current driver of climate change is not always proportionally linked to past changes in global temperatures underlines the importance of mechanisms beyond greenhouse gas drawdown that contributed to Cenozoic climate cooling. Several questions remain regarding mechanisms and drivers of climate evolution as reconstructed from Cenozoic proxy recorders: How can a low meridional temperature gradient be maintained at carbon dioxide concentrations that are in line with reconstructions and inference on relatively modest tropical tempatures? Which mechanisms contributed to extremely high deep sea temperatures ?

Here we propose that during the Miocene and the Pliocene enhanced vertical mixing in the ocean may provide potential explanations to some of these enigmas. We employ the global general circulation model, which contributed to PlioMIP, MioMIP, and DeepMIP (e.g., Stärz et al., 2017; Stepanek et al., 2020; Hossain et al., 2020), and study the impact of variations in vertical mixing in the ocean on large-scale climate patterns, meridional temperature gradient, and deep sea ocean temperatures. We find that both carbon dioxide and enhanced vertical mixing cause increased radiative feedback by reducing effective emissivity and surface albedo. For the Miocene, enhanced oceanic heat uptake due to invigorated vertical mixing causes intense warming of the deep ocean (5-10°C) and of the Arctic (>12°C). For the Pliocene we find that the impact of radiative forcing and enhanced vertical mixing is less relevant. This hints to a dependency of carbon dioxide and mixing sensitivity to background climate and ocean dynamics.

While our work is focused on climate modelling, we highlight that consideration of enhanced vertical mixing leads in our Miocene and Pliocene climate simulations to large-scale climate patterns that are in better agreement with specific aspects of proxy-based inference on past warm climates. To further corroborate our results we must compare our simulations with reconstructions of thermocline depth and seasonality - lower seasonality in reconstructions would be in line with higher heat capacity as facilitated by enhanced vertical mixing. Our ad-hoc enhanced mixing formulation for the Pliocene and Miocene (Lohmann et al., 2022) can be motivated by recent simulations with a strongly eddying ocean and an altered heat transport (Nooteboom et al, 2022). In the future, we make use of the eddy resolving model to evaluate the polar amplification of the system with respect to model resolution, gateway configuration, and background CO2.

How to cite: Lohmann, G., Knorr, G., Hossain, A., and Stepanek, C.: Effects of CO2 and Ocean Mixing on Miocene and Pliocene Temperature Gradients, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11669,, 2023.

On-site presentation
Peter Hopcroft, Diane Segalla, Gilles Ramstein, and Thomas Pugh

Past warm climates allow the evaluation model predictions of the response of the Earth System to elevated greenhouse gas levels. However, Earth System model simulations routinely underestimate high-latitude warmth for past states, meaning that the forcings provided to models, the models themselves or the climate reconstructions are in error. We focus on the first of these and review the potential role of varying levels of atmospheric trace gases besides carbon dioxide (non-CO2 trace gases), which has been investigated in relatively few studies to date. Using a combination of terrestrial biogeochemistry models and simplified atmospheric chemistry scheme we make first-order estimates of the radiative forcing by non-CO2 trace gases for the mid-Pliocene and compare these with new results for the Miocene. We will discuss the main uncertainties involved and review potential avenues for future research.

How to cite: Hopcroft, P., Segalla, D., Ramstein, G., and Pugh, T.: Potential role of methane and other non-CO2 trace gases in past warm climates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6667,, 2023.