. As global warming is proceeding due to rising greenhouse
gas concentrations, the Earth system moves towards climate states that
challenge adaptation. Past Earth system states are offering possible
modelling systems for the global warming of the coming decades. These
include the climate of the mid-Pliocene ( ∼  3 Ma), the last
interglacial ( ∼  129–116 ka) and the mid-Holocene
( ∼  6 ka). The simulations for these past warm periods are the
key experiments in the Paleoclimate Model Intercomparison Project (PMIP)
phase 4, contributing to phase 6 of the Coupled Model Intercomparison Project (CMIP6).
Paleoclimate modelling has long been regarded as a robust out-of-sample
test bed of the climate models used to project future climate changes. Here,
we document the model setup for PMIP4 experiments with EC-Earth3-LR and
present the large-scale features from the simulations for the mid-Holocene,
the last interglacial and the mid-Pliocene. Using the pre-industrial
climate as a reference state, we show global temperature changes,
large-scale Hadley circulation and Walker circulation, polar warming, global
monsoons and the climate variability modes – El Nino–Southern Oscillation
(ENSO), the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO).
EC-Earth3-LR simulates reasonable climate responses during past warm
periods, as shown in the other PMIP4-CMIP6 model ensemble. The systematic
comparison of these climate changes in past three warm periods in an
individual model demonstrates the model's ability to capture the climate
response under different climate forcings, providing potential implications
for confidence in future projections with the EC-Earth model.

/pdf/simulating-the-mid-holocene-last-interglacial-and-mid-32i1udjs40.pdf

Simulating the mid-Holocene, last interglacial and mid-Pliocene climate with EC-Earth3-LR

. Following the protocol of the fourth phase of the Paleoclimate
Modelling Intercomparison Project (PMIP4), we performed numerical experiments
targeting distinctive past time periods using the Model for
Interdisciplinary Research on Climate, Earth System version 2 for Long-term
simulations (MIROC-ES2L), which is an Earth system model. Setup and basic
performance of the experiments are presented. The Last Glacial Maximum was one of the most extreme climate states during
the Quaternary and conducting numerical modeling experiments of this period
has long been a challenge for the paleoclimate community. We conducted a
Last Glacial Maximum experiment with a long spin-up of nearly 9000 years.
Globally, there was reasonable agreement between the anomalies relative to the present day derived from model climatology and those derived from proxy data archives, while some regional discrepancies remained. By changing orbital and greenhouse gas forcings, we conducted experiments
for two interglacial periods: 6000 and 127 000 years before present. Model
anomalies relative to the present day were qualitatively consistent with
variations in solar forcing. However, anomalies in the model were smaller
than those derived from proxy data archives, suggesting that processes that
play a role in past interglacial climates remain lacking in this
state-of-the-art model. We conducted transient simulations from 850 to 1850 CE and from 1850
to 2014 CE. Cooling in the model indicated a clear response to huge volcanic
eruptions, consistent with paleo-proxy data. The contrast between cooling
during the Little Ice Age and warming during the 20th to 21st
centuries was represented well at the multidecadal timescale.

/pdf/pmip4-experiments-using-miroc-es2l-earth-system-model-sycbyeii4c.pdf

PMIP4 experiments using MIROC-ES2L Earth system model

The Last Interglacial period (LIG) is a period with increased summer insolation at high northern latitudes, which results in strong changes in the terrestrial and marine cryosphere. Understanding the mechanisms for this response via climate modelling and comparing the models' representation of climate reconstructions is one of the objectives set up by the Paleoclimate Modelling Intercomparison Project for its contribution to the sixth phase of the Coupled Model Intercomparison Project. Here we analyse the results from 16 climate models in terms of Arctic sea ice. The multi-model mean reduction in minimum sea ice area from the pre industrial period (PI) to the LIG reaches 50 % (multi-model mean LIG area is 3.20×106 km2, compared to 6.46×106 km2 for the PI). On the other hand, there is little change for the maximum sea ice area (which is 15–16×106 km2 for both the PI and the LIG. To evaluate the model results we synthesise LIG sea ice data from marine cores collected in the Arctic Ocean, Nordic Seas and northern North Atlantic. The reconstructions for the northern North Atlantic show year-round ice-free conditions, and most models yield results in agreement with these reconstructions. Model–data disagreement appear for the sites in the Nordic Seas close to Greenland and at the edge of the Arctic Ocean. The northernmost site with good chronology, for which a sea ice concentration larger than 75 % is reconstructed even in summer, discriminates those models which simulate too little sea ice. However, the remaining models appear to simulate too much sea ice over the two sites south of the northernmost one, for which the reconstructed sea ice cover is seasonal. Hence models either underestimate or overestimate sea ice cover for the LIG, and their bias does not appear to be related to their bias for the pre-industrial period. Drivers for the inter-model differences are different phasing of the up and down short-wave anomalies over the Arctic Ocean, which are associated with differences in model albedo; possible cloud property differences, in terms of optical depth; and LIG ocean circulation changes which occur for some, but not all, LIG simulations. Finally, we note that inter-comparisons between the LIG simulations and simulations for future climate with moderate (1 % yr−1) CO2 increase show a relationship between LIG sea ice and sea ice simulated under CO2 increase around the years of doubling CO2. The LIG may therefore yield insight into likely 21st century Arctic sea ice changes using these LIG simulations.

/pdf/a-multi-model-cmip6-pmip4-study-of-arctic-sea-ice-at-127-ka-3z97rhij8h.pdf

A multi-model CMIP6-PMIP4 study of Arctic sea ice at 127 ka: sea ice data compilation and model differences

The Last interglacial (LIG) is a period with increased summer insolation at high northern latitudes, which results in strong changes in the terrestrial and marine cryosphere. Understanding the mechanisms for this response via climate modelling and comparing the models&#8217; representation of climate reconstructions is one of the objectives set up by the Paleoclimate Modelling Intercomparison Project for its contribution to the sixth phase of the Coupled Model Intercomparison Project. Here we analyse the results from 12 climate models in terms of Arctic sea ice. The mean pre-industrial to LIG reduction in minimum sea ice area (SIA) reaches 59% (multi-model mean LIG area is 2.21 mill. km2, compared to 5.85 mill. km2 for the PI), and the range of model results for LIG minimum sea ice area (from 0.02 to 5.65 mill. km2) is larger than for PI (from 4.10 to 8.30 mill. km2). On the other hand there is little change for the maximum sea ice area (which is 12 mill. km2 for both the PI and the LIG, with a standard deviation of 1.04 mill. km2 for PI and 1.21 mill. km2 for LIG). To evaluate the model results we synthesize LIG sea ice data from marine cores collected in the Arctic Ocean, Nordic Seas and northern North Atlantic. South of 78oN, in the Atlantic and Nordic seas, the LIG was seasonally ice-free. North of 78oN there are some discrepancies between sea ice reconstructions based on dinocysts/foraminifers/ostracods and IP25: some sites have both seasonal and perennial interpretations based on the same core, but different indicators. Because of the con&#64258;icting interpretations it is not possible for any one model to match every data point in our data synthesis, or say whether the Arctic was seasonally ice-free. Drivers for the inter-model differences are: different phasing of the up and down short-wave anomalies over the Arctic ocean, associated with differences in model albedo; possible cloud property differences, in terms of optical depth; LIG ocean circulation changes which occur for some, but not all, LIG simulations. Finally we note that inter-comparisons between the LIG simulations, and simulations with moderate CO2 increase (during the transition to high CO2 levels), may yield insight into likely 21C Arctic sea ice changes using these LIG simulations.

A multi-model CMIP6 study of Arctic sea ice at 127 ka: Sea ice data compilation and model differences

During orbital precession minima, the Sahara was humid and more vegetated, providing potential corridors for Hominins migration. Uncertainties remain over the climatic processes controlling the initiation, demise and amplitude of these African Humid Periods. Here we study these processes using a series of transient simulations of the penultimate deglaciation and Last Interglacial period, and compare the results with a transient simulation of the last deglaciation and Holocene. We find that the strengthening of the Atlantic Meridional Overturning Circulation at the end of deglacial millennial-scale events exerts a dominant control on the abrupt initiation of African Humid Periods as the Atlantic Meridional Overturning Circulation modulates the position of the Intertropical Convergence Zone. In addition, residual Northern Hemispheric ice-sheets can delay the peak of the African Humid Period. Through its impact on Northern Hemispheric ice-sheets disintegration and thus Atlantic Meridional Overturning Circulation, the larger rate of insolation increase during the penultimate compared to the last deglaciation can explain the earlier and more abrupt onset of the African Humid Period during the Last Interglacial period. Finally, we show that the mean climate state modulates precipitation variability, with higher variability under wetter background conditions. A strengthening of the Atlantic Meridional Overturning Circulation was the main driver of the abrupt initiation of African Humid Periods during the last two deglaciations, according to transient simulations performed using an Earth system model.

Drivers of the evolution and amplitude of African Humid Periods

The modeling of paleoclimate, using physically based tools, is increasingly seen as a strong out-of-sample test of the models that are used for the projection of future climate changes. New to the Coupled Model Intercomparison Project (CMIP6) is the Tier 1 Last Interglacial experiment for 127 000 years ago (lig127k), designed to address the climate responses to stronger orbital forcing than the midHolocene experiment, using the same state-of-the-art models as for the future and following a common experimental protocol. Here we present a first analysis of a multi-model ensemble of 17 climate models, all of which have completed the CMIP6 DECK (Diagnostic, Evaluation and Characterization of Klima) experiments. The equilibrium climate sensitivity (ECS) of these models varies from 1.8 to 5.6 ∘C. The seasonal character of the insolation anomalies results in strong summer warming over the Northern Hemisphere continents in the lig127k ensemble as compared to the CMIP6 piControl and much-reduced minimum sea ice in the Arctic. The multi-model results indicate enhanced summer monsoonal precipitation in the Northern Hemisphere and reductions in the Southern Hemisphere. These responses are greater in the lig127k than the CMIP6 midHolocene simulations as expected from the larger insolation anomalies at 127 than 6 ka.

New synthesis for surface temperature and precipitation, targeted for 127 ka, have been developed for comparison to the multi-model ensemble. The lig127k model ensemble and data reconstructions are in good agreement for summer temperature anomalies over Canada, Scandinavia, and the North Atlantic and for precipitation over the Northern Hemisphere continents. The model–data comparisons and mismatches point to further study of the sensitivity of the simulations to uncertainties in the boundary conditions and of the uncertainties and sparse coverage in current proxy reconstructions.

The CMIP6–Paleoclimate Modeling Intercomparison Project (PMIP4) lig127k simulations, in combination with the proxy record, improve our confidence in future projections of monsoons, surface temperature, and Arctic sea ice, thus providing a key target for model evaluation and optimization.

/pdf/large-scale-features-of-last-interglacial-climate-results-2z88cp3kts.pdf

Large-scale features of Last Interglacial climate : Results from evaluating the lig127k simulations for the Coupled Model Intercomparison Project (CMIP6)-Paleoclimate Modeling Intercomparison Project (PMIP4)

Xaoxu Shi

Papers

Large-scale features of Last Interglacial climate : Results from evaluating the lig127k simulations for the Coupled Model Intercomparison Project (CMIP6)-Paleoclimate Modeling Intercomparison Project (PMIP4)