Last Interglacial CMIP6/PMIP6 proposed experiments

The Summary for Policymakers for both the IPCC WG1 AR4 and AR5 included statements on the Last Interglacial (LIG):

AR5: There is very high confidence that maximum global mean sea level during the last interglacial period (129,000 to 116,000 years ago) was, for several thousand years, at least 5 m higher than present, and high confidence that it did not exceed 10 m above present. During the last interglacial period, the Greenland ice sheet very likely contributed between 1.4 and 4.3 m to the higher global mean sea level, implying with medium confidence an additional contribution from the Antarctic ice sheet. This change in sea level occurred in the context of different orbital forcing and with high-latitude surface temperature, averaged over several thousand years, at least 2°C warmer than present (high confidence).

Yet the AR4 and AR5 had no coordinated simulations for the LIG to assess the interplay of polar amplification of temperature, seasonal memory of sea ice, and precipitation/storm track changes on the stability of the Greenland ice sheet and its contribution to the sea level high stand nor the interplay of oceanic and atmospheric temperatures and circulation on the stability of the Antarctic ice sheet. Climate model simulations for the LIG assessed in the AR5, although completed by many modeling groups, varied in their forcings and often were not made with the same model/same resolution as the CMIP5 future projections, thus providing a useful but incomplete means for assessment (IPCC AR5 WGI Chapter 5; Lunt et al., Climate of the Past, 2013). Similarly, Greenland ice sheet simulations assessed in the AR5 used offline models with a variety of climate forcing setups, not then allowing feedbacks among the Earth system components (Chapter 5). No simulations were available to assess the Antarctic ice sheet (particularly, the West Antarctic Ice Sheet) contribution to the LIG sea level high stand.

We propose two CMIP6 experiments for the LIG to determine the interplay of warmer atmospheric and oceanic temperatures, changed precipitation, and changed surface energy balance on ice sheet thermodynamics and dynamics during this period. Still uncertain are how well ice sheet-climate models can predict the stability of the ice sheets and if thresholds may be passed this century. A LIG simulation will be of high societal relevance because of implications for sea level changes as well as sea ice and monsoons. The LIG simulations will also provide an‘out-of-sample’ evaluation of new features of CMIP6 models: coupled climate-ice sheet models. The LIG is the most suitable of the warm interglacials for a CMIP6 assessment because of the wealth of data including: ice cores providing measurements of well-mixed greenhouse gases, aerosols including dust and sea salt, and stable water isotopes as a proxy for temperature, as well as for Greenland, ice sheet elevation and extent; marine records for ocean temperatures and geotracers that can be interpreted in terms of water masses and overturning strength; speleothems that provide indication of monsoon strength; and terrestrial records that indicate temperature and vegetation. As well, new records are refining our knowledge of sea ice extent, fire, and biodiversity.

The proposed CMIP6 simulations for the LIG are particularly relevant to the WCRP Grand Challenges: Changes in Cryosphere and Regional Sea-level Rise, but also to Regional Climate Information and Clouds, Circulation and Climate Sensitivity because of the large forcings and thus large regional responses as recorded in the data. It addresses well the broad scientific questions: 1. How does the Earth System respond to forcing? and 2. What are the origins and consequences of systematic model biases (especially at high latitudes and relevant to the stability of the ice sheets)? As part of PMIP, some groups will additionally perform transient coupled ice sheet-climate simulations that will provide rates of change for sea level, including regional sea level if offline GIA models applied, as well as a measure of the capability of these models to initiate the next glacial inception.

The CMIP6 experiments will evaluate systematic biases and their origins among models on their ability to simulate Arctic warmth and sensitivity of Greenland ice sheet to this warmth, and ocean warming and transmission of subsurface warming from North Atlantic to Southern Ocean, with implications for basal melting of West Antarctic Ice Sheet.

• Evaluating climate models for warm period, high sea-level stand.
• Impacts of smaller ice-sheets/higher sea-level on climate
• Comparison to climate responses (monsoons, temperature, sea ice, etc) to mid-Holocene simulation with smaller orbital forcing.

128ka simulations - large orbital forcing, large responses.

• Orbital parameters set to 128ka (Figure 1).
• Greenhouse gas concentrations well-known from ice cores (Figure 2), [CO2 275ppm; CH4 709ppb; N2O 266 ppb].
• Experiment 1 (Tier 1): AOGCM; modern geography, ice sheets, and vegetation; length: ≥ 100 years after spinup. Initialize from CMIP6 preindustrial DECK simulation. Save appropriate output to allow forcing of offline ice-sheet models.
• Experiment 2 (Tier 2; collaborative with ISMIP6): Coupled AOGCM-ISM; predictive Greenland ice sheet models, asynchronous coupling with 10 ISM years per 1 AOGCM years; length: AOGCM 300 years, ISM 3000 years. Initialize from Experiment 1.

• 125ka, 122ka time slices.
• Plus transient simulation: 128-118ka (possibly should be started earlier, then would need one more snapshot to initialize, ice sheet and meltwater reconstructions for remnant North American and Eurasian ice sheets).

• Relevant topics: climate, ice sheets and sea level, sea ice, monsoons, carbon cycle.
• LIG CMIP6 experiments, collaborative between PMIP6 and ISMIP6.
• The experiments should be well established for Tier 1 and at least tested for Tier 2 and beyond.
• A sufficient number of modelling centers (~8) are a part of the MIP and committed to simulating all of the MIP‘s Tier 1 experiments for a particular science question.

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