Objective 1: Drill a replicate ice core in the bottom part of the GRIP site
To supplement the existing ice core material available in the Copenhagen ice core freezer, we will, for the first time, drill a replicate ice core covering the bottom 200 m of the GRIP ice core (NewGRIP). We will continue into the bedrock and retrieve a rock core of the underlying sediments and bedrock. There is not sufficient ice core material left in the old GRIP ice core to apply all the novel dating methods to this basal material [Tison et al., 2019]. In addition, no rock core was originally drilled at GRIP and it is of large importance to retrieve new samples in this central part of the GrIS, a location that is ideal to document a complete GrIS collapse.

Objective 2: Reconstruct the history of the GrIS
We will extract the geochemical and biological archives preserved in both basal ice [Willerslev et al., 2007; Yau et al., 2016] and the underlying sediments [Christ et al., 2021; Schaefer et al., 2016]. These archives can provide age constraints, but at present the paucity of measurements limits the interpretation. We will date basal ice with δ⁴⁰Ar and ⁸¹Kr in ice bubbles, atmospheric ³⁶Cl/¹⁰Be in the ice, in-situ cosmogenic ²⁶Al and ¹⁰Be in rocks, ancient DNA, optical-stimulated luminescence on grains and pebbles. We will develop new dating approaches, such as solving vertical gas and water stable isotopes profiles in basal ice and cross dating with the timewise longer records from Antarctica using global proxies as CH₄, δ¹⁸O of O₂ in air, meteoric ¹⁰Be and volcanic eruptions in ice. Interpretation of these data will provide ages for the last ice-free and partial melting episodes in Greenland, as well as duration of these ice-free episodes.

Objective 3: Reconstruct past ecosystems during ice-free conditions
Green2Ice will evaluate the occurrence of terrestrial plant macrofossils, ancient molecules (DNA and lipid biomarkers) and organic matter composition (C/N ratio, δ¹³C and δ¹⁵N) in basal ice and underlying sediments. This will provide insights into the climatic conditions prevailing during ice-free intervals [Tison et al., 1998]. This information will be compared to marine and terrestrial sedimentary records surrounding the GrIS [De Vernal and Mudie, 1989; Funder et al., 2001]) to assess the regional impact of advances and retreats of the GrIS. The information will also determine the temperature and precipitation conditions needed to support the ecosystems found at the base at the time it was ice free and isostatically lifted [Cluett and Thomas, 2021; Willerslev et al., 2007].

Objective 4: Understand the subglacial environment
The discovery of microorganisms in subglacial environments confirms that glaciers and ice-sheet beds are not sterile but inhabited by metabolically active microbial populations [Christner et al., 2014; Sharp et al., 1999]. Green2Ice will evaluate to what extent there is an in-situ production of greenhouse gases (CO₂, CH₄, N₂O) and what the contribution of the subglacial environment to the atmospheric burden of these potent greenhouse gases. The redox cycling of nitrogen appears to be central in the acquisition of energy and the transfer of electrons in this ecosystem [Tranter, 2014]. We will also evaluate the extent of microbial nitrogen recycling by developing nitrate isotopes investigations (δ¹⁵N, δ¹⁸O, Δ¹⁷O) in the subglacial environment [Fripiat et al., 2021].

Objective 5: Model sensitivity and thresholds/tipping points for the GrIS
Based on input of the age of the GrIS from Objective 2, we will model the evolution of the GrIS on a long time scale (3 Ma). We will test the MPT theories on the influence of regolith under the GrIS and changing concentrations of greenhouse gases. We will test the sensitivity of the GrIS in relation to knowledge of past resilience of the ice sheet. For this, Objective 3 goals on the temperature conditions (warming amplitude and duration) needed to have an ice-free Greenland will be key. An outcome of this will also be a study of the hysteresis of the ice sheet, the difference in time scales to build or melt an ice sheet both in a 41 ka and a 100 ka world [Solgaard et al., 2011].

Objective 6: Use paleo reconstructions to predict future sea level rise from the GrIS
The final and integrating objective is to use the new knowledge of past relations between climate and the evolution of the GrIS in the context of future warming climate. Here results from objectives 3 and 5 on the time and temperature needed to melt the ice sheet are important, because an ice sheet rarely is in a steady state situation with response times of 10,000 years to fully react to climate changes. We will interact with the IPCC modelling groups and expect the impact of the Green2Ice can be part of the coming IPCC AR7 report.