The interplay between climate and CO2 is complex, especially amidst multiple environmental changes. Atmospheric CO2 influences climate over both human and geological timescales, yet key uncertainties remain. For instance, how the Antarctic Ice Sheet (AIS) responds to varying CO2 levels, particularly its melting and growth dynamics, is crucial for future sea level rise projections. To clarify these uncertainties, the ERC-funded HighBorG project will examine CO2-climate interactions during three critical geologic periods: 52-46 Ma (warmer Earth with high CO2), 39-23 Ma (establishment of the AIS), and 17-13 Ma (cooler Earth with dynamic AIS). By developing advanced purification and analysis techniques and integrating marine records with modelling approaches, HighBorG sheds light on past and future climate states.
The link between climate and CO2 is not always straightforward, particularly during periods of multiple climatic and environmental changes, but atmospheric carbon dioxide (CO2) plays an important part in determining climate on human and geological timescales. For example, state dependency of climate sensitivity, the response of Earth’s temperature to CO2, is a critical unknown for future climate projections and policy strategies. Another unknown is the response of the Antarctic Ice Sheet (AIS) at different CO2 thresholds for its melting and growth, “hysteresis”, despite the fact that this will ultimate determine the magnitude of future sea level rise. Existing records suggest that the ice sheet margin waxed and waned since the Eocene, ~50 million years ago (Ma), while CO2 varied within the likely range projected for year 2100 and Earth’s temperature progressively cooled. HighBorG aims to resolve climate-AIS-CO2 unknowns on three periods of the geologic past: 52-46 Ma (hot Earth, high CO2, likely ice free), largely unexplored 39-23 Ma (when AIS was established), and 17-13 Ma (cold Earth, low CO2, dynamic AIS). HighBorG focuses on reconstructing CO2-climate feedbacks at millennial/orbital timescales, a medium risk high gain opportunity necessary to resolve warming and cooling periods at different climate states. To achieve this, a new automated system for purification-analysis will be developed, accompanied by a laser ablation split-stream approach, utilising new marine archives and cutting-edge methodologies. Seasonal reconstructions from contemporaneous tropical corals will provide a novel way to constrain seawater composition, necessary to obtain accurate estimates of CO2. Earth system modelling incorporating reconstructions will provide a new understanding of the mechanisms driving state dependent interactions between Earth’s orbit, CO2, temperature, cryosphere and carbon cycling, increasing our confidence to sea level/temperature projections for the future.