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Sea level and extreme waves in the Last Interglacial

Periodic Reporting for period 2 - WARMCOASTS (Sea level and extreme waves in the Last Interglacial)

Reporting period: 2020-10-01 to 2021-10-31

The ERC StG "WARMCOASTS" aims at improving our knowledge of the Last Interglacial, a period of the Earth's history characterized by a slightly warmer climate than today. During the Last Interglacial (Marine Isotopic Stage 5e, peaking 125 thousand years ago), polar and global temperatures were slightly higher than pre-industrial. As a result, ice sheets were smaller, and the sea level was higher than today. While today's warming climate is due to increased carbon dioxide in the atmosphere to levels well above 400 ppm (parts per million), orbital forcing (causing more insolation to reach the Earth) was responsible for higher Last Interglacial Earth temperatures. For this reason, the Last Interglacial is considered a "process-analog" of a future warmer world. In short, regardless of the causes of warming, it provides a past analog of the consequences that a warmer climate might have on many Earth processes, such as ice sheets.

While we have a broad understanding of the Last Interglacial climate, some critical details are still uncertain and highly debated. In WARMCOASTS, we tackle the three most relevant open issues regarding sea level and coastal changes in the Last Interglacial.

First, the current state of the art is that the sea level during the Last Interglacial was 6-10m higher than today. However, recent work demonstrated that significant uncertainties might affect this estimate. These need addressing before confidently using the Last Interglacial sea level as a proxy for ice sheet melting in a warmer climate.

Second, some studies suggest the presence of relevant sea-level oscillations during a period of substantial climatic stability within the Last Interglacial. Other studies reject this hypothesis. These sea-level oscillations might be due to rapid ice-sheet melting during the interglacial. Therefore, understanding their existence and nature is paramount to gauge the sensitivity of ice sheets to even small changes in ocean and atmospheric polar temperatures.

Third, some studies suggest that the Last Interglacial was characterized, in some coastal regions, by stronger sea storms/hurricanes than today. The geological proofs for this hypothesis are heavily debated and limited to local contexts in the Bahamas and Bermuda. Finding new evidence related to the "stronger storms" hypothesis is crucial to gauge whether we can expect more coastal damage under future climate conditions.

These three open issues have one common thread: improving our knowledge on the Last Interglacial to gauge the extent of impacts our coastal areas will experience as global temperatures increase. In addition, a better understanding of these processes has direct relevance to society at large, which is preparing to face the consequences of climate change.
In WARMCOASTS, we have begun to tackle the issues above, merging geosciences and earth process modeling approaches. The first action was to compile a global Last Interglacial sea-level database to provide a complete account of the available information on the Last Interglacial coastal stratigraphies globally. We called this database "The World Atlas of Last Interglacial Shorelines" (WALIS). We are currently working with earth modelers to obtain an extensive array of models to predict vertical land motions caused by glacial isostatic adjustment and dynamic topography. Analyzing the database in light of these models will allow us to give a more precise answer to the question: "how high was sea level in the Last Interglacial?".

In addition to building the database, we have been collecting field data to obtain highly accurate information on coastal stratigraphy at different locations globally. We use high-precision Global Navigation Satellite System, and we reconstruct high-resolution virtual outcrops processing drone and land photography with Structure-from-Motion/Multi-View Stereo techniques. We couple these new technologies with classic geological interpretation and dating to unravel the history recorded in the rocks. To go beyond state of the art, we plan to use a set of models, called stratigraphic forward models, to help us understand how coastal systems might develop through an entire interglacial. With these models, we can gauge how a coastal system evolves through time, tuning the environmental conditions (such as hydrodynamics and sea level) to match what we observe in the field. The challenge to overcome is the small temporal and spatial scales at which we are working.

The compilation of WALIS is nearing completion. A large part of the Pleistocene paleo sea-level community has compiled standardized data into WALIS and wrote accompanying "data-description" articles in a Special Issue on the Earth System Science Data journal. Members of the WARMCOASTS project have been active both as editors and authors of manuscripts in this Special Issue. The database counts more than 4500 sea-level index points, each associated with one or more dated samples or chronostratigraphic constraints.

In parallel, we collected preliminary geological data from different areas globally. We carried out these activities in collaboration with local colleagues and scientists working on projects complementary to WARMCOASTS. As of October 2021, we are working on Argentina, Madagascar, Cape Verde, and Italy data. The work aims at analyzing the geological proxies surveyed in the field to establish a chronology, depositional patterns, and the potential to contribute new information on the main scientific questions of the project. Together with earth modelers, we are now working towards understanding the processes that caused departures from eustasy in these areas.

The results so far include six published manuscripts co-authored by WARMCOASTS team members in the WALIS Special Issue. In addition, we carried out other research activities in collaboration with colleagues working on complementary projects. These activities resulted so far in two papers. One (Gilford et al., 2020, JGR) suggests that increased knowledge on Last Interglacial global mean sea level might help constrain future sea-level projections. The second (Dyer et al., 2021, PNAS) indicates that peak Last Interglacial sea level might be lower than hitherto assumed (1.2 to 5.3 m instead of the typically cited 6-9 m range), opening the field for a new debate on the sensitivity of ice sheets to future climate change.
WALIS is arguably the most extensive compilation of Last Interglacial sea-level index points to date. The data insertion interface we built for WALIS has no equals in the paleo sea-level community and finds rare counterparts in the geological community at large. Thanks to the support provided by PAGES (the Past Global Changes project), WALIS is being updated to include Holocene sea-level data, following existing database templates and best practices. As of October 2021, the number of sea-level data points in WALIS has surpassed those included in previous global compilations. Furthermore, WALIS is not static, and it will remain open to both new contributions and corrections of existing records as new data becomes available.

As the project continues, data in WALIS will be used, together with new field data and earth models, to better gauge peak Last Interglacial sea level. Alongside this effort, we are also working on field data collection strategies to maximize their potential to support models of Last Interglacial sea level oscillations and extreme waves.
Redux of Last Interglcial temperature and atmspheric carbon dioxide.
ReadTheDocs page to help navigate the database interface.
Fieldwork on Last Interglacial sea-level indicators in Patagonia, Argentina.
Database interface for the World Atlas of Last Interglacial shorelines.