Anais Orsi

I am a polar climate scientist, combining fieldwork in key locations, innovation in the lab, and interactions with the modelling community to better quantify our understanding of polar climate variability at the decadal to millennial scale, and to better assess the risks associated with the current global warming. Polar regions are a key component of the Earth’s climate system, and are changing dramatically under anthropogenic climate change. The single greatest challenge for polar climate science is that there are not enough data to describe the climatology and trends. For example, in Greenland and Antarctica, historical weather stations were mostly situated on the coast; in Greenland, there were no inland stations at all before 1995. Satellite data help, but aside from their short record, they have their own limitations, like the presence of cold biases due to difficulties in separating the snow surface from low clouds. To be useful, satellite data also need ground validation. As a result, even though Greenland and Antarctica are perhaps the most sensitive regions on Earth to climate change, it is still very difficult to precisely answer questions like “How much have they been warming over the past 50 years?”, and even more difficult to put recent observations in the context of natural climate variability. I am addressing these issues by producing new records in key locations using paleoclimate tools. The originality of my work is to use a wide variety of temperature proxies in ice cores (inert gas isotopes, water isotopes, melt layers, firn thickness, borehole temperature), analyse them with proxy system models (gas and temperature diffusion, firn densification) into a single inverse problem to produce a well constrained temperature history. I then use atmospheric reanalyses and climate model outputs to investigate the mechanisms that can be responsible for the observed variability and change. As an example, at NEEM in North West Greenland, I used borehole temperature data, inert gas isotopes (d15N, d40Ar), and water isotope data to constrain the recent warming trend and quantify how exceptional it was in the context of the past 200 years of climate history. I demonstrated that the intense warming trend is not limited to coastal regions where melt is observed, but widespread on the ice sheet, implying that large scale dynamics are important. I then used the MAR regional model to show that longwave feedbacks associated with an increase in atmospheric humidity are the dominant cause of surface warming on the Greenland ice sheet. To expand the use of this type of datasets, I have worked to compile results from borehole temperature in Antarctica into easily useable metrics that can improve climate model evaluation in this data-limited region. To give a broader context to modern observations, I have produced new temperature reconstructions, and shown, for instance, that the strong warming observed in West Antarctica is within the range of natural variability at the centennial scale, and likely attributable to natural modes of variability in the Pacific Ocean, rather than a response to the anthropogenic forcing. I also work to bring many datasets together to get a broader view of regional climate variability, and I am the Antarctic ice core representative of large database efforts (PAGES temperature-2k, iso-2k, temperature-12k). Additionally, I keep challenging how the proxies we measure are interpreted, through detailed field-based process studies (firn air experiments, water vapour isotope monitoring), improvements of proxy system models (firn densification) and associated inverse methods, and investigating sampling biases using atmospheric reanalyses and isotope-enabled global climate models. For example, Servettaz et al. (2020) clearly demonstrated that the variability in the water isotopes of East Antarctic ice cores is attributable to the presence or absence of large precipitation events in winter, and we quantified precisely the biases in the representation of the mean annual climate. These types of studies are essential for improving paleoclimate reconstructions from ice core data. In the immediate future, I will continue to work on developing and exploiting new proxies, provide reconstructions, contribute to database work, develop collaborations with the modeling community to provide benchmarks for model evaluation, and use models to investigate mechanisms of climate variability and change. Specifically, my efforts will be centered on four projects: - Investigate the mechanisms driving modern changes in Arctic humidity, using new measurements of water vapour isotopic composition in the Western Canadian Arctic. The humidity increase is involved in a dominant feedback mechanism responsible for Arctic Amplification. - Investigate the temporal variability of Katabatic winds in Antarctica, including ice-core proxy development, and a field campaign in the high katabatic area of Adelie Land that I will lead in 2022. It has recently been shown that 17% of Antarctic snowfall is lost by re-evaporation and wind transport, and this is a crucial part of the surface mass balance and moisture budget that is understudied. - Expand temperature databases, and develop new metrics of Antarctic climate variability over the Holocene to improve climate reconstructions, and better understand when the Anthropogenic warming will become visible. - Develop a new absolute dating technique for ice cores using Argon isotopes. This is a powerful method that can date ice that is not in stratigraphic order, and is a key component to the search for the oldest ice in Antarctica. I am working to reduce the sample requirement by a factor of 5 which would make this technique useable broadly in precious deep ice core samples. Research Areas: Atmospheric Science, Climate Science, Geochemistry, Glaciology

2024: 50. *Harris Stuart, R., Landais, A., Arnaud, L., Buizert, C., Capron, E., Dumont, M., Libois, Q., Mulvaney, R., Orsi, A., Picard, G., Prié, F., Severinghaus, J., Stenni, B., and Martinerie, P.: On the relationship between δO2∕N2 variability and ice sheet surface conditions in Antarctica, The Cryosphere, 18, 3741–3763, https://doi.org/10.5194/tc-18-3741-2024, 2024. 49. Westhoff, J., Freitag, J., Orsi, A., Martinerie, P., Weikusat, I., Dyonisius, M., Faïn, X., Fourteau, K., and Blunier, T.: Combining traditional and novel techniques to increase our understanding of the lock-in depth of atmospheric gases in polar ice cores - results from the EastGRIP region, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1904, 2023. 2023: 48. *Davrinche, C., Orsi, A., Agosta, C., Amory, C