Dr. Valérie Masson-Delmotte is a senior scientist from Laboratoire des Sciences du Climat et de l'Environnement, Institut Pierre Simon Laplace, Université Paris Saclay / CEA / CNRS, France. She is the Co-chair of IPCC Working Group I for the AR6 cycle. Her research interests are focused on quantifying and understanding past changes in climate and atmospheric water cycle, using analyses from ice cores in Greenland, Antarctica and Tibet, analyses from tree-rings as well as present-day monitoring, and climate modelling for the past and the future. She has worked on issues such as the North Atlantic Oscillation, drought, climate response to volcanic eruptions, polar amplification, climate feedbacks, abrupt climate change and ice sheet vulnerability accross different timescales. She is active in outreach for children and for the general public and has contributed to several books on climate change issues (e.g. Greenland, climate, ecology and society, CNRS editions, 2016; in French). Her research was recognized by several prizes (European Union Descartes Prize for the EPICA project, 2008; Women scientist Irène Joliot Curie Prize, 2013; Tinker-Muse Prize for science and policy in Antarctica, 2015; Highly Cited Researcher since 2014). Abstract: Ice cores provide a wealth of insights into past climatic and environmental changes. Obtaining information on past polar temperature changes is important to document climate variations beyond scarce instrumental records, and to test our quantitative understanding of past climate variations. Water stable isotope ratios in ice core records have commonly been used as qualitative proxies for past changes in polar temperature and moisture source characteristics, but extracting quantitative signals is a major challenge. Initially, spatial relationships between surface snow isotopic composition and surface temperature were used to establish a modern "isotopic thermometer". Simulations performed with climate models equipped with water stable isotopes were subsequently used to assess the validity of this "isotopic thermometer calibration" for different climate states (e.g. glacial, interglacial), assuming that the ice core signal is a precipitation weighted deposition record. I will first present recent findings based on new capability to monitor water vapour isotopic composition in the North Atlantic / Greenland and several Antarctic regions. These new datasets challenge the classical interpretation of ice core records as just precipitation-weighted signals. Moreover, they challenge the ability of atmospheric models equipped with water stable isotopes to fully resolve the initial marine boundary layer isotopic composition spatial patterns. These are key limitations to our quantitative understanding of ice core signals. I will then illustrate major results obtained from water stable isotope records in Greenland and Antarctic ice cores at three time scales : (i) the documentation of polar climate variability during the last thousand years, and the challenge to separate intrinsic, spontaneous climate variability from the response to natural forcings; (ii) the bipolar structure of abrupt changes during the last climatic cycle, and its implications for the interplay between reorganizations in ocean circulation, sea ice extent and polar climate ; (iii) polar temperature trends during the current and last interglacial period, and their relevance for the assessment of ice sheet vulnerability. Over these three time scales, I will stress why quantifying past changes is relevant for the evaluation of climate models and for the assessment of future risks. The Colloquium will be livestreamed at ictp.it/livestream. Light refreshments will be served after the talk.