Towards improved polar climate reconstructions from ice-core stable water isotopes and borehole thermometry

Understanding the global climate system, e.g., drivers of temperature variability and regional feedbacks to greenhouse gas forcing, remains a major challenge, particularly because direct temperature observations are limited in both space and time. This limitation is especially pronounced in the polar regions. Consequently, climate proxies such as ice-core stable water isotopes and ice-sheet temperatures from Greenland and Antarctica are essential for extending our knowledge on past climates. Stable water isotope records (e.g., δ18O) from ice-cores offer information on past temperatures at high temporal resolutions and across multiple glacial cycles. However, they contain noise, which limits their usage, especially at short timescales. This dissertation shows that the magnitude of noise in these records varies systematically with depositional conditions and therefore across both space and time. Local stratigraphic noise, driven by wind-induced erosion and redistribution of surface snow, emerges as a particularly dominant factor. The impacts of this noise are more severe at low-accumulation rates, both across the East Antarctic Plateau and throughout the last 100,000 years as recorded in Greenland ice-cores. By separating noise and common signals in replicate ice-core isotope records, new estimates of past climate variability are derived. They reveal that climate variability in Greenland was generally larger during the last glacial period than during the Holocene, but with no systematic differences between stadials and interstadials. The studies on ice-core stable water isotopes thus suggest that the environmental conditions are important criteria for selecting new sampling locations, highlight the limitations inherent in single ice-core records, and provide refined estimates of climate variability across different climate states. As the noise in isotope records prevents their usage as a climate proxy at decadal timescales in low-accumulation areas, this work further employs borehole thermometry to investigate recent temperature changes on the East Antarctic Plateau. Despite the strong diffusive smoothing of temperatures in firn and ice, precise measurements in a 200 m deep borehole reveal surface temperature variations over the last century. The study site experienced a warming comparable in magnitude to the global mean between 1850 and 2017, but with a notably different temporal evolution, with a very gradual warming after 1850 followed by an acceleration only after 1990. This dissertation presents improved methods for borehole thermometry which can be applied to other boreholes across the East Antarctic Plateau to better quantify regional climate change. The combination of advanced sampling strategies for new replicate isotope records with continuous borehole temperature measurements provides a powerful framework for extracting a maximum amount of climate information from polar ice sheets to adress relevant questions of today’s climate research.