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Anthropogenic climate change is accentuated at high latitudes due to Polar Amplification, a process driven by interactions and feedback mechanisms among terrestrial, atmospheric, and oceanic systems. Arctic sea ice is in rapid decline while Antarctic sea ice has experienced recent extreme lows after relative stability, with global climatic and ecological responses and impacts. Multiple proxies have been developed for past sea ice reconstructions to assess modern trends and aid future forecasting. These include microfossil assemblages (dinocysts, foraminifera, ostracodes) and biomarker concentrations derived from ice-edge and open-water diatoms. Two sea ice biomarkers, IP25 (Ice Proxy with 25 carbon atoms) and IPSO25 (Ice Proxy for the Southern Ocean with 25 carbon atoms), can be used to produce semi-quantitative reconstructions of past sea ice extent when combined with phytoplankton derived biomarkers (e.g. phytosterols brassicasterol and dinosterol). However, there gaps remain in understanding past sea ice states and the critical processes that drive change, including at critical transitions that may provide insight into current and predicted future warming. We present an overview of synthesised sea-ice proxy records spanning key periods characterised by lower and higher than pre-industrial CO₂ background states: the Mid-Holocene (6ka; 8.2–4.2 ka BP), the Last Glacial Maximum (LGM; 21ka; ~ 19–23 ka), the Last Interglacial (LIG; 127ka; ~130–115 ka BP), and the Mid Pliocene Warm Period (mPWP: 3.264–3.025 Ma). These syntheses are feeding into model-data integration as a key component of the EU Horizon project Past-to-Future (P2F), which aims to radically advance our knowledge of past climatic conditions to better understand Earth’s climate response to different kinds of forcing. A better understanding of past sea ice states and stronger data–model integration are essential for improving our ability to anticipate the future trajectory of sea ice and its cascading effects on global climate.