Dramatic changes in sea ice have been observed in both poles in recent decades. However, the observational period for sea ice is short, and the climate models tasked with predicting future change in sea ice struggle to capture the current Antarctic trends. Paleoclimate archives, from marine sedimentary records and coastal Antarctic ice cores, provide a means of understanding sea ice variability and its drivers over decadal to centennial timescales. In this study, we collate published records of Antarctic sea ice over the past 2000 years (2 ka). We evaluate the current proxies and explore the potential of combining marine and ice core records to produce multi-archive reconstructions. Despite identifying 92 sea ice reconstructions, the spatial and temporal resolution is only sufficient to reconstruct circum-Antarctic sea ice during the 20th century, not the full 2 ka. Our synthesis reveals a 90 year trend of increasing sea ice in the Ross Sea and declining sea ice in the Bellingshausen, comparable with observed trends since 1979. Reconstructions in the Weddell Sea, the Western Pacific and the Indian Ocean reveal small negative trends in sea ice during the 20th century (1900–1990), in contrast to the observed sea ice expansion in these regions since 1979.

The dissolution of the delicate shells of sea butterflies, or pteropods, has epitomised discussions regarding ecosystem vulnerability to ocean acidification over the last decade. However, a recent demonstration that the organic coating of the shell, the periostracum, is effective in inhibiting dissolution suggests that pteropod shells may not be as susceptible to ocean acidification as previously thought. Here we use micro-CT technology to show how, despite losing the entire thickness of the original shell in localised areas, specimens of polar species Limacina helicina maintain shell integrity by thickening the inner shell wall. One specimen collected within Fram Strait with a history of mechanical and dissolution damage generated four times the thickness of the original shell in repair material. The ability of pteropods to repair and maintain their shells, despite progressive loss, demonstrates a further resilience of these organisms to ocean acidification but at a likely metabolic cost.

We present high‐resolution records of upper ocean temperatures derived from Mg/Ca ratios of surface‐dwelling Globigerina bulloides and subsurface‐dwelling Neogloboquadrina pachyderma sinistral and the relative abundance of N. pachyderma sinistral for the period 55–8 ka BP from NE Atlantic sediment core MD01‐2461. Millennial‐scale temporal variability and longer‐term trends in these records enable us to develop a detailed picture of past ocean conditions such as a weakening of thermocline intensity from marine isotope stage 3 (MIS 3) to the last glacial maximum (LGM). The correspondence of all temperature proxies and convergence of paired oxygen isotope (δ¹⁸O) records from both planktonic species implies a breakdown in the thermocline and year‐round mixing of the upper water column through the LGM, perhaps related to decreasing insolation and additional cooling in association with the expansion of the circum–North Atlantic ice sheets. Millennial‐scale divergence in surface and subsurface temperatures and δ¹⁸O across the last glacial correspond to meltwater release and the development of a strong halocline associated with both Heinrich (H) events and instabilities of the NW European ice sheet. During such episodes, G. bulloides Mg/Ca appears to record ambient, even warming summer sea surface temperatures across H events while the other proxies record maximum cooling.

We present high‐resolution benthic δ¹³C records from intermediate water depth core site MD01‐2461 (1153 m water depth), from the Porcupine Seabight, NE Atlantic, spanning 43 to 8 kyr B.P. At an average proxy time step of 160 ± 56 years this core provides information on the linkage between records from the Portuguese Margin and high‐latitude North Atlantic basin, allowing additional insights into North Atlantic thermohaline circulation (THC) variability during millennial‐scale climatic events of the last glacial. Together, these records document both discrete and progressive reductions in Glacial North Atlantic Intermediate Water (GNAIW) formation preceding Heinrich (H) events 1, 2, and 4, recorded through the apparent interchange of glacial northern and southern‐sourced intermediate water signatures along the European Margin. Close coupling of NW European ice sheet (NWEIS) instability and GNAIW formation is observed through transient advances of SCW along the European margin concurrent with pulses of ice‐rafted debris and meltwater release into the NE Atlantic between 27 and 16 kyr B.P., when the NWEIS was at maximum extent and proximal to Last Glacial Maximum convection zones in the open North Atlantic. It is such NWEIS instability and meltwater forcing that may have triggered reduced North Atlantic THC prior to collapse of the Laurentide ice sheet at H1 and H2. Precursory reduction in GNAIW formation prior to H4 may also be inferred. However, limited NWEIS ice volume prior to H4 and convection occurring in the Norwegian‐Greenland Sea require that if a meltwater trigger is invoked, as appears to be the case at H1 and H2, the source of meltwater prior to H4 is elsewhere, likely higher‐latitude ice sheets. Clarification of the sequencing and likely mechanisms of precursory decrease of the North Atlantic THC support theories of H event initiation relating to ice shelf growth during cold periods associated with reduced North Atlantic THC and subsequent ablation through subsurface warming and sea level rise associated with further reductions in meridional overturning.

We all know that carbon dioxide (CO₂) is produced from burning fossil fuels, and that it contributes to global warming. But have you heard about the “evil twin” of global warming, also caused by CO₂–ocean acidification? The ocean absorbs about 30% of the CO₂ that humans emit each year. As CO₂ dissolves into the ocean, it forms carbonic acid, effectively making the ocean acidic. Animals that use a substance called calcium carbonate to build their shells and skeletons are vulnerable to acidic conditions, as their hard parts may dissolve. Microscopic swimming snails called pteropods or sea butterflies are common in the Southern Ocean. With incredibly delicate shells thinner than a human hair, pteropods are often considered to be the organisms most vulnerable to ocean acidification. Using microscopes and X-rays, we took extremely detailed images of these tiny shells and found that pteropods have a couple of clever tactics to defend their “homes” against ocean acidification.