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The Antarctic Ice Sheet (AIS) is out of equilibrium with the current anthropogenic-enhanced climate forcing. Paleoenvironmental records and ice sheet models reveal that the AIS has been tightly coupled to the climate system during the past and indicate the potential for accelerated and sustained Antarctic ice mass loss into the future. Modern observations by contrast suggest that the AIS has only just started to respond to climate change in recent decades. The maximum projected sea level contribution from Antarctica to 2100 has increased significantly since the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report, although estimates continue to evolve with new observational and theoretical advances. This review brings together recent literature highlighting the progress made on the known processes and feedbacks that influence the stability of the AIS. Reducing the uncertainty in the magnitude and timing of the future sea level response to AIS change requires a multidisciplinary approach that integrates knowledge of the interactions between the ice sheet, solid Earth, atmosphere, and ocean systems and across time scales of days to millennia. We start by reviewing the processes affecting AIS mass change, from atmospheric and oceanic processes acting on short time scales (days to decades), through to ice processes acting on intermediate time scales (decades to centuries) and the response to solid Earth interactions over longer time scales (decades to millennia). We then review the evidence of AIS changes from the Pliocene to the present and consider the projections of global sea level rise and their consequences. We highlight priority research areas required to improve our understanding of the processes and feedbacks governing AIS change.

International audience; During the late Miocene (~11-5 Ma), atmospheric pCO may have been 1 to 3 times pre-industrial values, and mean global temperatures were likely 3−4°C higher than today with reduced latitudinal gradients. Major ecosystem changes that took place during the late Miocene are consistent with falling global temperatures and decreasing pCO , but proxy evidence for these physical climate changes remains sparse because few continuous, orbital-resolution palaeoclimate records cover this interval. Recently, geographically widespread and major (~5-10°C) late Miocene sea surface cooling was revealed for the mid-and high-latitudes based on the U sea surface temperature (SST) proxy, however existing tropical SST records suggest only minor cooling in this interval (1-2°C). Here we present new multi-proxy geochemical and micropaleontological records of surface and deep ocean variability from IODP Site U1443, drilled in the equatorial Indian Ocean (5°N, 90°E) during Expedition 353, which span the interval 9.5 to 5 Ma. Surface ocean records from Site U1443 have the potential to resolve SST change at a "warm pool" tropical site, whilst also providing insight into long-term and orbital-scale changes in the Indian monsoon subsystem. SST estimates derived from Mg/Ca ratios in the surface-dwelling planktic foraminifer Trilobatus trilobus show ~4°C of cooling (from 3.8 to 2.5 mmol/mol Mg/Ca) between 8.5 and 6 Ma. Following a SST minimum at 6 Ma, reconstructed temperatures increase from 6 to 5 Ma, consistent with published extratropical SST records. SST estimates combined with paired foraminiferal δ O measurements reveal a 2‰ decrease in seawater δ O (δ O , uncorrected for ice-volume changes) between 8.5 and 6 Ma, and a subsequent increase between 6 and 5 Ma. Consistent with records from other sites, Site U1443 benthic δ O data show no long-term increase concurrent with the Mg/Ca-SST decrease. We also observe dynamic changes in coccolith mass, a proxy for cell size and/or calcification, on both million-year and more abrupt timescales, sometimes correlated to SST changes. On orbital timescales, our records show coeval cyclicity in SST, δ O , and export productivity proxies, suggesting that changes in monsoon strength affected upper water column structure and properties on these timescales during the late Miocene. 2 2 k 37 18 18 18 sw 18 18 sw

Previous work on oxygen and hydrogen isotope data from Eastern Mediterranean water samples has defined a mixing relationship in this region that is different from the world surface ocean. This prompted speculations about the hydrological processes in the Mediterranean region. We present new δ18O and δD data from the Eastern Mediterranean region and the East Greenland Current system, spanning a wide salinity range. These data define δ18O:δD relationships for both regions that are consistent with the world surface ocean δ18O:δD relationship, despite the highly evaporative conditions that prevail in the Mediterranean region. These new geochemical data have suggested that the world surface ocean &delta18O:δD relationship holds throughout almost the entire global salinity range.

Over the last decade, our understanding of cli- mate sensitivity has improved considerably. The climate system shows variability on many timescales, is subject to non-stationary forcing and it is most likely out of equi- librium with the changes in the radiative forcing. Slow and fast feedbacks complicate the interpretation of geolog- ical records as feedback strengths vary over time. In the geological past, the forcing timescales were different than at present, suggesting that the response may have behaved differently. Do these insights constrain the climate sensitiv- ity relevant for the present day? In this paper, we review the progress made in theoretical understanding of climate sensitivity and on the estimation of climate sensitivity from proxy records. Particular focus lies on the background state dependence of feedback processes and on the impact of tipping points on the climate system. We suggest how to further use palaeo data to advance our understanding of the currently ongoing climate change.

The Mediterranean basin is sensitive to global sea-level changes and African monsoon variability on orbital timescales. Both of these processes are thought to be important to the deposition of organic-rich sediment layers or 'sapropels' throughout the eastern Mediterranean, yet their relative influences remain ambiguous. A related issue is that an assumed 3-kyr lag between boreal insolation maxima and sapropel mid-points remains to be tested. Here we present new geochemical and ice-volume-corrected planktonic foraminiferal stable isotope records for sapropels S1 (Holocene), S3, S4, and S5 (Marine Isotope Stage 5) in core LC21 from the southern Aegean Sea. The records have a radiometrically constrained chronology that has already been synchronised with the Red Sea relative sea-level record, and this allows detailed examination of the timing of sapropel deposition relative to insolation, sea-level, and African monsoon changes. We find that sapropel onset was near-synchronous with monsoon run-off into the eastern Mediterranean, but that insolation-sapropel/monsoon phasings were not systematic through the last glacial cycle. These latter phasings instead appear to relate to sea-level changes. We propose that persistent meltwater discharges into the North Atlantic (e.g., at glacial terminations) modified the timing of sapropel deposition by delaying the timing of peak African monsoon run-off. These observations may reconcile apparent model-data offsets with respect to the orbital pacing of the African monsoon. Our observations also imply that the previous assumption of a systematic 3-kyr lag between insolation maxima and sapropel midpoints may lead to overestimated insolation-sapropel phasings. Finally, we surmise that both sea-level rise and monsoon run-off contributed to surface-water buoyancy changes at times of sapropel deposition, and their relative influences differed per sapropel case, depending on their magnitudes. Sea-level rise was clearly important for sapropel S1, whereas monsoon forcing was more important for sapropels S3, S4, and S5.

The Mediterranean Sea provides a major route for heat and freshwater loss from the North Atlantic and thus is an important cause of the high density of Atlantic waters. In addition to the traditional view that loss of fresh water via the Mediterranean enhances the general salinity of the North Atlantic, and the interior of the eastern North Atlantic in particular, it should be noted that Mediterranean water outflowing at Gibraltar is in fact cooler than compensating inflowing water. The consequence is that the Mediterranean is also a region of heat loss from the Atlantic and contributes to its large-scale cooling. Uniquely, this system can be understood physically via the constraints placed on it by a single hydraulic structure: the Gibraltar exchange. Here we review the existing knowledge about the physical structure of the Gibraltar exchange today and the evidential basis for arguments that it has been different in the past. Using a series of quantitative experiments, we then test prevailing concepts regarding the potential causes of these past changes. We find that (1) changes in the vertical position of the plume of Mediterranean water in the Atlantic are controlled by the vertical density structure of the Atlantic; (2) a prominent Early Holocene "contourite gap" within the Gulf of Cadiz is a response to reduced buoyancy loss in the eastern Mediterranean during the time of "sapropel 1" deposition; (3) changes in buoyancy loss from the Mediterranean during MIS3 caused changes in the bottom velocity field in the Gulf of Cadiz, but we note that the likely cause is reduced freshwater loss and not enhanced heat loss; and (4) strong exchange at Gibraltar during Atlantic freshening phases implies that the Gibraltar exchange provides a strong negative feedback to reduced Atlantic meridional overturning. Given the very counterintuitive way in which the Strait of Gibraltar system behaves, we recommend that without quantitative supporting work, qualitative interpretations of how the system has responded to past external forcing are unlikely to be robust.

The climate sensitivity parameter S is defined as the equilibrium change in global annual mean surface temperature ∆T per radiative forcing ∆R, S= ∆T/∆R. We here combine a data set of radiative forcing ∆R of greenhouse gases and albedo changes (Köhler et al., 2010) with an estimate of ∆T based on the deconvolution of benthic δ18O into sealevel and temperature (Bintanja et al., 2005) for the last 800 kyr. We show how S varies depending on the radiative forcing considered, e.g. if only ∆R of CO2 or ∆R of CO2+CH4+N2O or additionally ∆R of the albedo changes are taken into account. Furthermore we find, that for the LGM all calculated S, independent on the considered forcing ∆R is about 10-15% smaller than if calculated for the whole 800 kyr time window. We propose that this difference between the rather stable climate of the LGM and the whole 800 kyr is caused by transient effects and the state dependency of S. We identify based on thresholds in temporal changes in ∆T and ∆R relatively stable climates and separate the transient effect from state dependency in S. In a final application it is shown how the state dependency of S and assumptions on various slow and fast feedbacks are important for the functional relationship between ∆T and CO2 for the range in CO2 observed in the past 800 kyr and proposed in the future (2×CO2).