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Arctic sea ice is a critical component of the climate system, known to influence ocean circulation, earth’s albedo, and ocean–atmosphere heat and gas exchange. Current developments in the use of IP25(a sea ice proxy with 25 carbon atoms only synthesized by Arctic sea ice diatoms) have proven it to be a suitable proxy for paleo-sea ice reconstructions over hundreds of thousands to even millions of years. In the NE Baffin Bay, off NW Greenland, Melville Bugt is a climate-sensitive region characterized by strong seasonal sea ice variability and strong melt-water discharge from the Greenland Ice Sheet (GIS). Here, we present a centennial-scale resolution Holocene sea ice record, based on IP25and open-water phytoplankton biomarkers (brassicasterol, dinosterol and HBI III) using core GeoB19927-3 (73° 35.26′ N, 58° 05.66′ W). Seasonal to ice-edge conditions near the core site are documented for most of the Holocene period with some significant variability. In the lower-most part, a cold interval characterized by extensive sea ice cover and very low local productivity is succeeded by an interval (~ 9.4–8.5 ka BP) with reduced sea ice cover, enhanced GIS spring melting, and strong influence of the West Greenland Current (WGC). From ~ 8.5 until ~ 7.8 ka BP, a cooling event is recorded by ice algae and phytoplankton biomarkers. They indicate an extended sea ice cover, possibly related to the opening of Nares Strait, which may have led to an increased influx of Polar Water into NE-Baffin Bay. The interval between ~ 7.8 and ~ 3.0 ka BP is characterized by generally reduced sea ice cover with millennial-scale variability of the (late winter/early spring) ice-edge limit, increased open-water conditions (polynya type), and a dominant WGC carrying warm waters at least as far as the Melville Bugt area. During the last ~ 3.0 ka BP, our biomarker records do not reflect the late Holocene ‘Neoglacial cooling’ observed elsewhere in the Northern Hemisphere, possibly due to the persistent influence of the WGC and interactions with the adjacent fjords. Peaks in HBI III at about ~ 2.1 and ~ 1.3 ka BP, interpreted as persistent ice-edge situations, might correlate with the Roman Warm Period (RWP) and Medieval Climate Anomaly (MCA), respectively, in-phase with the North Atlantic Oscillation (NAO) mode. When integrated with marine and terrestrial records from other circum-Baffin Bay areas (Disko Bay, the Canadian Arctic, the Labrador Sea), the Melville Bugt biomarker records point to close ties with high Arctic and Northern Hemispheric climate conditions, driven by solar and oceanic circulation forcings.

We present a comprehensive multi-proxy analysis spanning 550,000 years from the outer Labrador Sea region at the Integrated Ocean Drilling Program (IODP) Sites U1302/1303. We combine new benthic foraminiferal stable oxygen (δ18O) and carbon (δ13C) isotope records, with sediment elemental composition and authigenic neodymium isotope measurements, to provide insights into deep-water mass sourcing and changes of the Deep Western Boundary Current (DWBC), which exports North Atlantic Deep Water (NADW) into the wider North Atlantic as part of the lower limb of the Atlantic Meridional Overturning Circulation.

The geological record shows that abrupt changes in the Earth system can occur on timescales short enough to challenge the capacity of human societies to adapt to environmental pressures. In many cases, abrupt changes arise from slow changes in one component of the Earth system that eventually pass a critical threshold, or tipping point, after which impacts cascade through coupled climate–ecological–social systems. The chance of detecting abrupt changes and tipping points increases with the length of observations. The geological record provides the only long-term information we have on the conditions and processes that can drive physical, ecological and social systems into new states or organizational structures that may be irreversible within human time frames. Here, we use well-documented abrupt changes of the past 30 kyr to illustrate how their impacts cascade through the Earth system. We review useful indicators of upcoming abrupt changes, or early warning signals, and provide a perspective on the contributions of palaeoclimate science to the understanding of abrupt changes in the Earth system.

The Greenland Ice Sheet (GIS) is losing mass at a high rate1. Given the short-term nature of the observational record, it is difficult to assess the historical importance of this mass-loss trend. Unlike records of greenhouse gas concentrations and global temperature, in which observations have been merged with palaeoclimate datasets, there are no comparably long records for rates of GIS mass change. Here we reveal unprecedented mass loss from the GIS this century, by placing contemporary and future rates of GIS mass loss within the context of the natural variability over the past 12,000 years. We force a high-resolution ice-sheet model with an ensemble of climate histories constrained by ice-core data2. Our simulation domain covers southwestern Greenland, the mass change of which is dominated by surface mass balance. The results agree favourably with an independent chronology of the history of the GIS margin3,4. The largest pre-industrial rates of mass loss (up to 6,000 billion tonnes per century) occurred in the early Holocene, and were similar to the contemporary (ad2000–2018) rate of around 6,100 billion tonnes per century5. Simulations of future mass loss from southwestern GIS, based on Representative Concentration Pathway (RCP) scenarios corresponding to low (RCP2.6) and high (RCP8.5) greenhouse gas concentration trajectories6, predict mass loss of between 8,800 and 35,900 billion tonnes over the twenty-first century. These rates of GIS mass loss exceed the maximum rates over the past 12,000 years. Because rates of mass loss from the southwestern GIS scale linearly5with the GIS as a whole, our results indicate, with high confidence, that the rate of mass loss from the GIS will exceed Holocene rates this century.

Climate archives available from deep sea and marine shelf sediments, glaciers, lakes, and ice cores in and around Greenland allow us to place the current trends in regional climate, ice sheet dynamics, and land surface changes in a broader perspective. We show that, during the last decade (2000s), atmospheric and sea surface temperatures are reaching levels last encountered millennia ago, when northern high latitude summer insolation was higher due to a different orbital configuration. Records from lake sediments in southern Greenland document major environmental and climatic conditions during the last 10,000 years, highlighting the role of soil dynamics in past vegetation changes, and stressing the growing anthropogenic impacts on soil erosion during the recent decades. Furthermore, past and present changes in atmospheric and oceanic heat advection appear to strongly influence both regional climate and ice sheet dynamics. Projections from climate models are investigated to quantify the magnitude and rates of future changes in Greenland temperature, which may be faster than past abrupt events occurring under interglacial conditions. Within one century, in response to increasing greenhouse gas emissions, Greenland may reach temperatures last time encountered during the last interglacial period, approximately 125,000 years ago. We review and discuss whether analogies between the last interglacial and future changes are reasonable, because of the different seasonal impacts of orbital and greenhouse gas forcings. Over several decades to centuries, future Greenland melt may act as a negative feedback, limiting regional warming albeit with global sea level and climatic impacts. WIREs Clim Change2012 doi: 10.1002/wcc.186

Analyses of dinoflagellate cyst assemblages from the surface sediments of 371 sites from the North Atlantic Ocean were performed in order to define relationships with nitrate, phosphate, silica and productivity in the upper water layer. Statistical analyses reveal close links between dinocyst assemblages and nitrate content in February that provide a measurement of nitrate availability. Amongst the taxa in the assemblages, Nematosphaeropsis labyrinthus, which positively correlates with nutrients, shows an eutrophic relationship, whereas Impagidinium aculeatum, Impagidinium patulum, Spiniferites ramosus, Spiniferites mirabilisand Lingulodinium machaerophorumhave a rather oligotrophic relationship. On these grounds, transfer functions were tested using multiple regressions and the best analogue technique to reconstruct nitrate content. Both methods yielded reasonable estimates and allow the reconstruction of the nitrate concentrations in February with an accuracy better than 1.45μmolm−3. At the scale of the study area, there are also tenuous links between the dinocyst assemblages and productivity. Transfer functions were tentatively tested for the reconstruction of annual productivity and yield estimates with an accuracy of about 26.5gCm−2year−1. These results demonstrate that dinocyst assemblages can be used for the reconstruction of productivity and nitrates.

Palynological analyses performed on cores from the Norwegian Channel (Troll 8903) led to reconstruction of the late-glacial variations in sea-surface conditions using dinoflagellate cyst data and permitted direct correlation with the vegetation history of northwestern Europe derived from pollen assemblages. By ∼15,000 yr B.P., ice rapidly receded from the Norwegian shelf and relatively warm summer conditions prevailed in surface waters. A first late-glacial cooling marked by extensive seasonal sea–ice cover is dated at ca. 13,600–13,00014C yr B.P., which coincides with the Oldest Dryas interval. During the Bølling–Allerød interval, a rise in sea-surface temperature both in February (up to 3°C) and August (up to 15°C) led to the establishment of ice-free conditions in the northern North Sea, while pollen data reveal a densification of the vegetation cover. The beginning of the Younger Dryas interval is marked by an increase in nonarboreal pollen input indicative of the opening of the forest vegetation cover, concomitant with a cooling of surface waters during winter and development of sea–ice cover. However, sea-surface conditions remained relatively warm in summer until about 10,300 yr B.P., when extremely cold conditions and extensive sea–ice cover developed (up to 7 months/yr). Improving conditions are recorded in surface waters by ∼10,100 yr B.P., a few hundred years before the development of forest cover onshore, as shown by the pollen record. Such a discrepancy between marine and terrestrial indicators at the end of Younger Dryas time suggests a delayed response of the vegetation to regional climate warming.

We analysed 70 surface sediment samples collected in the southern Indian Ocean in order to document the distribution of dinoflagellate cyst assemblages. These organic-walled micro-organisms have a greater potential for preservation than carbonate or silicate microfossils. A total of 53 dinoflagellate cyst taxa were identified, and two new endemic species are described: Selenopemphix antarctica sp. nov. and Impagidinium variaseptum sp. nov. Dominant taxa allowed the recognition of assemblages which show a latitudinal distribution. The circum-Antarctic domain is characterized by assemblages dominated by S. Antarctica sp. nov. and accompanied by I. pallidum. The Subantarctic domain is marked by the dominance of Brigantedinium spp. accompanied by Nematosphaeropsis labyrinthus. The Subtropical domain shows high species diversity, taxa dominance varying along onshore to offshore gradient: the neritic assemblage is dominated by Brigantedinium spp., and Spiniferites spp., the outer neritic is characterized by Operculodinium centrocarpum and the oceanic assemblage is dominated by N. labyrinthus. Principal component analysis illustrates that the distribution of dinoflagellate cyst assemblages is controlled by temperature and salinity. Transfer functions based on the best analogues method are developed to reconstruct past sea-surface conditions.

A reference data base of dinoflagellate cyst assemblages was developed from the palynological analysesof surface sediment samples collected at ∼450 sites from middle to high latitudes of the North Atlantic and adjacent basins. The data set is representative of a wide range of sea-surface conditions with respect to sea-ice cover (0–12 months/year), temperature (up to 25°C), salinity (20–36 psu), and seasonality. The analyses reveal relatively high dinoflagellate cyst concentrations especially along the continental margins (up to 105 cysts/cm3). The distribution of species and assemblages show close relationships with the salinity, the temperature of the warmest and coldest months, and the seasonal duration of the sea-ice cover. On these grounds, transfer functions using the best analogue method were developed in order to reconstruct the following sea-surface parameters with a reasonable degree of accuracy, i.e. within the range of interannual variations: temperature in February (± 1.2°C) and August (± 16°C), salinity (± 0.7 psu) and sea-ice cover extent (± 1.1 months/year). The transfer functions based on dinoflagellate cysts are original and yield paleoceanographic and paleoclimatic data complementary to those produced currently on the basis of transfer function using planktonic foraminifera.