Expansion Of Sea Ice Research Articles

Long-term and millennial-scale climate variability in northwestern France during the last 8850 years

Vegetation and quantitative climate reconstructions from a northwestern France shelf core (VK03-58Bis) show orbital and suborbital climate variability for the last 8850 years in this region. A long-term cooling trend in summer temperatures, marked by gradual temperate and humid forest decline, parallels cooling in Greenland and the decrease of mid-latitude summer insolation reduction until at least 2000 cal. yr BP. At the long-term scale, the lowering in seasonal contrast revealed by vegetation changes follows the increase of precession. Corylus woodlands spread at the expense of deciduous Quercus forest, between 8740 and 8390 cal. yr BP, linked with the high seasonality conditions that, counterbalancing the long-term astronomical forcing trend, were amplified by the north Atlantic high-latitudes winter sea-ice expansion. High seasonality conditions resulted from the Agassiz and Ojibway final outburst episodes and consequent gradual reduction of the Meridional Overturning Circulation (MOC). Between 8390—8060 cal. yr BP, a sudden Corylus woodland decline marks the 8.2 kyr cold event in northwestern France probably triggered by the severe MOC reduction leading to the additional drop in winter temperature over Europe and Greenland. Nonetheless, seasonality remains high during this interval. The high seasonality conditions detected in `VK03-58Bis' between 8740 and 8060 cal. yr BP reflects the multicentennial-scale climate cooling 8.6—8.0 kyr episode of the North Atlantic. Following the Agassiz and Ojibway final outburst episodes, climate became more stable. However, millennial-scale climate cooling episodes are recorded in `VK03-Bis' and are characterized by weak winter cooling and increases in precipitation. Furthermore, dinocyst analysis and benthic gastropod Turritella communis occurrences indicate regional changes such as the southward migration of the Boreal biogeographical zone between 8740 and 8480 cal. yr BP and the subsequent opening of the English Channel at around 8480—8390 cal. yr BP.

Sea ice expansion in the Bering Sea during the Neoglacial: evidence from archaeozoology

The Neoglacial was a period of cold that lasted more than 2000 years during the mid-Holocene, from approximately 4700 to 2500 years ago. Although proxy data from a number of sources document the regional onset and duration of cold conditions in the Northern Hemisphere during this period, none have suggested an expansion of sea ice in the Bering Sea. Here we provide new evidence that Neoglacial sea ice expansion in the Bering Sea was substantial enough to have altered the distribution of North Pacific pinnipeds and cetaceans, using prehistoric skeletal remains recovered from an archaeological site on the island of Unalaska in the eastern Aleutians (Amaknak Bridge, occupied from c. 3500—2500 yr BP (radiocarbon years before present, uncalibrated)). Comprehensive archaeozoological analysis of the Amaknak Bridge fauna indicates that sea ice in the Bering Sea must have reached a more southerly position at the height of the Neoglacial and persisted longer than it does today. We infer from this evidence that for most of the Neoglacial period, sea ice must have surrounded the Pribilof Islands until early summer and blocked the Bering Strait until late summer. Such an expansion and seasonal persistence of sea ice would have prevented fur seals from using the Pribilofs as a summer breeding rookery and whales from making summer migrations into arctic waters to feed, as they do today. We suggest this expansion of sea ice in the Bering Sea during the Neoglacial may explain several unresolved phenomena of mammalian distributions, genetic partitioning and extinctions in the North Pacific.

Sea ice feedback and Cenozoic evolution of Antarctic climate and ice sheets

The extent and thickness of Antarctic sea ice have important climatic effects on radiation balance, energy transfer between the atmosphere and ocean, and moisture availability. This paper explores the role of sea ice and related feedbacks in the Cenozoic evolution of Antarctic climate and ice sheets, using a numerical climate model with explicit, dynamical representations of sea ice and continental ice sheets. In a scenario of decreasing Cenozoic greenhouse gas concentrations, our model initiates continental glaciation before any significant sea ice forms around the continent. Once variable ice sheets are established, seasonal sea ice distribution is highly sensitive to orbital forcing and ice sheet geometry via the ice sheet’s control on regional temperature and low‐level winds. Although the expansion of sea ice has significant climatic effects near the coast, it has only minimal effects in the continental interior and on the size of the ice sheet. Therefore the Cenozoic appearance of Antarctic sea ice was primarily a response to the growth of grounded ice sheets and was not a critical factor in episodes of Paleogene and Neogene glaciation. The influence of the East Antarctic Ice Sheet on sea ice, Southern Ocean surface temperatures, and winds has important implications for ocean circulation, the marine carbon cycle, and the development of the West Antarctic Ice Sheet. The sensitivity of sea ice to grounded ice sheets implies reconstructions of sea ice based on marine diatoms are good indicators of glacial conditions in the continental interior and may provide insight into the long‐term stability of Antarctic Ice Sheets.

Simulation of Holocene cooling events in a coupled climate model

Three potential mechanisms behind centennial-scale Holocene cooling events are studied in simulations performed with the coupled climate model ECBilt–CLIO: (1) internal variability, (2) solar forcing, and (3) freshwater forcing. In experiments with constant preindustrial forcings, three centennial-scale cooling events occur spontaneously in 15,000 years. These rare events represent an unstable internal mode of variability that is characterised by a weaker thermohaline circulation, a more southward location of the main site of deep-water formation, expanded sea-ice cover and cooling of 10 °C over the Nordic Seas. This mode is visited more frequently when the climate is cooled by abruptly reducing the solar constant by 5 or 3 Wm −2. Prescribing a solar forcing of the same magnitude, but following a sinusoidal function with a period of 100 or 1000 years, does not result in any centennial-scale cooling events. The latter forcing does however result in more frequent individual cold years in the North Atlantic region that are related to local weakening of the deep convection and sea-ice expansion. Adding realistic freshwater pulses to the Labrador Sea is also able to trigger centennial-scale cooling events with temperature anomalies resembling proxy evidence for the cooling event at 8.2 kyr BP, suggesting that freshwater forcing is a valid explanation for early Holocene cooling events.

Last global extinction in the deep sea during the mid‐Pleistocene climate transition

Twenty percent (19 genera, 95 species) of cosmopolitan, deep‐sea (500–4000 m), benthic foraminiferal species became extinct during the late Pliocene–Middle Pleistocene (3–0.12 Ma), with the peak of extinctions (76 species) occurring during the mid‐Pleistocene Climate Transition (MPT, 1.2–0.55 Ma). One whole family (Stilostomellidae, 30 species) was wiped out, and a second (Pleurostomellidae, 29 species) was decimated with just one species possibly surviving through to the present. Our studies at 21 deep‐sea core sites show widespread pulsed declines in abundance and diversity of the extinction group species during more extreme glacials, with partial interglacial recoveries. These declines started in the late Pliocene in southern sourced deep water masses (Antarctic Bottom Water, Circumpolar Deep Water) and extending into intermediate waters (Antarctic Intermediate Water, North Atlantic Deep Water) in the MPT, with the youngest declines in sites farthest downstream from high‐latitude source areas for intermediate waters. We infer that the unusual apertural types that were targeted by this extinction period were adaptations for a specific kind of food source and that it was probably the demise of this microbial food that resulted in the foraminiferal extinctions. We hypothesize that it may have been increased cold and oxygenation of the southern sourced deep water masses that impacted on this deep water microbial food source during major late Pliocene and Early Pleistocene glacials when Antarctic ice was substantially expanded. The food source in intermediate water was not impacted until major glacials in the MPT when there were significant expansion of polar sea ice in both hemispheres and major changes in the source areas, temperature, and oxygenation of global intermediate waters.

Reconstruction of millennial changes in dust emission, transport and regional sea ice coverage using the deep EPICA ice cores from the Atlantic and Indian Ocean sector of Antarctica

Continuous sea salt and mineral dust aerosol records have been studied on the two EPICA (European Project for Ice Coring in Antarctica) deep ice cores. The joint use of these records from opposite sides of the East Antarctic plateau allows for an estimate of changes in dust transport and emission intensity as well as for the identification of regional differences in the sea salt aerosol source. The mineral dust flux records at both sites show a strong coherency over the last 150 kyr related to dust emission changes in the glacial Patagonian dust source with three times higher dust fluxes in the Atlantic compared to the Indian Ocean sector of the Southern Ocean (SO). Using a simple conceptual transport model this indicates that transport can explain only 40% of the atmospheric dust concentration changes in Antarctica, while factor 5–10 changes occurred. Accordingly, the main cause for the strong glacial dust flux changes in Antarctica must lie in environmental changes in Patagonia. Dust emissions, hence environmental conditions in Patagonia, were very similar during the last two glacials and interglacials, respectively, despite 2–4 °C warmer temperatures recorded in Antarctica during the penultimate interglacial than today. 2–3 times higher sea salt fluxes found in both ice cores in the glacial compared to the Holocene are difficult to reconcile with a largely unchanged transport intensity and the distant open ocean source. The substantial glacial enhancements in sea salt aerosol fluxes can be readily explained assuming sea ice formation as the main sea salt aerosol source with a significantly larger expansion of (summer) sea ice in the Weddell Sea than in the Indian Ocean sector. During the penultimate interglacial, our sea salt records point to a 50% reduction of winter sea ice coverage compared to the Holocene both in the Indian and Atlantic Ocean sector of the SO. However, from 20 to 80 ka before present sea salt fluxes show only very subdued millennial changes despite pronounced temperature fluctuations, likely due to the large distance of the sea ice salt source to our drill sites.

A pre-Hirnantian (Late Ordovician) interval of global cooling – The Boda event re-assessed

Facies analysis suggests that the early–mid Ashgill (Late Ordovician) ‘Boda event’ of Fortey and Cocks [Fortey, R.A., Cocks, L.R.M., 2005. Late Ordovician global warming—The Boda event. Geology 33 (5), 405–408] was an episode of global cooling, not of warming as they proposed. Limestones are widely developed at this level from mid-latitude to high-latitude, commonly interrupting thick Ordovician successions of fine clastic sediments. Bryozoan carbonate mud mounds in a laterally extensive cool-water carbonate belt along the high palaeolatitude margin of W Gondwana are associated with glacial diamictites, and grew during glacial lowstand with sea ice prior to the Hirnantian glaciation. From high to mid-palaeolatitudes, contemporaneous cool-water carbonates with mud mounds, including stromatactis both in Gondwana and in Baltica, developed in mid-outer ramp settings during regression. In the dominantly clastic Welsh Basin of Avalonia, phosphatic facies on an Iapetus-facing carbonate ramp indicate upwelling cold waters down-ramp, yet up-ramp coralliferous-algal limestones suggest warm surface waters. We propose that the Late Ordovician cool-water carbonates developed in response to global cooling and expansion of Gondwanan terrestrial and sea ice that lowered eustatic sea level, and on the W Gondwanan shelf reduced or shut down fluvial clastic input. Previously poorly circulated, salinity-stratified oceans that had restricted benthic dispersal in mid–low latitudes were replaced by thermohaline ocean circulation that oxygenated bottom waters and dispersed benthic faunas. An earlier, Caradoc carbonate interval in low–mid palaeolatitudes that shares many facies characteristics with the Ashgill cool-water limestones, and correlates with a widely recognised isotope excursion, is interpreted as indicating an earlier global cooling episode.

Rates of thermohaline recovery from freshwater pulses in modern, Last Glacial Maximum, and greenhouse warming climates

Recovery rates of the thermohaline circulation after a freshwater pulse in the North Atlantic vary considerably depending on the background climate, as demonstrated in the Community Climate System Model. The recovery is slowest in a Last Glacial Maximum (LGM) climate, fastest in a modern climate, and intermediate between the two in a greenhouse warming (4XCO2) climate. Previously proposed mechanisms to explain thermohaline circulation stability involving altered horizontal freshwater transport in the North Atlantic are consistent with relative recovery rates in the modern and 4XCO2 climates, but fail to explain the slow LGM recovery. Instead, sea ice expansion inhibits deep‐water formation after freshening in the LGM climate by reducing heat loss to the atmosphere and providing additional surface freshwater. In addition, anomalous vertical freshwater transport across ∼1 km depth after freshening is most effective at weakening the stratification in the modern case but is negligible in the LGM case.

Millennial-scale fluctuations in seasonal sea-ice and deep-water formation in the Japan Sea during the late Quaternary

The East Asian winter monsoon is characterized by cold, dry air blowing from the Siberian high to the surrounding lows. This cold wind cools the surface water along the Russian coast of the Japan Sea. Modern oceanographic observations indicate that sea-ice formation along this coast during an extremely cold winter is an important process for deep-water formation. Thus, formation of both seasonal sea ice and deep-water masses in the Japan Sea are closely related to the East Asian winter monsoon. Two sediment cores were analyzed to reconstruct the history of sea-ice expansion and deep-water formation. Seasonal sea-ice history was reconstructed from the occurrence of ice-rafted debris (IRD) in a core from the northeastern Japan Sea, whereas the deep-water ventilation history was estimated on the basis of records of a deep-dwelling radiolarian species, Cycladophora davisiana, in a core from the southwestern Japan Sea. In the records of sea-ice expansion and deep-water ventilation for the last 160 ky, clear fluctuations on a millennial timescale were observed during marine oxygen isotope stages 3–5. Some of the peaks in sea-ice formation and C. davisiana abundance occurred contemporaneously, indicating strong surface cooling along the Russian coast by a strong winter monsoon. Decoupling of the two records might have occurred because of surface-water stratification caused by the freshening of surface water in relation to high precipitation in summer. Synchronicity of high IRD peaks with North Atlantic Heinrich events suggests that high-latitude climate changes related to the Bond cycle also occurred in the Asian monsoon region. On shorter timescales, IRD occurrence was not clearly related to Dansgaard–Oeschger cycles, which are revealed in the alternating dark and light layers in the late Quaternary Japan Sea sediments. Therefore, during the studied period, strong East Asian winter monsoons did not have a simple see-saw-like relationship with strong summer monsoons.

Coupled climate model simulation of Holocene cooling events: oceanic feedback amplifies solar forcing

Abstract. The coupled global atmosphere-ocean-vegetation model ECBilt-CLIO-VECODE is used to perform transient simulations of the last 9000 years, forced by variations in orbital parameters, atmospheric greenhouse gas concentrations and total solar irradiance (TSI). The objective is to study the impact of decadal-to-centennial scale TSI variations on Holocene climate variability. The simulations show that negative TSI anomalies increase the probability of temporary relocations of the site with deepwater formation in the Nordic Seas, causing an expansion of sea ice that produces additional cooling. The consequence is a characteristic climatic anomaly pattern with cooling over most of the North Atlantic region that is consistent with proxy evidence for Holocene cold phases. Our results thus suggest that the ocean is able to play an important role in amplifying centennial-scale climate variability.

Millennial-scale variations of sea-ice expansion in the southwestern part of the Okhotsk Sea during the past 120 kyr: Age model and ice-rafted debris in IMAGES Core MD01-2412

A 58-m-long sediment core IMAGES MD01-2412 was recovered in the southwestern part of the Okhotsk Sea for high resolution paleocenography. An age model of the core was obtained by accelerator mass spectrometry (AMS) 14C dating of planktonic foraminifer shells, oxygen–isotope stratigraphy of benthic foraminifer calcite, and tephrochronology, resulting in a core-bottom age of 115 kyr. Sea-ice expansion in the Okhotsk Sea was reconstructed by ice-rafted debris (IRD) based on measurement of dropstone, coarse fraction, sand fractions of terrigenous particles, and the magnetic properties. The SW Okhotsk Sea has not had perennial but seasonal sea-ice conditions during the 115 kyr. Seasonal sea ice fluctuated with large amplitudes on millennial scale during the glacials (Marine isotope stage: MIS 2, 3, and 4) and varied relatively little during the Holocene (MIS 1) and the last interglacial (MIS 5). Enhanced polar atmospheric circulation during the glacial resulted in strong wind fields over the Okhotsk Sea and accelerated the large sea-ice expansion during the glacials (MIS 2, 3, and 4). During the interglacials (MIS 1 and 5), sea ice also expanded by small amplitudes. During these periods, decrease of the Amur River discharge would be one of the possible factors for sea-ice expansion. The two main factors of polar atmospheric circulation and Amur River discharge would be responsible for sea-ice expansion during 120 kyr.

Rapid fluctuation of alkenone temperature in the southwestern Okhotsk Sea during the past 120 ky

Sea-ice expansion in the Okhotsk Sea in winter is sensitively affected by global warming and cooling. Regionally, the southwestern Okhotsk Sea is closely linked to climate change in East Asia, including Japan, because the cold sea surface temperature (SST) in the southwestern Okhotsk Sea influences directly the development of the Okhotsk atmospheric high-pressure system, and the activated Okhotsk high causes cold climatic conditions in northern Japan. Therefore, environmental change in the Okhotsk Sea indicates two-way interactions as a sensitive mirror reflecting global climate change and as a driving force of regional climate change. To better understand how surface environmental changes in the Okhotsk Sea can influence climate change in East Asia, SSTs were estimated in the southwestern Okhotsk Sea for the past 120 ky with millennial to centennial time resolution using the long-chain unsaturated alkyl ketone (alkenone) thermometer. The alkenone temperature, which corresponds to the SST to 20 m depth in autumn, showed repeated abrupt changes at a centennial timescale, especially during the last glacial period, 20–60 ky before present (BP). The alkenone temperature changed concurrently with changes from interstadials (warm events) to stadials (cold events) in the δ 18O record of the ice cores from Greenland, although some interstadials could not be identified in the alkenone temperature record. A wavelet power spectrum analysis showed that a periodicity of about 8 ky was prominent during 10–90 ky BP, and a 4- to 5-ky cycle was characteristic during 30–40 ky BP in the alkenone temperature records. These periodicities were both similar and dissimilar to those in the Polar Circulation Index, which is based on the atmospheric circulation intensity at high latitudes, as recorded by major-ion concentrations in GISP2. Both the similarity and dissimilarity imply that the SST in the southwestern Okhotsk Sea is controlled mainly by the atmosphere–ocean circulation system in the Northern Hemisphere; however, the relationship between the SST in the Okhotsk Sea and the climate in the Greenland is not linear. Anomalously high alkenone temperatures occurred repeatedly in the glacial period. These warm alkenone temperature episodes would have had multiple causes. In particular, high alkenone temperatures during the last glacial maximum (LGM) have been reported previously for locations near this study site. More investigations are necessary to understand what happened in the Okhotsk Sea and in adjacent seas at the time of the LGM.

Mechanisms for climate variability during glacial and interglacial periods

This paper suggests and explores mechanisms relevant to millennial‐scale climate variability during glacial periods. In particular, we present the results of model studies that are able to reproduce many aspects of observed glacial climate variability (e.g., Dansgaard‐Oeschger oscillations) without external forcing and that provide a natural explanation for the prevalence of high‐amplitude variability in glacial climates and the relative stability of the Holocene. We show that the role of sea ice is critical to cold climate variability because of the effective reduction in the high‐latitude meridional sea surface temperature gradient resulting from sea ice expansion and the associated role of sea ice in inhibiting heat flux from the ocean to atmosphere. Thus as sea ice expands in a cooler climate, the high‐latitude oceanic heat loss to the atmosphere is inhibited, the thermohaline circulation weakens, and the sinking regions move equatorward, leading to a shallower and weaker deep circulation. This weak circulation is unstable, and intermittent high‐amplitude oscillations occur on a timescale and with a spatial structure very similar to Dansgaard‐Oeschger cycles. Consistent results are found using both a three‐dimensional ocean circulation model coupled to an energy balance atmospheric model and with a much simpler ocean box model. In general, freshening plays a secondary role in the weakening of the North Atlantic thermohaline circulation. Significant freshening is required to alter the stable northern deepwater formation that occurs in a warm climate such as today's Holocene, but once this freshening threshold is achieved, the thermohaline circulation shifts to reverse overturning with sinking in the tropics.

Decadal-scale sea ice changes in the Canadian Arctic and their impacts on humans during the past 4,000 years

Climate warming of >1.5°C over three decades has diminished Arctic sea ice and forced drastic changes on Inuit people of the Canadian Arctic. Discontinuities in archaeological records also suggest that climatic changes may have caused site abandonment and life style shifts in Paleo- and Neo-eskimo societies. We therefore examine the decadal-scale palaeoclimatic changes recorded by quantitative palynological data in marine records from Coburg Polynya, near Palaeo- and Neo-eskimo settlements on the North Devon Lowlands, and from the North Water Polynya between Canada and Northwest Greenland. Palaeotransfer functions from dinoflagellate cyst assemblages provide quantitative estimates of changes in sea surface temperature (SST) and sea ice cover (SIC) with the accuracy of historical measurements.Both sites record temperature variations of 2–4°C corresponding to changes in hunting modes and occupation-abandonment cycles on Devon and Ellesmere Islands. Our data show that from ∼6500 to 2600 BP, there were large oscillations in summer SST from 2–4°C cooler than present to 6°C warmer and SIC ranged from 2 months more sea ice to 4 months more open water. The warmer interval corresponds to the period of pre-Dorset cultures that hunted muskox and caribou. Subsequent marine-based Dorset and Neo-eskimo cultures correspond to progressively cooler intervals with expanded sea ice cover. The warming took ∼50–100 years and lasted ∼300 years before replacement by colder intervals lasting ∼200–500 years. These climate oscillations are more rapid than the archaeological cultural changes, but are of similar length to successive Palaeoeskimo occupations in the Nares Strait region.

Interannual variation of particle fluxes in the eastern Bransfield Strait, Antarctica: A response to the sea ice distribution

Time-series sediment traps were deployed to investigate the temporal evolution of particle fluxes in the eastern Bransfield Strait over a 3-year period, from December 1998 to December 2001. Particle fluxes showed large seasonal and interannual variations. In 1999 and 2000, a seasonal trend of mass fluxes was characterized by highly elevated mass fluxes in the summer season, greater than 300 mg m −2 d −1, two orders of magnitude greater than winter mass fluxes. Such high summer fluxes were not observed in 2001, however, and seasonality was also significantly reduced. The absence of a high summer flux in 2001 might have been due to an abnormal expansion of sea ice to the trap mooring site during the 2001 summer season. The large interannual variation in particle flux is closely related to annual changes in sea ice cover in the eastern Bransfield Strait, which limits phytoplankton productivity in the surface waters. There is a lag of about 1 month between the surface water productivity and the export production in the eastern Bransfield Strait. The delay in the export production might have been due to the favorable environmental conditions for phytoplankton growth in the early summer when most primary production is stored in phytoplankton biomass, and few organic particles sink into the deep ocean. An average biogenic silica:organic carbon (Si:C) ratio of 1.5 was obtained from trap material collected at a depth of 678–1034 m, lower than the ratio obtained from the marginal winter sea ice zone of the Atlantic section and the Antarctic zone of the Pacific section. To produce a Si:C ratio of 1.5 at a depth of 678–1034 m from the surface water ratio of 0.52, at least 65% of the surface organic carbon must be remineralized during the descent to 678–1034 m. The 3-year mean of annual organic carbon fluxes measured in the eastern Bransfield Strait was 4.21±3.16 g C m −2, about two times higher than that of the central Bransfield Strait (2.84±3.33 g C m −2) and an order of magnitude higher than that of the western Bransfield Strait (0.35 g C m −2).

Ice-rafted debris (IRD)-based sea-ice expansion events during the past 100 kyrs in the Okhotsk Sea

Abstract Ice-rafted debris (IRD) from sediment traps, surficial sediments, and sediment cores were investigated in order to reconstruct the extent history of sea-ice cover during the last 100 kyrs in the Okhotsk Sea. The seasonal IRD buried into the sediments, during and just after sea-ice melting in spring, consists of silt- and sand- size terrigenous particles. The spatial distribution of IRD within surficial sediments clearly corresponds to the extent of sea-ice coverage. Sea-ice expanded to a maximum extent during the glacial periods, although sea-ice cover was not perennial. The Okhotsk Sea Sea-ice Expansion (OSIE) events, which are recognized by millennial-scale-abrupt peaks of IRD content, have occurred 13 times during the past 100 kyrs. These events were amplified during the glacial periods. A significant modification of the polar atmospheric circulation in the northern hemisphere is a key process in explaining these OSIE events.

The impact of precession changes on the Arctic climate during the last interglacial–glacial transition

Three sensitivity experiments using an Ocean Atmosphere General Circulation Model (OAGCM) are conducted to simulate the climate impact of precession. The relative contributions of components of the hydrological cycle including the albedo of Arctic sea ice, advection of atmospheric water vapor and sea surface temperature to the summer Arctic melt process are evaluated. Timing of the perihelion is varied in each experiment with meteorological spring (SP), winter (WP) and autumn (AP) perihelion corresponding to conditions at 110, 115 and 120 ky BP, respectively. Obliquity is unchanged at the 115 ky level which is lower than today. The experiments are assessed relative to the present day control, which has been shown to simulate current conditions based on observations. In the SP experiment, top of the atmosphere (TOA) insolation is weaker than today between the summer solstice and autumnal equinox. In the AP case representing the interglacial, it is less intense between vernal equinox and summer solstice but stronger during the remainder of the year. Although the incident solar radiation is reduced in summer in the SP experiment, increased melting of snow is found primarily as a result of feedbacks from the delayed seasonal cycle of hydrologic components. This is in contrast to both the WP and AP cases in which the perennial snow cover is simulated. At the time of the last glacial inception, 115 ky BP, the WP experiment shows lower insolation to the high northern latitudes in late spring and summer mainly as a result of lower obliquity than today. Dynamical ocean–atmosphere interactions in response to precession maintain the reduced sea ice melting in late spring, strengthen the annual equator-to-pole sea surface temperature (SST) gradient and increase atmospheric moisture convergence in glaciation-sensitive regions. In both the WP and AP experiments seasonal sea ice melting is weakened resulting in pronounced outgoing radiative flux at the locations of expanded sea ice. This leads to further cooling and increased snowfall due to the reduced atmospheric water holding capacity and increased atmospheric moisture convergence from the subtropical Atlantic. In agreement with Milankovitch theory, our results show favorable conditions for glacial inception at 115 ky BP but with obliquity unchanged, they also show perennial snow cover at 120 ky BP resulting from the reduced strength of spring insolation.

Simulating the Holocene climate evolution at northern high latitudes using a coupled atmosphere-sea ice-ocean-vegetation model

The response of the climate at high northern latitudes to slowly changing external forcings was studied in a 9,000-year long simulation with the coupled atmosphere-sea ice-ocean-vegetation model ECBilt-CLIO-VECODE. Only long-term changes in insolation and atmospheric CO2 and CH4 content were prescribed. The experiment reveals an early optimum (9-8 kyr BP) in most regions, followed by a 1-3degreesC decrease in mean annual temperatures, a reduction in summer precipitation and an expansion of sea-ice cover. These results are in general agreement with proxy data. Over the continents, the timing of the largest temperature response in summer coincides with the maximum insolation difference, while over the oceans, the maximum response is delayed by a few months due to the thermal inertia of the oceans, placing the strongest cooling in the winter half year. Sea ice is involved in two positive feedbacks (ice-albedo and sea-ice insulation) that lead regionally to an amplification of the thermal response in our model (7degreesC cooling in Canadian Arctic). In some areas, the tundra-taiga feedback results in intensified cooling during summer, most notably in northern North America. The simulated sea-ice expansion leads in the Nordic Seas to less deep convection and local weakening of the overturning circulation, producing a maximum winter temperature reduction of 7degreesC. The enhanced interaction between sea ice and deep convection is accompanied by increasing interannual variability, including two marked decadal-scale cooling events. Deep convection intensifies in the Labrador Sea, keeping the overall strength of the thermohaline circulation stable throughout the experiment.

Late Neogene to Quaternary environmental changes in the Antarctic Peninsula region: evidence from drift sediments

Clay-mineral composition and biogenic opal content in upper Miocene to Quaternary drift sediments recovered at two Ocean Drilling Program (ODP) sites from the continental rise in the Bellingshausen Sea had been analyzed in order to reconstruct the climatic and glacial history of the Antarctic Peninsula. The clay mineral composition at both sites is dominated by smectite, illite, and chlorite, and alternates between a smectite-enriched and a chlorite-enriched assemblage throughout the last 9.3 my. The spatial distribution of clay minerals in Holocene sediments west of the Antarctic Peninsula facilitates the identification of particular source areas, and thus the reconstruction of transport pathways. The similarity to clay mineral variations reported from upper Quaternary sequences suggests that the short-term clay-mineralogical fluctuations in the ODP cores reflect glacial–interglacial cyclicity. Thus, repeated ice advances and retreats in response to a varying size of the Antarctic Peninsula ice cap are likely to have occurred throughout the late Neogene and Quaternary. The clay minerals in the drift sediments exhibit only slight long-term variations, which are caused by local changes in glacial erosion and in supply of source rocks, rather than by major climatic changes. The opal records at the ODP sites are dominated by long-term variations since the late Miocene. We infer that the opal content in the drift sediments, although it is influenced by dissolution in the water column and the sediment column and by the burial with lithogenic detritus, provides a signal of paleoproductivity. Because the annual sea-ice coverage is regarded as the main factor controlling biological productivity, the opal signal helps to reconstruct paleoceanographic changes in the Bellingshausen Sea. Slightly enhanced opal deposition during the late Miocene indicates slightly warmer climatic conditions in the Antarctic Peninsula area than at present. During the early Pliocene, enhanced opal deposition in the Pacific sector of the Southern Ocean and coinciding high opal concentrations in sedimentary sequences from the Atlantic and Indian sectors document a strong reduction of sea-ice cover and relatively warm climatic conditions. Thereby, the early onset of the Pliocene warmth in the Bellingshausen Sea points to a positive feedback of regional Antarctic climate on the global thermohaline circulation. A decrease of opal deposition between 3.1 and 2.6 Ma likely reflects sea-ice expansion in response to reduced supply of northern-sourced deep-waters to the Southern Ocean, caused by the onset of Northern Hemisphere glaciation. Throughout the Quaternary, a relatively constant level of opal deposition on the Antarctic continental margin indicates relatively stable climatic conditions.

A Simulation of the Last Glacial Maximum climate using the NCAR-CCSM

The National Center for Atmospheric Research-Community Climate System Model (NCAR-CCSM) is used in a coupled atmosphere–ocean–sea-ice simulation of the Last Glacial Maximum (LGM, around 21,000 years ago) climate. In the tropics, the simulation shows a moderate cooling of 3 °C over land and 2 °C in the ocean in zonal average. This cooling is about 1 °C cooler than the CLIMAP sea surface temperatures (SSTs) but consistent with recent estimates of both land and sea surface temperature changes. Subtropical waters are cooled by 2–2.5 °C, also in agreement with recent estimates. The simulated oceanic thermohaline circulation at the LGM is not only shallower but also weaker than the modern with a migration of deep-water formation site in the North Atlantic as suggested by the paleoceanographic evidences. The simulated northward flow of Antarctic Bottom Water (AABW) is enhanced. These deep circulation changes are attributable to the increased surface density flux in the Southern Ocean caused by sea-ice expansion at the LGM. Both the Gulf Stream and the Kuroshio are intensified due to the overall increase of wind stress over the subtropical oceans. The intensified zonal wind stress and southward shift of its maximum in the Southern Ocean effectively enhances the transport of the Antarctic Circumpolar Current (ACC) by more than 50%. Simulated SSTs are lowered by up to 8 °C in the midlatitudes. Simulated conditions in the North Atlantic are warmer and with less sea-ice than indicated by CLIMAP again, in agreement with more recent estimates. The increased meridional SST gradient at the LGM results in an enhanced Hadley Circulation and increased midlatitude storm track precipitation. The increased baroclinic storm activity also intensifies the meridional atmospheric heat transport. A sensitivity experiment shows that about half of the simulated tropical cooling at the LGM originates from reduced atmospheric concentrations of greenhouse gases.