Ice Sheet Changes Research Articles

Abrupt glacial climate shifts controlled by ice sheet changes.

During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard-Oeschger (DO) events. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (BP) in the ice core record, and has drawn broad attention within the science and policy-making communities alike. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified. Here we show, by using a comprehensive fully coupled model, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere-ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses--including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw--are generally consistent with empirical evidence. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere-ocean-sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere-ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials.

Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum

This paper compiles and reviews marine and terrestrial data constraining the dimensions and configuration of the Antarctic Peninsula Ice Sheet (APIS) from the Last Glacial Maximum (LGM) through deglaciation to the present day. These data are used to reconstruct grounding-line retreat in 5 ka time-steps from 25 ka BP to present. Glacial landforms and subglacial tills on the eastern and western Antarctic Peninsula (AP) shelf indicate that the APIS was grounded to the outer shelf/shelf edge at the LGM and contained a series of fast-flowing ice streams that drained along cross-shelf bathymetric troughs. The ice sheet was grounded at the shelf edge until ∼20 cal ka BP. Chronological control on retreat is provided by radiocarbon dates on glacimarine sediments from the shelf troughs and on lacustrine and terrestrial organic remains, as well as cosmogenic nuclide dates on erratics and ice moulded bedrock. Retreat in the east was underway by about 18 cal ka BP. The earliest dates on recession in the west are from Bransfield Basin where recession was underway by 17.5 cal ka BP. Ice streams were active during deglaciation at least until the ice sheet had pulled back to the mid-shelf. The timing of initial retreat decreased progressively southwards along the western AP shelf; the large ice stream in Marguerite Trough may have remained grounded at the shelf edge until about 14 cal ka BP, although terrestrial cosmogenic nuclide ages indicate that thinning had commenced by 18 ka BP. Between 15 and 10 cal ka BP the APIS underwent significant recession along the western AP margin, although retreat between individual troughs was asynchronous. Ice in Marguerite Trough may have still been grounded on the mid-shelf at 10 cal ka BP. In the Larsen-A region the transition from grounded to floating ice was established by 10.7–10.6 cal ka BP. The APIS had retreated towards its present configuration in the western AP by the mid-Holocene but on the eastern peninsula may have approached its present configuration several thousand years earlier, by the start of the Holocene. Mid to late-Holocene retreat was diachronous with stillstands, re-advances and changes in ice-shelf configuration being recorded in most places. Subglacial topography exerted a major control on grounding-line retreat with grounding-zone wedges, and thus by inference slow-downs or stillstands in the retreat of the grounding line, occurring in some cases on reverse bed slopes.

Temperature trends during the Present and Last Interglacial periods – a multi-model-data comparison

Though primarily driven by insolation changes associated with well-known variations in Earth's astronomical parameters, the response of the climate system during interglacials includes a diversity of feedbacks involving the atmosphere, ocean, sea ice, vegetation and land ice. A thorough multi-model-data comparison is essential to assess the ability of climate models to resolve interglacial temperature trends and to help in understanding the recorded climatic signal and the underlying climate dynamics. We present the first multi-model-data comparison of transient millennial-scale temperature changes through two intervals of the Present Interglacial (PIG; 8–1.2 ka) and the Last Interglacial (LIG; 123–116.2 ka) periods. We include temperature trends simulated by 9 different climate models, alkenone-based temperature reconstructions from 117 globally distributed locations (about 45% of them within the LIG) and 12 ice-core-based temperature trends from Greenland and Antarctica (50% of them within the LIG). The definitions of these specific interglacial intervals enable a consistent inter-comparison of the two intervals because both are characterised by minor changes in atmospheric greenhouse gas concentrations and more importantly by insolation trends that show clear similarities.Our analysis shows that in general the reconstructed PIG and LIG Northern Hemisphere mid-to-high latitude cooling compares well with multi-model, mean-temperature trends for the warmest months and that these cooling trends reflect a linear response to the warmest-month insolation decrease over the interglacial intervals. The most notable exception is the strong LIG cooling trend reconstructed from Greenland ice cores that is not simulated by any of the models. A striking model-data mismatch is found for both the PIG and the LIG over large parts of the mid-to-high latitudes of the Southern Hemisphere where the data depicts negative temperature trends that are not in agreement with near zero trends in the simulations. In this area, the positive local summer insolation trend is counteracted in climate models by an enhancement of the Southern Ocean summer sea-ice cover and/or an increase in Southern Ocean upwelling. If the general picture emerging from reconstructions is realistic, then the model-data mismatch in mid and high Southern Hemisphere latitudes implies that none of the models is able to resolve the correct balance of these feedbacks, or, alternatively, that interglacial Southern Hemisphere temperature trends are driven by mechanisms which are not included in the transient simulations, such as changes in the Antarctic ice sheet or meltwater-induced changes in the overturning circulation.

Better measurements unlock new ice sheet mass loss estimates

Understanding the dynamics of the world's glaciers is integral to predicting global sea level rise in response to climate change. Greenland's rapidly shrinking ice sheet is a major factor of future sea level rise, but scientists are still unsure how much each component of the ice sheet's changes—surface accumulation, melting and runoff, and changes in ice discharged from the outlet of the glaciers draining into the ice sheet—is contributing to sea level rise.

Ice sheets as interactive components of Earth System Models: progress and challenges

One of the major impacts of anthropogenic climate change is sea level rise. Reliable estimates of the contribution of ice sheets to future sea level rise are important to policy makers and the civil society. In addition to sea level rise, ice sheet changes can affect the global climate through modified freshwater fluxes in the areas of deep‐water convection. Also, ice sheets modify local and large‐scale climate through changes in surface albedo and in their own topography. In the past, ice sheets have played a fundamental role in shaping climate and climate transitions. Despite their strong interactions with the climate system, they are not yet standard components of climate models. First attempts have been made in this direction, and it is foreseeable that in several years ice sheets will be included as interactive components of most models. The main challenges for this coupling are related to spatial and temporal resolution, ice sheet initialization, model climate biases, the need for explicit representation of snow/ice surface physics (e.g., albedo evolution, surface melt, refreezing, compaction), and coupling to the ocean component. This article reviews the main processes contributing to the ice sheet mass budget, the suite of ice sheet–climate interactions, and the requirements for modelling them in a coupled system. Focus is given to four major subjects: surface mass balance, ice sheet flow, ocean–ice sheet interaction, and challenges in coupling ice sheet models to climate models.This article is categorized under: Climate Models and Modeling > Model Components

Late Quaternary glacial history constrains glacio-isostatic rebound in Enderby Land, East Antarctica

Measurements of the loss or gain of ice mass from large ice sheets are presently achieved through satellite-based techniques such as GRACE (Gravity Recovery and Climate Experiment). The accuracy of these satellite-based measurements to changes in modern ice sheet mass depends on our knowledge of present-day glacio-isostatic crustal uplift rates caused by past ice sheet changes. To improve models of glacio-isostatic rebound in East Antarctica, we investigated ice histories along Rayner Glacier, Enderby Land, and a little explored sector of the ice sheet where GRACE data had suggested significant mass gain during the last decade. Observations from a recent glacial geomorphic reconnaissance coupled with cosmogenic nuclide dating indicate that in the lower part of the Rayner Glacier, Enderby Land, ice heights lowered by at least 300 m and the calving margin retreated by at least 10 km in the early Holocene (~6 to 9 ka B.P.). The magnitude and timing of deglaciation are consistent with ice histories used to model the postglacial rebound corrections for present-day GRACE mass trends. These observations strengthen the body of evidence that suggests ice mass gain in Enderby Land is presently partly offsetting mass loss in other parts of Antarctica.

Effect of Ice Shelf Changes on Ice Sheet Volume Change in the Amundsen Sea Embayment, West Antarctica

We derived elevation changes of the Geoscience Laser Altimeter System level-2 altimetry data from repeat tracks in the Amundsen Sea Embayment during 2004 to 2008. Then, we created four grid surfaces. Changes of ice shelf fronts in the study area were extracted from MODIS/Terra Calibrated Radiances Level-1B at 250-m resolution to analyze the relationship between ice shelf and ice sheet change. The results show the average volume change of the study area is about -92.8 km3/a. The obvious regions of the elevation decrease mainly located near the coastline, and the diversity of the ice volume loss corresponding to an elevation reduction exceeding 2 m was distinct between different periods. By analyzing the ice volume losses of the ice shelves and the large glaciers in the Amundsen Sea Embayment, we found the thinning of the ice shelf may result in the volume loss of the ice sheet, especially the thinning near the grounding line into which the glaciers flow. Because of this, the retreat of an ice shelf may cause the accelerated volume loss of the ice sheet, and the glacier buttressing ability of the fragmented ice shelf is weaker than that of unrifted or fractured large ice shelves.

Experimental results of elevation change analysis in the Antarctic ice sheet using DEMs from ERS and ICESat data

AbstractThe aim of this research is to investigate elevation changes in the Antarctic ice sheet by comparing two digital elevation models (DEMs) derived from satellite altimetry data covering the period 1994–2004. Data collected by ERS-1/2 satellite radar altimetry and by NASA GLAS/ICESat laser altimetry were used. After preprocessing and resampling at the same spatial resolution, both DEMs were compared in a pointwise fashion and elevation differences computed, which consisted of three main components: (1) actual elevation change, (2) errors in the original data sources and (3) interpolation errors that arose during generation of the DEMs. The objectives of the research were to analyze errors, attempt to mitigate systematic effects when possible, and draw some conclusions about the limitations of using DEM products for computing ice-sheet elevation change at local and continental scales. A linear correlation between errors in elevation differences and surface slope was found in the slope range [0°, 0.4°]. This trend was interpreted as residual slope-induced systematic error and compensated for. Finally, an elevation difference map of the Antarctic ice sheet was generated. Analysis of the derived elevation changes at the drainage basin was also made. Results are compared with the results of previous studies.

Simulation of Greenland ice sheet during the mid-Pliocene warm period

One of the key uncertainties in future sea-level projections is attributed to the Greenland ice sheet (GrIS). Studying the response of the GrIS to climate changes during the past warm periods is helpful for understanding future changes in the GrIS. In this study, using three global climate models (Community Atmosphere Model version 3.1 and version 4.0 and Norwegian Earth System Model) and a three-dimensional ice sheet model, we investigate the climate and ice sheet changes over Greenland during the mid-Pliocene warm period (~3 Ma bp). The results show that the regionally averaged summer temperature over Greenland is 9.4–13.4 °C higher during the mid-Pliocene period than during the pre-industrial era and the annual mean precipitation is 65.2–108.3 mm a−1 greater. In response to this warm-wet climate, the GrIS shows a substantial decrease in size during the mid-Pliocene, with little ice existing along the eastern coast of Greenland. Compared to that simulated in the control run, the global sea level is approximately 7.8–8.1 m higher during the mid-Pliocene due to the decrease in the size of the GrIS. In addition, paleoclimate proxies also indicate that it is unlikely that a large-scale ice sheet exists over Greenland during the mid-Pliocene warm period.

Long-term cycles in the carbon reservoir of the Quaternary ocean: a perspective from the South China Sea

AbstractIn recent years, long-term, high-resolution records from the deep sea and ice-cores have offered new research opportunities for Quaternary science. Paleoclimate studies are no longer restricted to individual glacial cycles, but extend to long-term (≥105 yr) processes across those cycles. Ocean Drilling Program Leg 184 of the South China Sea in 1999 uncovered well-preserved sediment sections, in which three long-term cycles in Pleistocene carbon isotope (δ13C) sequence have been found and demonstrated to be common in the global ocean. Subsequent discoveries confirm the existence of long-term processes of 105 yr in both the hydrologic (ice-sheet changes) and carbon (biogeochemical changes) cycles, posing the question whether the two processes are related. The present review shows that the long-eccentricity cycles prevail throughout the δ13C and other biogeochemical records in geologic history, and 400-kyr cycles in the oceanic δ13C sequence before the Quaternary can be hypothetically explained by changes in ratio between particulate and dissolved organic carbon (POC/DOC) in the ocean, depending on the monsoon-controlled nutrient supply. This is a ‘DOC hypothesis’. However, ocean restructuring at 1.6 Ma marked by the isolation of a sluggish abyss under the Southern Ocean has obscured the long-eccentricity 400-kyr signal in oceanic δ13C. The last million-year period has experienced two major changes in the climate regime, namely the mid-Pleistocene transition (MPT) centered at 0.9 Ma and the mid-Brunhes event (MBE) around 0.4 Ma. The MPT and MBE were preluded by δ13C maxima-III (δ13Cmax-III) ∼ 1.0 Ma and δ13Cmax-II ∼ 0.5 Ma, respectively. Together with similar hydroclimatic phenomena over corresponding glacial cycles, the two groups of hydrologic and biogeochemical events appear to have been driven largely by oceanographic changes in the Southern Ocean. Therefore, we interpret that the long-term biogeochemical processes originating from the Southern Ocean must have played a crucial role in Quaternary ice-sheet waxing and waning.

Warm climates of the past—a lesson for the future?

This Discussion Meeting Issue of the Philosophical Transactions A had its genesis in a Discussion Meeting of the Royal Society which took place on 10–11 October 2011. The Discussion Meeting, entitled ‘Warm climates of the past: a lesson for the future?’, brought together 16 eminent international speakers from the field of palaeoclimate, and was attended by over 280 scientists and members of the public. Many of the speakers have contributed to the papers compiled in this Discussion Meeting Issue. The papers summarize the talks at the meeting, and present further or related work. This Discussion Meeting Issue asks to what extent information gleaned from the study of past climates can aid our understanding of future climate change. Climate change is currently an issue at the forefront of environmental science, and also has important sociological and political implications. Most future predictions are carried out by complex numerical models; however, these models cannot be rigorously tested for scenarios outside of the modern, without making use of past climate data. Furthermore, past climate data can inform our understanding of how the Earth system operates, and can provide important contextual information related to environmental change. All past time periods can be useful in this context; here, we focus on past climates that were warmer than the modern climate, as these are likely to be the most similar to the future. This introductory paper is not meant as a comprehensive overview of all work in this field. Instead, it gives an introduction to the important issues therein, using the papers in this Discussion Meeting Issue, and other works from all the Discussion Meeting speakers, as exemplars of the various ways in which past climates can inform projections of future climate. Furthermore, we present new work that uses a palaeo constraint to quantitatively inform projections of future equilibrium ice sheet change.

Rapid ice retreat in Disko Bugt supported by 10Be dating of the last recession of the western Greenland Ice Sheet

Due to rising sea levels and warming ocean currents, marine-based sectors of the Greenland and Antarctic ice sheets are particularly vulnerable to warming climate. Reconstructions of the timing of marine-based ice margin fluctuations in Greenland during the early Holocene can provide context for historical and modern observations of ice-sheet change. Here, we generate a 10Be chronology of ice-sheet retreat through Disko Bugt, western Greenland. Our new chronology, consisting of twelve 10Be ages from sites surrounding and within Disko Bugt, fills a gap in the history of the western margin of the Greenland Ice Sheet and allows for a continuous composite record of ice-margin recession between the continental shelf break and the current margin. We constrain the onset of ice-margin retreat from outer Disko Bugt to 10.8 ± 0.5 ka. When combined with previous chronologies, these results place the final Greenland Ice Sheet retreat out of Disko Bugt onto land at Jakobshavn Isfjord and Qasigiaanguit at 10.1 ± 0.3 ka, and later at 9.2 ± 0.1 ka in southeastern Disko Bugt. The rate of retreat during this time period is between ∼50–450 m a−1 for central Disko Bugt and ∼50–70 m a−1 along the southern coast of Disko Bugt. Deglaciation of Disko Bugt occurred ∼1000 years later than in neighboring Uummannaq Fjord to the north. This asynchrony in the timing of deglaciation suggests that local ice dynamics played an important role in the retreat of the Greenland Ice Sheet from large marine embayments in western Greenland.

Glaciology and geological signature of the Last Glacial Maximum Antarctic ice sheet

Dynamical changes in contemporary ice sheets account for significant proportions of their current rates of mass loss, but assessing whether or not these processes are a natural part of ice-sheet evolution requires inference from palaeo-glaciological records. However, a robust mechanism for translating sparse geological data into meaningful interpretations of past glacier dynamics at the continental scale is lacking, since geological archives can be ambiguous, and often their chronology is only poorly constrained. To address this, we combine the interpretation of high-resolution Antarctic ice sheet model results with continent-wide geological evidence pertinent to the dynamical configuration of the ice sheet during the last, and possibly preceding, glacial maxima. We first focus on the thermal regime of the ice sheet, its pattern and velocity of flow, variability in likely subglacial erosion and sediment transport, and how these characteristics evolve during glacial transitions. We show that rapid basal sliding was restricted to discrete outlets that eroded and advected sediment toward and across the continental shelf primarily during the early stages of advance and retreat of the ice sheet, highlighting the need to consider time-transgressive behaviour in the interpretation of geological archives. Secondly, we present new modelling that attempts to improve the fit of our numerical model to geologically-based reconstructions in the Ross Sea. By accounting for locally-enhanced ablation in McMurdo Sound, our new simulation achieves a much closer fit to empirically-derived flow patterns than previously. Growth of the modelled Last Glacial Maximum ice sheet takes place primarily by marine ice accretion in the major embayments, as a consequence of cooler ocean temperatures and reduced sub-ice-shelf melting, and at its maximal extent represents a grounded ice volume excess above present of approximately 8.3 m sea-level equivalent. This figure thus provides an upper bound on the possible Antarctic contribution to deglacial meltwater pulses.

Late Holocene ice-flow reconfiguration in the Weddell Sea sector of West Antarctica

Here we report Late Holocene ice sheet and grounding-line changes to the Weddell Sea sector of West Antarctica. Internal radio-echo layering within the Bungenstock Ice Rise, which comprises very slow-flowing ice separating the fast-flowing Institute and Möller ice streams, reveals ice deformed by former enhanced flow, overlain by un-deformed ice. The ice-rise surface is traversed by surface lineations explicable as diffuse ice-flow generated stripes, which thus capture the direction of flow immediately prior to the creation of the ice rise. The arrangement of internal layers can be explained by adjustment to the flow path of the Institute Ice Stream, during either a phase of ice sheet retreat not longer than ∼4000 years ago or by wholesale expansion of the grounding-line from an already retreated situation not sooner than ∼400 years ago. Some combination of these events, involving uplift of the ice rise bed during ice stream retreat and reorganisation, is also possible. Whichever the case, the implication is that the ice sheet upstream of the Bungenstock Ice Rise, which currently grounds over a >1.5 km deep basin has been, and therefore may be, susceptible to significant change.

Time-dependent climate sensitivity and the legacy of anthropogenic greenhouse gas emissions

Climate sensitivity measures the response of Earth's surface temperature to changes in forcing. The response depends on various climate processes that feed back on the initial forcing on different timescales. Understanding climate sensitivity is fundamental to reconstructing Earth's climatic history as well as predicting future climate change. On timescales shorter than centuries, only fast climate feedbacks including water vapor, lapse rate, clouds, and snow/sea ice albedo are usually considered. However, on timescales longer than millennia, the generally higher Earth system sensitivity becomes relevant, including changes in ice sheets, vegetation, ocean circulation, biogeochemical cycling, etc. Here, I introduce the time-dependent climate sensitivity, which unifies fast-feedback and Earth system sensitivity. I show that warming projections, which include a time-dependent climate sensitivity, exhibit an enhanced feedback between surface warming and ocean CO2 solubility, which in turn leads to higher atmospheric CO2 levels and further warming. Compared with earlier studies, my results predict a much longer lifetime of human-induced future warming (23,000-165,000 y), which increases the likelihood of large ice sheet melting and major sea level rise. The main point regarding the legacy of anthropogenic greenhouse gas emissions is that, even if the fast-feedback sensitivity is no more than 3 K per CO2 doubling, there will likely be additional long-term warming from slow climate feedbacks. Time-dependent climate sensitivity also helps explaining intense and prolonged warming in response to massive carbon release as documented for past events such as the Paleocene-Eocene Thermal Maximum.

Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier

Ice shelves are critical features in the debate about West Antarctic ice sheet change and sea level rise, both because they limit ice discharge and because they are sensitive to change in the surrounding ocean. The Pine Island Glacier ice shelf has been thinning rapidly since at least the early 1990s, which has caused its trunk to accelerate and retreat. Although the ice shelf front has remained stable for the past six decades, past periods of ice shelf collapse have been inferred from relict seabed “corrugations” (corrugated ridges), preserved 340 km from the glacier in Pine Island Trough. Here we present high‐resolution bathymetry gathered by an autonomous underwater vehicle operating beneath an Antarctic ice shelf, which provides evidence of long‐term change in Pine Island Glacier. Corrugations and ploughmarks on a sub‐ice shelf ridge that was a former grounding line closely resemble those observed offshore, interpreted previously as the result of iceberg grounding. The same interpretation here would indicate a significantly reduced ice shelf extent within the last 11 kyr, implying Holocene glacier retreat beyond present limits, or a past tidewater glacier regime different from today. The alternative, that corrugations were not formed in open water, would question ice shelf collapse events interpreted from the geological record, revealing detail of another bed‐shaping process occurring at glacier margins. We assess hypotheses for corrugation formation and suggest periodic grounding of ice shelf keels during glacier unpinning as a viable origin. This interpretation requires neither loss of the ice shelf nor glacier retreat and is consistent with a “stable” grounding‐line configuration throughout the Holocene.

Climate sensitivity in the Anthropocene

AbstractClimate sensitivity in its most basic form is defined as the equilibrium change in global surface temperature that occurs in response to a climate forcing, or externally imposed perturbation of the planetary energy balance. Within this general definition, several specific forms of climate sensitivity exist that differ in terms of the types of climate feedbacks they include. Based on evidence from Earth's history, we suggest here that the relevant form of climate sensitivity in the Anthropocene (e.g. from which to base future greenhouse gas (GHG) stabilization targets) is the Earth system sensitivity including fast feedbacks from changes in water vapour, natural aerosols, clouds and sea ice, slower surface albedo feedbacks from changes in continental ice sheets and vegetation, and climate–GHG feedbacks from changes in natural (land and ocean) carbon sinks. Traditionally, only fast feedbacks have been considered (with the other feedbacks either ignored or treated as forcing), which has led to estimates of the climate sensitivity for doubled CO2 concentrations of about 3°C. The 2×CO2 Earth system sensitivity is higher than this, being ∼4–6°C if the ice sheet/vegetation albedo feedback is included in addition to the fast feedbacks, and higher still if climate–GHG feedbacks are also included. The inclusion of climate–GHG feedbacks due to changes in the natural carbon sinks has the advantage of more directly linking anthropogenic GHG emissions with the ensuing global temperature increase, thus providing a truer indication of the climate sensitivity to human perturbations. The Earth system climate sensitivity is difficult to quantify due to the lack of palaeo‐analogues for the present‐day anthropogenic forcing, and the fact that ice sheet and climate–GHG feedbacks have yet to become globally significant in the Anthropocene. Furthermore, current models are unable to adequately simulate the physics of ice sheet decay and certain aspects of the natural carbon and nitrogen cycles. Obtaining quantitative estimates of the Earth system sensitivity is therefore a high priority for future work.

Paleoclimate simulations of the mid-Holocene and last glacial maximum by FGOALS

Paleoclimate simulations of the mid-Holocene (MH) and Last Glacial maximum (LGM) by the latest versions of the Flexible Global Ocean-Atmosphere-Land System model, Spectral Version 2 and Grid-point Version 2 (FGOALS-s2 and g2) are evaluated in this study. The MH is characterized by changes of insolation induced by orbital parameters, and the LGM is a glacial period with large changes in greenhouse gases, sea level and ice sheets. For the MH, both versions of FGOALS simulate reasonable responses to the changes of insolation, such as the enhanced summer monsoon in African-Asian regions. Model differences can be identified at regional and seasonal scales. The global annual mean surface air temperature (TAS) shows no significant change in FGOALS-s2, while FGOALS-g2 shows a global cooling of about 0.7°C that is related with a strong cooling during boreal winter. The amplitude of ENSO is weaker in FGOALS-g2, which agrees with proxy data. For the LGM, FGOALS-g2 captures the features of the cold and dry glacial climate, including a global cooling of 4.6°C and a decrease in precipitation by 10%. The ENSO is weaker at the LGM, with a tendency of stronger ENSO cold events. Sensitivity analysis shows that the Equilibrium Climate Sensitivity (ECS) estimated for FGOALS ranges between 4.23°C and 4.59°C. The sensitivity of precipitation to the changes of TAS is ∼2.3% °C−1, which agrees with previous studies. FGOALS-g2 shows better simulations of the Atlantic Meridional Overturning Circulation (AMOC) and African summer monsoon precipitation in the MH when compared with FGOALS-g1.0; however, it is hard to conclude any improvements for the LGM.

From circumpolar deep water to the glacial meltwater plume on the eastern Amundsen Shelf

The melting of Pine Island Ice Shelf (PIIS) has increased since the 1990s, which may have a large impact on ice sheet dynamics, sea-level rise, and changes in water mass properties of surrounding oceans. The reason for the PIIS melting is the relatively warm (∼1.2°C) Circumpolar Deep Water (CDW) that penetrates into the PIIS cavity through two submarine glacial troughs located on the Amundsen Sea continental shelf. In this study, we mainly analyze the hydrographic data obtained during ANTXXVI/3 in 2010 with the focus on pathways of the intruding CDW, PIIS melt rates, and the fate of glacial meltwater. We analyze the data by dividing CTD profiles into 6 groups according to intruding CDW properties and meltwater content. From this analysis, it is seen that CDW warmer than 1.23°C (colder than 1.23°C) intrudes via the eastern (central) trough. The temperature is controlled by the thickness of the intruding CDW layer. The eastern trough supports a denser CDW layer than the water mass in Pine Island Trough (PIT). The eastern intrusion is modified on the way into PIT through mixing with the lighter and colder CDW from the central trough. Using ocean transport and tracer transport calculations from the ice shelf front CTD section, the estimated melt rate in 2010 is ∼30myr−1, which is comparable to published values. From spatial distributions of meltwater content, meltwater flows along the bathymetry towards the west. When compared with earlier (2000) observations, a warmer and thicker CDW layer is observed in Pine Island Trough for the period 2007–2010, indicating a recent thickening of the CDW intrusion.

Millennial-scale climate variability during the mid-Pleistocene transition period in the northern South China Sea

Stable isotopes of foraminiferal multispecies (surface dwelling Globigerinoides ruber, thermocline dwelling Pulleniatina obliquiloculata, and benthic Uvigerina peregrina) and Mg/Ca ratios of G. ruber shells were analyzed with an average time resolution of ∼370 yr for reconstructing the orbital- and millennial-scale changes in the East Asian monsoon and associated upper water structure during the mid-Pleistocene period of 800–1060 ka at ODP Site 1144 in the northern South China Sea (SCS). It has been found that generally the sea surface temperature (SST) were lower and the depth of thermocline (DOT) was deeper during glacial stages and vice versa during interglacial stages, indicating a strengthened winter monsoon and weakened summer monsoon during glacial stages. The sea surface salinity (SSS) was relatively lower during glacials, induced by the greatly reduced distance of this site to the shore during times of low sea level. Further, spectral analyses have revealed significant semi-precessional and/or precessional cycles in the planktic δ18O, SST and the proxies of SSS and DOT, showing the typical characteristics of tropical climate change. This means that during the mid-Pleistocene climate transition (MPT) period the East Asian monsoon evolution and associated changes of upper water column structure in the northern SCS were probably driven partly by tropical forcing, like in the southern SCS, and should not be related only to the high latitude ice sheet changes. Particularly, millennial-scale climate fluctuations (mainly ∼1.4 kyr cycle) were found to exist throughout the glacial and interglacial stages during the MPT period in the northern SCS, but slightly different from the sawtooth-like D/O events recorded in late Quaternary Greenland ice cores. During the mid-Pleistocene terminations, the SST warming was synchronous with the northern ice sheet retreat, indicated by benthic δ18O, in the northern SCS, which is consistent to the previous findings in the late Quaternary SCS and apparently different from that in the western equatorial Pacific Ocean, providing new insights into the studies of East Asian monsoon evolution and rapid climate change.