Warm Intervals Research Articles

North Pacific deep-sea ecosystem responses reflect post-glacial switch to pulsed export productivity, deoxygenation, and destratification

Deep-sea ecosystems are highly sensitive to changes in organic matter export and oxygenation driven by climate change. Here we document past ecological changes in benthic foraminiferal assemblages indicative of deglacial changes in deep-sea oxygenation and the character of organic matter fluxes from sedimentary records retrieved at intermediate (692 m) and abyssal (3667m) depths in the Gulf of Alaska. Constrained multivariate ordination combining faunal and geochemical data over the past ~22,000 years distinguishes the impacts of pulsed productivity, which exports carbon to the abyss, from extreme dysoxia across the deglacial warming transition. At both depths, opportunistic species are more prevalent in interglacial conditions, reflecting higher pulsed organic matter export to the seafloor developed in response to warming and reduced sea-ice cover. Benthic foraminiferal species tolerant of low-oxygen conditions increased in abundance during the deglacial transition at both intermediate and abyssal depths. Authigenic trace metals reveal sulfidic sedimentary conditions indicative intermittent anoxia, but only at intermediate-depths. Benthic foraminiferal richness and evenness are also highest during this deglacial low-oxygen interval, likely due to high food availability. Last Glacial Maximum faunas were distinctly different at the two sites, consistent with a more stratified deep Pacific, but the faunas become more similar during Holocene time, suggesting destratification of the abyss during deglaciation. These ecosystem responses support that carbon fluxes increased during warm intervals in subpolar regions and underscores the importance of considering the effects of transient biological blooms on paleoceanographic interpretations and in model projections of future deep carbon export.

Trends in Winter Warm Spells in the Central England Temperature Record

AbstractAn important impact of climate change on agriculture and the sustainability of ecosystems is the increase of extended warm spells during winter. We apply crossing theory to the central England temperature time series of winter daily maximum temperatures to quantify how increased occurrence of higher temperatures translates into more frequent, longer-lasting, and more intense winter warm spells. We find since the late 1800s an overall two- to threefold increase in the frequency and duration of winter warm spells. A winter warm spell of 5 days in duration with daytime maxima above 13°C has a return period that was often over 5 years but now is consistently below 4 years. Weeklong warm intervals that return on average every 5 years now consistently exceed ~13°C. The observed changes in the temporal pattern of environmental variability will affect the phenology of ecological processes and the structure and functioning of ecosystems.

The North Pacific Diatom Species Neodenticula seminae in the Modern and Holocene Sediments of the North Atlantic and Arctic

The paper presents micropaleontological information and observations of the North Pacific diatom species Neodenticula (N.) seminae (Simonsen and Kanaya) Akiba and Yanagisawa in the surface and Holocene sediments from the North Atlantic, Nordic, and Arctic Seas. The compilation of previously published data and new findings of this study on N. seminae in the surface sediments shows its broad occurrence as a usual element of the modern diatom microflora in the Nordic, Labrador, and Irminger Seas. The recent migration of N. seminae from its native area, the Subarctic Pacific, reflects the oceanographic shift in the late 1990s as greater transport of the warmer surface Pacific water to the Arctic causes Arctic sea-ice reduction. Micropaleontological studies of the Holocene sediments document the multiple events of N. seminae appearance in the Arctic during the latest Pleistocene and Holocene warming intervals. These observations can suggest the events of the increased influence of the North Pacific water on the Arctic environments in the past, not just during the recent warm climate amplification.

Pollen-based Holocene quantitative temperature reconstruction on the eastern Tibetan Plateau using a comprehensive method framework

Quantitative climate reconstruction on long timescales can provide important insights for understanding the climate variability and providing valuable data for simulations. Unfortunately, the credibility of some attempts was hampered by incomplete reconstruction procedures. We here establish a comprehensive framework resting on high-quality Chinese modern pollen database, including modern pollen data screening, calibration set selection, major climate factor analysis, appropriate model selection, strict statistical assessment of results and ecological interpretation. The application of this framework to three high-resolution pollen records from the eastern Tibetan Plateau allows accurate quantitative inferences of Holocene temperature changes, which is the major control of regional vegetation. The results show that the mean warmest month temperature (MTwa) during the early Holocene was ca. 10.4°C and reached the highest value at 8.5–6 ka BP (ca. 11°C). The early and mid-Holocene (11–5 ka BP) warmth was followed by 1.2°C temperature decrease, culminating in the coolest temperatures of the Holocene during the Neoglacial cooling. Superimposing on the general cooling trend, MTwa reveals a significant 500-yr periodicity with varying intensities through time, showing that warm (cold) intervals are in phase with solar maxima (minima) periods. This spectral similarity indicates a possible connection of multi-century scale climate fluctuations with solar forcing.

Warming favors subtropical lake cyanobacterial biomass increasing

Subtropical lakes are increasingly subject to cyanobacterial blooms resulting from climate change and anthropogenic activities, but the lack of long-term historical data limits understanding of how climate changes have affected cyanobacterial growth in deep subtropical lakes. Using high-resolution DNA data derived from a sediment core from a deep lake in southwestern China, together with analysis of other sedimentary hydroclimatic proxies, we investigated cyanobacterial biomass and microbial biodiversity in relation to climate changes during the last millennium. Our results show that both cyanobacterial abundance and microbial biodiversity were higher during warmer periods, including the Medieval Warm Period (930–1350 CE) and the Current Warm Period (1900 CE–present), but lower during cold periods, including the Little Ice Age (1400–1850 CE). The significant increases in cyanobacterial abundance and microbial biodiversity during warmer intervals are probably because warm climate not only favors cyanobacterial growth but also concentrates lake water nutrients through water budgets between evaporation and precipitation. Furthermore, because rising temperatures result in greater vertical stratification in deep lakes, cyanobacteria may have exploited these stratified conditions and accumulated in dense surface blooms. We anticipate that under anthropogenic warming conditions, cyanobacterial biomass may continue to increase in subtropical deep lakes.

FORAMINIFERI PLANCTONICI ED OSCILLAZIONI CLIMATICHE NELLA CAROTA BAN 84-27 (MEDITERRANEO ORIENTALE)

Planktonic foraminifera from Core BAN 84-27, recovered from a Mediterranean Ridge site located north of the Katia Basin (Eastern Mediterranean) were quantitatively investigated. The sediments are Late Pleistocene to Holocene in age; the sequence of sapropels, from S-1 to S-9, is almost complete, except S-2 and S4, which are missing and provides the lithostratigraphic framework. One tephra layer (Y-5) was identified.
 Peaks of abundance of Neogloboquadrina eceri were recognized in sapropels S-6 and S-8; these findings, recorded also in the reference cores, support the peculiarity of S-6 and S-8, which were deposited during cold intervals.A climatic curve, based on the relative abundance of climatically significant planktonic foraminifera, was constructed; its warm and cold peaks are correlatable to the fluctuations recorded in other Eastern Mediterranean cores (Core EASTW 29, Cobblestone Area 3; Core EASTW 6 and 45, Cobblestone Area 4; Cores KS-09 and RC9-181). The maximum diameter of Orbulina universa increases in accordance with relative abundance of climatically warmer species. This simple parameter seems to be an useful tool for paleoclimatic interpretations of Quaternary sediments, with larger sizes correlatable to warm intervals.
 Peaks of abundance of Neogloboquadrina eggeri were recognized in sapropels S-6 and S-8; these findings, recorded also in the reference cores, support the peculiarity of S-6 and S-8, which were deposited during cold intervals.
 The distribution of planktonic gastropods (Thecosomata Euthecosomata and Heteropoda) was investigated and correlated with the foraminiferal curve. The paleoclimatic value of pteropods is also evident in Core BAN 84-27: well diversified assemblages, characterized by a great abundance of warm water species (Limacina inflata), were observed in sapropels S-1 and S-9 and in other warm episodes. Poor assemblages, with sparse specimens of cold water species (L. retroversa) are present in cold episodes and in sapropel S-8. Intervals correlative to isotopic stages 1 to 9, substages 5a, 5c, 5e and substages 7a, 7c, 7d could be recognized. Moreover, climatic zone Z, Y, X, W and V (subzones V" and V') were identified. Average sediment accumulation rate is approximately 1.5 cm/1000 yr. Core BAN 84-27 represents the sedimentary expression of the last 320,000 yr in a low sedimentation rate area.

Climate signal shift under the influence of prevailing climate warming – Evidence from Quercus liaotungensis on Dongling Mountain, Beijing, China

Increasing climate warming is inducing drought stress and resulting in forest growth decline in many places around the world. The recent climate of northern China has shown trends of both warming and drying. In this study, we obtained tree ring width chronology of Quercus liaotungensis Koidz. from Dongling Mountain, Beijing, China. We divided the temperature series of the study area into cooling (1940–1969) and warming intervals (1970–2016). The climate–tree growth response analysis showed that temperature exerted a limiting impact on the annual radial growth of Q. liaotungensis during the cooling period, whereas the influence of temperature was lower during the warming period. The moving correlation analysis showed that the influence of summer temperature decreased with the warming climate since the 1970s, and that the influence of winter and spring temperatures decreased since the 2000s. The correlation values between the chronology and precipitation decreased during the cooling period, whereas spring and early summer precipitation correlations began to increase in the 1970s and reached significance (p < 0.05) in the 1990s. Our results show that the positive influence of temperature on radial growth of Q. liaotungensis in the study area has weakened, whereas precipitation has become the dominant regulator with climate warming. These findings suggest that forest growth on Dongling Mountain will decline if climate warming continues in the future.

Millennial-scale variations in sedimentary oxygenation in the western subtropical North Pacific and its links to North Atlantic climate

Abstract. The deep-ocean carbon cycle, especially carbon sequestration and outgassing, is one of the mechanisms to explain variations in atmospheric CO2 concentrations on millennial and orbital timescales. However, the potential role of subtropical North Pacific subsurface waters in modulating atmospheric CO2 levels on millennial timescales is poorly constrained. An increase in the respired CO2 concentration in the glacial deep-ocean due to biological pump generally corresponds to deoxygenation in the ocean interior. This link thus offers a chance to study oceanic ventilation and coeval export productivity based on redox-controlled sedimentary geochemical parameters. Here, we investigate a suite of geochemical proxies in a sediment core from the Okinawa Trough to understand sedimentary oxygenation variations in the subtropical North Pacific over the last 50 000 years (50 ka). Our results suggest that enhanced mid-depth western subtropical North Pacific (WSTNP) sedimentary oxygenation occurred during cold intervals and after 8.5 ka, while oxygenation decreased during the Bölling-Alleröd (B/A) and Preboreal. The enhanced oxygenation during cold spells is linked to the North Pacific Intermediate Water (NPIW), while interglacial increase after 8.5 ka is linked to an intensification of the Kuroshio Current due to strengthened northeast trade winds over the tropics. The enhanced formation of the NPIW during Heinrich Stadial 1 (HS1) was likely driven by the perturbation of sea ice formation and sea surface salinity oscillations in the high-latitude North Pacific. The diminished sedimentary oxygenation during the B/A due to a decreased NPIW formation and enhanced export production, indicates an expansion of the oxygen minimum zone in the North Pacific and enhanced CO2 sequestration at mid-depth waters, along with the termination of atmospheric CO2 concentration increase. We attribute the millennial-scale changes to an intensified NPIW and enhanced abyss flushing during deglacial cold and warm intervals, respectively, closely related to variations in North Atlantic Deep Water formation.

Magnetic, granulometric and geochemical characterizations of loess sections in the eastern Arid Central Asia: Implication for paleoenvironmental interpretations

Loess deposits in the Arid Central Asia (ACA) contain sensitive messages concerning the evolution of the Westerlies and the interior aridification of Asia. In this study, we conducted a combined study of geophysical and geochemical characterization on loess sections in the eastern ACA, to discuss possible paleoclimate development since the late Pleistocene Era. Results show that the ACA loess chiefly exhibited similar geophysical and geochemical compositions with loess deposits in other regions, like the Chinese Loess Plateau. There are various accumulation rates that would have been responsible for the depositional difference within the ACA loess profiles. In addition, there are three magnetic responses to the bulk grains and geochemical proxies in this study. At the bottom segment of the section, magnetic enhancement was associated with coarse grain sizes and water leaching. Magnetic enhancement responded to stronger pedogenesis and moderate paleo-wind intensity in the uppermost parts. Moreover, there is a transitional segment in the middle part of the section, characterized by inordinately detrital inputs and weak pedogenic contributions. Thus, it may be better to evaluate the magnetic climatology of the ACA loess through different depositional stages. Furthermore, this study generally reveals that the paleoclimate pattern in the ACA was probably characterized by relatively low moisture during warm intervals, and vice versa, in the middle and bottom parts of the profiles. But relatively enhanced moisture was indicated during the Holocene period. Further studies are still necessary to obtain reliable proxies and chronology for a detailed paleoclimate interpretation of the ACA loess.

Secular temperature variations and the spatial disparities of war in historical China

Studies about climate change and the variation of the spatial pattern of war are extremely scarce in academia at present. Therefore, the temperature series and battle coordinates in imperial China from AD 5 to 1911 are integrated in this research, and their long-term quantitative relationship is examined by employing mathematical statistics such as one-way ANOVA, as well as the spatial analytical tool, standard deviational ellipse (SDE) in ArcGIS. Meanwhile, the temperature sequence is divided into three multicentennial warm–cold cycles, which are combined with different types of war (all war, agri-nomadic conflict, and rebellion) to reveal the spatial disparity of war under the influence of secular and periodic temperature change. Results show that (1) battle longitude and battle latitude are significantly different between warm and cold phases. (2) SDEs stretch toward the north/west/northwest in warm intervals but retreat south-/east-/southeastward in cold stages. (3) SDEs generally shift southeastward over time, and the variation of latitude is more evident than that of longitude, which corresponds to the overall cooling trend throughout the past 2000 years. Based on these research findings, we conclude that temperature fundamentally regulates the spatial difference of war in imperial China via controlling agricultural and pastoral productivity. This innovative study provides a robust climatological explanation of the historical conundrum why wars in ancient China distribute with specific directions, and it also lays a foundation for spatiotemporal investigations of climate change and human responses at long-term scales in the future.

Similar Glacial‐Interglacial δ15N Variations in Two MIS 13–10 Sediment Sequences in the Western North Atlantic Ocean: Changes in Nitrogen Sources, Denitrification, or Diagenesis?

AbstractWe have determined total organic carbon‐mass accumulation rates (TOC‐MARs) and δ15N values in Marine Isotope Stages (MIS) 13–10 sediments from two oligotrophic parts of the western North Atlantic Ocean—Ocean Drilling Program (ODP) Site 1063 on the Bermuda Rise and Site 1058 on the Blake Outer Ridge. Both TOC‐MARs and δ15N values vary significantly in these depositional records. TOC‐MARs are highest in sediments deposited at Site 1063 during MIS 12 and MIS 10, implying marine productivity or TOC preservation increased at this location during glacial stages, and low TOC‐MARs during most of MIS 11 in Site 1063 sediment imply low rates during this warm interval. In contrast to the Site 1063 record, productivity or preservation appears to have been greater at Site 1058 during parts of the MIS 11 interglacial than during the glacial intervals. Unlike the TOC‐MAR records, δ15N values at both locations exhibit similar glacial‐interglacial alternations. Values peak during substage 11.3 and gradually decrease toward the terminations of MIS 12 and MIS 10. The similarity of the δ15N patterns at these two sites implies that nitrogen cycling at these locations is largely independent of changes in local surface production of organic matter, and the higher δ15N values are unlikely to result from denitrification. Instead, the δ15N alternations likely result from glacial‐interglacial alternations between higher rates of sediment delivery to these western North Atlantic locations during glacial periods that favored TOC preservation and lower rates during interglacials that allowed more TOC degradation and associated diagenetic alteration of its nitrogen isotopic composition, leading to higher sediment δ15N values.

Reproductive limitation mediates the response of white spruce (Picea glauca) to climate warming across the forest–tundra ecotone

Shifts in the extent of the boreal forest during past warm intervals and correlations between climate and the position of the forest–tundra ecotone suggest that recent temperature increases will facilitate forest expansion into tundra ecosystems. In this study, we used a unique set of high-resolution repeat photographs to characterize white spruce (Picea glauca (Moench) Voss) populations in 1980 and 2015 at 52 sites across the forest–tundra transition in the Northwest Territories, Canada. We also conducted field inventories at eight sites to examine mapping accuracy, construct age distributions, and assess cone production and seed viability. Our analysis shows that stand density in the forest–tundra has increased significantly since 1980 but that the density of spruce at sites in the tundra has not changed. Age distributions constructed from field sampling also indicate that recent recruitment has occurred in the forest–tundra but not at tundra sites. The nonlinear relationship between summer temperature and seed viability suggests that recent warming has facilitated recruitment in the northern Subarctic but that cold temperatures still limit recruitment at higher latitude tundra sites. Additional research to determine the extent of changes in forest density across the northern Subarctic should be conducted to determine if similar changes are occurring across this ecotone.

Ocean-atmosphere interconnections from the last interglacial to the early glacial: An integration of marine and cave records in the Iberian region

This study explores the climatic variability in the Iberian Peninsula (IP) and its surrounding seas from 140 to 65 kyr BP. Marine sediment cores and cave speleothems are used to reconstruct changes in sea surface water conditions, deep sea current intensities and atmospheric moisture availability based on stable isotopes, trace elements, grain size and XRF-elemental analysis. Oxygen isotopes in both terrestrial and oceanic archives recorded a large-scale precessional climatic rhythm that suggests a common modulation. This signal is interpreted as changes of the isotopic composition in the rainwater source area that is later transmitted into the cave. In terms of millennial-scale variability, the records trace the sequence of events defined for the North Atlantic region. The marine records show an intense sea-surface freshening and cooling related to Heinrich Event 11. During the Last Interglacial (LIG), the sea surface temperature evolution was heterogeneous around the IP with gradients larger than those from today. The LIG ended earlier in the Cantabrian Sea than in the western Mediterranean Sea, which it was coincident with an accelerated aridification phase that marked the glacial inception in a Minorca speleothem at 116.5 kyr BP and preceding the GS25. This was the first of a series of stadials that punctuated the early glaciation and where the sea thermal gradient almost disappeared around the IP. These intense cooling during stadials led the development of drier but intense westerlies over southern European latitudes, reflected in a Pyrenees speleothem record, and favouring enhanced deep convection in the western Mediterranean Sea. In contrast to this stadial regional homogeneity among the studied records, the interstadials periods are distinguished by their rather heterogeneous patterns around the IP pointing to much complex ocean-atmosphere interconnections during warm intervals.

Environmental changes in Arctic Chukotka during Marine Isotope Stages 38‐31: implications for the Early to Middle Pleistocene transition

Lake El'gygytgyn in far northeastern Russia provides one of the few records that can improve understanding of terrestrial responses to climatic changes during the Early to Middle Pleistocene transition (EMPT; 1400–400 ka). Palynological data from MIS 38‐31 (~1260 to 1060 ka) help clarify the complex nature of that transition. In contrast to the evergreen conifer forest of the MIS 31 ‘super’ interglaciation, Larix‐Betula‐Alnus forest with shrub tundra characterized MIS 37 and MIS 35. The Larix forest‐tundra of MIS 33 represents an even cooler interglaciation. The landscapes of MIS 38, MIS 36 and early MIS 34 were a tundra mosaic with areas of Larix‐forest tundra. Although marine records suggest that MIS 38 and MIS 36 were exceptionally harsh glaciations, the decrease in woody taxa (particularly Alnus and Pinus) indicate the coolest conditions at El'gygytgyn occurred during MIS 32 and late MIS 34. When combined with other palynological data from El'gygytgyn, the EMPT portion of the record indicates that Betula was important in the vegetation during both cool and warm periods prior to MIS 24. Following the build‐up of global ice volumes ~900 ka, the taxon was characteristic of warm intervals only, marking the establishment of the herb‐dominated tundra, which typifies the remainder of the Pleistocene glaciations. The more complex successional patterns in post‐900 ka vegetation suggest greater climate variability as ice volume increased. Pinus pumila became a significant component of interglacial vegetation following the shift from a 41 ka to a quasi‐100 ka orbital cycle, indicating an increase in snow cover and a lessening of aridification, at least in this part of the Arctic.

Temperature variation at the low‐latitude regions of East Asia recorded by tree rings during the past six centuries

AbstractThe understanding of past millennium climate change is very important. Due to the lack of continuously annual resolution records, the temperature characteristics in the low‐latitude regions of East Asia (LLREA) during past several centuries are still unknown. Using tree‐ring width chronology from Taiwan, an island located in the western Pacific region and in southeastern China, February–October mean temperature from 1380 to 2007 AD was reconstructed. Spatial correlation analysis indicated that the reconstructed temperature could represent the sea‐land temperature change in LLREA. During the past six centuries, both the warmest intervals and the largest variability of temperature appeared in the twentieth century. This is probably related to human activities. Temperature in LLREA provided good contrast with temperature changes in the Tibet Plateau and elsewhere in the Northern Hemisphere. The LLREA entered into the Little Ice Age around the year of 1450, approximately 100 years earlier than the Tibet Plateau. The 10‐year cycle contained in the reconstructed temperature is close to the 11‐year period of solar activity, which revealed there was a certain relationship between them. The temperature in LLREA significantly correlated with solar activity at the decadal to centennial scales. The strong/weak solar irradiance corresponded to the great/small temperature variability.

Legacy of Holocene Landscape Changes on Soil Biogeochemistry: A Perspective From Paleo‐Active Layers in Northwestern Canada

AbstractRecent climate warming is contributing to permafrost degradation and vegetation change; however, little is known about the legacy of Holocene landscape change on contemporary soil biogeochemical conditions. In permafrost soils of northwestern Canada, widespread permafrost degradation occurred during the early Holocene warm interval and its impacts on soil biogeochemistry are archived in the paleo‐active layer. Here we show contrasting profiles of soil soluble chemistry and organic carbon composition at sites affected by different types of permafrost degradation. At sites that experienced increased depth of thaw, the relict active layer contained a lower abundance of soluble ions than the underlying undisturbed permafrost; however, both the relict active layer and undisturbed permafrost contained mainly old recalcitrant organics suggesting that minor microbial degradation of organics had occurred. At sites that experienced past thaw slumping, the relict active layer had a higher solute content and contained both young‐degradable and old‐recalcitrant organics due to the integration of slumped surface organic mats into the colluvial soils or vegetation re‐colonizing the surface of the former slump. Our results show that permafrost degradation that occurred during the early to mid‐Holocene have preconditioned the biogeochemical conditions in near‐surface permafrost soils such that significant local variability exist following past landscape changes. Thus, determining the state of soil soluble chemistry and SOC in permafrost should be done within a paleo‐landscape change framework to better forecast CO2–CH4 emissions and solutes release from thawing permafrost.

Abrupt changes in Indian summer monsoon strength during the last ~900 years and their linkages to socio-economic conditions in the Indian subcontinent

Abrupt shifts in Indian summer monsoon (ISM) precipitation largely affected regional hydrometeorology, accelerating socio-economic and cultural adaptations of agrarian economies in South Asia in the last millennium. Linking ISM variability to human history in the Indian subcontinent is still limited by the scarcity of high-resolution paleoclimatic data for the period encompassing the last 1000 years or earlier. We present a new high resolution oxygen isotope proxy record from the Wah Shikar cave, northeastern India and identified abrupt shifts in the ISM that had profound impact on human societies during the last ~900 yrs. The ISM was strong during the Medieval Climate Anomaly (MCA) and the Current Warm Period (CWP), and show multiple shifts during the Little Ice Age (LIA). The abrupt shifts in the ISM were more frequent during warm intervals, showing a secular trend with few exceptions during the cold periods. Intervals of prolonged summer monsoon weakening are correlated to the decline of major dynasties in India, possibly due to decrease in agriculture productivity, declining economy and societal crisis. Changes in atmospheric temperature due to volcanic eruptions and sun spot activity have played major role in the onset of the LIA and other prolonged weak intervals of the ISM during the last ~900 yrs.

Modelling ice sheet evolution and atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; during the Late Pliocene

Abstract. In order to investigate the relation between ice sheets and climate in a warmer-than-present world, recent research has focussed on the Late Pliocene, 3.6 to 2.58 million years ago. It is the most recent period in Earth's history when such a warm climate state existed for a significant duration of time. Marine Isotope Stage (MIS) M2 (∼3.3 Myr ago) is a strong positive excursion in benthic oxygen records in the middle of the otherwise warm and relatively stable Late Pliocene. However, the relative contributions to the benthic δ18O signal from deep ocean cooling and growing ice sheets are still uncertain. Here, we present results from simulations of the Late Pliocene with a hybrid ice-sheet–climate model, showing a reconstruction of ice sheet geometry, sea level and atmospheric CO2. Initial experiments simulating the last four glacial cycles indicate that this model yields results which are in good agreement with proxy records in terms of global mean sea level, benthic oxygen isotope abundance, ice-core-derived surface temperature and atmospheric CO2 concentration. For the Late Pliocene, our results show an atmospheric CO2 concentration during MIS M2 of 233–249 ppmv and a drop in global mean sea level of 10 to 25 m. Uncertainties are larger during the warmer periods leading up to and following MIS M2. CO2 concentrations during the warm intervals in the Pliocene, with sea-level high stands of 8–14 m above the present day, varied between 320 and 400 ppmv, lower than indicated by some proxy records but in line with earlier model reconstructions.

Eemian Greenland ice sheet simulated with a higher-order model shows strong sensitivity to surface mass balance forcing

Abstract. The Greenland ice sheet contributes increasingly to global sea level rise. Its history during past warm intervals is a valuable reference for future sea level projections. We present ice sheet simulations for the Eemian interglacial period (∼130 000 to 115 000 years ago), a period with warmer-than-present summer climate over Greenland. The evolution of the Eemian Greenland ice sheet is simulated with a 3-D higher-order ice sheet model, forced with a surface mass balance derived from regional climate simulations. Sensitivity experiments with various surface mass balances, basal friction, and ice flow approximations are discussed. The surface mass balance forcing is identified as the controlling factor setting the minimum in Eemian ice volume, emphasizing the importance of a reliable surface mass balance model. Furthermore, the results indicate that the surface mass balance forcing is more important than the representation of ice flow for simulating the large-scale ice sheet evolution. This implies that modeling of the future contribution of the Greenland ice sheet to sea level rise highly depends on an accurate surface mass balance.

Challenges and research priorities to understand interactions between climate, ice sheets and global mean sea level during past interglacials

Quaternary interglacials provide key observations of the Earth system's responses to orbital and greenhouse gas forcing. They also inform on the capabilities of Earth system models, used for projecting the polar ice-sheet and sea-level responses to a regional warmth comparable to that expected by 2100 C.E. However, a number of uncertainties remain regarding the processes and feedbacks linking climate, ice-sheet and sea-level changes during past warm intervals. Here, we delineate the major research questions that need to be resolved and future research directions that should be taken by the paleoclimate, sea-level and ice-sheet research communities in order to increase confidence in the use of past interglacial climate, ice-sheet and sea-level reconstructions to constrain future predictions. These questions were formulated during a joint workshop held by the PAGES-INQUA PALSEA (PALeo constraints on SEA level rise) and the PAGES-PMIP QUIGS (QUaternary InterGlacialS) Working Groups in September 2018.