mid-Pliocene Warming Research Articles

Coccolith dissolution versus productivity changes during the Plio-Pleistocene (3.14–1.80 MA) in the South Atlantic (ODP site 1090)

Calcareous nannofossil assemblages and fragmentation ratios of two coccolithophore species (Coccolithus pelagicus and Calcidiscus leptoporus) were analysed in samples from ODP Site 1090, South Atlantic (42° 54.8′S, 8° 53.9′E), to obtain valuable insights into the paleoceanographic evolution and carbonate preservation trends during the late Pliocene and Early Pleistocene (3.14–1.80 Ma). Coccolithophore species were identified in a total of 232 samples. Calcareous nannofossil counts revealed shifts in the assemblages that can be correlated with the climatic evolution of the Southern Ocean. Between 3.2 and 2.85 Ma, i.e. during the mid-Pliocene Warmth, the assemblage was dominated by small Gephyrocapsa, a group of species typically associated with upwelling conditions. The dominance of the opportunistic small Gephyrocapsa group can be linked to a more intense mixing of the upper water column most likely due to the more southward position of the Subtropical Front (STF) compared with its present position. After 2.85 Ma, a gradual increase in the cold water species C. pelagicus marked a progressive cooling of the South Atlantic coupled with a northward shift of the (STF). The changes in the assemblage composition co-varied with the δ18O record. Periods with isotopically heavier values (glacial) were dominated by C. pelagicus and periods with lighter values (interglacial) were dominated by very small reticulofenestrids (VSR; <3 μm). After 2.7 Ma, an intensification of the glacial-interglacial patterns of C. pelagicus and VSR was registered. The interval between 2.44 and 2.34 Ma was characterised by low abundance of C. pelagicus and higher biodiversity which are interpreted to be due to a temporal southward movement of the STF interrupting the previously registered cooling. After 2.0 Ma, the decreasing abundance of C. pelagicus suggests a slowdown of the relatively intense cooling registered since 2.7 Ma. The fragmentation ratios of C. pelagicus and C. leptoporus suggest moderate preservation along our interval, with a general trend toward better preservation after 2.2 Ma. This enhanced preservation has been related to a greater input of the less-corrosive northern component water mass into the Cape Basin after 2.2 Ma. The lack of significant glacial-interglacial variations in the fragmentation ratios suggests that the low amplitude of the glacial cycles did not have a substantial impact on the carbonate preservation throughout the entire interval. During MIS 99–98 (approximately 2.5 Ma), a remarkably high accumulation of VSR was found and identified as a short (9 ka) event of high productivity. The timing of this event coincides with that of the Eltanin impact (approximately 2.5 Ma), suggesting that, albeit speculatively, the two events may be related.

Sustained mid-Pliocene warmth led to deep water formation in the North Pacific

Geologic intervals of sustained warmth such as the mid-Pliocene Warm Period can inform our understanding of future climate change, including the long-term consequences of oceanic uptake of anthropogenic carbon. Here we generate carbon isotope records and synthesize existing records to reconstruct the position of water masses and determine circulation patterns in the deep Pacific Ocean. We show that the mid-depth carbon isotope gradient in the North Pacific was reversed during the mid-Pliocene compared with today, which implies water flowed from north to south and deep water probably formed in the subarctic North Pacific Deep Water. An isotopically enabled climate model that simulates this North Pacific Deep Water reproduces a similar carbon isotope pattern. Modelled levels of dissolved inorganic carbon content in the North Pacific decrease slightly, although the amount of carbon stored in the ocean actually increases by 1.6% relative to modern due to an increase in dissolved inorganic carbon in the surface ocean. Although the modelled Pliocene ocean maintains a carbon budget similar to the present, the change in deep ocean circulation configuration causes pronounced downstream changes in biogeochemistry. Marine carbon isotope patterns point to substantial deep water formation in the North Pacific during the mid-Pliocene Warm Period, according to a synthesis of carbon isotope records and isotope-enabled climate modelling.

Enhanced Arctic Stratification in a Warming Scenario: Evidence From the Mid Pliocene Warm Period

AbstractGlobal warming is most pronounced in the Arctic as evident from the massive sea ice loss during the past few decades. The Mid‐Pliocene Warm Period (MPWP), 3.264 – 3.025 million years ago with similar CO2 levels, is the nearest analogue for understanding the impacts of future global warming. High‐resolution studies of relative nutrient utilization and productivity from the Atlantic‐Arctic Gateway (AAG) can provide insight into the nutrient availability governed by stratification strength during past warm climates. Here, we present relative nutrient utilization and productivity variability during the MPWP using sediments collected during the Ocean Drilling Program (ODP) Leg 151 from Fram Strait, AAG. We find that the relative nutrient utilization was high (low) implying stronger (weaker) stratification during warm (cold) periods during the MPWP. Stronger stratification inhibits the nutrient influx from intermediate water depths into the surface leading to higher utilization of available nutrients. It existed during warm periods likely due to enhanced summer sea ice melt and river discharge from the hinterland. As a consequence, the freshened surface layer could have stored more heat and accelerated the sea ice melt further implying that in the present‐day warm scenario, stronger stratification and upper layer freshening may lead to more sea ice melt in the Arctic Ocean.

Life history patterns of modern and fossil Mercenaria spp. from warm vs. cold climates

Recent work projects significant increases in sea surface temperature by the end of the 21st century. The biological consequences of such temperature increases are poorly understood. Study designs using a conservation paleobiology approach combined with sclerochronology methods can provide a powerful framework in which to assess these consequences. This study focuses on the ecological and economically important hard clam, Mercenaria, from modern and fossil settings that grew during climates that were warmer than or comparable to today. We compared lifespans and growth rates (von Bertalanffy k) of modern Mercenaria spp. populations to those from the Mid Pliocene Warm Period (MPWP) and early Pleistocene to better understand the influences of temperature on life history. We found that growth rates tend to increase with increasing temperature both through space and time. However, the relationship between lifespan and climate state is not as clear. Further, we observe that mid-to high-latitude individuals seem to be more impacted by changes in climate state than low latitude individuals. We suggest in response to increased seawater temperatures, mid- and high-latitude individuals might experience significant shifts towards faster growth rates whereas low latitude individuals might not see as much change. These findings provide insight to how growth rates and lifespans of Mercenaria might shift in response to future increases in seawater temperature. Understanding such impacts are critical for the development of management strategies and policies for future environmental change.

Paradox Negative Effects of the mid-Pliocene Warming on the Climatic Suitability of Six Mediterranean Sandfly Species in Europe

The Pliocene era could be the last time when sandfly (Diptera: Psychodidae) species were widespread in Europe. Within the Pliocene, the mid-Pliocene period is an important model period in the investigation of the future effects of anthropogenic climate change. In this study, the mid-Pliocene potential distribution of six Mediterranean sandfly species was modelled based on the M2 mid-Pliocene cold and mid-Pliocene warm paleoclimatic reconstructions. It was found that the cold period’s potential occurrence of sandfly species could be notably more extended than the distribution of the taxa in the warm period. The difference is less expressed in the case of the West Mediterranean species, but it is particularly visible in the circum-Mediterranean and East Mediterranean taxa. It can be concluded that not the changes in the mean annual temperature, but the increase of the precipitation patterns and the wetter climate of the mid-Pliocene warm period resulted in the observed differences. The results imply that the use of mid-Pliocene warming as a model of the present climatic changes can be handled with caution in the performing of biogeographic proxies for vector sandflies related to the anthropogenic climate change.

Contribution of the coupled atmosphere–ocean–sea ice–vegetation model COSMOS to the PlioMIP2

Abstract. We present the Alfred Wegener Institute's contribution to the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) wherein we employ the Community Earth System Models (COSMOS) that include a dynamic vegetation scheme. This work builds on our contribution to Phase 1 of the Pliocene Model Intercomparison Project (PlioMIP1) wherein we employed the same model without dynamic vegetation. Our input to the PlioMIP2 special issue of Climate of the Past is twofold. In an accompanying paper we compare results derived with COSMOS in the framework of PlioMIP2 and PlioMIP1. With this paper we present details of our contribution with COSMOS to PlioMIP2. We provide a description of the model and of methods employed to transfer reconstructed mid-Pliocene geography, as provided by the Pliocene Reconstruction and Synoptic Mapping Initiative Phase 4 (PRISM4), to model boundary conditions. We describe the spin-up procedure for creating the COSMOS PlioMIP2 simulation ensemble and present large-scale climate patterns of the COSMOS PlioMIP2 mid-Pliocene core simulation. Furthermore, we quantify the contribution of individual components of PRISM4 boundary conditions to characteristics of simulated mid-Pliocene climate and discuss implications for anthropogenic warming. When exposed to PRISM4 boundary conditions, COSMOS provides insight into a mid-Pliocene climate that is characterised by increased rainfall (+0.17 mm d−1) and elevated surface temperature (+3.37 ∘C) in comparison to the pre-industrial (PI). About two-thirds of the mid-Pliocene core temperature anomaly can be directly attributed to carbon dioxide that is elevated with respect to PI. The contribution of topography and ice sheets to mid-Pliocene warmth is much smaller in contrast – about one-quarter and one-eighth, respectively, and nonlinearities are negligible. The simulated mid-Pliocene climate comprises pronounced polar amplification, a reduced meridional temperature gradient, a northwards-shifted tropical rain belt, an Arctic Ocean that is nearly free of sea ice during boreal summer, and muted seasonality at Northern Hemisphere high latitudes. Simulated mid-Pliocene precipitation patterns are defined by both carbon dioxide and PRISM4 paleogeography. Our COSMOS simulations confirm long-standing characteristics of the mid-Pliocene Earth system, among these increased meridional volume transport in the Atlantic Ocean, an extended and intensified equatorial warm pool, and pronounced poleward expansion of vegetation cover. By means of a comparison of our results to a reconstruction of the sea surface temperature (SST) of the mid-Pliocene we find that COSMOS reproduces reconstructed SST best if exposed to a carbon dioxide concentration of 400 ppmv. In the Atlantic to Arctic Ocean the simulated mid-Pliocene core climate state is too cold in comparison to the SST reconstruction. The discord can be mitigated to some extent by increasing carbon dioxide that causes increased mismatch between the model and reconstruction in other regions.

Sensitivity of mid-Pliocene climate to changes in orbital forcing and PlioMIP's boundary conditions

Abstract. We compare results obtained from modeling the mid-Pliocene warm period using the Community Earth System Models (COSMOS, version: COSMOS-landveg r2413, 2009) with the two different modeling methodologies and sets of boundary conditions prescribed for the two phases of the Pliocene Model Intercomparison Project (PlioMIP), tagged PlioMIP1 and PlioMIP2. Here, we bridge the gap between our contributions to PlioMIP1 (Stepanek and Lohmann, 2012) and PlioMIP2 (Stepanek et al., 2020). We highlight some of the effects that differences in the chosen mid-Pliocene model setup (PlioMIP2 vs. PlioMIP1) have on the climate state as derived with COSMOS, as this information will be valuable in the framework of the model–model and model–data comparison within PlioMIP2. We evaluate the model sensitivity to improved mid-Pliocene boundary conditions using PlioMIP's core mid-Pliocene experiments for PlioMIP1 and PlioMIP2 and present further simulations in which we test model sensitivity to variations in paleogeography, orbit, and the concentration of CO2. Firstly, we highlight major changes in boundary conditions from PlioMIP1 to PlioMIP2 and also the challenges recorded from the initial effort. The results derived from our simulations show that COSMOS simulates a mid-Pliocene climate state that is 0.29 ∘C colder in PlioMIP2 if compared to PlioMIP1 (17.82 ∘C in PlioMIP1, 17.53 ∘C in PlioMIP2; values based on simulated surface skin temperature). On the one hand, high-latitude warming, which is supported by proxy evidence of the mid-Pliocene, is underestimated in simulations of both PlioMIP1 and PlioMIP2. On the other hand, spatial variations in surface air temperature (SAT), sea surface temperature (SST), and the distribution of sea ice suggest improvement of simulated SAT and SST in PlioMIP2 if employing the updated paleogeography. Our PlioMIP2 mid-Pliocene simulation produces warmer SSTs in the Arctic and North Atlantic Ocean than those derived from the respective PlioMIP1 climate state. The difference in prescribed CO2 accounts for 0.5 ∘C of temperature difference in the Arctic, leading to an ice-free summer in the PlioMIP1 simulation, and a quasi ice-free summer in PlioMIP2. Beyond the official set of PlioMIP2 simulations, we present further simulations and analyses that sample the phase space of potential alternative orbital forcings that have acted during the Pliocene and may have impacted geological records. Employing orbital forcing, which differs from that proposed for PlioMIP2 (i.e., corresponding to pre-industrial conditions) but falls into the mid-Pliocene time period targeted in PlioMIP, leads to pronounced annual and seasonal temperature variations. Our result identifies the changes in mid-Pliocene paleogeography from PRISM3 to PRISM4 as the major driver of the mid-Pliocene warmth within PlioMIP and not the minor differences in forcings.

Modelling the mid-Pliocene warm period using HadGEM2

Abstract Here, for the first time, we present simulations of mid-Pliocene climate using a UK IPCC AR5-class model (HadGEM2). The global annual mean surface air temperature increases by 3.4∘C compared to the preindustrial control, with warming amplified towards the poles. The overall sensitivity of surface air temperature and polar amplification in response to the specification of the Pliocene boundary conditions is greater in HadGEM2 than in a previously utilised UK model (HadCM3). The simulated temperature anomaly is also at the upper range of that produced by the first phase of the Pliocene Model Intercomparison Project ensemble. Energy balance analysis indicates that the polar amplification of the mid-Pliocene warming in HadGEM2 is due to greenhouse gas emissivity changes and surface albedo changes. Approximately 5 × 106km2 of Arctic sea-ice is lost in the HadGEM2 Pliocene simulation and the global precipitation increases by 0.18 mm/day, these anomalies are approximately twice as large as seen in HadCM3. HadGEM2 can retain a much larger amount of soil moisture than HadCM3, such that the amount of evaporation (and precipitation) over the land surface in the mid-Pliocene simulation is not as strongly constrained by water availability. These results highlight the importance of using more recently developed climate and Earth System Models to simulate the past. They further underline that our appreciation of Pliocene climate is model dependant and ultimately limited by our physical understanding of the climate and the way this is represented in models.

Pollen evidence for Late Pliocene – Early Pleistocene vegetation and climate change in the North Caucasus, North-Western Caspian Region

Pollen and spores have been analysed in deposits of the Akchagylian–Apsheronian in the north-western Caspian Sea region, providing a picture of past vegetation and climate change for the late Pliocene to early Pleistocene. On the basis of pollen assemblages in sediment cores and outcrops from the North Caucasus (the Caucasus Mineral'nyye Vody region, the Tersko-Sunzhensky area and the foothills of Dagestan), climatic fluctuations, and related changes in vegetation can be recognized for the time from 3.6 to 0.8 Ma. The lower Akchagylian is characterized at first by an open landscape dominated by steppe vegetation. In the middle of the lower Akchagylian, the transgression of the palaeo-Caspian spread, and the treeless landscapes of the earliest Akchagylian were replaced by forests with thermophilic relicts. During the middle-upper Akchagylian and Apsheronian periods, the vegetation cover of the North Caucasus gradually changed: forests were replaced with steppe and semi-desert vegetation in response to episodes of aridification, and changes were evident in the structure of the dendroflora. The vegetation of the North Caucasus shows changes consistent with climatic warming at around 3.2 Ma which coincides with a period of warming in the Mediterranean and probable represents the “Mid Pliocene Warm Period”.

The Slanicul de Buzau section, a unit stratotype for the Romanian stage of the Dacian Basin (Plio-Pleistocene, Eastern Paratethys)

Climatic changes cause large paleoenvironmental responses in semi-isolated basins. We analyze here the sedimentary successions of the Dacian Basin (Romania) to evaluate Late Pliocene and Early Pleistocene paleoenvironmental changes through macro- and micropaleontology. These changes are dated by creating a magnetostratigraphic time frame for two long and continuous sections with a combined total thickness of 2850m. The studied succession spans the time interval between 4.7Ma and 1.6Ma and records both the mid Pliocene Warm Period (3.3–2.9Ma) and the onset of large-scale glaciations on the Northern Hemisphere (~2.7Ma). Due to progressive basin infill, the paleoenvironment changes from brackish to fluvio-lacustrine with a major extinction event of lymnocardiine bivalves around 4.15Ma. Rich and dominantly freshwater mollusk and ostracod faunas develop from this moment onwards. Between 3.2Ma and 2.95Ma, the reappearance of lymnocardiines identifies a short moment of higher salinities, the previously identified Plescoi event. In time, this correlates closely to the warmest interval of the Pliocene, and is therefore most likely related to connectivity to the Black Sea during maximum sea-level. After the climatic optimum, deposition continues in a fluvio-deltaic setting with only scarce finds of fauna. Increasing amounts of coarse grained fluvial sediments show a close relationship with the progressive cooling during the Pleistocene.

The impact of dynamic topography change on Antarctic ice sheet stability during the mid-Pliocene warm period

The evolution of the Antarctic ice sheet during the mid-Pliocene warm period (MPWP) remains uncertain and has important implications for our understanding of ice sheet response to modern global warming. The extent to which marine-based sectors of the East Antarctic Ice Sheet (EAIS) retreated during the MPWP is particularly contentious, with geological observations and geochemical analyses being cited to argue for either a relatively minor or a significant ice sheet retreat in response to mid-Pliocene warming. The stability of marine-based ice sheets is intimately linked to bedrock elevation at their grounding lines, and previous ice sheet modeling assumed that Antarctic bedrock elevation during the MPWP was the same as today with the exception of a correction for the crustal response to ice loading. However, various processes may have perturbed bedrock elevation over the past 3 m.y., most notably vertical deflections of the crust driven by mantle convective flow, or dynamic topography. Here we present simulations of mantle convective flow that are consistent with a wide range of present-day observables and use them to predict changes in dynamic topography and reconstruct bedrock elevations during the MPWP. We incorporate these elevations into a simulation of the Antarctic ice sheet during the MPWP and find that the correction for dynamic topography change has a significant effect on the stability of the EAIS within the marine-based Wilkes Basin, with the ice margin in that sector retreating considerably further inland (200–560 km) relative to simulations that do not include this correction for bedrock elevation.

Mountain building and the initiation of the Greenland Ice Sheet

The effects of a new hypothesis about mountain building in Greenland on ice sheet initiation are investigated using an ice sheet model in combination with a climate model. According to this hypothesis, low-relief landscapes near sea level characterised Greenland in Miocene times until two phases of km-scale uplift in the late Miocene and in the latest Miocene–Pliocene (beginning at 10 and ~5Ma, respectively) initiated the formation of the present-day mountains. The topography of Greenland, prior to these uplift events is reconstructed from the present-day, isostatically compensated bedrock by mapping the two main steps in the landscape that resulted from the two uplift phases. Ice sheet initiation is studied using the topography before uplift and after each phase of uplift by applying different forcing conditions relevant for the late Cenozoic, which was characterised by long-term cooling superimposed by cold and warm excursions. The modelling results show that no ice initiates in the case of the low-lying and almost flat topography prior to the uplifts. However, the results demonstrate a significant ice sheet growth in response to the orographically induced increase in precipitation and the cooling of surface temperatures accompanying the uplift. Large amounts of ice are able to form after the first uplift event, but the ice sheet is sensitive to changes in climate. The results show that the second phase of uplift facilitates ice sheet build-up further and increases the stability of the ice sheet by providing anchoring points which are not available to the same extent in the lower topographies. However, the results also reveal a Föhn effect that inhibits ice sheet expansion into the interior Greenland and thus shifts the threshold of formation of inland ice towards colder temperatures. Under conditions that are colder than the present, the ice can overcome the Föhn effect, flow into the interior and form a coherent ice sheet. The results thus indicate that the Greenland Ice Sheet of today is a relict formed under colder conditions. The modelling results are consistent with the observed climatic variability superimposed on the general cooling trend in the late Cenozoic: e.g., ice rafted debris in late Miocene deposits off southeast Greenland and the mid-Pliocene Warmth. The late Cenozoic mountain building in Greenland augments the effects of the climatic deterioration leading to the Northern Hemisphere glaciations, and without the second phase of uplift, the Greenland Ice Sheet would have been more sensitive to the changes in climate over the past millions of years.

Mid-pliocene Atlantic Meridional Overturning Circulation not unlike modern

Abstract. In the Pliocene Model Intercomparison Project (PlioMIP), eight state-of-the-art coupled climate models have simulated the mid-Pliocene warm period (mPWP, 3.264 to 3.025 Ma). Here, we compare the Atlantic Meridional Overturning Circulation (AMOC), northward ocean heat transport and ocean stratification simulated with these models. None of the models participating in PlioMIP simulates a strong mid-Pliocene AMOC as suggested by earlier proxy studies. Rather, there is no consistent increase in AMOC maximum among the PlioMIP models. The only consistent change in AMOC is a shoaling of the overturning cell in the Atlantic, and a reduced influence of North Atlantic Deep Water (NADW) at depth in the basin. Furthermore, the simulated mid-Pliocene Atlantic northward heat transport is similar to the pre-industrial. These simulations demonstrate that the reconstructed high-latitude mid-Pliocene warming can not be explained as a direct response to an intensification of AMOC and concomitant increase in northward ocean heat transport by the Atlantic.

Mid-Pliocene climate modelled using the UK Hadley Centre Model: PlioMIP Experiments 1 and 2

Abstract. The Pliocene Model Intercomparison Project (PlioMIP) is a sub-project of the Paleoclimate Modelling Intercomparison Project (PMIP) whose objective is to compare predictions of the mid-Pliocene climate from the widest possible range of general circulation models. The mid-Pliocene (3.3–3.0 Ma) is the most recent sustained period of greater warmth and atmospheric carbon dioxide concentration than the pre-industrial times and as such has potential to inform predictions of our warming climate in the coming century. This paper describes the UK contribution to PlioMIP using the Hadley Centre Model both in atmosphere-only mode (HadAM3, PlioMIP Experiment 1) and atmosphere-ocean coupled mode (HadCM3, PlioMIP Experiment 2). The coupled model predicts a greater overall warming (3.3 °C) relative to the control than the atmosphere-only (2.5 °C). The Northern Hemisphere latitudinal temperature gradient is greater in the coupled model with a warmer Equator and colder Arctic than the atmosphere-only model, which is constrained by sea surface temperatures from Pliocene proxy reconstructions. The atmosphere-only model predicts a reduction in equatorial precipitation and south Asian monsoon intensity, whereas the coupled model shows an increase in the intensity of these systems. We present sensitivity studies using alternative boundary conditions for both the Pliocene and the control simulations, indicating the sensitivity of the mid-Pliocene warming to uncertainties in both pre-industrial and mid-Pliocene climate.

Paleoceanographic changes in the Eastern Equatorial Pacific over the last 10 Myr

To examine the Late Neogene evolution of tropical Pacific oceanography, we determined multiple geochemical proxy records for temperature (U37K′and TEX86Hindices) and primary productivity (algal biomarkers and diol indices) in sediments recovered at ODP Site 1241 in the East Equatorial Pacific (EEP) spanning a record of the last 10 Myr. The TEX86Htemperatures are lower than those recorded by U37K′indices, exhibiting large fluctuations and suggesting strong warming during the Mid Pliocene Warm Period (MPWP; 4.5–3.2 Ma) and significantly colder temperature during the Late Miocene cooling period (7–5 Ma) and after the Middle Pliocene Warm Period (MPWP). Such variations could reflect changes in the EEP thermocline temperatures, but we suggest that they instead reflect changes in the depth of export production of glycerol dialkyl glycerol tetraether lipids in response to changes in the upper ocean structure. A combination of temperature records, inferred to represent different and likely varying depths in the water column, as well as algal biomarker records for export production and ecosystem structure, suggest that both productivity and inference upwelling were reduced in the EEP during warmer periods, such as the MPWP and prior to 7 Ma. In contrast, stronger upwelling conditions and associated increased productivity likely prevailed from 7 to 5 Ma and for the past 3 Myr, both corresponding to globally cool intervals. A further increase in EEP productivity occurred at ca 1.8 Ma, coincident with the development of the E‐W Pacific SST gradient. These results confirm previous work that protracted El Niño–like conditions prevailed during warmer intervals of the Pliocene before ultimately descending into the current climate state.

On the causes of mid-Pliocene warmth and polar amplification

The mid-Pliocene (~3 to 3.3Ma ago), is a period of sustained global warmth in comparison to the late Quaternary (0 to ~1Ma ago), and has potential to inform predictions of long-term future climate change. However, given that several processes potentially contributed, relatively little is understood about the reasons for the observed warmth, or the associated polar amplification. Here, using a modelling approach and a novel factorisation method, we assess the relative contributions to mid-Pliocene warmth from: elevated CO2, lowered orography, and vegetation and ice sheet changes. The results show that on a global scale, the largest contributor to mid-Pliocene warmth is elevated CO2. However, in terms of polar amplification, changes to ice sheets contribute significantly in the Southern Hemisphere, and orographic changes contribute significantly in the Northern Hemisphere. We also carry out an energy balance analysis which indicates that that on a global scale, surface albedo and atmospheric emmissivity changes dominate over cloud changes. We investigate the sensitivity of our results to uncertainties in the prescribed CO2 and orographic changes, to derive uncertainty ranges for the various contributing processes.

Snapshots of the Greenland ice sheet configuration in the Pliocene to early Pleistocene

AbstractThe geometry of the ice sheets during the Pliocene to early Pleistocene is not well constrained. Here we apply an ice-flow model in the study of the Greenland ice sheet (GIS) during three extreme intervals of this period constrained by geological observations and climate reconstructions. We study the extent of the GIS during the Mid-Pliocene Warmth (3.3–3.0 Ma), its advance across the continental shelf during the late Pliocene to early Pleistocene glaciations (3.0–2.4 Ma) as implied by offshore geological studies, and the transition from glacial to interglacial conditions around 2.4 Ma as deduced from the deposits of the Kap København Formation, North Greenland. Our experiments show that no coherent ice sheet is likely to have existed in Greenland during the Mid-Pliocene Warmth and that only local ice caps may have been present in the coastal mountains of East Greenland. Our results illustrate the variability of the GIS during the Pliocene to early Pleistocene and underline the importance of including independent estimates of the GIS in studies of climate during this period. We conclude that the GIS did not exist throughout the Pliocene to early Pleistocene, and that it melted during interglacials even during the late Pliocene climate deterioration.

Mid-Pliocene equatorial Pacific sea surface temperature reconstruction: a multi-proxy perspective

The Mid-Pliocene is the most recent interval of sustained global warmth, which can be used to examine conditions predicted for the near future. An accurate spatial representation of the low-latitude Mid-Pliocene Pacific surface ocean is necessary to understand past climate change in the light of forecasts of future change. Mid-Pliocene sea surface temperature (SST) anomalies show a strong contrast between the western equatorial Pacific (WEP) and eastern equatorial Pacific (EEP) regardless of proxy (faunal, alkenone and Mg/Ca). All WEP sites show small differences from modern mean annual temperature, but all EEP sites show significant positive deviation from present-day temperatures by as much as 4.4 degrees C. Our reconstruction reflects SSTs similar to modern in the WEP, warmer than modern in the EEP and eastward extension of the WEP warm pool. The east-west equatorial Pacific SST gradient is decreased, but the pole to equator gradient does not change appreciably. We find it improbable that increased greenhouse gases (GHG) alone would cause such a heterogeneous warming and more likely that the cause of Mid-Pliocene warmth is a combination of several forcings including both increased meridional heat transport and increased GHG.

Pliocene climate and seasonality in North Atlantic shelf seas

This paper reviews North Atlantic shelf seas palaeoclimate during the interval 4-3Ma, prior to and incorporating the 'Mid-Pliocene warm period' (ca 3.29-2.97Ma). Fossil assemblages and stable isotope data demonstrate northwards extension of subtropical faunas along the coast of the Carolinas-Virginia (Yorktown and Duplin Formations) relative to the present day, suggesting a more vigorous Florida Current, with reduced seasonality and warm water extending north of Cape Hatteras (reconstructed annual range for Virginia 12-30 degrees C). This interpretation supports conceptual models of increased meridional heat transport for the Pliocene. Sea temperatures for Florida (Lower Pinecrest Beds) were similar to or slightly cooler than (summers 25-27 degrees C) today, and were probably influenced by seasonal upwelling of cold deep water. Reduced seasonality is also apparent in the Coralline Crag Formation of the southern North Sea, with ostracods suggesting winter sea temperatures of 10 degrees C (modern 4 degrees C). However, estimates from Pliocene bivalves (3.6-16.6 degrees C) are similar to or cooler than the present day. This 'mixed' signal is problematic given warmer seas in the Carolinas-Virginia, and climate model and oceanographic data that show warmer seas in the 'Mid-Pliocene' eastern North Atlantic. This may be because the Coralline Crag Formation was deposited prior to peak Mid-Pliocene warmth.

Surface temperatures of the Mid-Pliocene North Atlantic Ocean: implications for future climate

The Mid-Pliocene is the most recent interval in the Earth's history to have experienced warming of the magnitude predicted for the second half of the twenty-first century and is, therefore, a possible analogue for future climate conditions. With continents basically in their current positions and atmospheric CO2 similar to early twenty-first century values, the cause of Mid-Pliocene warmth remains elusive. Understanding the behaviour of the North Atlantic Ocean during the Mid-Pliocene is integral to evaluating future climate scenarios owing to its role in deep water formation and its sensitivity to climate change. Under the framework of the Pliocene Research, Interpretation and Synoptic Mapping (PRISM) sea surface reconstruction, we synthesize Mid-Pliocene North Atlantic studies by PRISM members and others, describing each region of the North Atlantic in terms of palaeoceanography. We then relate Mid-Pliocene sea surface conditions to expectations of future warming. The results of the data and climate model comparisons suggest that the North Atlantic is more sensitive to climate change than is suggested by climate model simulations, raising the concern that estimates of future climate change are conservative.