Aisling M. Dolan

Sea Surface Temperature of the mid-Piacenzian Ocean: A Data-Model Comparison

The mid-Piacenzian climate represents the most geologically recent interval of long-term average warmth relative to the last million years, and shares similarities with the climate projected for the end of the 21st century. As such, it represents a natural experiment from which we can gain insight into potential climate change impacts, enabling more informed policy decisions for mitigation and adaptation. Here, we present the first systematic comparison of Pliocene sea surface temperature (SST) between an ensemble of eight climate model simulations produced as part of PlioMIP (Pliocene Model Intercomparison Project) with the PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project mean annual SST field. Our results highlight key regional and dynamic situations where there is discord between the palaeoenvironmental reconstruction and the climate model simulations. These differences have led to improved strategies for both experimental design and temporal refinement of the palaeoenvironmental reconstruction.

Are there pre-Quaternary geological analogues for a future greenhouse warming?

Given the inherent uncertainties in predicting how climate and environments will respond to anthropogenic emissions of greenhouse gases, it would be beneficial to society if science could identify geological analogues to the human race’s current grand climate experiment. This has been a focus of the geological and palaeoclimate communities over the last 30 years, with many scientific papers claiming that intervals in Earth history can be used as an analogue for future climate change. Using a coupled ocean–atmosphere modelling approach, we test this assertion for the most probable pre-Quaternary candidates of the last 100 million years: the Mid- and Late Cretaceous, the Palaeocene–Eocene Thermal Maximum (PETM), the Early Eocene, as well as warm intervals within the Miocene and Pliocene epochs. These intervals fail as true direct analogues since they either represent equilibrium climate states to a long-term CO2 forcing—whereas anthropogenic emissions of greenhouse gases provide a progressive (transient) forcing on climate—or the sensitivity of the climate system itself to CO2 was different. While no close geological analogue exists, past warm intervals in Earth history provide a unique opportunity to investigate processes that operated during warm (high CO2) climate states. Palaeoclimate and environmental reconstruction/modelling are facilitating the assessment and calculation of the response of global temperatures to increasing CO2 concentrations in the longer term (multiple centuries); this is now referred to as the Earth System Sensitivity, which is critical in identifying CO2 thresholds in the atmosphere that must not be crossed to avoid dangerous levels of climate change in the long term. Palaeoclimatology also provides a unique and independent way to evaluate the qualities of climate and Earth system models used to predict future climate.

On the identification of a Pliocene time slice for data–model comparison

The characteristics of the mid-Pliocene warm period (mPWP: 3.264–3.025 Ma BP) have been examined using geological proxies and climate models. While there is agreement between models and data, details of regional climate differ. Uncertainties in prescribed forcings and in proxy data limit the utility of the interval to understand the dynamics of a warmer than present climate or evaluate models. This uncertainty comes, in part, from the reconstruction of a time slab rather than a time slice, where forcings required by climate models can be more adequately constrained. Here, we describe the rationale and approach for identifying a time slice(s) for Pliocene environmental reconstruction. A time slice centred on 3.205 Ma BP (3.204–3.207 Ma BP) has been identified as a priority for investigation. It is a warm interval characterized by a negative benthic oxygen isotope excursion (0.21–0.23‰) centred on marine isotope stage KM5c (KM5.3). It occurred during a period of orbital forcing that was very similar to present day. Climate model simulations indicate that proxy temperature estimates are unlikely to be significantly affected by orbital forcing for at least a precession cycle centred on the time slice, with the North Atlantic potentially being an important exception.

Integrating geological archives and climate models for the mid-Pliocene warm period

The mid-Pliocene Warm Period (mPWP), analogous to future climate conditions, is considered a test-bed for the predictive capability of climate models. Here, Dowsett et al. review our understanding of the mPWP and discuss recent and future advances in the context of proxy data/model integration.

Sea surface temperature control on the distribution of far‐traveled Southern Ocean ice‐rafted detritus during the Pliocene

The flux and provenance of ice-rafted detritus (IRD) deposited in the Southern Ocean can reveal information about the past instability of Antarcticas ice sheets during different climatic conditions. Here we present a Pliocene IRD provenance record based on the 40Ar/39Ar ages of ice-rafted hornblende grains from Ocean Drilling Program Site 1165, located near Prydz Bay in the Indian Ocean sector of the Southern Ocean, along with the results of modeled sensitivity tests of iceberg trajectories and their spatial melting patterns under a range of sea surface temperatures (SSTs). Our provenance results reveal that IRD and hence icebergs in the Prydz Bay area were mainly sourced from (i) the local Prydz Bay region and (ii) the remote Wilkes Land margin located at the mouth of the low-lying Aurora Subglacial Basin. A series of IRD pulses, reaching up to 10 times background IRD flux levels, were previously identified at Site 1165 between 3.3 and 3.0 Ma. Our new results reveal that the average proportion of IRD sourced from distal Wilkes Land margin doubles after 3.3 Ma. Our iceberg trajectory-melting models show that slower iceberg melting under cooling SSTs over this middle Pliocene interval allowed Wilkes Land icebergs to travel farther before melting. Hence, declining SSTs can account for a large part of the observed IRD provenance record at Site 1165. In early Pliocene IRD layers, sampled at suborbital resolution around 4.6 Ma, we find evidence for significant increases in icebergs derived from Wilkes Land during very warm interglacials. This is suggestive of large-scale destabilization of the East Antarctic Ice Sheet in the Aurora Subglacial Basin, as far-traveled icebergs would have to overcome enhanced melting in warmer SSTs. Our results highlight the importance of considering SSTs when interpreting IRD flux and provenance records in distal locations.

Can uncertainties in sea ice albedo reconcile patterns of data-model discord for the Pliocene and 20th/21st centuries?

General Circulation Model simulations of the mid-Pliocene warm period (mPWP, 3.264 to 3.025 Myr ago) currently underestimate the level of warming that proxy data suggest existed at high latitudes, with discrepancies of up to 11°C for sea surface temperature estimates and 17°C for surface air temperature estimates. Sea ice has a strong influence on high-latitude climates, partly due to the albedo feedback. We present results demonstrating the effects of reductions in minimum sea ice albedo limits in general circulation model simulations of the mPWP. While mean annual surface air temperature increases of up to 6°C are observed in the Arctic, the maximum decrease in model-data discrepancies is just 0.81°C. Mean annual sea surface temperatures increase by up to 2°C, with a maximum model-data discrepancy improvement of 1.31°C. It is also suggested that the simulation of observed 21st century sea ice decline could be influenced by the adjustment of the sea ice albedo parameterization.

The transient response of ice volume to orbital forcing during the warm Late Pliocene

Examining the nature of ice sheet and sea level response to past episodes of enhanced greenhouse gas forcing may help constrain future sea level change. Here, for the first time, we present the transient nature of ice sheets and sea level during the late Pliocene. The transient ice sheet predictions are forced by multiple climate snapshots derived from a climate model set up with late Pliocene boundary conditions, forced with different orbital forcing scenarios appropriate to two Marine Isotope Stages (MISs), MIS KM5c, and K1. Our results indicate that during MIS KM5c both the Antarctic and Greenland ice sheets contributed to sea level rise relative to present and were relatively stable. Insolation forcing between the hemispheres was out of phase during MIS K1 and led to an asynchronous response of ice volume globally. Therefore, when variations of precession were high, inferring the behavior of ice sheets from benthic isotope or sea level records is complex.

Icebergs in the Nordic Seas throughout the Late Pliocene

Abstract The Arctic cryosphere is changing and making a significant contribution to sea level rise. The Late Pliocene had similar CO2 levels to the present and a warming comparable to model predictions for the end of this century. However, the state of the Arctic cryosphere during the Pliocene remains poorly constrained. For the first time we combine outputs from a climate model with a thermodynamic iceberg model to simulate likely source regions for ice‐rafted debris (IRD) found in the Nordic Seas from Marine Isotope Stage M2 to the mid‐Piacenzian Warm Period and what this implies about the nature of the Arctic cryosphere at this time. We compare the fraction of melt given by the model scenarios with IRD data from four Ocean Drilling Program sites in the Nordic Seas. Sites 911A, 909C, and 907A show a persistent occurrence of IRD that model results suggest is consistent with permanent ice on Svalbard. Our results indicate that icebergs sourced from the east coast of Greenland do not reach the Nordic Seas sites during the warm Late Pliocene but instead travel south into the North Atlantic. In conclusion, we suggest a continuous occurrence of marine‐terminating glaciers on Svalbard and on East Greenland (due to the elevation of the East Greenland Mountains during the Late Pliocene). The study has highlighted the usefulness of coupled climate model‐iceberg trajectory modeling for understanding ice sheet behavior when proximal geological records for Pliocene ice presence or absence are absent or are inconclusive.

High Climate Model Dependency of Pliocene Antarctic Ice-Sheet Predictions

The mid-Pliocene warm period provides a natural laboratory to investigate the long-term response of the Earth’s ice-sheets and sea level in a warmer-than-present-day world. Proxy data suggest that during the warm Pliocene, portions of the Antarctic ice-sheets, including West Antarctica could have been lost. Ice-sheet modelling forced by Pliocene climate model outputs is an essential way to improve our understanding of ice-sheets during the Pliocene. However, uncertainty exists regarding the degree to which results are model-dependent. Using climatological forcing from an international climate modelling intercomparison project, we demonstrate the high dependency of Antarctic ice-sheet volume predictions on the climate model-based forcing used. In addition, the collapse of the vulnerable marine basins of Antarctica is dependent on the ice-sheet model used. These results demonstrate that great caution is required in order to avoid making unsound statements about the nature of the Pliocene Antarctic ice-sheet based on model results that do not account for structural uncertainty in both the climate and ice sheet models.Ice sheet models forced by climate model output indicate ice-sheet retreat during the Pliocene, yet concerns remain regarding potential model bias. Here, the authors present results from the Pliocene Ice-sheet Modelling Intercomparison Project, and show that results are highly dependent on the model forcing used.

Climate model simulations of the mid-Pliocene: Earth's last great interval of global warmth

Pliocene Model Intercomparison Project Workshop; Reston, Virginia, 2–4 August 2011 The Pliocene Model Intercomparison Project (PlioMIP), supported by the U.S. Geological Surveys (USGS) Pliocene Research, Interpretation and Synoptic Mapping (PRISM) project and Powell Center, is an integral part of a third iteration of the Paleoclimate Modelling Intercomparison Project (PMIP3). PlioMIPs aim is to systematically compare structurally different climate models. This is done in the context of the mid-Pliocene (∼3.3–3.0 million years ago), a geological interval when the global annual mean temperature was similar to predictions for the next century.