NSFGEO-NERC Pliocene sea level amplitudes (PLIOAMP)

It is estimated that 5% of the world's population lives on land which is less than 5 metres above current sea level, in communities that are vulnerable to the impacts of sea level rise, either from direct loss of land, or increased flood risk. Society more broadly may be impacted by disruption to key infrastructure which is located on the coast e.g. power stations, and by the movement of displaced communities. The Antarctic ice sheet is the largest potential contributor to future sea level rise and projections of Antarctic ice sheet change in the future also have the largest range of estimates. This makes it difficult to accurately determine the risks of future sea level rise. Because sea level rise from Antarctic ice loss is not evenly distributed across the oceans, retreat of the Antarctic Ice Sheet will disproportionately affect coastlines that are furthest away, such as those in Europe and North America. In this proposal we will improve projections of Antarctic ice sheet change by reconstructing how the ice sheet changed during past warm intervals during the mid-Pliocene (approximately 3 million years ago). The mid-Pliocene is the last geological interval when atmospheric CO2 was similar to present day.

The proposal will focus on reconstructing the amplitudes of mid-Pliocene sea level change between colder glacial stages and warmer interglacial states. We will use these data as a constraint for two types of ice sheet models. Recent work has used Pliocene interglacial sea level maxima as a constraint for Antarctic ice sheet models and has led to much higher projections of future sea level rise from Antarctica under anthropogenic warming. However, subsequent work has suggested that it may not be possible to accurately determine absolute Pliocene sea level maxima, such that the value of using these data has been questioned. The main source of uncertainty on these estimates comes from attempts to quantify them relative to a modern-day reference (i.e. as metres above present).

An alternative approach that we will propose and one that can greatly improve past sea level estimates is to focus on the Pliocene glacial-interglacial sea level amplitude. We will reconstruct the glacial-interglacial sea level amplitude for 3 intervals in the mid-Pliocene using analysis of sediments recovered from the drilling of ocean sediment cores. Specifically, we will measure the geochemical composition (the isotopes of oxygen, magnesium and calcium) of calcite microorganisms (benthic foraminifera) to reconstruct past ice volume. In the absence of a modern-day reference we will simulate both the Pliocene glacial (cooler climate intervals) and interglacial (warmer climate intervals) extent of the Antarctic and Northern Hemisphere Ice Sheets (principally the Greenland Ice Sheet) and compare this with the sea level data that we will produce. We will then be able to determine what was the magnitude of Antarctic ice sheet melt during the past.

Combining two groups based in the UK and US, the ice sheet models used will include the Penn State Ice Sheet Model (PSU-ISM) and the BISICLES ice sheet model. The treatment of the grounding line physics (the point at which grounded ice becomes floating ice shelf) is very different in these two models. The PSU-ISM requires additional processes (ice shelf hydrofracture and ice cliff failure) to simulate Antarctic retreat that was consistent with Pliocene sea level maxima. By using the BISICLES model, which has much higher resolution at the grounding line, we will be able to test whether these processes are needed to simulate ice retreat consistent with our measured Pliocene sea level amplitudes. Finally, we will use what we learn to produce a new set of future sea level estimates that are constrained using the palaeoclimate data. These will have tighter constraints than previous future sea level projections, enabling a more accurate estimate of the risk of future sea level rise from Antarctica.

The principal benefit to society from this research will be an improved assessment of the risks of sea level rise from retreat of the Antarctic ice sheet in the coming decades to centuries.

Who will benefit from this research?

1. Government and policy makers involved with policy relating to the impacts of climate change, particularly the vulnerability of coastal communities and infrastructure, and with flood risk.
2. Communities at risk of disruption or displacement due to sea level rise, for example coastal and island communities specifically, but also the population more widely who may be impacted by disruptions to coastal infrastructure and an increased risk of flooding.
3. Non-Governmental Organisations (NGOs) who work with displaced or disaster affected communities and who may need to plan for the future effects of future sea level rise.

How will they benefit from this research?

This project will enhance the ability of these groups to better plan and mitigate for the impacts of sea level rise by providing new estimates of the magnitude of the change by reducing the range of uncertainty on the estimate when compared to current values. Thus policy makers will be able to better anticipate and budget for the communities which will be most strongly impacted and the findings will contribute to evidence based policy making.

Future sea level estimate studies of this kind have a track record of receiving widespread attention in the media. By this means, it will capture the attention of the public and enable them to better understand the widespread impacts of climate change and how it may affect their local communities or infrastructure. Members of the general public will ultimately benefit from the enhanced quality of life that will result from development and adoption of policies that will minimise the negative impacts of sea level rise.

Summary of impact activities

All members of the research team have a strong track record of research dissemination to members of the public and other relevant stakeholders. Members of the team have been intensely involved with past and current Intergovernmental Panel on Climate Change reports including as chapter lead authors. We will communicate our research to members of the public through public lectures (with an aim of delivering 1-2 public lecture per researcher per year) and participation at science fairs, such as the Bristol Festival of Nature. As part of these outreach activities and to aid the visualisation of our results we will build an ice model of the Antarctic ice sheet using a 3d printed mould. We will commission a short film produced by Griott Creative about the research project, which will be shown at these events and distributed online. Building on an existing collaboration we will disseminate our results via climatecentral.org, an independent organisation which aims to communicate the facts of climate change and the impacts on the public. We will be supported in these activities by the University of Bristol Public Engagement Team and will also work with PolicyBristol in order to develop a strategy for communicating policy relevant aspects of our research to government agencies.