Topography under Antarctic ice crucial for predicting future sea level

The Antarctic Ice Sheet is currently losing ice and contributing to sea level rise. However, the amount of ice being discharged to the ocean now and in the future remains deeply uncertain because of limited data on ice thickness and bed topography in the coastal regions of Antarctica.

By: Kenichi Matsuoka and Geir Moholdt // Norwegian Polar Institute

The life of Antarctic ice starts when snow accumulates over the continent’s vast inland and gradually compresses into ice. It slowly moves towards the coast over thousands of kilometres until it reaches the ocean and floats out onto the water. The place where it begins to float is called the grounding line. The ice pushed across this grounding line contributes directly to increased sea level, in accordance with Archimedes’ principle (Eureka!).

To estimate the amount of ice discharge, it is necessary to know ice thickness and flow speed at the grounding line. Flow speed is monitored routinely by satellites, with great precision. However, satellites cannot measure ice thickness directly. To calculate ice thickness, bed topography needs to be measured separately using airborne or ground-based ice-penetrating radar – a significant challenge given the continent’s remoteness and inaccessibility. This is why bed topography at the grounding line is incompletely measured despite being critically important to Antarctica’s contribution to global sea level rise.

Important

Estimates of future Antarctic ice discharge use computer models to simulate how ice flows over bedrock. Detailed features of bed topography further inland don’t really matter in calculations of flow speed at the grounding line, but bed topography within tens of kilometres upstream of the grounding line has a profound effect on ice motion. Therefore, inland bed topography in the vicinity of the grounding line also needs to be known.

The floating fringes of the Antarctic Ice Sheet, known as ice shelves, make no difference to sea level, but their behaviour is nonetheless highly important for future ice discharge from the ice sheet to the ocean. Ice shelves act like the cork of a wine bottle, keeping the upstream grounded ice in place and reducing ice discharge to the ocean. Ice shelves can shrink from iceberg calving or melting from below caused by warmer seawater underneath.

When more ice melts from the shelf than is added by local snowfall and ice discharge across the grounding line, the ice shelf thins and becomes less resistive, potentially causing increased ice discharge through the grounding line. This is proven by theory and observations. Basal melting of ice shelves is largely driven by ocean circulation under the ice shelves, which depends on the seabed topography under the ice shelves and over the continental shelf. The bed topography seaward of the grounding line is thus also important.

Limited

Following this logic, detailed knowledge of bed topography at the grounding line and around it on the landward and seaward sides is critically needed to monitor current and future Antarctic contribution to the sea level rise. To this end, we recently analysed the availability of radar data near the grounding line, focusing exclusively on data collected after 2007, when GPS technology allowed more accurate positioning. Our analysis of available bed topography data showed that their coverage is limited all around the Antarctic Ice Sheet.

The distribution of the data gaps tells us a simple story. As we move away from research stations, the data gaps become more significant. This means that international coordination is truly needed. Moreover, coordinating survey efforts yields much better final results than compilation of results collected separately. So, together with international colleagues we proposed a new action group, RINGS, to the Scientific Committee on Antarctic Research.

Rings

The name RINGS refers to the group’s plan to survey the Antarctic coast in three rings, at the grounding line, and on its inland and seaward sides. The action group has started to plan coordinated pan-Antarctic airborne surveys to map bed topography, and to address other key scientific questions in the coastal regions, where small climate changes may lead to large, rapid, and possibly irreversible ice sheet changes with multiple tipping mechanisms.
The RINGS initiative has high relevance for society. IPCC’s recent climate assessment report identifies the Antarctic Ice Sheet as the largest uncertainty in terms of future sea level rise, and we directly address this issue by collecting critical data needed to better predict how the Antarctic Ice Sheet will change and affect the global sea level in the coming centuries.