This article provides a brief overview of the motivation and results of our study on MPWP sea level that’s just been published in Science Advances.
As the concentration of greenhouse gasses in the atmosphere continues to increase, melting of polar ice caps is accelerating. All of that water ends up in the oceans, driving sea-level rise at various rates around the planet. One of the biggest uncertainties when predicting future sea level is understanding how the major ice sheets will likely respond to continued global warming. Many scientists from a whole range of different backgrounds spend much of their time studying these ice-sheet sensitivities.
To understand the potential sea-level implications of melting different ice sheets, we use a calculation called meters of sea-level equivalent (m of SLE), which in simple terms roughly corresponds to the average sea-level rise expected if all of that ice sheet was to melt away (see AntarcticGlaciers.org for a helpful introduction). It is estimated that all of the remaining glaciers in mountainous areas, such as the Himalaya, Andes, Rockies and Alps, contain approximately 0.3 m of SLE (Farinotti et al. 2019). Greenland contains ~7.4 m SLE (Morlighem et al. 2017), whilst Antarctica contains a whopping ~58 m of SLE (Fretwell et al. 2013). From these numbers, it is clear that any major change in the Antarctic Ice Sheet would have catastrophic consequences for the more than 230 million people around the world who currently live less than 1 meter above high tide (Kulp & Strauss, 2019).
So how do we go about predicting how much future melting might occur in Antarctica? One common approach is to turn to the geological record. By targeting previous periods in Earth’s history when carbon dioxide levels were as high as today and temperatures were as hot as we are expecting later this century, we can try to figure out how high sea level used to be and, therefore, which ice sheets might have melted. As the handy graphic below from Dutton et al. (2015) shows, the most recent such period occurred approximately 3.3–2.7 million years ago and is called the Mid-Pliocene Warm Period. It is sea-level markers from this time period that we have investigated in our study.
Australia has a number of useful geological markers of former sea level dating from the Pliocene period, including several features in Western Australia at Cape Range, onshore Perth Basin, the Roe Plain and off the northwest coast of Port Hedland. The problem is that, when we compare their elevations with respect to present-day sea level, they are not in agreement at all! They range all the way from -100 m (i.e. below present sea level) to +50 m above it. The reason for this variation is that, in addition to sea levels rising and falling as ice sheets shrink and grow, the land surface itself also moves up and down in a process called dynamic topography. If we want to see back to the original Pliocene sea level, we need to carefully account for and remove this additional process.
The team of researchers involved in this study work at institutions across the UK, US and Australia and all of them have spent considerable time studying dynamic topography. What we’ve done is to use new models of dynamic topography to reconstruct vertical motions of Australia over the intervening period since these sea-level markers were formed. Remarkably, when we subtract out this process, the different sites suddenly fall into agreement with one another. Our revised estimate of sea level in the Mid Pliocene Warm Period comes out at +16 m (with uncertainty range of +10.4 m to +21.5 m), providing an estimated contribution from extra melting of the Antarctic Ice Sheet of ~10 m SLE in comparison to its present-day extent.
While these revised numbers still have uncertainties, they substantially improve previous uncertainty ranges and we can use them to update projections for future ice melting in Antarctica. Using the forecasts of DeConto et al. (2021), we now find that Antarctica is likely to contribute 7 cm (6–9 cm) to global mean sea-level rise by 2100, and approximately +2.5 m by 2300. The good news is that these revised numbers are smaller than the initial projections. Nevertheless, they still represent the beginnings of destabilisation of an ice sheet that, if left unchecked, has the potential to wreak havoc on coastal communities in our ever-warming world.
