In conversation with Fangyi Tan, PhD student, Sea Level Research team at the Earth Observatory of Singapore

1. Will melting ice sheets in such quantities pose a threat to Southeast Asia in the future? 

A recent study found that the Earth has lost a staggering 28 trillion tonnes of ice between 1994 and 2017. The scientists commented in a related news article that the melting of glaciers and ice sheets could cause sea levels to rise by as much as a metre by the end of this century.

One metre may not sound like a lot, but there is a positive feedback when we melt ice. Ice is very reflective; when we melt ice and replace the reflective surface with a darker surface that absorbs more heat, this leads to greater warming that further enhances melting.

The current level of ice loss revealed in this report on Greenland by Professor Andy Shepherd, director of Leeds University’s centre for Polar Observation and Modelling, showed that this level of ice loss matches the worst-case-scenario predictions outlined by the Intergovernmental Panel on Climate Change.

Here in Southeast Asia, we are far from the melting ice bodies which may make us wonder – how can the melting of ice sheets affect us?

Sea-level rise from the melting of glaciers and ice sheets will not be distributed evenly across the globe. This may seem counter-intuitive, but it is the places that are furthest away from the ice sheets that will be most greatly impacted by rising sea levels.

When ice on land melts, places where the melting is happening actually experience sea-level fall. This is because the land is relieved of the weight of the former ice mass and starts to rebound – much like when we lift our hand off a memory foam pillow, the parts that were indented start to rise and the pillow regains its original shape.

As the land goes up, sea level relative to the coastline falls. In contrast, sea levels will rise by a disproportionately large amount in places distal from the ice because water that melts from the ice at higher latitudes flows away from them towards the equator. 

Therefore, even though we are physically removed from the source of the melting, we will experience the impact of rising sea levels which could reach as much as 30% above the global average in this region.  

In addition, there is a lot of uncertainty surrounding the future behaviour of these ice sheets. Greenland and Antarctica, if they were to melt completely, would contribute potentially 6 to 8 metres and ~58 metres of global mean sea-level rise respectively. 

When and by how much these ice sheets will melt, and how quickly they will melt, remains uncertain because we still do not fully understand how ice sheets interact with the climate. 

The ice sheets could reach a tipping point that could potentially trigger dynamic and irreversible ice mass loss.  This would affect millions of people in Southeast Asia living in low-lying regions, giving significant incentive for this region to be concerned about the rate at which glaciers are melting at the poles. 

2. Why is it so important to identify where sea-level rise has impacted the region in the past?

To better understand how the ice sheets will respond to a warming climate in the coming decades, we need to understand the processes that govern the waxing and waning of ice sheets and whether they differed when the climate was warmer than it is today.

To do this, we must look back at how ice sheets have responded to different climatic conditions in the past. Since water that melts from the poles flows towards us near the equator, being able to reconstruct how sea levels have changed in this region will help us better understand past and, therefore, future ice-sheet behaviours. 

Sea-level rise associated with the melting of ice sheets involves a complex interplay of different processes apart from the increase in ocean mass and volume, which has to do with the deformation of the land surface and changes in the gravitational field and rotation of the Earth.  Identifying where and by how much sea levels have changed in the past in different regions will enable us to constrain these processes and, consequently, better predict and prepare for sea-level rise in the foreseeable future.

3. How do you reconstruct sea levels in the past and where do you find historic sea-level records in nature? 

The way we reconstruct past sea levels is to rely on the relationship that geological proxies have with the sea. For instance, we know that mangroves are coastal plants that need sea water to survive and so live within a certain elevation range relative to the tides. If we find evidence in sediments (e.g. decomposed mangrove peat or macro-fossils like pieces of leaves and barks) that mangroves could once live at a higher elevation, but today they can only live at a lower elevation, then we know that the sea must have been higher in the past and that sea levels have since fallen.

Our sea-level group at the Earth Observatory of Singapore (EOS) is currently working on locating mangrove sediments at Pulau Ubin. By taking cores of sediments from the ground, we are able to analyse changes in the paleo-environment of Pulau Ubin over the past hundreds to thousands of years – when the sea transgressed, for instance, allowing mangroves to inhabit what was previously an upland forest.

We have also been studying a special group of corals in Sentosa, known as coral microatolls. These are coral colonies that have concentric rings with relatively flat tops and look like domes or mini islands (I like to think that is why they are called microatolls). The reason for their flat upper surfaces is that their upward growth is restricted by the lowest of tides. In this way, microatolls, like mangroves, also have established relationships with the tides that allow us to reconstruct past sea levels. 

Since corals develop annual growth bands and grow at the interface between the land and the sea, microatolls act like tide gauges, recording variations in the sea-surface height relative to the land. The ability to analyse the coral growth bands enables us to develop high-resolution (annual) chronologies of sea-level change through time – much like rings in a tree, and allows us to extend the modern tide gauge record back in time, before tide gauge observations were made available. 

4.  How will this work help communities and governments in Southeast Asia in the future?

It has never been a more important time to be an earth scientist in Southeast Asia, to be pursuing the knowledge that will guide communities in preparing for sea-level rise in the coming decades. Our work here at EOS will contribute to a better understanding of the different processes that drive sea-level change – both in the long-term and short-term, and I hope this will one day benefit the lives of the people living in this region.

(Source of thumbnail image: Roxanne Desgagnés/Unsplash)