Have you ever wondered how rising sea levels could reshape the world’s coastlines? Or how cities like Singapore, nestled by the ocean, can prepare for this challenge? Sea-level rise is a major threat to low-lying coastal areas and islands. It can disrupt communities, threatens ecosystems, and endangers critical infrastructure. Planning for it requires projections that are not only accurate but also actionable.
Writing in the scientific journal Earth’s Future, Dr. Benjamin Grandey, a Senior Research Fellow, and Professor Chew Lock Yue both at the School of Physical and Mathematical Sciences (SPMS) at NTU Singapore, together with collaborators present a novel approach that combines both ice-sheet modelling as well as expert assessment into a single projection. Their work goes beyond previous methodology by providing a “very likely” range (a 5th–95th percentile projection) of future sea-level rise under both low- and high-emissions scenarios, bridging gaps in earlier assessments.
Filling the Gaps in Sea-Level Projections
Scientists often rely on climate models to predict future sea-level rise. Such models track well-understood physical processes such as glacier melting to make projections. However, other ice sheet processes, that include extreme events such as abrupt ice shelf collapse or sudden shifts in ice sheet behaviour, are poorly understood. This makes it challenging to incorporate these processes in reliable estimates of sea level rise. As a result, projections of sea-level rise can differ from model to model depending on the specific method employed. For example, under a high-emissions scenario, high-end projections of global mean sea-level rise during the 21st century differ substantially. One model-based approach projects up to 0.9 m while another model-based approach projects up to 1.3 m. Furthermore, structured expert judgment projects up to 2.3 m, much higher than the model-based projections.
This ambiguity in projections from distinct methods can make it difficult to make well-informed policy decisions and undermines effective planning on climate change. For instance, the intergovernmental panel on climate change (IPCC) has only been able to provide “likely” range projections of sea level rise. The “likely” range represents scenarios within one standard deviation of the most-likely projection of sea level rise (i.e. “1-sigma” projection or between the 17th and 83rd percentiles of possible scenarios). As a result of ambiguity in projections from distinct methods, the IPCC has been unable to provide a “very likely” range (i.e. between the 5th and 95th percentiles of possible scenarios) — a gold standard in managing risk.
Dr. Benjamin Grandey and his collaborators tackled this head-on by developing a “fusion” method that combines the strengths of different sea-level projections. For example, “medium confidence” projections sample the most-probable scenarios but may often miss out the full range of low-likelihood events that might occur. In contrast, “low-confidence” projections can provide critical information about low-likelihood events.
The team’s key innovation was to use expert assessment in combining different classes of projections. Different “medium confidence” and “low confidence” projections were combined together with expert assessment to incorporate difficult-to-model processes such as sudden ice sheet behaviour shifts.
“Our new approach combines the strengths of different sea-level projections,” explains Dr. Grandey. “This means we can estimate the uncertainty associated with future sea-level rise. To complement the most recent IPCC report, we quantify the very likely range of sea-level rise.”
The “very likely” range of predictions encompasses the 5th and 95th percentiles of possible sea-level rise scenarios, offering projections that help decision making. This is particularly important for planning activities that have very low risk tolerance.
“By combining the best knowledge on sea-level information at different confidence levels, we have discovered a novel way to project the full uncertainty range of future sea levels,” adds Prof. Chew Lock Yue.
What Could Sea Levels Look Like by 2100?
The study reveals that, under a low-emissions scenario, global mean sea level is very likely to rise between 0.3 and 1.0 meters by 2100. Under a high-emissions scenario, the range expands to 0.5 to 1.9 meters. This broader range suggests that previous estimates may have understated the potential for extreme outcomes, including up to 90 cm higher than earlier projections under high emissions.
This refined understanding is crucial for cities like Singapore, where even small changes in sea levels could have significant consequences for infrastructure and communities.
The new projections are designed to support coastal planning and adaptation efforts. With a more comprehensive range of possibilities, decision-makers can design flexible strategies to accommodate uncertainty.
“The high-end projection of 1.9 meters can support decision-making for critical infrastructure,” says Dr. Grandey. “These results also demonstrate the importance of climate mitigation by reducing our carbon dioxide emissions.”
Beyond coastal cities, the research has far-reaching applications, from better climate risk assessments to informing global policy discussions on climate adaptation and mitigation.
Looking ahead
Prof. Benjamin Horton, a co-author of the study and the director of the Earth Observatory of Singapore as well as faculty at the Asian School of the Environment NTU Singapore underscored the study’s significance: “This NTU research is an important breakthrough in sea-level science. It highlights the most severe impacts on coastal communities, infrastructure, and ecosystems, emphasizing the urgency to address the climate crisis.”
As this method gains traction, the researchers envision its application in assessing climate risks globally. They also emphasize the urgent need for the world to act on climate mitigation to reduce the likelihood of catastrophic outcomes, such as rapid ice loss in Antarctica.
“As we enter an era of unprecedented climate change, we need improved climate projections to help communities plan and adapt,” says Dr. Grandey. “As a global community, it is vital that we rapidly reduce our carbon dioxide emissions to reduce the risk of catastrophic possibilities.”
Read the article on Straits Times here.