Yves here. It seems that actions to mitigate the risks of population and infrastructure exposure to rising sea levels are not progressing as swiftly as one might hope. Many major urban areas are already at risk, and the article below suggests that the crisis is unfolding more rapidly than previously anticipated.
By Fred Pearce, a freelance author and journalist based in the U.K. He is a contributing writer for Yale Environment 360 and has authored several books. Originally published at Undark
The Rising Tide: An Urgent New Reality
Recent studies reveal that global sea levels are significantly higher than previously thought. Existing flood-risk assessments, based on inadequate global models, are proving to be unreliable. Adding to the urgency, many coastal regions are experiencing subsidence at rates that exceed the rise in sea levels.
Two pivotal studies are reshaping our understanding of the dangers posed by rising tides and sinking land, highlighting the imminent threat of flooding for millions living in some of the largest cities worldwide. According to experts who weren’t part of these studies, the findings emphasize a troubling urgency.
Underestimating the Threat
“The impacts of sea level rise under climate change have been systematically underestimated,” states Matt Palmer, a sea-level specialist at the U.K. Met Office’s Hadley Centre for Climate Science. “We could see devastating impacts much earlier than predicted—especially in the Global South.”
Franck Ghomsi, an oceanographer from the University of Cape Town, echoes these concerns, saying, “Taken together, these studies present a far more alarming picture than either would if considered alone. We are witnessing an evolving body of research that rewrites our understanding of coastal vulnerability.”
Today, approximately 80 million people are residing in coastal areas that are below sea level—almost double previous estimates.
New Discoveries on Sea Levels
The variation in global sea levels is well-known, but a groundbreaking analysis from the Netherlands has found that much of the scientific literature has significantly underestimated actual sea levels measured by tidal gauges. Geographers Katharina Seeger and Philip Minderhoud from Wageningen University & Research assert that sea levels are, on average, nearly one foot higher than traditional estimates, which rely on calm seas and disregard factors such as ocean currents and wind effects.
In their examination of 385 locations, they found that accepted sea levels were off by over three feet in many instances—typically too low. This means around 80 million individuals now live in coastal areas below sea level, amplifying the risk as sea levels are projected to rise in the coming decades.
Forecasts indicating when these areas may flood could be decades too late, thereby necessitating more urgent planning from policymakers and funding organizations, such as the World Bank, who depend on scientific assessments of flooding risk.
The Issue of Sinking Deltas
The second study concentrated on the world’s river deltas. Although it’s long been known that many deltas are sinking due to groundwater extraction, inconsistency and crudeness in prior data have hindered accurate assessments. According to Robert Nicholls, a climate adaptation researcher at the University of East Anglia, “Now we finally have a consistent dataset with high spatial resolution.”
Leonard Ohenhen, an earth system scientist at the University of California, Irvine, used satellite-mounted radar to generate 3D maps of subsidence across 40 of the globe’s largest river deltas. His findings show that more than half of these deltas are sinking, with some experiencing subsidence rates that double or even multiply the local effective rise in sea levels.
This revelation places millions of people, previously thought to be safe from flooding, in immediate danger, particularly those residing in places like the Nile Delta in Egypt, the Mekong Delta in Vietnam, and the Yellow River in China. If current subsidence rates persist, these regions are likely to be flooded much sooner than originally predicted.
The Need for Revised Assessments
Researchers argue that many other deltas and low-lying coastal regions not included in these studies are at even higher risk than acknowledged and urgently require in-depth analysis of actual sea levels and land subsidence rates.
Data from tidal gauges indicate that real sea levels are, on average, 9.4 to 10.6 inches higher than model predictions.
Confronting the Gaps in Research
The studies underscore notable flaws in previous climate impact assessments. Seeger points out that most peer-reviewed studies and reports by the Intergovernmental Panel on Climate Change rely on flawed methodologies.
A major issue is that over 90% of local studies on current sea levels and future projections have simply replicated results from mathematical models of the “geoid,” which determines Earth’s shape based on its rotation and gravitational fields. While these models yield approximate sea levels, they overlook crucial factors, leading to significant local errors, according to Seeger.
In particular, she emphasizes that this modeling approach neglects ocean dynamics—including currents, thermal expansion, and wind effects—that impact local sea levels. Her meticulously compiled real-world data from tidal gauges indicate that global sea levels average 9.4 to 10.6 inches higher than geoid model predictions.
This discrepancy alone surpasses the total global sea level rise since the early 20th century, as pointed out by Jonathan Bamber, a glaciologist at the University of Bristol.
Regional Differences and Implications
The unexpected rise in sea levels is most pronounced in the Global South, where ocean dynamics are often stronger. For instance, tide levels along Southeast Asia’s coasts can exceed model predictions by over three feet. Only a few areas, such as parts of Antarctica and the northern Mediterranean, report lower sea levels.
“Our corrected calculations indicate that up to 37% more area and up to 68% more people will fall below sea level after a predicted 3.3-foot rise,” states Seeger. This amounts to an additional area vulnerable to flooding approximately the size of the United Kingdom, home to 132 million people—comparable to Mexico’s population.
Seeger believes the “methodological blind spots” identified in their research were overlooked, partly because geoid estimates tend to be most accurate in Europe and North America’s eastern seaboard, where the majority of researchers are situated.
Although current higher sea levels do not directly alter global future projections, they do raise the baseline from which future rises will occur, suggesting that some areas may face more severe increases than others.
Significantly, some cities have seen dramatic subsidence: certain parts of Shanghai have sunk more than 6 feet, Bangkok by over 5 feet, and cities like Osaka and Tianjin by around 10 feet.
Minderhoud notes that reliance on geoid data contributes to complacency by suggesting that heavily populated coastal regions have more time before crossing critical flooding thresholds than now seems likely.
Specific Regional Case Studies
An example is the Mekong Delta in Vietnam, where Minderhoud had begun scrutinizing the reliability of existing sea level data a decade ago. Published reports indicated that inundation would begin at a sea level rise of 5 to 6.6 feet. However, he observed that surface water levels were already significantly higher in many areas. Urgent protective measures were essential to prevent imminent flooding.
Multiple deltas are encountering similar harsh realities; many are sinking as they face a rising ocean. Some naturally sink if rivers do not deliver enough sediment to replenish what is lost to erosion. However, natural subsidence rarely exceeds 0.1 inches per year; current rates of subsidence are primarily human-induced, according to Nicholls.
In Jakarta, Indonesia’s capital, much of the city has experienced a decline of up to 13 feet since 1970, continuing to sink ten times faster than the adjacent bay’s rising sea levels. This looming threat of widespread flooding was a key motivator behind the Indonesian government’s decision to relocate its capital to Borneo in 2019.
Causes and Consequences of Subsidence
Ohenhen’s assessment shows that the subsidence of deltas is mainly a result of groundwater extraction aimed at supplying residential needs and industrial use. The depletion of subsurface water leads to loss of volume, resulting in widespread sinking at the surface.
Jakarta serves as a prominent example among many coastal cities facing threats from groundwater extraction on the island of Java, home to the world’s largest population. Semarang, a bustling coastal city of 2 million residents, pumps so much water that subsidence levels reach 20 to 50 times the rate of sea level rise, as indicated by Ohenhen’s analysis. Recent floods have devastated neighborhoods, forcing residents to build their homes on increasingly higher stilts to avoid inundation, yet entire communities are disappearing beneath the waves.
Another significant factor causing subsidence is the construction of dams and levees on rivers. These structures obstruct sediment supplies critical for maintaining delta integrity. Following the establishment of over 20 large dams on its primary branch, China’s Yellow River no longer carries the sediment it once did. Ohenhen found that due to this reduced sediment flow, the delta is sinking up to ten times faster than the projected rise of the Yellow Sea into which it discharges.
Conclusion: A Call for Urgency and Action
In some areas, governments are initiating protective measures against rising sea levels without fully understanding how high current sea levels are already. Examples like the Po Delta in Italy, which is sinking at rates of 2 to 4 inches per year, reflect this oversight. Simultaneously, the Mississippi Delta in North America has lost 1,900 square miles in the past century alone, constantly sinking by an average of 2 inches per year due to levees restricting the river’s natural sediment flow.
In certain regions, different factors contribute to subsidence. The Nile Delta, which encompasses two-thirds of Egypt’s agricultural land, faces sediment deprivation caused by the High Aswan Dam, built in the 1960s, while groundwater extraction is exacerbating subsidence. New analyses reveal much higher subsidence rates than previously reported, placing millions in danger.
These new findings signal urgent challenges for governments and humanitarian agencies. There’s an immediate need for thorough assessments of both current sea levels and land subsidence rates in order to better protect vulnerable populations. While rising sea levels can only be addressed through comprehensive global climate initiatives, local actions can halt subsidence quickly—such as ceasing groundwater extraction. The experience of Tokyo, which stabilized land levels after banning groundwater pumping, illustrates the potential for immediate impact. Similarly, reconstructing sediment supplies through changes in dam operations can mitigate the effects of rising waters. However, the looming threats, especially in Southeast Asia and Africa, remain daunting. Ghomsi summarizes the perilous situation: “The sea level baseline is higher than assumed; land is sinking in many crucial regions; the sea is rising faster; and extreme events add up to consequences that exceed mere sums.”
