Rising Sea Levels: Decades of Underestimation?

The Hidden Error in Sea Level Measurements

More than one in ten people globally reside within three miles of the coast, and roughly 40 percent live within an hour’s drive of the ocean. These coastal regions are economic powerhouses – in the United States alone, they generate one-third of the nation’s GDP. But this proximity to the sea comes with inherent risks, amplified by the increasing threat of tidal floods and storm surges. Water remains the most destructive element of many natural disasters, and with growing coastal populations, the economic toll is only expected to rise.

Global average sea levels have already risen approximately 9 inches since 1880, a tangible consequence of a warming planet as ice sheets melt and ocean water expands. This rise isn’t uniform; sea levels are accelerating in some areas due to land subsidence. Now, research published in the journal Nature reveals that conventional sea level estimates are, on average, a foot too low – and in some regions, the error exceeds three feet.

This isn’t merely an academic concern. Sea level data forms the foundation for hazard maps, guiding decisions about where people can live, where protective infrastructure should be built, and how insurance companies assess risk. An inaccurate baseline means existing infrastructure may be more vulnerable than previously thought. Coupled with the inevitable sea level rise projected by climate models – an additional 9 inches to over 3 feet by 2100, depending on emission reductions – the situation demands urgent attention.

The Complexity of Measuring the Ocean

Measuring global sea levels is deceptively complex. The ocean is in constant motion, influenced by waves, tides, and varying water densities due to temperature and salinity. Land itself isn’t static; it can sink over time. Even a precise measurement in one location doesn’t necessarily translate to another, due to the Earth’s shape and gravitational variations.

Scientists typically rely on a model of the Earth called a “geoid,” which simulates how oceans would settle under the influence of gravity and rotation, excluding factors like currents and winds. This geoid serves as a reference point for elevation. However, as explained by Katharina Seeger, a co-author of the new study from the University of Padua in Italy, the geoid model doesn’t incorporate actual sea level measurements. This simplification introduces significant errors, particularly at a regional level.

Researchers Philip Minderhoud of Wageningen University & Research and others observed these discrepancies firsthand while studying land subsidence in the Mekong River Delta in Vietnam, an area prone to routine flooding. Minderhoud noted, “Being in the delta itself, I witnessed there that the water levels were much higher than those maps implied.” These observations sparked a multi-year investigation into the extent of the problem.

Seeger and Minderhoud analyzed 385 peer-reviewed articles published between 2009 and 2025, revealing that over 90 percent of coastal hazard assessments underestimated sea levels due to the failure to correct geoid calculations with local sea level measurements. The largest errors were found in less-studied regions like Southeast Asia and the Indo-Pacific, areas with densely populated coastlines and disproportionately high vulnerability.

“The findings are pretty stunning,” says Torbjörn E. Törnqvist, a professor at Tulane University who was not involved in the study. “We’ve dropped the ball a little bit. It’s such a basic thing that a lot of us haven’t really paid much attention to.”

Correcting the Course: A Path Forward

While scientists have long recognized the limitations of geoid models, the connection between these inaccuracies and real-world coastal risks hadn’t been fully appreciated. The issue stems, in part, from a lack of communication between different scientific disciplines. As Törnqvist explains, “A lot of the issues that this paper points to is in the translation between these two communities.”

A relatively simple solution exists: update existing hazard calculations with accurate, locally measured sea levels. Minderhoud suggests, “It could be almost as simple as that. It’s not that these studies are methodologically wrong, but they have relied on an assumption that wasn’t acceptable.” Further investment in local measurements is crucial to establish accurate baseline sea levels and improve adaptation planning.

Fortunately, advancements in disaster forecasting and early warning systems have helped reduce the number of deaths from natural disasters, even as their costs increase. The World Meteorological Organization reports a decline in disaster-related deaths from 170 per day in the 1970s to 40 per day in the 2010s.

However, protecting infrastructure remains a significant challenge. Even selecting a sea level rise scenario for planning purposes can be politically charged. Ultimately, the future of coastal communities will depend on a combination of scientific accuracy and proactive human adaptation. Robert Kopp, a professor at Rutgers University, emphasizes that future coastal risk is determined as much by human actions as by sea level rise itself.

What steps will communities take to mitigate these risks? Will they retreat from the coastline, invest in seawalls and floodgates, or pursue other innovative solutions? The answers to these questions will shape the fate of millions.

Frequently Asked Questions About Sea Level Rise

• What is the primary finding of the new Nature study regarding sea levels?

  The study reveals that current sea level estimates are, on average, one foot too low, with some regions experiencing errors exceeding three feet. This underestimation has significant implications for coastal hazard assessments and infrastructure planning.

• Why have sea levels been underestimated for so long?

  Scientists have traditionally relied on a “geoid” model to estimate sea levels, which simplifies complex ocean dynamics. This model doesn’t incorporate actual sea level measurements, leading to inaccuracies, particularly at a regional level.

• Which regions are most affected by these inaccurate sea level estimates?

  Southeast Asia and the Indo-Pacific region, including densely populated island nations like Indonesia and the Philippines, are disproportionately affected due to a lack of local sea level measurements.

• How does climate change contribute to rising sea levels?

  Climate change causes sea levels to rise through two primary mechanisms: the melting of ice sheets and glaciers, and the thermal expansion of water as it warms.

• What can be done to improve the accuracy of sea level predictions?

  Investing in more local sea level measurements and incorporating this data into hazard assessments is crucial. Updating existing models with accurate data will provide a more realistic picture of coastal risks.

• Are there any benefits to improved disaster preparedness, even with rising sea levels?

  Yes, despite the increasing costs of disasters, improved forecasting and early warning systems have significantly reduced the number of deaths caused by natural events.