Underestimated Rise: New Analysis Reveals Sea Level Projections May Be Significantly Low

Global sea level rise, already a pressing concern for coastal communities worldwide, may be happening at a faster rate than previously understood. A new analysis, published today in Nature, reveals that hundreds of studies assessing the risks of rising seas and coastal flooding have likely underestimated actual sea levels by an average of 20 to 30 centimeters (roughly 8 to 12 inches). The findings suggest the potential impact on coastal populations is even greater than current models predict, with as many as 132 million people potentially facing displacement with a one-meter rise – a figure up to 68 percent higher than previous estimates.

The discrepancy stems from a common methodological flaw: the reliance on geoids – complex digital representations of Earth’s gravity – instead of direct sea level measurements when assessing coastal vulnerability. While geoids are useful tools, they can be inaccurate, particularly in regions with limited gravitational data, and fail to account for dynamic ocean conditions like currents and tides.

What are Geoids and Why Do They Matter?

Imagine the Earth not as a perfect sphere, but as an uneven, undulating surface. A geoid is essentially a map of this surface, representing the mean sea level if the oceans were at rest and extended under the continents. It’s based on measurements of Earth’s gravitational pull. However, gravity varies across the globe due to differences in density and mass distribution. This means geoids can be off by several meters in certain areas. Crucially, they don’t reflect the actual, fluctuating height of the ocean, which is influenced by factors like ocean currents, wind, and temperature.

Researchers Katharina Seeger and Philip Minderhoud of Wageningen University in the Netherlands discovered that approximately 90 percent of the 385 studies they evaluated – including 45 referenced by the United Nations’ Intergovernmental Panel on Climate Change (IPCC) – relied on geoids without sufficient correction using direct sea level measurements. Another 9 percent improperly aligned sea level and land elevation data. Less than 1 percent of the studies properly integrated both types of data. Read more about the study in Nature.

Impact on Coastal Populations

The implications of this underestimation are significant. A one-meter (approximately 3.3 feet) rise in sea level, a scenario increasingly likely this century, could submerge areas currently inhabited by tens of millions of people. The new analysis suggests that figure could be as high as 132 million, a substantial increase from previous projections. Low-lying island nations and densely populated coastal regions in Asia are particularly vulnerable.

“Sea level rise is slow but dangerous if you ignore it,” says climate scientist Anders Levermann of the Potsdam Institute for Climate Research in Germany, as reported by UN News. “That’s basically what we’ve done unknowingly. These estimates now tell us that we are much further in the future than we thought we were.”

How the Discrepancy Was Identified

Seeger and Minderhoud’s analysis involved evaluating hundreds of studies on sea level rise and related coastal hazards, including storm surges and tsunamis. They compared the methods used for determining land elevation and sea level, identifying the widespread reliance on geoids without adequate correction. To quantify the underestimation, they utilized a global dataset of sea levels based on satellite measurements, allowing them to compare the reported sea levels in the studies with actual measurements. The average underestimation was found to be 24 to 27 centimeters (approximately 10 inches), though the discrepancy varied significantly by region.

Regional Variations in Underestimation

The study revealed that the underestimation of sea levels was particularly pronounced in Southeast Asia and the Indo-Pacific region, where sea levels were found to be more than one meter higher than previously estimated in some areas. Conversely, some regions, such as the northern Mediterranean, Antarctica, and certain islands in the Atlantic and Pacific, experienced slight overestimations. The smallest discrepancies were observed in eastern North America and northern and western Europe.

What Does This Mean for Coastal Planning?

The findings underscore the need for caution when interpreting and applying the results of studies that rely on geoid-based sea level estimates. Coastal planners and policymakers should prioritize the apply of direct sea level measurements whenever possible and consider the potential for underestimation when making decisions about coastal protection and adaptation strategies. Patrick Barnard, a coastal geologist at the University of California, Santa Cruz, emphasizes the importance of verifying findings from large-scale studies with local data before incorporating them into adaptation plans.

Moving Forward: Improved Data and Collaboration

To facilitate more accurate assessments of sea level rise and coastal hazards, Seeger and Minderhoud have made their coastal sea level data publicly available. This dataset integrates the most recent measurements and provides a valuable resource for researchers and planners. The researchers hope their work will encourage a more consistent and accurate approach to sea level assessments within the scientific community. The IUCN (International Union for Conservation of Nature) has also been actively addressing the threat of sea level rise, developing policy briefs to guide international efforts.

The United Nations is also actively addressing the issue, with a high-level meeting held in September 2024 to discuss strategies for tackling the existential threats posed by rising seas. The UN Framework Convention on Climate Change (UNFCCC) continues to facilitate the Paris Agreement, aiming to limit global warming and mitigate the extent of future sea level rise. Learn more about the UN’s efforts on their website.

The scientific community is now focused on refining sea level projections and improving the integration of direct measurements with geoid data. Ongoing research and monitoring efforts will be crucial for providing more accurate and reliable information to inform coastal planning and adaptation strategies.