Many studies on the impact of future sea level rise have neglected to acknowledge that current sea levels are higher than previously estimated, resulting from a significant “methodological blind spot.” This oversight indicates that flooding and erosion may commence sooner than anticipated.

Katarina Seeger and Philip Minderhoud, researchers at Wageningen University in the Netherlands, evaluated 385 peer-reviewed studies addressing coastal vulnerability. They found that 90% of these studies failed to consider critical factors—such as ocean currents, tides, temperature, salinity, and wind—when assessing sea level variations. This oversight led to an average underestimation of coastal water levels by 24 to 27 centimeters.

Addressing this gap could potentially increase projections of individuals likely to experience flooding, estimating an increase of up to 68%, impacting approximately 132 million people by 2100. Areas significantly affected include Southeast Asia and Oceania, which often feature sea levels averaging a meter higher than previously calculated, with some regions seeing several meters’ difference.

“If representatives from these vulnerable regions attend global discussions to seek assistance, it may be frustrating, as their risks are grossly underestimated. This scientific miscalculation could affect outcomes for future generations,” Minderhoud stated during a briefing.

While predictions suggest that sea levels may rise by as much as 1 meter by the century’s end, many studies begin with baselines that are inaccurately low. Thus, the adverse effects will likely manifest sooner than expected.

Of the studies evaluated, 46 were referenced in the latest report from the Intergovernmental Panel on Climate Change (IPCC), the premier source on global warming impacts, including rising sea levels.

The Earth’s rotation causes it to bulge at the equator, while denser mantle sections exert a greater gravitational pull on overlying water. To accurately determine the elevation of a specific area, measurements must be compared to the geoid, which depicts mean sea level worldwide.

However, in some regions, actual sea levels can be several meters above the geoid due to wind and ocean currents accumulating water or thermal expansion caused by rising temperatures. Additionally, coastlines may shift due to sediment deposition in rivers or groundwater extraction beneath coastal areas.

Instead of comparing satellite observations to the geoid for insight into coastal water levels and land elevation, many researchers relied on unadjusted geoid sea levels. Even those who attempted calculations often encountered errors caused by differing geoid models for land and ocean elevations. Alarmingly, less than 1% of surveys accurately determined the current sea level at the coastline in question.

“The Coastal Research Community may not have full access to these critical sea level datasets as we are primarily focused on the coastal land aspect,” Seeger remarked during the briefing.

Climate scientists and oceanographers must collaborate more closely with geographers and environmental scientists who assess coastal impacts, emphasized Matt Palmer from the UK’s Met Office.

“It could be said that the crucial final details got lost in translation,” he noted. “Ensuring that the last mile of information is handled adeptly is vital; otherwise, the integrity of the entire effort is compromised.”

The implications of this issue extend to matters of climate justice, said Palmer. The underestimation of sea levels is particularly critical in low-income nations, including various deltas in Africa and Asia. Limited data on gravity fluctuations and lower geoid accuracy contribute to this challenge in regions that are most susceptible to rising sea levels.

The scientific community advocates for enhanced data collection in low-income regions, particularly through the installation of tide gauges for accurate sea level measurements, according to Joan Williams from the UK National Marine Centre.

“Coastal sea levels are influenced by various local factors, necessitating long-term, well-calibrated regional measurements as the gold standard,” she stated.