Sea level rise risk may be higher as studies underestimate baseline coastal water levels

A new study in Nature reports that many sea level rise impact assessments underestimate baseline coastal water heights, often by about 30 cm. The authors cite mismatched sea and land altitude measurements, especially affecting hazard estimates in the Global South, the Pacific, and Southeast Asia, and potentially raising projected coastal flooding risk.

A new study warned that millions more people may face coastal flooding as seas rise. The research said earlier studies often started from the wrong coastal water level. That mistake could underplay future land loss and population risk. The study appeared on Wednesday in the journal Nature.

Researchers reviewed hundreds of scientific papers and hazard assessments. The team found about 90% set the coastal baseline too low. On average, the baseline was underestimated by about 30 cm. The study said the error appeared more in the Global South, the Pacific, and Southeast Asia.

Sea level rise study finds baseline errors in coastal planning

The authors linked the problem to how land and sea heights get measured. Philip Minderhoud, from Wageningen University & Research, said methods do not align well. Satellite tools and land models may each work in isolation. Yet extra factors near the shoreline often get missed where water meets land.

Katharina Seeger of the University of Padua said many impact studies skip real sea level readings. As a result, researchers treat "zero-metre\" in land data as sea level. Minderhoud said some Indo-Pacific sites sit far above that assumed mark. In a few places, the difference is close to 1 metre.

Minderhoud and Seeger said coastal waters rarely match a calm, flat assumption. They cited wind, tides, waves, and currents as constant influences. They also pointed to temperature shifts and events like El Niño. These processes can raise water at the shore above what simple datasets suggest.

Sea level rise projections show higher land and population exposure

Using a higher and more realistic baseline changed future estimates in the study. If seas rise a little more than 1 metre by century’s end, impacts grew. The paper said up to 37% more land could flood. It added that 77 million to 132 million more people could face threats.

The authors said such shifts could strain budgets and planning for climate impacts. Anders Levermann, from the Potsdam Institute for Climate Impacts Research, said risk may be larger than expected. \"You have a lot of people here for whom the risk of extreme flooding is much higher than people thought,\" Levermann said.

Levermann said Southeast Asia showed the biggest gap in baseline estimates. The region also has many people already exposed to sea level rise, according to the study. Minderhoud also highlighted island nations across the Indo-Pacific. The study said the baseline mismatch is especially common in the Pacific.

Sea level rise in Vanuatu highlights daily impacts on coastal communities

For 17-year-old climate activist Vepaiamele Trief, the findings matched lived experience. Trief said the shoreline on Vanuatu has pulled back in a short time. Beaches have eroded and coastal trees have been uprooted. Some homes now sit barely 1 metre from the sea at high tide.

On Trief’s grandmother’s island of Ambae, a key road has shifted inland. The route from the airport to the village was changed due to advancing water. Graves have been submerged, Trief said. \"These studies, they arent just words on a paper. They arent just numbers. Theyre peoples actual livelihoods,\" Trief said.

Trief urged readers to picture what coastal residents may lose. \"Put yourself in the shoes of our coastal communities - their lives are going to be completely overturned because of sea level rise and climate change,\" Trief said. Minderhoud said the study aimed to reflect what communities see at ground level.

Sea level rise experts debate how much the baseline issue changes results

Seeger and Minderhoud said the issue lies at the shore boundary. They said ocean-wide or land-only measures can look correct alone. Yet the overlap point can carry hidden offsets. Ben Strauss, CEO of Climate Central, said coastal height needs both land and water elevations for comparison.

\"To understand how much higher a piece of land is than the water, you need to know the land elevation and the water elevation. And what this paper says the vast majority of studies have done is to just assume that zero in your land elevation dataset is the level of the water. When in fact, its not,\" Strauss said.

Strauss said the main issue was the starting reference level. \"Its just the baseline that you start from that people are getting wrong,\" Strauss said. The new paper said Strauss’s 2019 work was among the few that handled the baseline correctly.

Other scientists said the authors may have overstated what the issue changes. \"I think theyre exaggerating the implications for impact studies a bit - the problem is actually well understood, albeit addressed in a way that could probably be improved,\" said Goneri Le Cozannet of the French geological survey.

Rutgers University expert Robert Kopp said many local officials already know their shore risks. Planners often build around local measurements, Kopp said. Minderhoud agreed this can apply in some areas. Minderhoud said Vietnam, in the high-impact zone, has a solid understanding of elevation.

Sea level rise concerns grow as UNESCO flags ocean carbon sink gaps

The study arrived alongside a UNESCO report on ocean carbon uptake. UNESCO warned of gaps in knowledge about how much carbon the ocean absorbs. The report said models vary by 10% to 20% on the size of this carbon sink. That uncertainty may affect climate projections based on those models.

Together, the findings raised questions about how complete risk planning may be. The reports suggested governments could be working with partial information on ocean change. Thompson Natuoivi, a Save the Children Vanuatu climate advocate, described the loss tied to rising seas. \"When the ocean comes closer, it takes away more than just the land we used to enjoy,\" Natuoivi said.

Natuoivi said the issue already affects daily life for some families. \"Sea level rise is not just changing our coastline, its changing our lives. We are not talking about the future - were talking about the right now,\" Natuoivi said. The Nature study said better baseline data could help improve coastal risk estimates.

With inputs from PTI