The sea is higher than we thought and millions more are at risk, study finds

Rising sea levels due to climate change could threaten tens of millions more people than scientists and government planners initially thought because of faulty research assumptions about the height of coastal waters, a new study suggests.

Researchers studied hundreds of scientific studies and risk assessments, calculating that about 90% of them underestimated the base heights of coastal waters by an average of 30 centimeters, according to the study published Wednesday in the journal Nature. This is a much more common problem in the Global South, the Pacific and Southeast Asia, and less so in Europe and along the Atlantic coasts.

The cause is a mismatch between how sea and land altitudes are measured, said Philip Minderhoud, co-author of the study and professor of hydrogeology at Wageningen University and Research in the Netherlands. And he attributed this to a “methodological blind spot” between the different ways these two things are measured.

Each lane measures its own areas correctly, he said. But where sea meets land, many factors are often not taken into account when satellites and land models are used. Studies that calculate the impact of sea level rise “usually don’t look at the actual measured sea level, so they used that figure of zero meters” as a starting point, said lead author Katharina Seeger of the University of Padua in Italy. In some places in the Indo-Pacific it reaches almost a meter, Minderhoud said.

A simple way to understand this is that many studies assume sea levels without waves or currents, when the reality at the water’s edge is oceans constantly churned by wind, tides, currents, temperature changes and phenomena like El Niño, Minderhoud and Seeger said.

Adapting to a more precise baseline of coastal height means that if the sea rises just over 1 meter – as some studies suggest by the end of the century – the waters could inundate up to 37% more land and threaten an additional 77 million to 132 million people, according to the study.

This would lead to problems in planning and financing the impacts of global warming.

People at risk

“There are a lot of people here for whom the risk of extreme flooding is much higher than we thought,” said Anders Levermann, a climatologist at the Potsdam Institute for Climate Impact Research in Germany, who was not involved in the study. And Southeast Asia, where the study finds the largest gap, has the largest number of people already threatened by sea level rise, he said.

Minderhoud highlighted the island nations of this region as an area where the reality of differences is felt.

For Vepaiamele Trief, a 17-year-old climate activist, the projections are not abstract. On its home island in the South Pacific archipelago of Vanuatu, the coastline has visibly receded in its short life, with eroded beaches, uprooted coastal trees and some houses now barely 3 feet (about 1 meter) from the sea at high tide. On Ambae Island, her grandmother’s island, a coastal road linking the airport to her village was diverted inland due to encroaching waters. Graves have been submerged and entire ways of life appear threatened.

“These studies, they’re not just words on a paper. They’re not just numbers. They’re people’s real livelihoods,” she said. “Put yourself in the shoes of our coastal communities: their lives are going to be completely upended by sea level rise and climate change. »

Pay attention to the starting point

This new study focuses on the truth on the ground.

Calculations that may be correct for all seas or for land are not entirely correct at this key intersection point between water and land, Seeger and Minderhoud said. This is particularly true in the Pacific.

“To understand how much higher than water a piece of land is, you need to know the land elevation and the water elevation. And what this article says, the vast majority of studies have just assumed that zero in your land elevation data set is the water level. When in fact, that’s not the case,” said sea level rise expert Ben Strauss, CEO of Climate Central. His 2019 study was one of the few, according to the new journal, that was right.

“It’s just the basis from which people get it wrong,” said Strauss, who was not involved in the research.

Maybe not so bad, some scientists say

Other outside scientists said Minderhoud and Seeger may be exaggerating the problem.

“I think they’re exaggerating the implications for impact studies a bit – the problem is actually well understood, although it’s approached in a way that could probably be improved,” said Gonéri Le Cozannet, a scientist at the French geological survey. Most local planners know their coastal issues and plan accordingly, said Robert Kopp, a sea level expert at Rutgers University.

This is true in Vietnam, in the high-impact zone, Minderhoud said. They have a precise sense of elevation, he says.

The findings come as a new UNESCO report warns of major gaps in understanding how much carbon the oceans are absorbing. That report says models differ by 10 to 20 percent in estimating the size of this carbon sink, raising questions about the accuracy of global climate projections that rely on them.

Together, the studies suggest that governments may be planning for coastal and climate risks with an incomplete picture of the changing ocean.

“When the ocean gets closer, it takes away more than just the land we previously enjoyed,” said Thompson Natuoivi, climate advocate for Save the Children Vanuatu.

“Sea level rise isn’t just changing our coastline, it’s changing our lives. We’re not talking about the future, we’re talking about the now.”

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