Sea levels around the world are much higher than we thought

Almost all research on the impacts of future sea-level rise has assumed today’s sea levels are lower than they actually are due to a “methodological blind spot”. That means flooding and erosion will happen sooner than expected.

Katharina Seeger and Philip Minderhoud, both at Wageningen University in the Netherlands, analysed 385 peer-reviewed studies on coastal vulnerability and found that 90 per cent failed to consider the effects of ocean currents, tides, temperature, salinity and winds on sea level. As a result, they underestimated the water level at the coast by an average of 24 to 27 centimetres.

Once corrected, the number of people whose homes will be underwater by 2100 could increase by up to 68 per cent, or 132 million additional people. The bulk of them are in South-East Asia and Oceania, where sea levels are on average 1 metre higher than previously thought, and several metres in some areas.

“If a representative from a place like that comes to a global gathering and tries to gather support… it can be quite frustrating if there’s this type of scientific assessment that actually says… it will only be affected in the next century, while in reality the area is actually much more exposed,” said Minderhoud at a briefing.

Projections that sea levels will rise by up to 1 metre by the end of the century are still accurate. But since most research on the consequences of this rise started with a baseline that was too low, these impacts will happen sooner than forecast.

Forty-six of the studies were cited in the latest round of reports by the Intergovernmental Panel on Climate Change, the most authoritative statement on warming effects, including sea-level rise.

Earth bulges at the equator due to its rotation, and denser parts of its mantle exert a greater gravitational pull on any water above them. So determining the elevation of a given point requires a computer model of the geoid, an irregular shape that represents mean sea level around the world.

But actual sea level can be several metres higher than the geoid in places where winds or currents pile up water, for example, or where warmer temperatures cause water to expand. And coastlines can move up or down as rivers build up sediment or cities suck groundwater out from underneath them.

Rather than figuring out the elevation of water and land at the coast by comparing satellite measurements with the geoid, the majority of researchers simply used the unadjusted geoid sea level. And even those who did the calculations mostly botched them, often because they determined land and sea elevations with different geoid models. Less than 1 per cent of the studies found the correct current sea level for the coastlines they were studying.

“The coastal research community [is] really focusing on the coastal land, and therefore may not be aware of these sea-level data sets,” said Seeger at the briefing.

Climate scientists and oceanographers who study sea-level rise should work more closely with the geographers, environmental scientists and others who study coastal impacts, says Matt Palmer at the Met Office, the UK’s national weather service.

“You could call it lost in translation,” he says. “It’s important that that last mile is done well, otherwise it kind of undermines the whole enterprise.”

The problem is one of climate justice, says Palmer. Underestimates of sea level have been particularly egregious in parts of lower-income countries, such as river deltas in Africa and Asia, partly because data on variations in gravity is sparser there, making the geoid less accurate. But these are also the areas most vulnerable to sea level rise.

The scientific community should gather more data in lower-income countries, especially by installing tide gauges to measure sea level, says Joanne Williams at the UK National Oceanography Centre.

“There are many factors that affect sea level at the coast, some of which are quite localised, so the gold standard remains local, long-term, well-calibrated measurements,” she says.