Sea levels around the world are much higher than we thought

Almost all research on the effects of future sea level rise has assumed that current sea levels are lower than they actually are due to a “methodological blind spot”. This means that flooding and erosion will occur faster than expected.

Katharina Seeger and Philip Minderhoud, both at Wageningen University in the Netherlands, analyzed 385 peer-reviewed studies on coastal vulnerability and found that 90 percent failed to assess the effects of ocean currents, tides, temperature, salinity and wind on sea level. As a result, they underestimated coastal water levels by an average of 24 to 27 centimeters.

When corrected, the number of people whose homes will be underwater by 2100 could increase by up to 68 percent, or an additional 132 million people. The bulk of them are in Southeast Asia and Oceania, where the sea level is on average 1 meter higher than previously thought, and several meters in some areas.

“If a representative from such a place comes to a global gathering and tries to rally support … it can be quite frustrating if there are these kinds of scientific assessments that actually say … it will only be affected in the next century, when in reality the area is actually much more at risk,” Minderhoud said at a briefing.

Estimates that sea levels will rise by up to 1 meter by the end of the century remain accurate. However, since most of the research on the consequences of this increase started with a baseline that was too low, these impacts will occur faster than forecast.

46 of the studies were cited in the latest round of reports from 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 the mantle exert a greater gravitational pull on water above them. So determining the height of a given point requires a computer model of the geoid, an irregular shape that represents the average sea level around the world.

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

Instead of finding the height of coastal water and land by comparing satellite measurements with the geoid, the majority of researchers simply used the unadjusted geoid of sea level. And even those who did the calculations mostly got them wrong, often because they determined land and sea heights with different geoid models. Less than 1 percent of the studies found the correct current sea level for the coastlines they studied.

“The coastal research community (is) really focusing on the littoral, and so may not be aware of these sea level datasets,” Seeger said at the briefing.

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

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

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

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

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