Global Sea Level Rise Has Been Underestimated For A Very Long Time

Perito Moreno Glacier, in Argentina. K_Boonnitrod/Shutterstock

On November 4, the Paris agreement will officially come into force. While this is something that is worth celebrating, two caveats need to be seriously considered.

The first is that it needs to be strengthened over time, or the 2°C (3.6°F) warming limit will be breached as early as 2050. The second – as a new study in Geophysical Research Letters underscores – is that we may be underestimating the frightening effects of man-made climate change.

Most estimates place the last century’s global sea level rise average at around 14 centimeters (5.5 inches). However, according to data gathered from NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites, which measure precise changes in mass across the world, it is likely to be around 17 centimeters (6.7 inches).

This may not sound like much, but this represents a 21 percent underestimation of the true value. As a point of comparison, this new sea level rise represents about 9.1 quadrillion liters (about 2 quadrillion gallons) of ice melt, enough to cover both North and South America in about 30 centimeters (1 foot) of water.

“It’s not that there’s something wrong with the instruments or the data,” lead author Philip Thompson, associate director of the University of Hawaii Sea Level Center, said in a statement. “But for a variety of reasons, sea level does not change at the same pace everywhere at the same time.”

Sea level change as derived from GRACE measurements. Black circles depict the locations of historical water level gauges. University of Hawaii/NASA/JPL-Caltech

It seems, then, that global sea level rise has been underestimated for some time now because the gauges measuring it are in the wrong place.

Back in the 20th century, gauges were placed mostly around North America and Europe to measure just how quickly the waters were ascending. Other parts of the world, particularly the Southern Hemisphere, were neglected in this regard. Although these gauges were considered to be quite accurate, these parts of the world actually experienced less sea level rise than others – but why?

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