About 71 percent of the Earth is covered by water, so measuring sea level changes around the world is no small feat. Up until now, scientists believed they knew how much global sea level had risen during the 20th century. This number has hovered around 0.6 inches per decade since 1900, according to the National Oceanic and Atmospheric Administration, and has been partly driven by warming ocean temperatures.
But a new study, published this month to Geophysical Research Letters, found evidence to suggest that historical sea level records have been off—way off in some areas—by an underestimation of 5 to 28 percent. Global sea level, the paper concluded, rose no less than 5.5 inches over the last century, and likely saw an increase of 6.7 inches.
The reason for this discrepancy was uncovered by earth scientists at NASA’s Jet Propulsion Laboratory and the University of Hawai'i at Mānoa. By comparing newer climate models with older sea level measurements, the team discovered that readings from coastal tide gauges may not have been as indicative as we thought. These gauges, located at more than a dozen sites across the Northern Hemisphere, have been a primary data source for estimating sea level changes during the last several decades.
“It's not that there's something wrong with the instruments or the data, but for a variety of reasons, sea level does not change at the same pace everywhere at the same time,” said Philip Thompson, the study’s lead author and associate director of the University of Hawai'i Sea Level Center, in a statement.
“As it turns out, our best historical sea level records tend to be located where past sea level rise was most likely less than the true global average.”
Coastal tide gauges generally work by monitoring sea levels compared to a fixed point on land. They’re often placed on piers, and can measure other variables such as barometric pressure and wind speed, which can sometimes impact water level changes.
The only problem was where these gauges were situated. According to the study, most of the gauges were located in the Northern Hemisphere where global ice melt has occurred at a faster rate over the past century. In fact, the disproportionate sea ice loss between the Arctic and Antarctic has been befuddling climate scientists for years, although several theories stand to explain the discrepancy.
GIF: YouTube/NASA Goddard
In terms of sea level changes, however, the study found the effects of melting are more drastically felt in regions farther from its source. According to InsideClimate News, melting sea ice in the Northern Hemisphere might raise water levels more in “the southern Pacific Ocean and equatorial regions.”
Other factors the team considered were “ice melt fingerprints,” which can influence why sea levels due to sea ice melt vary from place to place. Changes in China, for example, might differ from those seen in the United States or in Africa. Things like tides, winds, ocean currents, and gravity, explain NASA, are why melting ice sheets don’t cause global sea level to rise evenly, as if it were a bathroom sink.
Sea level change resulting from Greenland ice melt. Black circles show best water level records. Image: University of Hawai'i at Mānoa
“This is really important, because it is possible that certain melt fingerprints or the influence of wind on ocean circulation might cause us to overestimate past sea level rise,” said Thompson. “But these results suggest that is not likely and allow us to establish the minimum amount of global sea level rose that could have occurred during the last century.”
So far, this year has continued to shatter atmospheric and environmental records. As Motherboard’s Canada Editor Kate Lunau wrote: Every month this year has been the hottest in recorded history. NASA is now reporting that summer sea ice is covering 40 percent less surface area than it did just three decades ago. And if the West Antarctic ice sheet continues to melt, sea levels around the world could rise as much as 12 feet, or enough to wipe out entire coastal cities.
But, like Thompson seemed to suggest, there’s a silver lining to all of this. Like all disciplines, climate science can be imperfect, and it’s better we learn about of our errors now, before we’re in deep water.