By Nanci Bompey

Global mean sea level is rising 25 percent faster now than it did during the late 20th century largely due to increased melting of the Greenland Ice Sheet, a new study shows.

Satellites first started measuring sea level rise in 1993. The new study revisits how well these measurements agree with independently observed changes in the various components contributing to sea level rise, from the melting of glaciers to the amount of water vapor in the atmosphere.

Using this approach, the new study finds the global mean sea level, or the average height of the world’s oceans, has been increasing by 3 millimeters (.1 inches) per year on average during the satellite period.

The research also finds the rate of global mean sea level rise increased by 0.8 millimeters (.03 inches) per year during the second half of the satellite period, from 2004 to 2015. The increase is mainly due to accelerated melting of the Greenland Ice Sheet, according to the new research.

The new study is not the first to estimate sea level rise, but the comprehensive approach used by the authors provides perhaps the best estimate yet of how fast global mean sea level is changing, said Anny Cazenave, a space geodesist at the Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS) in Toulouse, France, and co-author of the new study published in Geophysical Research Letters, a journal of the American Geophysical Union.

“We came up with new numbers and a new, more robust view of what is going on,” Cazenave said.

Global mean sea level is one of the best indicators of climate change because it integrates many changes occurring in the climate system, from the melting of glaciers to the amount of water stored on land to the amount of water vapor in the atmosphere, Cazenave said.

The ocean is currently storing most of the additional heat generated by greenhouse gas warming and because of the ocean’s slow response to this additional heat, sea levels will continue to rise for hundreds of years, even if emissions of greenhouse gases ceased immediately, Cazenave said. Because of this, accurate sea level rise information like that detailed in the new study will be needed to model future sea levels for centuries, she said.

“It is very important to accurately know how sea level is evolving, what is the current rate of sea level rise and if there have been changes in this rate of sea level rise during the last decade,” Cazenave said. “Now we have in hand a new number that seems to be robust and says, yes, global mean sea level is accelerating and we can say this is because of accelerating land ice melt.”

A comprehensive look

The new research considers nearly all of the available data used to estimate the sea level budget from January 1993, when satellites first started measuring sea level, until December 2015.

The data includes information for all the components contributing to sea level rise, including the melting of glaciers and ice sheets, liquid water storage on land and the amount of water vapor in the atmosphere, among other factors. These components are quantified using various methods, including floats to measure ocean properties and mass measurements of glaciers and ice sheets.

The researchers analyzed information from 26 data sets to recalculate the sea level budget and come up with a new estimate of the global mean sea level rise rate. They were also able to account for drift from an instrument used to measure sea level in the 1990s that scientists had thought could be throwing off sea level rise estimates.

This approach gave the researchers a more accurate estimate of sea level rise, which was slightly lower than previous estimates. It also allowed them to detect an increase in the rate of sea level rise since the mid-2000s, which they attributed to accelerated melting of the Greenland Ice Sheet.

To ensure satellite sea level observations are accurate, scientists aim to have the satellite observations agree with the sum of the observations of the individual components. In the past, there has been a gap in this equation, which some scientists thought could be attributed to warming in the deep ocean, which is difficult to measure.

But the new study reconciles this gap and shows the satellite measurements are accounting for all potential factors influencing sea level rise. Warming of the deep ocean currently has a negligible contribution to sea level rise, according to Cazenave.

The new study re-estimates the global mean sea level change using the newly calibrated data that accounts for the instrument drift, which was recently discovered, said Jianli Chen, a senior research scientist at the University of Texas at Austin, who was not involved with the new study. He added that there could be additional uncertainty in the global sea level rise estimates not accounted for in the paper but that the new study gives scientists a better picture of sea level rise over the whole satellite altimeter era.

“It is one of the most comprehensive analyses considering different components contributing to sea level change,” Chen said.

— Nanci Bompey is the manager of AGU’s public information department. Follow her on twitter at @nbompey.