The report, published Monday in the journal Proceedings of the National Academy of Sciences, finds that rather than swelling at a consistent rate of 3 millimeters a year, global sea level rise is accelerating by roughly 0.08 millimeters each year and could exceed 10 millimeters a year, or more, by 2100.
If the oceans continue to change at this rate, sea levels will rise 26 inches by the end of the century – enough to cause major problems for coastal cities, according to the study.
“And this is almost certainly a conservative estimate,” said lead author Steve Nerem, an aerospace engineer at the University of Colorado, Boulder.
“Our extrapolation assumes that sea level continues to change in the future as it has over the last 25 years. Given the large changes we are seeing in the ice sheets today, that’s not likely.”
Increasing concentrations of greenhouse gases in Earth’s atmosphere elevate water and air temperatures, which produces sea level rise in two ways.
First, the natural expansion of warm water has contributed to roughly half of the 2.76 inches of global average sea-level rise the planet has experienced over the past 25 years, Nerem said. Second, melting land-ice flows into the ocean, heightening sea levels.
These surges have been measured using satellite altimeter measurements since 1992. However, detecting acceleration – even over a long period of time – is difficult. Volcanic eruptions, for example, can create variability: the Mount Pinatubo eruption in 1991 decreased global mean sea levels immediately before the Topex/Poseidon satellite launch.
Global sea levels can also be affected by climate patterns like El Nino and La Nina, the opposing periods of the El Nino Southern Oscillation which impact global precipitation patterns and ocean temperatures.
Using climate models to account for these factors, Nerem’s team eventually found the underlying sea level rate and acceleration over the past 25 years. Satellite data revealed that the acceleration stems from melting ice in Greenland and Antarctica.
The researchers used tide gauge data to identify potential errors in the altimeter estimate.
“The tide gauge measurements are essential for determining the uncertainty in the GMSL (global mean sea level) acceleration estimate,” said co-author Gary Mitchum, an oceanography professor at the University of South Florida. “They provide the only assessments of the satellite instruments from the ground.”
While other researchers have used tide gauge data to evaluate the acceleration in global sea-level rise, they have struggled to identify other important details from this information including changes over the past couple of decades stemming from more active ice sheet melting.
“This study highlights the important role that can be played by satellite records in validating climate model projections,” said co-author John Fasullo, a climate scientist at the National Center for Atmospheric Research.
“It also demonstrates the importance of climate models in interpreting satellite records, such as in our work where they allow us to estimate the background effects of the 1991 eruption of Mount Pinatubo on global sea level.”
The team says their findings will become more robust as altimetry satellites extent the examined time series beyond 25 years.
The research is critical as it provides a data-driven assessment of how sea levels have been changing that largely supports projections that use independent methods.
“The acceleration predicted by the models has now been detected directly from the observations,” Mitchum said.
“I think this is a game-changer as far as the climate change discussion goes.”