Global sea level is rising faster now than it was in the 1990s, according to a mammoth analysis of 26 different data sets, published last month in Geophysical Research Letters.
The study uses a ‘sea level budget’ approach, which places the mass of the oceans in the context of the entire hydrological cycle. The amount of water on Earth is constant, so the sum of changes in all forms of water—chiefly oceans, glaciers, the Greenland and Antarctic ice sheets, terrestrial liquid water such as lakes and rivers, atmospheric water vapor, and snowpack—should be zero.
To track those changes, the researchers drew on multiple sources of data for each element of the hydrological cycle, integrating the work of various research groups and government and international initiatives spanning multiple continents. This yields a more accurate picture than any single data set, they found, because random sources of error tend to average out.
They focused on the sea level budget from January 1993 to December 2015. This is because a major impetus of the study was to correct for inaccuracies in recordings of sea level during the early- to mid-1990s. Since late 1992, researchers have been measuring sea level by satellite altimetry, a method that involves timing how long it takes a radar pulse to travel between an orbiting satellite and a beacon on the ocean surface.
At first, that beacon was an altimeter known as TOPEX A. But it was old, and its measurements were known to be off by just a hair, so it was retired in early 1999. Two recent studies have attempted to quantify the impact of this instrument glitch, but with fairly divergent results. The sea level budget approach offers a more robust, reliable methodology to do this, the researchers say.
According to their calculations, correcting for the TOPEX A inaccuracies reveals that global mean sea level rose 3.0 millimeters per year between 1993 and 2015. That’s slightly lower than generally accepted estimates of about 3.3 millimeters per year.
But finer-grained analysis reveals a more complex picture, and a disheartening one. Sea level rose 2.7 millimeters per year from 1993 to 2004. And it accelerated to 3.5 millimeters per year during 2004 to 2015—an increase of about 25 percent.
Previous studies had suggested a slowdown of sea level rise during the 2000s due to shorter-term climate cycles known as La Niña events. “Here we show that in spite of the several temporary sea level drops caused by La Niña events, the rise has increased during the last decade,” the researchers write.
The accelerated sea level rise is primarily due to increased melting of the Greenland ice sheet, they found. (And that’s before taking into account the latest dire projections about Arctic sea ice melt.)
Even without the issue of correcting for past measurement errors, the new study would represent an impressively comprehensive analysis. Yet there’s something poignant about the story of TOPEX A, a lone beacon out there riding the waves. Is it a metaphor for humanity trying to understand our collective predicament with limited powers of perception?
Or maybe the lesson is a more straightforward and practical one, about the vulnerability of our understanding when we have limited sources of data to rely on. That, in turn, implies a need for multiple monitoring efforts using a diversity of approaches.
To paraphrase an old saying: Measure twice—six times, even. You only get to save the climate system once.