Dr. Jeff Masters ·

Above: Scuba diver Nicola Brischigiaro poses on the flooded St Mark's Square in Venice, Italy on December 3, 2010, when the 'acqua alta' (high water) reached the highest level of the year, leaving 55 percent of the city under water. Shortly before dawn, sirens rang out across the city to warn inhabitants that the water level had risen above 1.10 meters (3.6 feet). Later in the morning, the level had risen to 1.40 meters above sea level. Sea level rise is an increasing threat to this historic city. Image credit: Marco Sabadin/AFP/Getty Images.

Not only is sea level rising, it is rising at an increasing amount each year, found a hugely important study of global sea level published on Monday in Proceedings of the National Academy of Sciences. The paper, Climate-change–driven accelerated sea-level rise detected in the altimeter era, looked at 25 years of satellite-based global sea level measurements taken by four satellites. The researchers found that global sea levels rose by an average of 3 millimeters per year, plus or minus 0.4 mm/yr, but this rate has been accelerating by 0.084 ± 0.025 mm/y2 over the past 25 years. If this acceleration were to continue through the end of the century, global sea level rise between 2005 and 2100 would be about 26 inches (65 centimeters), which is more than double the rise of 11 inches (28 centimeters) that would occur if sea level rise stayed constant at 3 mm/yr.

Figure 1. The change in global sea level as measured by satellites since 1993, from the University of Colorado Sea Level Research Group. The curve has been corrected for the seasonal cycle and replotted with a smoothed-fit line added by statistician Grant Foster (AKA Tamino, author of the excellent Open Mind blog). The acceleration in the curve is apparent, as it rises more steeply at the end.

Figure 2. The variation in the rate of sea level rise from 1993 – 2017 (thick red line, with error bars shown in thinner lines), as computed from data from the University of Colorado Sea Level Research Group by statistician Grant Foster in his Open Mind blog. He finds that the average rate is 3.1 mm/y, but the present rate is closer to 4.8 mm/y. His estimates are based on the raw data, and do not remove the estimated influence of the natural El Niño/La Niña and Pacific Decadal Oscillation (PDO) cycles, or effect of the Mt. Pinatubo volcanic eruption in 1991. The leveling off of the curve at the end is due to the 2015/2016 El Niño event; if that influence were to be removed, sea level rise would be seen to still be accelerating.

Uncertainties

There are many corrections that must be made to the 25-year period of the data in order to precisely determine global sea level acceleration. In particular, potential drifts in the instruments over time must be accounted for, as well as changes in water storage on land due to the natural El Niño and La Niña cycle, ice sheet mass loss that might masquerade as a long-term acceleration over a 25-year record, and episodic variability driven by large volcanic eruptions. After correcting for all these factors, the researchers concluded that the probability that the acceleration in sea level rise is zero was less than 1%.

Future sea level rise likely to be higher

The scientists commented that their estimate might represent a conservative lower bound on future sea level change, since rapid changes in the stability of the Greenland and Antarctic ice sheets could be expected to cause higher accelerations of sea level rise in the future. They added: “In contrast, few potential processes exist to suggest that this estimate is too high.”

Predictions of sea level rise from the IPCC and National Climate Assessment

The prediction by the scientists that global sea level rise would reach 26” by 2100 is well within the estimates of sea level rise from the 2013 Intergovernmental Panel on Climate Change (IPCC) report and the Climate Science Special Report from the 2017 U.S. Fourth National Climate Assessment (NCA). According to the 2017 NCA report, global mean sea level rose about 4 - 5 inches (11 - 14 cm) from 1901 to 1990, and about 3 inches (7 cm) in the comparatively brief period since 1990. Here are the four scenarios the report gave for sea level rise (the low end is comparable to a linear extension of the recent rate of 3 mm/year, while the high end is a very-bad-case scenario, including rapid ice loss in Antarctica):



Projected rises in global mean sea level from 2000 to the year shown:

Low: 0.2’ by 2020, 0.5’ by 2050, 1.0’ by 2100

Intermediate: 0.3’ by 2020, 1.1’ by 2050, 3.3’ by 2100

High: 0.4’ by 2020, 1.8’ by 2050, 6.6’ by 2100

Extreme: 0.4’ by 2020, 2.1’ by 2050, 8.2’ by 2100

(with further rises expected after 2100)

Video 1. Global sea level rise is accelerating incrementally over time rather than increasing at a steady rate, as previously thought, according to a new study based on 25 years of NASA and European satellite data.

Commentary: Ruh-Roh

Acceleration in sea level rise is something climate models have long predicted, but it is a Scooby-Doo “Ruh-Roh” moment to see it present itself so early this century. Sea level rise is going to cause massive upheavals to civilization in coming decades, forcing millions of people to abandon the coast as rising seas invade cities, knock out transportation systems and sewage treatment plants, swallow prime agricultural land and barrier islands, and infiltrate aquifers with salt water.

But the impacts of sea level rise are not limited to future decades—they’re happening right in front of us, right now. Hurricane Sandy’s storm surge in New York City in 2012 caused an extra $2 billion in damage, due to the higher water levels the city experienced due to sea level rise over the 20th century. “Nuisance” flooding has become a growing problem in places like Miami Beach, Norfolk and San Francisco. For example, in Maryland, both Annapolis and Baltimore now get more than nine times the number of flood days they experienced in the 1960s. I review many more examples in my December review of Jeff Goodell’s new must-read book on sea level rise,The Water Will Come: Rising Seas, Sinking Cities, and the Remaking of the Civilized World. In response to the news that a long-predicted acceleration of sea level is already underway, we should heed the words of Duke University sea level rise expert Dr. Orrin Pilkey and co-authors in their excellent 2016 book, Retreat From a Rising Sea; Hard Choices in an Age of Climate Change:

“Like it or not, we will retreat from most of the world’s non-urban shorelines in the not very distant future. Our retreat options can be characterized as either difficult or catastrophic. We can plan now and retreat in a strategic and calculated fashion, or we can worry about it later and retreat in tactical disarray in response to devastating storms. In other words, we can walk away methodically, or we can flee in panic.”

Bob Henson contributed to this post.