5 august 2019

Global sea level rise began accelerating ‘30 years earlier’ than previously thought

Carbon Watch, August 5, 2019

Global sea level rise began to accelerate in the 1960s, 30 years earlier than suggested by previous assessments, a new study finds.

The study, published in Nature Climate Change, introduces a new technique to more accurately determine historical global sea levels by combining two different statistical approaches.

It was found that the southern hemisphere, home to many developing small island nations, experienced the majority of the observed sea level rise, the lead author tells Carbon Brief.

The implication of this work is that ocean heat uptake will “likely increase again in the near future, further increasing the rate of current sea level rise”, another scientist tells Carbon Brief.
Measuring our rising seas

Global sea levels rose by around 20cm over the 20th century. This is primarily due to melting ice and ocean warming, which causes the “thermal expansion” of water.

Rising sea levels present a significant threat to coastal regions, putting people at risk of suffering severe economic costs, or being forced to migrate as tides rise.

Increasing sea levels are particularly dangerous for small islands in the southern hemisphere, where residents have limited ability to migrate.

To study sea level rise, scientists have historically used tide gauges. However, reliance on tide gauges can be problematic, says Dr Sönke Dangendorf, a sea level rise researcher from the University of Siegen and lead author of the study. He tells Carbon Brief:

“There are only maybe 70 or 80 tide gauges from before the 1950s that are still measuring sea level and most of those are located in the northern hemisphere. Furthermore, most of these are located at the continental coastlines and only a very limited number are on islands in the open ocean, so we are only measuring along the boundaries.”

In 1993, “satellite altimetry” was introduced as an additional way to measure sea levels across the globe.

Altimeters are instruments attached to satellites that send high-frequency pulses down to Earth. By measuring the time taken for each pulse to bounce off the surface of the ocean and return back, the altimeter can calculate the sea level.

Using this method, an accurate uninterrupted record of sea level changes has been made from 1993 (when altimetry was introduced) to the present day. Before this date, however, only tide gauge data exists.
Best of both worlds

In order to determine global mean sea levels before the introduction of satellite altimetry, there are two techniques predominantly in use – “probabilistic techniques” and the “empirical orthogonal approach”.

Probabilistic techniques use tide-gauge records together with factors that contribute to sea level rise in known spatial patterns (known as “fingerprints”). From this, they are able to produce a smooth reconstruction of long-term changes in sea level, but are unable to reproduce short-term variability caused by natural phenomena, such as the El Niño Southern Oscillation (ENSO).

The “empirical orthogonal approach”, on the other hand, is able to reconstruct interannual and decadal variations well, but has limited ability in estimating long-term trends.

By combining these two “complementary” techniques, the team developed a “hybrid reconstruction” incorporating the advantages of both approaches.


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