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.
Recreating past oceans
Using their new hybrid reconstruction, the team calculated the increase in average global sea level each year from 1900 to the present day.
The results show that global sea level rise began to accelerate persistently in the 1960s. As Dangendorf explains in the video below, this persistent acceleration “did not start, as widely suggested, with the advent of satellite altimetry” in the 1990s – but instead began in the 1960s.
The charts below show the rate of sea level rise. The top chart shows the annual increase in global average sea from 1900 to present. The blue line shows data from the tide gauges, while the red is from satellite altimetry.
The researchers also calculated the “acceleration constant” by finding the annual rate of change of sea level rise. This is shown in the bottom chart.
Top: chart showing annual increase in global average sea from 1900 to present. The blue line shows data from the tide gauges, while the red is from satellite altimetry. Bottom: Change in ‘Acceleration constant’ over time. Chart by Carbon Brief using Highcharts.
(Note that in the bottom chart, a negative acceleration constant does not signify a decrease in global average sea level in that year. It signifies a drop in the rate of sea level increase compared to prior years.)
The charts show that global average sea level has increased every year since 1900, although the size of this increase has fluctuated.
Wind from the west
The study also explored the reasons for the increase in sea level and found that the dominant cause is thermal expansion in the Indo-Pacific and South Atlantic oceans, caused by high-intensity “westerly winds”.
Westerly winds (also known as “westerlies”) are prevailing winds that blow from west to east in two bands across the mid-latitudes – one in the northern hemisphere, and one in the southern hemisphere.
When westerly winds in the southern hemisphere blow over the Pacific ocean, the warm surface layer of the ocean is displaced
As the warm top layer of the ocean is moved, colder, denser water from deeper down in the ocean wells up to take its place. This cold water absorbs more heat and so it expands more, raising sea levels. As Dangendorf explains in the video below, one of the main findings of this work is that the pace of global sea level increase is “driven by these winds in an indirect effect”.
This effect has led to a significant increase in sea levels in the southern hemisphere, the study finds. It shows that a westerly wind intensification of 15% results in a 40mm increase in average global sea level over 70 years.
It was also found that while ice melt contributed to increased acceleration in the 1930s (due to a late response from a mini-ice Age), as well as more recently, it had very little impact on acceleration from the 1940s to 1990s.
It is still not clear to what extent climate change could have driven the intensification of westerly winds. However, says Dangendorf in the video below, it is possible that “it is…related to human causes”.
Due to the activity of westerly winds, the southern hemisphere has seen greater sea level rise acceleration than the northern hemisphere, says Dangendorf:
“We are observing that acceleration in the southern hemisphere, in particular, and in the tropical regions – and that is not good news for all the island nations which are located there.”
The study reinforces the need to understand past changes in sea level rise, says Dr Natasha Barlow, a climate change and sea level rise researcher at the University of Leeds, who was not involved in the study. She tells Carbon Brief:
“Studies like this are an important way of placing recent climate change observations in context so that we don’t just focus on the last 25 years.”
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