Sea level: accelerated increase since 1960

Researchers determine the beginning and cause of the ever-increasing level increases

Sea levels are not only steadily increasing, their rise is also accelerating. © Moorefam / iStock
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Earlier than expected, global sea level rise picked up earlier than previously thought. New evaluations prove that the levels have been rising ever faster since the 1960s. The main reasons for this were changed winds in the South Pacific, which drove the warming and thermal expansion of seawater. From the 1990s onwards, meltwater was added, as the researchers report in the journal Nature Climate Change.

The advancing global warming not only causes glaciers to melt and heat waves increase, it also increases sea levels. This leads to more frequent flooding, but also to creeping land loss on many islands and coastal areas. If the trend continues, more than two billion people could become climate refugees by the year 2100, due to rising sea levels alone.

To make matters worse, sea-level rise is accelerating: the sea-level rise is increasing by just 0.1 millimeters per year, according to researchers in 2018, based on satellite measurements. As a result, the current annual increase in level of around three millimeters could already be four millimeters per year in ten years' time.

When and where did the acceleration begin?

So far, however, was open when this trend began: "What we could not say so far, was when this acceleration has begun, from which region it was based - and which processes have mainly contributed to it", explains Sönke Dangendorf of the Research Institute of Water and Environment (fwu) of the University of Siegen. Because for the time before 1992, there is still no reliable satellite data, but only the data of local tide measuring stations. However, they were previously considered too inaccurate to capture the tiny changes in the rate of increase.

Now, however, Dangendorf and his team have been able to calibrate the data from the various level measurements to produce a more accurate image back to the early 20th century. "In our study, we combined two computational methods to find a way to reconstruct sea-level evolution using the level data as accurately as satellite measurements, " said co-author Carling Hay of Boston College. display

Beginning in the South Pacific

The result: The calculations show that the acceleration of the sea-level rise began at the end of the 1960s and has therefore been going on for around 50 years. "The sea level has also been increasing at times accelerated, for example, in the 1930s, " says Dangendorf. "However, the fact that the current acceleration has been going on for 50 years is very unusual."

The new evaluations also show where the acceleration of sea-level rise began: "Based on our data, we found that the accelerated increase mainly originates in the southern hemisphere, especially in the subtropical region South Pacific, east of Australia and New Zealand, "says Dangendorf. In this region, researchers reported a five-fold faster acceleration than the global average.

Winds, thermal expansion and then the melt

The researchers see two processes as the cause for the accelerated pace of the increase: From the 1960s, climatic changes led to a widening of the westerly winds in the southern hemisphere. As a result, more warm water was transported to the north, which increased the sea level rise in subtropical regions. Further south, the winds made the seawater more mixed.

"This allowed more heat to be pumped out of the atmosphere into the ocean, causing the water body to expand and the sea level to rise, " explains Dangendorf. According to the researchers, this thermal expansion in the 1960s was the main driving force for the accelerated level rise. At the beginning of the 1990s, the increased melting of glaciers in Greenland and the Antarctic was added.

The increase could be even faster

Both processes are fueled by climate change and both contribute to accelerating sea-level rise, the researchers explain. In their view, it is not excluded that the thermal expansion of seawater in the future will increase the levels even faster and more strongly than previously assumed. (Nature Climate Change, 2019; doi: 10.1038 / s41558-019-0531-8)

Source: University of Siegen

- Nadja Podbregar