The rate of modern global sea level rise began accelerating in the 1960s and not the 1990s as many previously thought, according to new research.
In a study published in Nature today, researchers have used historic tide gauge data combined with modern satellite and Argo float data to reconstruct sea level rise throughout the 1900s to the present.
They concluded that the jump in the rate of sea level rise in the 1960s was mostly caused by thermal mass expansion — the expanding of the oceans as they warm up. And if trends continue, Australia’s region could be hit harder than most.
Thermal expansion continued to drive sea level rise through to the 1990s, when glacial melting overtook thermal expansion as the dominant forcing, according to lead researcher Sonke Dangendorf from the University of Siegen in Germany.
“The acceleration detected over the recent satellite period already started in the 1960s [and] has proceeded to present day,” he said.
“The sea level acceleration detected over the recent satellite period seems to be strongly dominated by ice melt, and in particular by the accelerated mass loss of the Greenland ice sheet.
Global sea levels rose by an average of 3.1 millimetres per year between 1993 and 2015.
The average rate of sea level rise increased from 2.1mm to 3.4mm per year during that period.
But that hasn’t been uniform worldwide. The waters off the east of Australia have undergone some of the greatest sea level rise according to Dr Dangendorf.
“A further very interesting and important result is the finding that the global sea level acceleration has mainly been dominated by the southern hemisphere, with largest values obtained in the subtropical Pacific, east of Australia and New Zealand.”
Before the 1990s, there were far fewer points across the globe that were consistently collecting sea level data, and most were tide gauge records positioned along the coast.
This makes projecting past sea levels complicated, according to Xuebin Zhang from the CSIRO’s Ocean and Atmosphere Division, who wasn’t involved with the study.
“When we do past sea level construction, it’s like playing a game. The only problem is we only have a few pieces and we need to fill a lot of missing pieces,” Dr Zhang said.
The difficulty is that ocean levels behave differently in different places.
But researchers can use knowledge of contemporary sea level behaviour to fill in some of those missing pieces, Dr Zhang said.
“For example, the eastern tropical Pacific is connected to the western tropical Pacific. So if you know the information in the east you can [work out] the west.”
Sea-level rise could hit 10mm per year by end of the century
The researchers used knowledge of how sea levels rise in different places around the world today, to work out how they would have risen in the 1900s based on available historic tide gauge data.
As well as the acceleration in the ’60s, they also found that there was an earlier increase in the rate of sea level rise in the 1930s, before climate change was a strong driver.
This was also likely due to thermal expansion.
But the big difference in sea level rise since the 1960s is that it has continued until present day, despite thermal expansion slowing in the 1990s, according to Dr Dangendorf.
“The eruption [of Mt Pinatubo in 1991] led to a quick reduction in ocean heat content and therefore thermal expansion in the few years following, but this period of slowing was superseded by a recovery period with faster than usual thermal expansion from the mid to late 1990s,” he said.
Thermal expansion then slowed down again in the 2000s.
Forecasts predict that both thermal expansion and increasing ice melt will push sea levels higher at an accelerating rate in the near future.
But how much more acceleration we will see depends on what we do with emissions today, Dr Zhang said.
“For the business-as-usual scenario, we will see some acceleration. The rate of rise will be more than 10mm per year by the end of the century.”