Humans are responsible for around 70% of sea-level rise since 1970, new research shows.
Climate scientist Aimée Slangen and colleagues from Australia and Europe found greenhouse gas emissions overtook natural climate variations and drivers such as El Nino and volcanic eruptions in mid- to late last century.
The work was published in Nature Climate Change.
Across the globe over the past century, sea levels have risen by an average of “15 to 20 centimetres”, Slangen says. But even her calculated proportion due to humans surprised her: “It was more than I thought it would be.”
A 2014 Science study showed around a quarter of glacier melt from 1851-2010 was due to anthropological factors, and that figure rose to 69% from 1991 to 2010.
But no one had attempted the same calculations with sea-level rise. Expanding warm water and melting ice are affected by greenhouse gases such as carbon dioxide and methane that warm the atmosphere, and aerosols such as sulfur emissions or ash thrown out by volcanoes that reflect heat from Earth, and cool it.
So Slangen, based at the University of Tasmania in Hobart, Australia at the time, and her colleagues collected sea-level data from 1900 to 2005.
The later years were simple, she says. “Satellite measurements are awesome. They cover most of the Earth. From 1993, we have a good regional image of what’s happening.”
But before the satellite era, sea levels were measured using tidal gauges: “basically sticks attached to the [ocean] floor,” she says. “If you take away the tides you can see what’s happening to the mean sea level.”
'The beauty of models is that you can just switch on greenhouse gases or you can switch them off.'
There are a couple of problems with tidal gauges, she admits. They only take coastal measurements, not in the middle of the ocean. They were also mostly in the northern hemisphere too.
“So some people have been comparing satellite measurements with tidal gauge measurements when they overlapped,” she says. This way, she and her team were able to extrapolate global sea level throughout the early part of the 20th century – “within a relatively large error margin, but it gives us an idea”.
They then combined climate models of ice sheet and glacier melt, and thermal expansion – warm water is more voluminous, and “taller”, than cool.
“The beauty of models is that you can just switch on greenhouse gases or you can switch them off,” Slangen says. “You can switch on volcanoes, or aerosols. And we use those results to see what happens to thermal expansion, see what happens to the glaciers, look at the ice sheets.”
By teasing out how much sea-level rise was caused by natural forcing, based on climate modelling of the pre-Industrial Era, they found prior to 1950 sea-level rise was mostly (67%, plus or minus 23%) due to past climate variations and natural forcing.
But this was quickly overtaken by human-induced sea-level rise, which by the '70s had crept up to 69% (plus or minus 31%). And by 2000, that figure was 72% (plus or minus 39%).
Given the effects of atmospheric carbon dioxide can take decades to appear, Slangen thinks that number is still “coming up”.
Climate scientist Soenke Dangendorf from the University of Siegen in Germany agrees. In an accompanying News and Views article, he writes that not only does the work highlight that “current changes in the climate system will have serious impacts on many future generations”, it also emphasises the role of natural variations, which can amplify or conceal human-induced effects.