A group of scientists say they have now reconstructed the history of the planet’s sea levels arcing back over some 3000 years, and for our climate future they have come up with some disturbing conclusions.
The research has led them to conclude that the rate of increase experienced in the 20th century was “extremely likely” to have been faster than during nearly the entire period.
“We can say with 95 per cent probability that the 20th-century rise was faster than any of the previous 27 centuries,” said Professor Bob Kopp, a climate scientist at Rutgers University who led the research with nine colleagues from several United States and global universities.
Professor Kopp said it was not that seas rose faster before that, they probably didn’t, but merely that the ability to say as much with the same level confidence declines.
The Washington Post newsapaper reports the study was published in the Proceedings of the National Academy of Sciences.
Seas rose about 14 centimetres from 1900 to 2000, the new study suggests, for a rate of 1.4 millimetres a year.
The newspaper reports unsurprisingly, the study blames the anomalous 20th-century rise on global warming, and not just that.
It also calculates that, had humans not been warming the planet, there’s very little chance that seas would have risen so much during the century, finding that instead of a 14 centimetre rise, we would have seen somewhere between a three centimetre fall and a seven centimetre rise.
The new work is particularly significant because, in effect, the sea level analysis produces a so-called “hockey stick” graph, showing a long and relatively flat sea level “handle” for thousands of years, followed by a “blade” that turns sharply upwards in very recent times.
The discovery of such patterns itself has a long history, going back to a 1998 study by climate researcher Dr Michael Mann of Penn State University and two colleagues, who found a “hockey stick” graph for the planet’s temperature, rather than for its sea level.
Since then the “hockey stick,” in its various incarnations, has come in for voluminous criticism from sceptics and doubters of human-caused climate change, even as multiple scientists have continued to affirm the conclusion that the last 100 years or so are way out of whack with what the planet has seen in the past thousand or more.
Nonetheless, choices made today could have a big impact.
As Professor Kopp acknowledges, the major contributors to sea level rise in the 20th century were the melting of mountain glaciers around the globe and the natural expansion of ocean water as it warms.
However, in the 21st century, researchers think that the truly major players in potential sea level rise, the huge ice sheets of Greenland and Antarctica, will come to play a larger role.
The new study follows in the footsteps of a 2011 study that looked at the ocean and climate records contained in salt marshes in North Carolina to infer the history of sea level rise over the past 2100 years, research that had many of the same authors.
That study, too, found that the recent sea level rise is unprecedented over that time period.
The new paper emerges even as another study, also published on Monday in the Proceedings of the National Academy of Sciences, produced very similar 21st-century projections of sea level rise.
That paper, led by Professor Matthias Mengel of the Potsdam Institute for Climate Impact Research in Germany, also calculated that with unconstrained emissions, the Earth could see a maximum of more than a metre of sea level rise by 2100.
But it too acknowledged that the approach “cannot cover processes” like the possible collapse of the oceanfront glaciers of the West Antarctic ice sheet, which, it said, “is hypothesised to be already underway”.
Such calculations are roughly in line with the 2013 projections of the UN’s Intergovernmental Panel on Climate Change (IPCC) for expected sea level rise by 2100, although they rely on a different, more simple type of model than that body used.
Capping a major day for sea level rise news, Professor Kopp also released a report a report on Monday along with Dr Benjamin Strauss of Climate Central and two other researchers, using the current study’s approach to determine that thousands of coastal “nuisance” floods in the United States over the 20th century would not have happened without human-caused global warming.