WASHINGTON (AP) — The world's oceans are now rising far faster than they did in the past, a new study says.
The study found that for much of the 20th century — until about 1990 — sea level was about 30 percent less than earlier research had figured. But that's not good news, scientists say, because about 25 years ago the seas started rising faster and the acceleration in 1990 turns out to be more dramatic than previously calculated.
The current sea level rise rate — which started in 1990 — is 2.5 times faster than it was from 1900 to 1990, according to a study published Wednesday in the journal Nature. Scientists say that faster pace of sea level rise is from melting ice sheets in Greenland and West Antarctica and shrinking glaciers, triggered by man-made global warming.
"We're seeing a significant acceleration in the past few decades," said study lead author Carling Hay, a geophysical researcher at Harvard University. "It's concerning for cities along the U.S. East Coast" where water levels are rising even faster than the world average.
"It's definitely something that can't be ignored," Hay said.
Previous research said that between 1900 and 1990, the seas rose about two-thirds of an inch a decade. The new study recalculates the 1900-1990 rate to less than half an inch a decade.
Old and new research both say that since 1990 seas are rising at about 1.2 inches a decade.
While hundreds of tide gauges around the world have been measuring sea levels since 1900, they have mostly been in Europe and North America with few in the polar regions or the middle of the oceans, Hay said. So past estimates of 20th century sea level rise gave an incomplete picture of the global effect, said study co-author Jerry Mitrovica, a geophysics professor at Harvard.
The new method uses statistical analysis and computer models to better simulate the areas in the gap, Mitrovica said.
Outside scientists praised the new study, but were still cautious about adopting the estimates until more studies could be done.
"The implications are troubling — accelerated ocean warming, ice sheet collapse and sea level rise — all point to more and more sea level rise in the future, perhaps at a faster rate than previously thought," said Jonathan Overpeck, co-director of the Institute of the Environment at the University of Arizona. "This will make adaptation to climate change more difficult and costly."
Sea level rise quickens more than thought in threat to coasts
Alister Doyle PlanetArk 15 Jan 15;
Sea level rise in the past two decades has accelerated faster than previously thought in a sign of climate change threatening coasts from Florida to Bangladesh, a study said on Wednesday.
The report, reassessing records from more than 600 tidal gauges, found that readings from 1901-90 had over-estimated the rise in sea levels. Based on revised figures for those years, the acceleration since then was greater than so far assumed.
The report said the earlier readings were incomplete or skewed by local factors such as subsidence.
The new analysis "suggests that the acceleration in the past two decades is 25 percent higher than previously thought," Carling Hay, a Canadian scientist at Harvard University and lead author of the study in the journal Nature, told Reuters.
The study said sea level rise, caused by factors including a thaw of glaciers, averaged about 1.2 millimeters (0.05 inch) a year from 1901-90 - less than past estimates - and leapt to 3 mm a year in the past two decades, apparently linked to a quickening thaw of ice.
Last year, the U.N.'s Intergovernmental Panel on Climate Change (IPCC) estimated the 1901-90 rate at 1.5 mm a year, meaning less of a leap to the recent rate around 3 mm.
The Harvard-led study said the new findings might affect projections of the future pace of sea level rise, especially those based on historical trends.
John Church, a top IPCC author at the Commonwealth Scientific and Industrial Research Organization in Australia, told Reuters he did not expect any impact on the IPCC's core sea level projections, which are not based on past trends.
IPCC scenarios, which range from a sea level rise of 28 to 98 cms this century, are based on the processes driving sea level change, for instance how ice in Greenland reacts to rising temperatures or the expansion of water as it warms, he said.
Stefan Rahmstorf, of the Potsdam Institute for Climate Impact Research and a world expert in past sea levels, said further analysis was needed to pin down 20th century sea level rise.
The new findings confirm that "sea level is rising and ... the rise has accelerated, with the most recent rates being the highest on record," he told Reuters.
Sea level rise is gnawing away at shores from Miami to Shanghai. In cities such as Jakarta, the rise is aggravated by big local subsidence.
Study at: nature.com/articles/doi:10.1038/nature14093
(Editing by Dominic Evans)
Rate of sea-level rise 'steeper'
BBC News 14 Jan 15;
The rate at which the global oceans have risen in the past two decades is more significant than previously recognised, say US-based scientists.
Their reassessment of tide gauge data from 1900-1990 found that the world's seas went up more slowly than earlier estimates - by about 1.2mm per year.
But this makes the 3mm per year tracked by satellites since 1990 a much bigger trend change as a consequence.
It could mean some projections for future rises having to be revisited.
"Our estimates from 1993 to 2010 agree with [the prior] estimates from modern tide gauges and satellite altimetry, within the bounds of uncertainty. But that means that the acceleration into the last two decades is far worse than previously thought," said Dr Carling Hay from Harvard University in Cambridge, Massachusetts.
"This new acceleration is about 25% higher than previous estimates," she told BBC News.
Dr Hay and colleagues report their re-analysis in this week's edition of the journal Nature.
Tide gauges have been in operation in some places for hundreds of years, but pulling their data into a coherent narrative of worldwide sea-level change is fiendishly difficult.
Historically, their deployment has been sparse, predominantly at mid-latitudes in the Northern Hemisphere, and only at coastal sites. In other words, the instrument record is extremely patchy.
What is more, the data needs careful handling because it hides all kinds of "contamination".
Scientists must account for effects that mask the true signal - such as tectonic movements that might force the local land upwards - and those that exaggerate it - such as groundwater extraction, which will make the land dip.
Attention needs to be paid also to natural oscillations in ocean behaviour, which can make waters rise and fall on decadal timescales.
Previous efforts to untangle the record concluded that sea levels rose through much of the last century by around 1.6-1.9mm per year.
These figures were included in the most recent Intergovernmental Panel on Climate Change (IPCC) report on the state of the planet.
But these numbers have been somewhat problematic because they are at odds with the calculated contributions to global ocean rise - namely, the volumes of water coming from melting land ice, the expansion of the seas from global warming, and changes in the amount of global water held on the continents. Simply put, the calculated contributions were about 0.5mm per year short of what previous tide-gauge assessments were suggesting they should be.
Dr Hay's and colleagues' study makes another attempt to sort through the instrument record, and they find the 1900-1990 rises to have been overstated.
Their rate for this period is 1.2mm per year, which neatly closes the contributions "budget gap".
Dr Hay said: "What we have done, which is a bit different from past studies, is use physical models and statistical models to try to look for underlying patterns in the messy tide gauge data observations.
"Each of the different contributions actually produces a unique pattern, or fingerprint, of sea-level change. And what we try to do is model these underlying patterns and then use our statistical approach to look for the patterns in the tide gauge observations. That allows us to infer global information from the very limited records."
Modern sea level monitoring station
The modern tide gauge is now a highly sophisticated tool. Coastal instruments have recorded sea level change at some locations for more than 200 years
In the last IPCC report, global mean sea-level rise for 2081−2100 was projected to be between 26cm (at the low end) and 82cm (at the high end), depending on the greenhouse emissions path this century.
If the Hay analysis is reproduced by peer groups, it may prompt the scientific community to revisit these future sea-level projections and some of the other estimates that envisage even larger changes in the decades ahead.
Commenting, Dr Paolo Cipollini at the UK's National Oceanography Centre, said the Nature study was an important new contribution to the field.
Having a good view of historical change, he explained, would allow researchers to test their models of the processes driving sea-level rise by permitting them to do "hindcasts" - to check whether those models could reproduce the past before making confident projections of the future.
"But let's not lose sight of the central message that at the moment we have a very strong consensus on the 3.2mm per year of sea-level rise coming from satellites and modern tide gauges, and that any future projection should be based mainly on our understanding of the processes of sea-level rise, which really we need quantify better for later IPCC reports."
The "gold standard" satellite record of sea-level rise is maintained by the Jason series of spacecraft, which have an unbroken record of measurements stretching back to 1992.
Jason-3, the latest incarnation, launches this year, along with the EU's Sentinel-3 spacecraft, which has been tasked with starting another continuous - and independent - sequence of observations.