Finding fingerprints in sea level rise

May 18, 2012

It was used to help Apollo astronauts navigate in space, and has since been applied to problems as diverse as economics and weather forecasting, but Harvard scientists are now using a powerful statistical tool to not only track sea level rise over time, but to determine where the water causing the rise is coming from.

As described in an April 23 paper in the (PNAS), graduate students Eric Morrow and Carling Hay demonstrate the use of a called a Kalman smoother to identify " fingerprints" – tell-tale variations in – in a synthetic data set. Using those fingerprints, scientists can determine where glacial melting is occurring.

"The goal was to establish a rigorous and precise method for extracting those fingerprints from this very noisy signal," Professor of Geophysics Jerry Mitrovica, who oversaw the research, said. "What Carling and Eric have come up with is very elegant and it provides a powerful method for detecting the fingerprints. In my view everyone is soon going to be using this method."

At the heart of the new technique is the idea, first proposed by Mitrovica and others more than a decade ago, that variability in sea level changes amount to a "fingerprint" researchers can use to identify the source of water pouring into the oceans.

With the public unconvinced about the effects of climate change, Mitrovica proposed the fingerprint idea as a way to refute the argument that melting ice sheets would cause a uniform sea level rise, he said, "like what you see when you turn on the tap in the bathtub – it all goes up uniformly." Rather than a uniform rise in sea level, skeptics pointed to records that showed levels rising in some areas and dropping in others as evidence that man-made climate change was a myth.

But that variability is exactly what researchers expect to see, Mitrovica said.

"As ice sheets around the world melt, there is an extremely variable effect on sea level," Mitrovica explained. "That variation is actually very beautiful – it has information embedded in it. By looking at the differences in sea level changes around the globe, we should, in principle, be able to determine if the changes are the result of melting in Alaska, Greenland, Antarctica, or elsewhere."

That variation in sea level change is the result, in part, of the sheer size of the ice sheets, which are so massive they draw water to them, creating their own tides. Though a melting glacier can dump millions of gallons of water into the ocean, it also reduces the size of the , relaxing that tidal effect. The "highly counterintuitive" result, Mitrovica said, is that while sea levels rise in some parts of the world, within 2,000 kilometers of the ice sheet, the melting would case sea levels to drop.

"There are other effects as well – close to the ice sheet the sea floor rebounds a bit because it has less mass on top of it," Morrow added. "What's interesting is that the pattern of sea-level change will be different for each and every ice sheet – and that is why these patterns have come to be known as sea-level fingerprints."

However, while scientists have been able to model the fingerprint associated with each ice sheet on the globe, the far trickier question is how much each contributes to the current picture of rising sea levels.

"What we're really doing is detective work," Mitrovica said. "We're trying to say how much of the Antarctic fingerprint, plus the Greenland fingerprint, plus the Alaska fingerprint, plus the others – how much of each do you need in order to get precisely the variation we see today.

"The challenge in doing that is that the ocean is a noisy place," he continued. "We're talking about identifying very small signals and separating them from the waves, the changes in salt content, circulation changes, the temperatures effects and more. What Carling and Eric have developed is a way to detect these fingerprints in the sea-level variations that we observe with tide gauges and satellites, and their method takes into account the fact that those variations may change over time."

To test the new tool, Hay and Morrow created a data set of hundreds of sea level records, then added the type of noise seen in real-world data. Such tests against "synthetic" data are a typical first step, Mitrovica said, to ensure that statistical tools work.

As described in the PNAS paper, the tool they developed was able to consistently identify in the synthetic data, and tease out how much each was contributing to the rise in sea levels worldwide.

The next step, Mitrovica said, will be to test the tool against real-world data. That work is still ongoing, and will be published in a forthcoming paper.

Though it is clearly a powerful way to highlight the continuing problem of glacial melting, Mitrovica said the ultimate power of the tool might lie in bringing to light the true costs and dangers of sea level rise.

"We need to get the person living on the Maryland coast to understand that the sea-level change they will observe in their area will depend on which ice sheets are melting and by how much," he said. "When I give public talks, one of the questions I'm invariably asked is 'Where should I buy property?' It's tongue in cheek, but it does belie a certain sense that this is going to hit home."

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User comments : 11

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NotParker
1.9 / 5 (14) May 18, 2012
Tide Gauges say sea level rise is decelerating.
MR166
2.4 / 5 (18) May 18, 2012
Lets see, they gave gone from NYC being covered by 20 feet of water to trying to measure local gravity effects as proof of man made global warming. Give it up guys the CO2 warming hoax has finally died. Get with the program people, the "In" catastrophe is now shrinking bio diversity.
tadchem
4.2 / 5 (5) May 18, 2012
This would seem to imply that "mean sea level' is a local phenomenon, and that one could map contours of sea level - so the sea is not really level?
NotParker
1.9 / 5 (14) May 18, 2012
tadchem, thats what they say. The trouble is that the satellite derived contour maps don't really agree with tide gauge data.

http://www.aviso....just.png
Shelgeyr
3.4 / 5 (9) May 18, 2012
tadchem: Correct - the sea is not really level. "Level" is an averaging concept we calculate. The seas themselves are affected by all sorts of things, from being thrown about by tidal forces, to covering countless gravitational anomalies, to even the very basic issue of being on a spinning oblate spheroid rather than a perfect ball.

Depending on how detailed an area or time frame you're looking at, you could probably add "constructive and destructive deep wave patterns" (but we probably shouldn't go there). It would also be reasonable to factor in seasonal thermal expansion effects, and in fact I'd be surprised if that wasn't already done.
gregor1
1.6 / 5 (11) May 18, 2012

"With the public unconvinced about the effects of climate change, Mitrovica proposed the fingerprint idea as a way to refute the argument that melting ice sheets would cause a uniform sea level rise,This is not science but a classic case of confirmation bias. Being motivated by the desire to convince the public is dodgy indeed. Lets just stick to the scientific method and find out what's actually happening.
verkle
1.2 / 5 (6) May 19, 2012
Have to take into account the moving tectonic plates as well. In northern Japan since the great earthquake last year, the seashore has sunk 2-3 feet. From a casual observer, this looks like the see level has risen by 2-3 feet, and many piers are now under water at high tide.
kaasinees
2.9 / 5 (16) May 19, 2012
ParkerTard why are you not banned yet?
MarkyMark
4.2 / 5 (5) May 19, 2012
Have to take into account the moving tectonic plates as well. In northern Japan since the great earthquake last year, the seashore has sunk 2-3 feet. From a casual observer, this looks like the see level has risen by 2-3 feet, and many piers are now under water at high tide.
Err somehow i suspect this obviouse variable is taken into account. Afterall its so obviouse that uninformed armchair scientists like Verkie can point it out.
NotParker
1.7 / 5 (12) May 19, 2012
ParkerTard why are you not banned yet?


Typical VD/AGW cult member drops in to try and ruin a civilized discussion.

Why don't you and VD and the other trolls grow up. Adults are having a discussion.
NotParker
1.4 / 5 (10) May 19, 2012
Have to take into account the moving tectonic plates as well. In northern Japan since the great earthquake last year, the seashore has sunk 2-3 feet. From a casual observer, this looks like the see level has risen by 2-3 feet, and many piers are now under water at high tide.
Err somehow i suspect this obviouse variable is taken into account. Afterall its so obviouse that uninformed armchair scientists like Verkie can point it out.


How would someone "take this into account" from a satellite?

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