New study: Antarctic and Greenland ice sheet melt may be natural event, no consensus on cause

Ice sheets are the largest potential source of future sea level rise – and they also possess the largest uncertainty over their future behaviour

Those ice sheets again

Those ice sheets again (Photo credit: swan-scot)

From the University of Bristol

Continuous satellite monitoring of ice sheets needed to better predict sea-level rise

The findings, published in Nature Geoscience, underscore the need for continuous satellite monitoring of the ice sheets to better identify and predict melting and the corresponding sea-level rise.

The ice sheets covering Antarctica and Greenland contain about 99.5 per cent of the Earth’s glacier ice which would raise global sea level by some 63m if it were to melt completely. The ice sheets are the largest potential source of future sea level rise – and they also possess the largest uncertainty over their future behaviour. They present some unique challenges for predicting their future response using numerical modelling and, as a consequence, alternative approaches have been explored. One common approach is to extrapolate observed changes to estimate their contribution to sea level in the future.

Since 2002, the satellites of the Gravity Recovery and Climate Experiment (GRACE) detect tiny variations in Earth’s gravity field resulting from changes in mass distribution, including movement of ice into the oceans. Using these changes in gravity, the state of the ice sheets can be monitored at monthly intervals.

Dr Bert Wouters, currently a visiting researcher at the University of Colorado, said: “In the course of the mission, it has become apparent that ice sheets are losing substantial amounts of ice – about 300 billion tonnes each year – and that the rate at which these losses occurs is increasing. Compared to the first few years of the GRACE mission, the ice sheets’ contribution to sea level rise has almost doubled in recent years.”

Yet, there is no consensus among scientists about the cause of this recent increase in ice sheet mass loss observed by satellites. Beside anthropogenic warming, ice sheets are affected by many natural processes, such as multi-year fluctuations in the atmosphere (for example, shifting pressure systems in the North Atlantic, or El Niño and La Niña events) and slow changes in ocean currents.

“So, if observations span only a few years, such ‘ice sheet weather’ may show up as an apparent speed-up of ice loss which would cancel out once more observations become available,” Dr Wouters said.

The team of researchers compared nine years of satellite data from the GRACE mission with reconstructions of about 50 years of mass changes to the ice sheets. They found that the ability to accurately detect an accelerating trend in mass loss depends on the length of the record.

At the moment, the ice loss detected by the GRACE satellites is larger than what we would expect to see just from natural fluctuations, but the speed-up of ice loss over the last years is not.

The study suggests that although there may be almost enough satellite data to detect a speed-up in mass loss of the Antarctic ice sheet with a reasonable level of confidence, another ten years of satellite observations is needed to do so for Greenland. As a result, extrapolation of the current contribution to sea-level rise of the ice sheets to 2100 may be too high or low by as much as 35 cm. The study, therefore, urges caution in extrapolating current measurements to predict future sea-level rise.

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The paper:

Limits in detecting acceleration of ice sheet mass loss due to climate variability

B. Wouters, J. L. Bamber, M. R. van den Broeke, J. T. M. Lenaerts & I. Sasgen

Nature Geoscience (2013) doi:10.1038/ngeo1874

The Greenland and Antarctic ice sheets have been reported to be losing mass at accelerating rates1, 2. If sustained, this accelerating mass loss will result in a global mean sea-level rise by the year 2100 that is approximately 43 cm greater than if a linear trend is assumed2. However, at present there is no scientific consensus on whether these reported accelerations result from variability inherent to the ice-sheet–climate system, or reflect long-term changes and thus permit extrapolation to the future3. Here we compare mass loss trends and accelerations in satellite data collected between January 2003 and September 2012 from the Gravity Recovery and Climate Experiment to long-term mass balance time series from a regional surface mass balance model forced by re-analysis data. We find that the record length of spaceborne gravity observations is too short at present to meaningfully separate long-term accelerations from short-term ice sheet variability. We also find that the detection threshold of mass loss acceleration depends on record length: to detect an acceleration at an accuracy within ±10 Gt yr−2, a period of 10 years or more of observations is required for Antarctica and about 20 years for Greenland. Therefore, climate variability adds uncertainty to extrapolations of future mass loss and sea-level rise, underscoring the need for continuous long-term satellite monitoring.

http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1874.html

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36 thoughts on “New study: Antarctic and Greenland ice sheet melt may be natural event, no consensus on cause

  1. If all this glacial ice melt from Greenland and the Antarctic continent at the south is acceleralting so much, how come sea level rise has slowed down so much? Where is all the water going? Massive inland lakes are not increasing. In fact in areas of rapidly increasing local populations, such fresh water sources in some regions are becoming much smaller. So if the 99% of the ice cover which could increase sea levels, really is melting at an increased rate, surely the rate of sea level rise should be accellerating, but it is not. Why not?

  2. “They present some unique challenges for predicting their future response using numerical modelling and, as a consequence, alternative approaches have been explored.” Not more modelling, surely?

    Well 10 years research + for Antarctica & 20 years research for Greenland, that should just about secure the pension plan, thanks for the cheque taxpayers!!!!! :-)

  3. The paper is about the greenland ice sheet , not ‘arctic ice’ which most people will assume is the sea ice.

    Interesting paper though . Shows that changes to the ice sheet mass could be ‘weather’ rather than ‘climate’

  4. first, let’s find some gauges that actually show rise..
    ..and not some satellites falling out of orbit and constantly being adjusted

  5. Aren’t the NASA ICESAT satellites newer and show a net gain of ice on Antarctica? The study looking at both GRACE and ICESAT sure interpreted a net gain. I’d hope these researchers are familiar with that study.

  6. Quote from article: “Yet, there is no consensus among scientists about the cause of this recent increase in ice sheet mass loss observed by satellites. Beside anthropogenic warming, ice sheets are affected by many natural processes, such as multi-year fluctuations in the atmosphere (for example, shifting pressure systems in the North Atlantic, or El Niño and La Niña events) and slow changes in ocean currents.”

    That actually sounds like science. How refreshing to recognize the effect of “many natural processes,” even though a reference to “anthropogenic warming” had to be thrown in there somewhere.

  7. And there goes the JAXA. As expected its left its above-2000s perch and plunging toward a new low. I had hopes for a talking point, but the natural variability seems to be unidirectional for now.

  8. Anthony,
    I haven’t read in detail but I think it’s the Antarctic and Greenland. My advanced apologies if I missed it.

    • Apologies for “Arctic” in the headline rather than “Antarctic”, it’s a condition of being interrupted this morning by a phone call then being late for an appointment. Corrected.

  9. “increase in ice sheet mass loss”

    Awkward. Must be paid by the word. How about just “decrease in ice sheet mass”.

  10. Sounds like an honest assessment of the state of the science.

    We really are just in the very early stages of collecting data, and some of the tools are still being developed, adjusted and refined.

    I suspect we may have this whole issue pretty well sorted in another twenty years.

  11. As the ice melts the land underneath rises due to isostatic rebound: net result for the GRACE project = confusing problem.

    Picture of clearly delineated isostatic rebound:

    It’s quite clear from the picture that there is a substantial fall in sea level where isostatic rebound occurs.

    Exactly where are the effects of Arctic and Antarctic ice melt felt?

  12. And another thing. Why are these people so averse to altimetry? If they are so worried about the Greenland ice cap why don’t they use satellite altimetry to measure the size of the ice cap. Surely, if you can measure sea level to within a few millimeters, a giant chunk of static ice must be a piece of cake.

    Or might it show ‘the wrong result’?

  13. It must be really hard for the Ice of the Antarctic ice sheet to melt these days, since it is not only getting colder down there, but the Antarctic sea ice cover is ON A STEADY RISE, too.

    I would advise the authors of the study to check the facts at least once, before they start publishing results of computer models.

  14. As usual, so-called “climate” scientists have not done their homework. In 2011 I showed [1] that Arctic warming is caused by warm water carried into the Arctic Ocean by currents. There was nothing but slow cooling in the Arctic for 2000 years until warming suddenly started at the turn of the twentieth century. It paused for thirty years in mid-century, then resumed, and is still going strong. There was no increase of atmospheric carbon dioxide when the warming started which rules out carbon dioxide as a cause. The only possibility left is a relatively rapid change of North Atlantic current system at the turn of the century that started carrying warm Gulf Stream water north. Because of this steady supply of warm water the Arctic today is the only part of the world that is still warming. As is well known, the amount of carbon dioxide in the air is the highest ever but there is no warming of any kind and there has been none for the last 16 years. This just about kills the theory that carbon dioxide greenhouse effect is the cause of global warming. Luckily, Arctic warming is not dependent upon it and can be expected to keep going as long as the present pattern of North Atlantic currents persists. That is of course commercially important. Unfortunately there is no guarantee that the flow will remain constant. It is very likely that the warming pause in mid-century was due to the temporary return of the original flow pattern. Nature is fickle and what has happened before can happen again. If we have another such cooling period it will have a large negative influence on the exploitation of the Arctic.
    [1] Arno Arrak E&E 22(8):1069-1083 (2011)

  15. With Antarctic sea ice increasing I can’t see how the ice sheets can be melting, especially with temps around -50c most of the year, it doesn’t stack up.

    Grace is impressive but not designed to measure ice thickness, depth or height, it’s a contour mapper designed to probe the ocean bed measuring changes in gravity to indicate depth. Once models become involved anything can happen, and they have a track record of failure with Grace data already.

  16. just wondering how GRACE gravity measurements account for any core changes/movements deep within the earth in order to discern gravity changes as purely on top of the crust – i.e. the icesheets?

  17. The ice sheets covering Antarctica and Greenland contain about 99.5 per cent of the Earth’s glacier ice which would raise global sea level by some 63m if it were to melt completely.

    The important word is “if“, as this is not a realistic scenario in foreseeable future. Isn’t there any “professional” that is capable to do the math correct? Hint: it’s not linear math, it’s exponential …

    *

  18. Let’s not forget the recently-discovered photographs of Greenland from the 1930s (that troublesome decade for the warmists) that show much less ice than today.

    Let’s also not forget the U.S. Navy photographs of a nearly-ice free North Pole in the late 1950s.

    Unless I’m mistaken, the world’s coastlines didn’t seem to be flooded in either era.

  19. It baffles me that your readers cannot see the blatant sleight of hand in play here. The title of this post may lead you to think that this new piece of research is saying that ice melt in the ice sheets might be natural. This seems to be the impression on most commenters.

    However, it’s clear from the abstract and the press release that the scientists are talking about the current ACCELERATION of such melt. That’s why the text reads “increase in ice sheet mass loss” and not “decrease in ice sheet mass” as KevinM suggested.

    To put it in maths terms, they are NOT talking about the first derivative (rate of melt), they are talking about the second derivative (acceleration of melt). While the first one “is larger than what we would expect to see just from natural fluctuations”, the second one is not.

  20. “… the ice sheets’ contribution to sea level rise has almost doubled in recent years.”

    From teensie to teeny.

  21. At least it is science. Lewandowski must be quaking in his boots to be in Bristol when denialism is rife.

  22. “””””””…….Ice sheets are the largest potential source of future sea level rise – and they also possess the largest uncertainty over their future behavior…….”””””””

    Is this where the lawyer objects to the question, on the grounds that it “refers to facts not in evidence.” or maybe the mediaeval Roman translation of that, which lawyers seem to want to converse in.

    Just what is their basis for assuming “future sea level rise.” ?? Isn’t it also plausible that ice sheets are the largest potential source of future sea level FALL; that being the likelihood in the event of a potential future coming ice age ??

    These “scientists” have more potential weasel words to describe the non result of a not yet performed experimental observation; not on the present list of potential important future possible events.

    As for “””””…..– and they also possess the largest uncertainty over their future behavior…..”””””

    Compared to what list of other potential future natural phenomena ??

    People can get grant money, for this kind of non research ??

  23. Sorry, I don’t believe them a word.
    The southern hemisphere sea-ice area rises for decades. Therefore it is really hard to believe the even more poleward Antarctica icesheet – in the environment of several dozens of degrees below zero mean surface temperature – looses ice due to global warming. It just doesn’t make any sense and even much less if one implies the ice melts there and rises the sea level.

  24. Eustace Cranch [July 16, 2013 at 8:53 am] says:

    Only ice sheets on land, melting into the oceans, can cause sea level rise.

    Causes land rise too. We should call it ‘Continental Lift’.

  25. I think they have it wrong. The material for that isostatic rebound must come from somewhere. Let’s assume it yield sinking at other places. 70% of the surface is under water. How to calculate the net effect of rising/sinking solid surface?

  26. As has been pointed out in previous articles the GRACE data is suspect due to the absence of a TRF. The most recent data from an article in The Cryosphere shows gains in land ice in Antarctica (covered by the Hockeyschtick and Jo Nova).

    http://hockeyschtick.blogspot.com/2013/02/new-paper-finds-antarctica-has-been.html

    In addition, the sea ice gains tend to contradict any claims of land ice melting. Finally, last summer Zwally used the IceSat to measure the Antarctic land ice and once again found the ice increasing.

    I can see why Greenland may have had some ice loss due to the warmer temperatures in the Arctic. However, just the opposite is happening in Antarctica.

  27. I like what the poster said about the core of the eath. Assuming gravity changes due to ice may be wrong. Lets let one of their scientist blindly interpret data for a point over Kansas without telling them the locating and see what we get. If they say zero than I’ll by the results. I doubt it as I doubt an instument is accurate enough to measure such a thing over 10 years. 100 maybe, but not 10

  28. It’s been proven that at one point in time, long before humans were a blip in evolution, that Antarctica was a virtual jungle. As for the Arctic in the north melting, it’s my understanding that studies on melting there weren’t even started until around 1980. That’s many years unaccounted for. However, they do have instruments that take extremely long tube samples of ice in both arctic regions. One tube was actually several hundred feet long. They drill so far with a tube filling and bring it back up. When the drill is inserted again, they don’t start the next tube collection until they hit the bottom spot of the previous drilling and continue downwards with a new tube, and so on. In this way, they can drill quite deeply and the different layers they bring up are studied and discoveries and reports made.

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