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A basic overview of melting ice around the globe

Posted on 16 February 2011 by James Wight

Contrary to contrarian claims, ice is melting at accelerating rates in the Arctic, Antarctica, Greenland, and glaciers all over the world. Arctic summer sea ice has shrunk by an area equal to Western Australia, and might be all gone in a decade.

Ice sheets are beginning to shrink

An ice sheet is a huge layer of land ice. The only ice sheets are in Antarctica and Greenland.

The Greenland ice sheet is losing mass at an accelerating rate. In recent years the ice loss has spread from the south coast around to the northwest.

(Image source: Climate Signals.)

Similarly, Antarctica is also losing ice at an accelerating rate. Antarctica is basically divided into two distinct ice sheets, the West Antarctic and East Antarctic. The East Antarctic ice sheet, which is much bigger than the West Antarctic one, was until recently considered stable, but has also begun losing ice.

(Image source: NASA.)

Ice shelves are collapsing

Ice shelves are thick, floating platforms of ice formed when glaciers flow from the land onto the ocean surface.

The Antarctic Peninsula is warming rapidly. Several ice shelves have collapsed completely, including one covering 3,250 km2, almost twice the area of urban Sydney.

 

(Image source: National Snow and Ice Data Center.)

Glaciers are retreating

Glaciers are retreating around the globe. Although one can point to particular glaciers that are growing, glaciologists look for trends in the total mass of glaciers worldwide. It turns out the world’s glaciers are losing ice at an accelerating rate.

(Image data source: Cogley 2009.)

And despite all the hype about a certain mistake in the 2007 IPCC report, the Himalayan glaciers are in fact melting.

Southern sea ice not doing much

Sea ice floats on the ocean surface, and is not to be confused with ice sheets on land. Even though the Antarctic ice sheet is losing mass, the extent of sea ice around the coast of the continent has grown slightly.

This is because of a complex variety of factors, and despite the warming of the Southern Ocean. The trend is expected to reverse in coming decades as the Antarctic continues to warm.

Arctic sea ice in a death spiral?

Arctic sea ice grows and shrinks seasonally, with an annual minimum in September. In 1979, when satellites first measured it, September Arctic sea ice extent was roughly equivalent to the area of Australia. Since then it has declined by about a third, equivalent to losing Western Australia – outstripping all projections.

(Image source: Copenhagen Diagnosis.)

2010 had the third lowest minimum on record (after 2007 and 2008). Two expeditions successfully circumnavigated the Arctic Ocean in a single summer, something that would have been impossible just a few years earlier or any time in recorded history.

Contrarians claim Arctic sea ice has “recovered” since the record low extent of 2007. But sea ice exists in three dimensions, and it has continued to thin rapidly. Ice volume data paints a picture even more dire: the Arctic has actually lost not one third but two thirds of September sea ice. What’s more, the volume reached a record low in 2010 – not an encouraging sign of recovery.

2010 set the stage for continued melting. At the end of the summer, a record-breaking 86% of ice cover was less than two years old; ice older than five years has all but disappeared. The remaining new ice is thinner and much easier to melt than older ice.

 

(Image source: National Snow and Ice Data Center.)

Ice-free summers are now probably inevitable, but it’s not clear how soon because the Arctic is melting much faster than any model predicted. Mark Serreze, Director of the US National Snow and Ice Data Center, says we’re “looking at a seasonally ice-free Arctic in twenty to thirty years.” A few scientists argue that September sea ice could be essentially gone within the next decade.

This blog post is the Basic version by James Wight of the intermediate rebuttal Ice Isn't Melting.

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Comments 1 to 50 out of 57:

  1. I'm a little confused by that first plot of Greenland ice balance - specifically, that it seems to show that Greenland was in fact gaining ice until about 2007 (if the y-axis shows what I think it does).

    This does not sync up with the GRACE data shown in this previous sks post: http://www.skepticalscience.com/Why-is-Greenlands-ice-loss-accelerating.html

    By the way, that's a post I hadn't read before, and the graph showing how surface mass balance used to track precipitation until runoff started increasing at a rapidly accelerating rate around 1995-2000. What an eye-opener.
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  2. Oops, ignore that. I see from here that the figure shows:

    "Greenland ice mass anomaly - deviation from the average ice mass over the 2002 to 2010 period. Black line shows monthly values. Orange line shows long-term trend (John Wahr)"

    So it tells you that Greenland has lost about 2000Gt total since 2002 - that's not yearly mass change. Sorry for the confusion.
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  3. The first figure is from GRACE data. 2007 is about the mid-point of the reference range, so they used it for a zero point. Mass loss has continued from 2002 (inception of GRACE data).

    Central Greenland is gaining mass through increased precipitation, principally in the form of snow, but that gain is vastly offset by the thinning margins and marine-terminating outlet glaciers.

    The Yooper
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  4. I suppose its amazing really to see the changes in the world today, and to see an ice free artic regions would be something this world has not seen for many thousands of years. What wonderful times we live in, in that we can see and monitor these changes.

    As they say there is nothing new under the sun, it's just we have never seen it before with the naked eye and mankind has to re-educate himself that nothing is permanent and he is going to have to uproot and move with the times. No wonder animal species (as we are) tend to migrate seasonally.
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  5. HuggyPopsBear: You're right, we're going to see some amazing things over the next few decades / centuries, sights that humans have never seen before.

    However, it puts me in mind of the Chinese saying: "May you live in interesting times."

    Remember, that's a curse, not a blessing...
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  6. "What wonderful times we live in, in that we can see and monitor these changes."

    I'm not sure the hundreds of millions of people who depend on glacier melt for drinking water consider glacial retreat so wonderful, nor the other hundreds of millions who will be displaced by the one to two metres sea level rise predicted over the next century.
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  7. And that's assuming that Hansen's estimate of a 10-year doubling period for ice mass loss isn't correct... 5m by the end of the century? 25m sea level rise in the next few centuries? Nasty stuff!

    Will it be amazing? Yes, absolutely. In the same sense that a plane crash, or a train wreck, or a massive pile-up on a freeway is amazing.

    Some forms of amazement I can do without. Watching those ice mass loss charts get steeper by the year is just scary!
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  8. Re: Bern (7)

    Hansen 2011 (p. 17) makes the case for a best-fit 5-6 year doubling time for mass loss in both Greenland and Antarctica due to non-linear ice sheet losses, and that we have already equaled the temperature levels of the Holocene Altithermal (Holocene Maximum).

    Pass the popcorn.

    The Yooper
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  9. It would be much more informative if the change in polar ice were ratioed to the total ice content. This is important for two reasons: (1) The ratio is in fact small and (2) It is the polar ice that puts a brake on global warming.

    The total heat capacity of polar ice is huge, greater than the heat capacity of the world's oceans. It important to understand the role polar ice plays in stabilizing climate over the course of scores of years, and possibly hundreds of years.
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  10. So, Daniel, does a doubling of Greenland and Antarctic ice mass loss every 5 years mean 4x current rate of loss in a decade? 16x in 20 years? 64 times the current rate of loss in 30 years? (I'll be dead then, my grandchildren should be at the start of their life careers then) 256 times in 2 score years? What would that do to the "small" ratio of loss in a mere two score years, Peter?
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  11. Contrarians claim Arctic sea ice has “recovered”

    What makes you think "contrarians" aren't in favor of global warming? ...as if one must not only believe warming is real, but that it can only bring negative consequences. I am sure deep down there are "hundreds of millions of people" who wouldnt mind seeing some changes in their lives,... and more likely than not, the well-to-do who do not.

    In any event, since we seem to have fair warning here, (out to 50 years or so), when exactly should society begin backing away for coast lines and building dikes? Supposedly, even if we stop burning fossil fuels the warming is suppose to continue since "equilibrium" hasnt been reached yet. Why are we just gawking at the ice melt, and not spending money on preparing for "the worst"?
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  12. RSVP: I don't know where you live, but in the UK there are plans being drawn up to cope with initial changes (unless emissions are cut then long term changes are not defendable against).

    Most large companies, organisations, authorities etc, have been required to produce climate adaptation plans.

    http://www.bbc.co.uk/news/science-environment-12302555
    http://www.bbc.co.uk/news/science-environment-12384389
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  13. RSVP@11 wrote "I am sure deep down there are "hundreds of millions of people" who wouldnt mind seeing some changes in their lives,... and more likely than not, the well-to-do who do not."

    That is very unlikely to be correct. The well-to-do are unlikely to be substantially bothered by the direct effects of warming - they have the resources to adapt. However, there are billions living at subsistence level around the world who have difficulty providing enough food for themselves; any disruption to food supply as existing agricultural methods become unviable will cause a big problem, as they don't have the resources to adapt. You have it entirely the wrong way round.

    If you were a Bangladeshi, I suspect you might not be that keen on a bit of warming and an increase in sea level.

    "Why are we just gawking at the ice melt, and not spending money on preparing for "the worst"? "

    In short because of politics. Science can demonstrate there is a problem that needs attention, and suggest solutions. Action however depends on politics, and when was raising taxes ever a vote winner with the electorate?

    However, this is not the thread (or perhaps the site) for that discussion
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  14. 11: RSVP: "What makes you think "contrarians" aren't in favor of global warming?"

    Clearly no one can assert or deny that either way. For "contrarians" point of view, we always have the choice of:
    - Nothing's happening
    - It's warming (not due to human action), but not by enough to worry about.
    - It's warming (not due to human action), but it's good
    - It's warming (due to human action), but not by enough to worry about.
    - It's warming (due to human action), but it's good
    - It's cooling (not due to human action), etc. etc.

    (That's not to say that all projections from the real world are in total agreement, of course not, but they vary by degree and technical detail)
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  15. The Ville 12
    "Most large companies, organisations, authorities etc, have been required to produce climate adaptation plans. "

    "have been required" sounds a lot different from enthusiastic support, which basically supports my micro thesis.

    les 14
    You missed one..
    -It's warming, and at this point, a colder climate might cause just as much havoc as a warmer one.

    or

    -Yes, yes, yes its warming, but the change is so slight compared to so many other problems.


    And frankly, if your only concern is Global Warming, you must not have "real" problems.
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  16. Ah, RSVP, you're once again proving that you have nothing worthwhile to add to this conversation. You want to provide the basis for your belief that change is slight "compared to other problems"? Given how many other problems warming will either create or exacerbate, I think you're on very "thin ice" (pardon the pun). When all the refugees created by global warming start muscling into your neighborhood-in the *millions*-then you'll know what *real* problems are. Of course, some of us are not so self centered, as you are, to only be interested in our own problems. Many of us are very concerned about the plight of those living in poverty-& how those people will be impacted by global warming-all because some of us can't be bothered using fuel efficient vehicles or catching public transport more often.
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  17. 15: RSVP
    As you say, I missed one - or two? - or a few. Still clearly we agree there is no coherent view of this from the "contrarian" world.

    My version of your "so many other problems" one is:
    - Whether or not it's warming, money is better spent on X.
    where X = any example of human misery that the author is prepared to make political capital out of.

    Which has been argued well and badly but is certainly off topic as the ice does not give a fig whether or not it's cost effective to tackle CO2 emissions and such like.



    As for your last comment "And frankly, if your only concern is Global Warming, you must not have "real" problems."
    What can I say but get a grip.
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  18. Re: Gordon (10)

    Fair questions, all. Given the non-linearity of the ice sheet mass losses that have occurred in the paleo record & the anticipated warming "in the pipeline" (even if emissions are magically capped at zero indefinitely), we are clearly past the point of no return with our current shorelines.

    Whether it's 5 meters by 2100 or 10: the all-important first 2 meters of SLR is what matters. 2 meters will be enough to destabilize the 3rd world coastal nations & most of the developing world as well. If it occurs with the anticipated increase in droughts worldwide (a forthcoming blog post here details that), then the impact on today's society passes reliable calculation.

    Getting pretty far off-topic here, but I'll close with this: the focus on Greenland is a red-herring. What matters is the impending summer loss of Arctic sea ice cover followed by what happens with the WAIS. Those are the things to monitor.

    The Yooper
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  19. re Daniel Bailey @ 8 - yes, that's the paper I was thinking of - it's compelling reading. I was taking the 10-year period as a 'conservative' approach (and that thought is scary all by itself!)

    Some people in many wealthy nations complain about the refugee problems we have today. What's it going to be like when sea levels rise by 5 metres? I saw one estimate (can't recall where) that that much rise puts half of Bangladesh (and most of it's arable land) under water - there's about 80 *million* potential refugees. How are developed economies going to deal with that, while also trying to move trillions of dollars worth of cities & infrastructure to higher ground?

    The federal gov't estimates from 2009 suggest Queensland alone faces losing between $10-16 billion worth of residential buildings if sea level rises 1.1 metres. That doesn't include the cost of the real estate (potentially as much again), the cost of infrastructure (roads, power, water, comms) to supply those residences, or commercial/industrial property. Or losses due to economic disruption.

    Clearly, the ice mass loss described in this article is of concern. The 5m plus increase Hansen is suggesting would be catastrophic, even just from an economic point of view. From a humanitarian point of view, it almost doesn't bear thinking about.
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  20. Peter Offenhartz: "much more informative if the change in polar ice were ratioed "

    Look at the Arctic ice extent graph; values have declined from a more-or-less steady ~8 to approaching 4. That's approaching a -50% change.

    "understand the role polar ice plays in stabilizing climate"

    It seems to be failing at that role; are you implying that as Arctic ice continues declining, things will get worse at an increasing rate?
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  21. @ Bern (19)

    One site to see the visual impact of various sea level rises is here. From there, this is 5 meters:



    (Larger version here)

    The Yooper
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  22. @Daniel Bailey (21)

    That is a nice map, but it does not tell the real scope of a 5 meter sea level rise. Looking at that map you only sea some red dots and a few red blodges. Not even 1 percent of the total available land area. That's why I do not like those maps.

    It does not show that all the seaports, a lot of densely populated area and quite a lot of farm land are in those red areas.
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  23. Daniel Bailey - could you give a brief rational for why Greenland is the red herring? I am assuming because of the potential positive feedback from the lack of arctic summer ice?

    thanks
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  24. Re: Arjen (23)

    Then try this one, couresy of Robert A. Rohde's Global Warming Art:



    Or this one, courtesy of Alex Tingle @ firetree.net:



    Better?

    The Yooper
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  25. Re; actually thoughtfull (24)

    Greenland is and has been the primary focus of the blogosphere for a long time as the source of sea level rise. Why? People live there. Now and in the past (Vikings). It's part of the denier mythos that "Greenland was warmer in the past" (see here and here, among others). Crap, all of it. Yeah, I know, it IS farther from the pole and thus receives more energy from the Sun than does Antarctica (and is in the NH so will be affected more by polar amplification).

    What they fail to take into account, when it comes to dynamic, nonlinear destabilization of ice sheets, is the underlying geomorphology of Greenland:



    You have a central, bowl-shaped, depression ringed by coastal mountains. Marine-terminating outlet glaciers vent the ice mass through gaps in the coastal mountains. Chance of "catastrophic" ice sheet demise in this century? Zilch.

    The West Antarctic Ice Sheet (WAIS) has no such free publicity or underlying protections. The sheet itself terminates in the Southern Ocean, which has undercut the ice edge sufficiently enough already that the edge has retreated back of its terminal moraine grounding points into deeper waters, particularly in the area of the Pine Island Glacier (PIG) and the Thwaites Glacier.

    Ice sheet flows have risen dramatically in the PIG. Once it goes, you have a 10,000 foot high ice mass basically ready to slide downhill into the sea. Oh, yeah, the base of the WAIS is below sea level, so warmer waters will always be lapping at its edges.

    The recent warming of the Antarctic Circumpolar Current raises the spectre of nonlinear ice sheet losses in the PIG and the WAIS significantly. Hansen 2011 (linked in an earlier comment here) covers this as well.



    Others may have different opinions, of course.

    Apologies to James & the mod's if I've derailed this thread any.

    The Yooper
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  26. Peter,

    You are completely wrong when you say that the heat capacity of the ice is greater than the heat capacity of the ocean. The ocean absorbs enough energy to melt ice equal to several meters of sea level rise every 10 years. Currently most of the heat goes into expansion of the water, which only raises sea level a little. If the energy starts to go into melting ice the sea level rise will very rapidly increase.
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  27. @michael sweet(27):
    @MuonCounter(20): The volume of ice is much more important than the extent. Approximately 3% of all water is locked up in the form of ice. (I'm talking about land ice, i.e., Greenland and Antarctica, not sea ice.) Given the heat of melting/fusion of ice, this is a huge amount of heat storage. To understand the rate of global warming, it is necessary to understand the RATIO of the annual ice melting to the total ice mass. I ask michael sweet to think again about the relative heat capacities of ocean and ice; I don't think I am wrong.
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  28. A few comments on some or all of the above:

    Heidi Cullen talks about Bangladesh in her book "The Weather of the Future", and points out that that country has about half the population of the US in an area the size of the state of Iowa. And a disturbingly high percentage of it is very low-lying coastal land.

    Daniel makes an excellent point about the relative threat from Greenland vs. that of the Arctic ice or the WAIS. As Arctic ice dwindles every summer, the Arctic amplification, a.k.a. the "albedo flip" will become a very big deal. The change in albedo from ice to open ocean is very large, and it will happen over million of sq. km. And as for the WAIS, I can barely think about the implications of it doing anything even remotely "interesting".

    Daniel also pointed out something that I think a lot of people (including me) have been negligent in stressing: The wildly non-linear nature of human impacts from sea level rise. A small amount of SLR results in occasional problems from storm surges and high tides in a few places. A little more SLR and the problems happen more often and in more places, but can still be treated as anomalies. But as you approach a SLR of meters it suddenly becomes a challenge of moving large coastal cities like Miami to higher ground, permanently, or dealing with the NY City sewer system that can't operate during sufficiently high sea levels. (Cullen also talks about the NYC issue in her book.)

    There's also a freshwater connection here: In some places in the southeast US they've already had to stop using some municipal wells for groundwater because of saltwater incursion. Despite its costs, desalination is going to be a very important technology around the world, much more so than it is already.
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  29. Daniel Bailey - thank you - you are a fantastic resource on this blog (among others, it must be noted, but you and you alone post links with serious sounding titles to hilarious web pages with cartoons and songs. And the value of that is beyond measure).
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    Moderator Response: [Daniel Bailey] Thank you for the kind words; muoncounter & others here are wicked funny, as well. When dealing with such sobering subject matter, I find humor helps keep me (relatively) sane.
  30. Amplification/correction: When I said "the volume of ice is much more important than the extent" I was writing only about heat capacity/storage. Clearly "extent" is more important than "volume" when it comes to changing albedo. I regret any confusion. My point is solely that total heat storage, via ice volume vs melt rate, has an important effect on slowing global warming; the total volume of ice is HUGE compared to the annual melt.
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  31. Peter: "total heat storage, via ice volume vs melt rate, has an important effect on slowing global warming"

    I don't understand why you are focused on this point. We have undeniable evidence that the Arctic ice melt seasons are growing more aggressive. The Arctic is the part of the globe with the most rapid temperature increase. Where and when is this 'slowing' of global warming going to kick in?
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  32. @muoncounter(32):
    I agree completely with your comment except to note that the "slowing" I refer to has kicked in a long time ago. Global warming would be much faster if it were not for the vast volume of ice holding things back.

    What I'd like is for you (and others) to do the math. Have a look at the annual ice melt vs the total ice volume. The result is something of a surprise, I think.
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  33. Peter,
    According to Wikipedia, the ocean masses about 1.3 billion gigatons. Greenland masses about 3 million gigatons of ice. It takes about 334 kj to melt one kg of ice and about 4 kj to raise one kilogram of water one degree celcius. I calculate it would raise the ocean about 0.2 C to melt all the ice in Greenland. The Antarctic is about 10 times bigger than Greenland so about 2C to melt all the ice in both ice sheets. I think the ocean can absorb more heat than the ice.
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  34. Peter: "the "slowing" I refer to has kicked in a long time ago. Global warming would be much faster if it were not for the vast volume of ice holding things back."

    I can see that working for Antarctica, as perhaps a reason the southern hemisphere is not warming as fast as the northern. I liken it to a house with the attic on fire; it's still pretty cool in the basement. And I think we agree that once the Arctic goes to minimal ice, things will be much worse.

    But the increasing melt rate of Arctic ice is a symptom of global warming, so I don't think one can say the ice is 'holding things back'.
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  35. @michael sweet (34)
    You did the math but you incorrectly assumed the Antarctic is ten times the size of Greenland. Nope. Total ice volume is about 3% of total water volume. Multiply 1.3 billion gigatonnes by .03. Now take the 334 kJ and the 4 kJ. See what I mean? Takes a lot more than 2C. But that wasn't my main point. Look at the annual melt rate. Look at the total volume. Ask how long the ice will last. Assume any melt acceleration you like.

    Yes,yes, the increasing melt rate is a symptom of global warming. No argument. But ask how long the ice will last. Please.
    @muoncounter(35) The trouble with the "house on fire" analogy is the rate of heat transfer. But I think you know that. Heat transfer along the earth's surface is relatively efficient.
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  36. @michael sweet(34) Whoops! My error. You are right about the 2 degrees. 2.4, actually. Still a lot more than the current <0.1. But in any case look at the melt rate vs volume.
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  37. Peter,
    It is not necessary for all the ice to melt (that would raise sea level about 75 meters) for it to be a disaster. If Greenland alone melted it would raise sea level 7 meters and put millions of people out of their homes. If half the ice in Greenland melted much of the sea level rise would occur since the bowl in the middle of the island would need to still be full of ice. That would require 0.1C of ocean heat, which has already occured.

    If you assume a 10 year doubling time for ice melt it is 5 meters of sea level rise by 2100. That would put Miami and 8 million other people out of their homes in Florida alone. For me that is a disaster. How much sea level rise do you need for it to count as a disaster? Of course the data indicates a doubling time of around 6 years at present. How do you feel about that for the next 50 years?
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  38. It is important to distinguish between the East (EAIS) and West (WAIS) ice sheets, the latter lying to the west of the Trans-Antarctic Mountains. The EAIS is a land based ice sheet loosing mass in a similar way to the Greenland ice sheet. The WAIS covers the land of a far-flung archipelago but is predominantly a marine ice sheet resting on the seabed.

    The WAIS is therefore far more susceptible to melting by coming into contact with the warming waters of the Southern Ocean and warmer currents flowing directly on to the ice from equatorial regions of the Pacific Ocean. This is why it is now loosing ice at a rate almost three times greater (132 Gt/annum) than the much larger EAIS (54 Gt/annum).

    Contact of warmer seawater with the WAIS face rising from the seabed makes it particularly vulnerable (far more so than EAIS or Greenland) to erosion of its footing. Were this to occur, large areas of the marine ice shelf could, in a relatively short time, loose contact with the seabed and become floating ice. This would result in more rapid sea level rise than present levels of ice loss from land based ice sheets.

    Sorry if this sounds nit-picking.
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  39. Please, please, no more doomesday scenarios! Here's the math that no one will do: The total ice volume is about 40 x 10^6 GT. The rate of melting is currently well under 1000 GT per year, or 0.001 x 10^6 GT/yr. (I hope this time I've done the math correctly!)
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  40. Peter: "ask how long the ice will last. Please."

    We have, several times. See the thread on Ice free Arctic, among others.

    I don't understand why you seem to be insisting that Arctic ice isn't in jeopardy. You've agreed that the Arctic is warming precipitously and that Arctic ice melt is increasing. There's no need for any specific heat calculation; the figures in the ice-free thread -- and several other threads on the same topic -- show very clearly how long the ice will last. The Flanner thread and papers linked on that thread amplify this topic with the increase in heat absorption of open Arctic waters vs. ice.
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  41. I think also a few people have tried to point out that we don't have to melt all the ice before we see significant sea level rise.

    We don't even have to melt the ice to see *any* sea level rise - it just has to start floating, where it's presently sitting on the seabed. It can do this by dynamically thinning to the point where it starts floating, which can also happen a long, long time before it actually melts.

    That Hansen paper Daniel Bailey referenced way back at comment #8 also makes the point that, once sea level rise reaches a rate of about a metre per decade, that we will start to see significant negative feedback from all those chunks of ice floating around in the oceans.

    That's pretty cold comfort, though (no pun intended!) - 1 metre per decade will be disastrous, as will the cumulative rise before we get to that point.
    BTW, for some sea level rise maps of selected areas of Australia, check out this site. It only has 0.5, 0.8, and 1.1m scenarios, but it's a starting point.
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    Moderator Response: [muoncounter] fixed link
  42. Bern @ 42 ... We don't even have to melt the ice to see *any* sea level rise - it just has to start floating

    That is exactly the point I make at 39. It seems to me that the real threat to sea level is the potential for WAIS to loose its footing on the seabed.
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  43. Re: Ice sheets and floating

    This is probably a good time to add some clarity to the mechanisms behind what happens at the transition between a "stable" grounded ice sheets, ice shelves and sea ice. The reality is that ice is a viscous fluid and "flows" from higher to lower altitudes.

    James describes ice shelves and sea ice in the post at top. When a "grounded" ice sheet meets the sea it will either unground and become an ice tongue/ice shelf, or it will calve. Ice tongues/shelves act as buttresses, slowing the advance of the ice sheet towards the sea.

    When an ice sheet no longer has this protective formation, gravity (because gravity is a you-know-what) is able to exert its full effects and the speed of flow nearest the sea increases. This downhill delta/vector is then propagated "upglacier" until a new dynamic equilibrium (in terms of rate of loss) is reached. The ice edge at the sea:ice junction calves, sometimes steadily, sometimes catastrophically (see video at bottom of this comment).

    The faster the ice sheet moves downhill, the faster the calving rate. This is the Jakobshavn Effect. Enough basal lubrication (the Zwally Effect) can increase this rate of downhill flow somewhat, but the primary mechanism of transport is the Jacobshavn Effect.

    At this point it's the sheer scale of the ice sheet that makes it difficult to understand why, if the basal melt causes an ungrounding of the ice sheet leading edge, why the whole sucker doesn't just pop up like a cork?

    Unlike a floating iceberg, where 90% of the berg is below the water line, on the ice sheets the majority of the ice is above sea level, especially at the 20th Century terminus points. If the calving front retreats landward enough, the basal edge is in much deeper water. So eventually you might see big bergs bobbing up, but in the interim (at least the next 20 years or so, even for the WAIS) thinning & outlet transport will rule the day.

    This doesn't mean that existing & future calving events are not and will not be spectacular, because they are. Check this out (pay special attention to the 12:45-13:05, 15:20-16:00 and 16:05-17:00 segments):



    Hope this is more clear than mud,

    The Yooper
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  44. ^^^ what he said!

    I think you explained it much better than I did, Daniel - and thanks for the link to that video - amazing stuff!
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  45. Peter:
    I am sorry, I thought you said "Assume any melt acceleration you like." I chose the conservative 10 year doubling time. The data shows a doubling time of six years which results in much higher sea level rise. You assume no acceleration, which is certainly not correct. You claim that we do not need to worry until the last ice cube is gone, which will be several hundred years. At that time sea level will be 75 meters higher than now and billions of people will have no homes. That is too much of a disaster for me to contemplate, even several hundred years from now. Please tell me what you consider a disaster so that we can only measure the time until that happens. For me 5 meters of sea level rise by 2100 is a disaster. That is the course we are currently on with BAU.
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  46. Peter,

    You seem to thing that this is all a storm in a tea cup. I do wish that were the case, but unfortunately research has determined how the planet responded to warming in the past, and the results are to say the least a little worrying. Others here have cited Hansen et al's recent paper. The figures below were sourced here.



    Also,



    These graphs are based on the work of Rohling et al. (2009). These data suggest that the increase in global sea level by 2100 could be way too low. Although we know that there will be quite a delay between ice loss and the initial temperature increase. So those are more likely expected sea level increases at equilibrium, and are not valid at the actual time the global surface air temperature anomaly reaches +3 K.

    Now they could be wrong of course, but if they are even remotely close.....then we have set the scene for a whole lot of misery down the road.
    0 0
  47. One meter is a disaster if you're Bangladeshi. Two meters is a disaster in a lot of the world.

    I wonder what the over/under is on the date Venice is abandoned.
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  48. For everyone trying to nail down sea level rise based on ice melt, it should be remembered that thermal expansion of the oceans is contributing a factor roughly equal to that of ice loss. The best one could say is that at least, for the moment, that factor is not accelerating.

    But for the foreseeable near future, one must double the ice-loss-caused sea-level rise estimates, to account for accompanying thermal expansion.
    0 0
  49. @48 GFW

    One meter is a disaster in Oregon, where we have a coastal mountain range that crowds our fishing and tourism towns close to the Pacific, as well as the single road that links all coastal towns north and south, and that provides sole access to most of them. One meter rise will cut that road in many places, requiring expensive roadwork, higher up existing slopes, many new bridges, and will destroy countless buildings, and will require huge coastal engineering projects to maintain access to fisheries. We can hardly pay for schools as it is, the additional burden may be impossible to meet. We may see coastal towns, and north south road travel abandoned on our coast.

    @ #40, Peter, you claim people here refuse to do calculations.
    "Please, please, no more doomesday scenarios! Here's the math that no one will do: The total ice volume is about 40 x 10^6 GT. The rate of melting is currently well under 1000 GT per year, or 0.001 x 10^6 GT/yr."

    Why do you ignore post #8, which suggests we may see a doubling of ice mass loss every 5-6 years? That means a rate of loss 1,000 times as great as now in 60 years, or equal to 1/40 of existing ice at that time, if I did my calculations correctly.
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  50. Dan Bailey poses "
    At this point it's the sheer scale of the ice sheet that makes it difficult to understand why, if the basal melt causes an ungrounding of the ice sheet leading edge, why the whole sucker doesn't just pop up like a cork?". The accelerations observed for the Ice sheet from meltwater lubrication have mostly been inland, not at the ice edge. It not the amount of meltwater that is key but the basal water pressure. Near the margin there has always been plenty of meltwater, increasing this by a percentage means little. The accleration from meltwater on all glaciers tends to be highest early in melt season or when a glacial lake drains in either case the meltwater exceeds the ability of the basal plumbing system to drain the water. After the drainage system develops added meltwater no longer lubricates motion. This is why Shephard (2011) noted the potential of added meltwater to reduce GIS flow. It also is why the acceleration is short lived, spatially limited and often is followed by a slower than usual flow some weeks after. We also must remember that there are alpine glaciers in warm environments of New Zealand, Chile and Alaska for example where melting occurs most of the year at the terminus and this bountiful meltwater does not lead to their collapse. Instead they just develop effective sub-glacial conduit systems.
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    Moderator Response: [DB] Thanks, Mauri!

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