RealClimate logo

Glacier Mass Balance: equilibrium or disequilibrium response?

Filed under: — group @ 24 May 2007

Guest Commentary from Mauri S. Pelto

I get asked at least once a day about the future prognosis for alpine glaciers and whether they have a future. I will focus here on North American glaciers whose mass balance measurements in the West from 1984-2005 indicate a declining trend. The trend suggests that all of the glaciers are out of balance and that some will disappear. The question is determining which glaciers are merely out of equilibrium and can retreat to a position of equilibrium, and which are in complete disequilibrium and will melt away? Let me explain.

Glaciers have long been recognised as sensitive and reliable indicators of climate. This together with their wide distribution has made them a key marker of current climate change. The World Glacier Monitoring Service (WGMS) collects from alpine glaciers around the world annual mass balance data, which is the difference between annual accumulation and annual ablation (from melting and sublimation). A glacier is divided into an ablation zone where all accumulated snow is lost from the winter and an accumulation zone where snowpack is retained to the end of the summer. The alpine glaciers discussed here are not calving, nor do they have superimposed ice, two factors that complicate measurement. For an alpine glacier to have an equilibrium balance it typically must have upwards of 60% of its area in the accumulation zone at the end of the summer melt season.

Glaciers respond to climate change in an attempt to achieve a new equilibrium. A glacier advances due to a climate cooling/snowfall increase that causes positive mass balance. The advance increases the glaciers area at low elevation where ablation is highest, returning the glacier to equilibrium. A climate warming/snowfall decrease leads to negative mass balances and glacier retreat. An equilibrium response occurs when a new equilibrium is reached by a retreating glacier losing enough of its high ablating sections, usually at its lowest elevations, so that accumulation once again balances ablation. This is a glacier equivalent to jettisoning bad debt. If a glacier cannot retreat to a point where equilibrium is established, it is in disequilibrium with the current climate. A glacier that is in disequilibrium will melt away with a continuation of the current climate.

Therefore, it is important to note whether glaciers are undergoing an equilibrium or disequilibrium response to climate change. It was noted in the Tropical Glacier Retreat post that if the annual equilibrium line rises above the mountain top this will lead to glacier loss. The equilibrium line is the elevation on a glacier at which annual accumulation equals ablation. This is not the only means to an end for a glacier, nor is it a robust means for identifying glacier disequilibrium. A glacier that is approaching equilibrium during retreat will thin mostly near the terminus, and at some elevation above the terminus, usually near the equilibrium line or in the lower accumulation zone, there will be comparatively little thinning (Schwitter and Raymond, 1993). This indicates that at least a portion of the glacier is healthy. A glacier that is not approaching equilibrium will thin appreciably along the entire length of the glacier (Pelto, 2006). There are two means to identify glacier disequilibrium. The first is substantial thinning along entire longitudinal profile of the glacier. The second is frequent years with no accumulation area at all – that is, the equilibrium line is above the glacier.

Figure 1. CRYSYS (CRYospheric SYStem in Canada) mass balance program (M. Demuth) monitors the Place and Helm Glacier in the British Columbia, Coast Range north of Vancouver. The USGS benchmark program monitors South Cascade Glacier, North Cascades, WA (R. Krimmel), Wolverine Glacier, Kenai Mountains, AK and Gulkana Glacier, Alaska Range, AK (R. March). The Juneau Icefield Research Program monitors the mass balance of Lemon Creek Glacier, Coast Range, AK (M. Miller and M. Pelto). The North Cascade Glacier Climate Project monitors the mass balance of Columbia, Daniels, Easton, Foss, Ice Worm, Lower Curtis, Lynch, Rainbow, Sholes, and Yawning Glacier, North Cascades, WA (M. Pelto).
Figure 2. The annual mass balance signal is remarkably coherent from glacier to glacier.

Annual mass balance measurements over at least 20 years are available on 16 glaciers in North America which are submitted to the WGMS. The annual balance and cumulative balance records of these glaciers illustrate the adjustment of these glaciers to climate change from 1984-2005 (Figure 1-2). The annual balance record, in meters of water equivalent, 1984-2005 indicates that despite continued significant retreat of all 16 glaciers, mass balances are declining. The mean annual mass balance has declined from -0.35 m/a 1984-1990, to -0.59 m/a in 1991-1997, to -0.91 m/a from 1998-2005. Instead of approaching equilibrium after several decades of dominantly negative annual balance, and significant retreat, mass balance is becoming more negative. This indicates the glaciers are getting further away from equilibrium, not approaching it. The consistency of the cumulative balance trends further indicates the glaciers mass balance record is dominated by large scale regional to global climate change.

Which glaciers are experiencing an equilibrium response? Glacier thinning in the accumulation zone is limited on Lemon Creek (Miller and Pelto, 1999), Gulkana and Wolverine (Sapiano, et. al, 1998), and Easton, Lynch and Rainbow Glacier (Pelto, 2006). Each appears able to retreat to a new point of equilibrium with the current climate. On Columbia, Daniels, Foss, Ice Worm, Lower Curtis, Sholes, and Yawning Glacier thinning is not notably less in the accumulation zone than the ablation zone, indicating disequilibrium (Pelto, 2006). Two North Cascade glaciers where mass balance measurements were conducted the Spider and the Lewis Glacier disappeared during this interval (Pelto, 2006). On Helm and Place Glacier the loss of more than 20% of their entire volume, since 1984 suggest they too are in disequilibrium. South Cascade Glacier has lost 70% of its volume since 1890 (Josberger et al., 2006), and is still experiencing large negative balances indicating disequilibrium. In Glacier National Park, North Cascades, Helm Glacier and Place Glacier frequent loss of the entire snowcover by the end of the ablation season has become commonplace (WGMS, 2005). The result is in net ablation throughout the accumulation area causing thinning of the glacier in the accumulation zone.

Are the glaciers responding primarily to climate changes of the last 30 years, or to the post Little Ice Age conditions? In the North Cascades Rainbow, Yawning, Easton and Lower Curtis Glacier all advanced during the 1950-1975. Sholes, Columbia, Ice Worm, did not advance or retreat significantly during this period. Thus, each of these glaciers is definitively responding to the climate of the last 30 years.

Figure 3. Lower Curtis Glacier in 2003 and in 1908 Photograph by namesake Asahel Curtis

In the North Cascades the warmer temperatures have increased summer ablation on the glaciers (Pelto, 2006). The glaciers are losing mass not just because of increased summer melting. In the winter despite a small increase in precipitation, retained snowpack at the end of the winter has declined. The reduced accumulation is due to rain and melt events in the winter. On Lemon Creek Glacier it has been noted that fewer ice lenses are forming (Miller and Pelto, 1999). This is an indication that the spring snowpack is not cold enough to lead to substantial refreezing of meltwater. This is important information regarding the 1998 mass balance, because 7-11% of the retained annual accumulation is typically in ice lenses. Failure to retain this meltwater increases net ablation. The reason for the change is warmer snowpack winter temperatures, +4oC above the long term mean at the Juneau Airport seven miles from the glacier at sea level in 1997 and 1998. The result of the failure to recapture meltwater as internal accumulation is increased ablation and negative annual balances (Kaser and Osmaston, 2002).

Alpine glaciers are losing income during the winter (snowpack) and have increased expenses (ablation) in the summer, and there is no financial safety net for recovery. In the North Cascades of Washington the United States Geologic Survey (led by Austin Post) completed an inventory identifying 756 glaciers in the range based on photographs from the 1950’s and early 1960’s. In 2005 an examination of these same glaciers, indicates that at least 53 are no longer present, illustrated in google earth tour.

If we had an endangered glacier act, we would need to invoke it for alpine glaciers in many of our world’s glaciated mountain ranges.

82 Responses to “Glacier Mass Balance: equilibrium or disequilibrium response?”

  1. 51
    Jim Crabtree says:

    #46: John:

    Thanks for the info.

    # 48: Mauri:

    This was not my original source, but it says the same thing.

    “Rapidly melting glaciers on the Qinghai-Tibet Plateau could cause a major flood on the Yangtze River this summer, says Cai Qihua, director of Yangtze River Water Resources Committee.

    We should be vigilant for a comparatively big flood on the Yangtze,” said Cai, emphasizing that meteorological conditions are similar to those of 1998 when a major flood killed more than 1,000 people.

    With plenty of snow on the Qinghai-Tibet plateau, higher than normal temperatures could lead to rapid melting and run off, said Cai.

    Meanwhile, water levels have remained low along the entire mainstream of the Yangtze and its tributaries since last winter and many areas in the Yangtze River Valley are suffering drought.

    No serious floods have occurred on the Yangtze since 1998, so it is quite possible for a major one on the river according the natural cycle,” said Cai.

    She also noted that the damage that might be caused by rainstorms and possible floods and other natural disasters in the Yangtze River Valley should not be underestimated.”

    On reading it closer, the third statement says it is a lot of snow that is also adding to the problem. I understand there are over 40 large glacial lakes in the Himalayas that are going to be causing some serious problems in the not too far distant future. Thanks for clearing up the misconception I had about glacier melting and flooding.

  2. 52
    Chuck Booth says:

    Re # 40 Harry,
    I’m curious: Do you critique the arguments put forth by AGW skeptics and denialists as carefully as you do the arguments of mainstream climatologists that AGW is real?

    And if the scientific consensus was that there is NO anthropodenic global warming, would you, as a random contrarian, argue that AGW IS occuring?

  3. 53
    mauri pelto says:

    #50- The response time of glaciers is something that Tomas Johannesson of Iceland first quantified. The faster a glacier flows, the smaller it is and the higher the amount of ablation and accumulation the faster it responds. In the North Cascades for example a climate shift toward wetter cooler conditions began in 1946, and by 1949 the first glacier to advance the Coleman Glacier on Mount Baker was noted. By 1956 all of the ten main glaciers on the mountain were advancing so in that case 3-10 years response. Similarly in 1977 a shift to a warmer-drier climate occurred, this led to retreat of all of the above glaciers by 1988, within 12 years. Further details on response time, and why not all of the glaciers advanced after 1946 are at J of Glaciology paper .

  4. 54
    George Collins says:

    I found Mauri Pelto’s guest comment excellent (missing only the definition of melt rate m/a supplied later). It seems to support much of what I have read. I recommend Mark Bowen’s book “Thin Ice” largely describing the work of Lonnie Thompson who has been all over the low and mid latitudes gathering ice cores and has seen glaciers shrinking. Then there are the continuing reports from mountineers and ice climbers reporting the same. Then there is Gore’s selection of glaciers from all over the globe. If that is “cheery picking” it is one hell of a tree. I believe the IPCC report (WG2) put the Himalayan Glaicers as the fastest retreating glaciers in the world and are expected to be 80% gone by 2030. There is also a report in Bowen’s book that the Chinese expect all their glaciers to be gone by the end of the century. This is a world wide problem.The alpine glacier has been called the canary in the coal mine. How dead does that canary have to get before folks realize we have a major problem? – George C.

  5. 55
    John Mashey says:

    #50, #53, #54

    bjc #50: if you look at the Holzhauser paper I mentioned in #15:

    They think the Great Aletsch glacier has a reaction time of about 24 years (Ts in Prof. Pelto’s paper) and the Gorner glacier 19 years, but the response time (Tm) is much longer, on the order of 50-100 years. So, they say:
    “The present-day position of the glacier front is therefore a reflection of the climactic conditions of past decades.”

    Putting all these together, glaciers act as a good set of time-filters. Some respond fairly quickly, and others take much longer to even notice a change, so there is a *natural* mechanism that smooths out gyrations of different durations, depending on the characteristics of the glacier. One doesn’t need to calculate N-year rolling averages, because the glaciers do it themselves :-)

    It is worth looking at the chart of Great Aletsch:

    and read the actual data:

    This is relatively smooth retreat, with recent acceleration reflecting the warming of the 1980s, I guess. If I understand this, Aletsch has barely noticed the last decade’s heat yet.

    It is also worth looking at:
    which sorts the Swiss glaciers by increasing length. The shorter ones tend to have much quicker advance/retreat jiggles, whereas the longer ones have filtered that out. Of course, there are variations, for the sorts of reasons described in Prof. Pelto’s nice paper in #53.

    Anyway, it’s quite useful to study these things … while they last.

  6. 56
    ajg says:

    Re Comment #47,
    I was going to mention Palisade Glacier, but you beat me to it! Of the glaciers in the Sierra Nevada, that is the only appreciable one left. There are only a few sets of air photos (USGS and privately), but it’s reasonable to estimate ~30% mass loss since mid-century. The SNOTEL records are really key to that study- and yes, we did use snow-water equivalent. A very interesting neighboring glacier just to the south appears to react more slowly, but it is mantled with a thick cover of rock debris (~1m).

  7. 57

    As some may recall, Pat Michaels tried to claim that Kilimanjaro’s alpine glacier retreat had nothing to do with global warming in 2002. The more reasonable explanation is that intensified convection over the Atlantic and Indian Oceans is altering the African monsoon and delivering less precipitation to the continent – which, in combination with rising surface temperatures, presents a double whammy to power leveling

    I would have preferred it if Skeptic Tracker’s comments had been dealt with in a similar fashion and in accordance with your comment policy. Skeptic Tracker’s comments, left as written, are clearly an ad hominem and entirely without foundation, i.e., I have never had anything to do with any think tank or interest group

  8. 58
    mauri pelto says:

    I can’t let the discussion end on that note. On the glacier noted above in western North America, the declining mass balance in recent years has been in spite of higher precipitation rates. Thus, the negative balances fall squarely on the shoulders of higher temperatures. If you are interested in a map based view of temperature changes examine this Goolge Earth file that has over 6000 stations with graphs of temp. changes for each.

  9. 59


    If you are accept that the glacier melting is due to CO2, what do you suggest we do? Reduce atmospheric CO2 instead of increasing it? That would seem to be the only solution, since we know at this level of CO2 the glaciers will continue to melt, and if we continue burning fossil fuels then they will melt faster.

    When are you scientists going to admit that we have passed the tipping point?

    When are you scientists going to tell GWB that his call to postpone action for another 18 months means that we will have another 18 months of his destruction to repair after he has gone?

  10. 60
    Hank Roberts says:

    Dr. Peito, perhaps the Contributors here can keep the discussion open but set a (very, very) high bar on approving postings to it, so you can continue to provide information as you do your research. (Gavin, consider this a plea.)

    Hoping for a “scientist talk, we read” approach to this, and other scientist-specific topics, on a selective basis, I am.

  11. 61
    mauri pelto says:

    Sorry the link for the GISS global temperature stations is GISS

  12. 62
    SomeBeans says:

    #60 Hank Roberts

    I’m with you on this one – it would be nice to see the comments attached to articles consisting of Q&A arising from the article (with answers supplied by the climate scientists) and perhaps a separate area for discussion by the variously informed.

  13. 63

    Dear Dr Peito,

    You wrote “The question is determining which glaciers are merely out of equilibrium and can retreat to a position of equilibrium, and which are in complete disequilibrium and will melt away?”

    In the spirit of Hank’s #60 post, can you make an estimate about how many more glaciers will have changed from a state of “out of equilibrium” to “complete disequilibrium” duting the next 18 months?

    Regards, Alastair.

  14. 64
    wacki says:

    In terms of continuous measurements only two Lemon Creek and South Cascade were begun in the 1950’s.

    Pelto, thanks for the insightful post. What response would you give to someone that claims this USGS website on the South Cascade as proof that global warming happened started before man? This USGS website seems to contradict your claims.

  15. 65
    Ike Solem says:

    Alastair, I don’t understand why you make a habit of attacking scientists who are simply trying to present an accurate picture of their research to the public. As far as your demand for an 18 month prognosis from ‘you scientists’, keep in mind that there are unavoidable uncertainties in the near-term climate predictions – for example, we could have another Pinatubo three months from now that would result in a short-term aerosol cooling.

    Secondly, what do you mean by “out of equilibrium” vs. “complete disequilibrium”? It’s obvious that alpine glaciers all over the world are melting, and it appears that the main culprit is increased surface air temperature at higher altitudes, though changes in precipitation play a role in some areas as well.

    I think your energy would be better spent trying to convince investors and politicians to begin truly large-scale renewable energy projects, as well as to institute a hefty carbon emissions tax.

    RE#57, on Kilimanjaro, the facts seem to be that Over the 20th century, the areal extent of Kilimanjaro’s ice fields has decreased ~80%, and if current climatological conditions persist, the remaining ice fields are likely to disappear between 2015 and 2020. (from Thompson et al 2002)

    So, there appears to be some debate as to what the causes of this particular glacial retreat are, and not much of it seems convincing. It ties into the question of what is causing the expansion of the subtropical dry zones in Africa (ongoing drought since the 70s). Cullen et. al GRL 2006 divides the mountain into ‘slope’ and ‘plateau’ glaciers, and claim that the fate of glaciers on Kilimanjaro has nothing to do with modern climate change, since there was ‘rapid retreat of slope glaciers in the early 20th century’.

    This may not be entirely fair – but these authors claim that changes in the large-scale circulation dating back to the 19th century are responsible for the glacier retreat on Kilimanjaro – which just seems to be unsupported.

    This paper has been uncritically trumpeted by Roger Pielke Sr., the World Climate Report, and other skeptic web sites… which have a long history of selectively choosing only those papers which support their views. However, the fact is that Kilimanjaro has been glaciated throughout the entire Holocene, and yet it is predicted to be bare within a few decades. Despite this, the contrarians have loudly proclaimed that glacial retreat on Kilimanjaro has nothing to do with global warming, based on a handful of papers by the same authors.

    This claim has been widely used to attack Al Gore’s “An Inconvenient Truth”, so it seems worth rebutting.

    There are really just three positions that the skeptics hold. First was “global warming isn’t happening”, and then it was “global warming is happeing due to strictly natural causes”, and the next one will be “yes, we are causing global warming, but it’s going to be a good thing!”.

  16. 66
    Hank Roberts says:

    Ah, Ike, Dr. Pelto said he’s here to talk about that sort of question. Look again at the original post, and Alastair’s question.

    Dr. P: “I get asked at least once a day about the future prognosis for alpine glaciers and whether they have a future. I will focus here on North American glaciers whose mass balance measurements in the West from 1984-2005 indicate a declining trend. …. The question is determining which glaciers are merely out of equilibrium and can retreat to a position of equilibrium, and which are in complete disequilibrium and will melt away? Let me explain.”

    Backing, slowly, away from the keyboard myself here …..

  17. 67
    mauri pelto says:

    #64 In the paper on glacier response time, I note that the South Cascade Glacier is slow to respond to climate and never fully adjusted to the post Little Ice Age climate before the current warming began. However, note that the it is the USGS mass balance record that shows it is the current warming that is the problem. Note the mean annual balance for the glacier from 1959-1976 -0.43 m/a, from 1977-1997 -0.84 m/a was and from 1998-2005 was -1.2 m/a. Thus as the graph for all three USGS glacier show and the cumulative balance curves in this commentary indicate mass balance is becoming more negative. On South Cascade Glacier this is after getting rid of the lowest elevation, highest melting section it is doing even worse. SC Glacier

    #63 The present climate is not new, we have experienced the current warm conditions for two decades. Thus, either a glacier is in disequilibrium or it is not, 18 months is not going to change that. A glacier may disappear in that period. The point is we can diagnose glaciers in disequilibrium, before they are nearly gone. We do not then have to reexamine every year to see if they have changed, glacier equilibrium is a longer term process.

  18. 68
    Ike Solem says:

    Thanks Hank for the reminder.

    Maybe the question is this – if these notions of glacial equilibriuma and disequilibrium are good descriptions of the alpine glacier system, do they also apply to larger ice masses like Greenland and the West Antarctic ice sheet? It seems that glaciologists were predicting that these systems were stable a decade or so ago, but now it looks like some kind of dynamic response is occurring – something that no glaciologist predicted.

    I suppose my question for Dr. Pelto is whether or not similar issues arise in the case of alpine glaciers?

  19. 69
    mauri pelto says:

    The reason we often use smaller alpine glaciers as reliable climate indicators is because their dynamics are simple. Thus, their response is directly to climate and glacier dynamics plays little role. That is obviously not the case. Dynamics become much more important if a glacier is calving, if it is not a temperate glacier, that is at the pressure melting point throughout and if it has a buttressing ice shelf. I would argue having worked on Pine Island Glacier and Jakobshavns Glacier 20 years ago, that what we are seeing today, was anticipated and was even the basis for the funding we used. That is we (U of Maine) were investigating the speed of Jakobshavns and measuring its characteristics to determine the role of various forces, including the water supply. The latter was expected to increase with warming and was of concern. Hence, the term Jakobshavns Effect was coined by Terry Hughes. Further Pine Island was seen as the weak underbelly of the West Antarctic Ice Sheet by Hughes and Thomas (NASA), and that acceleration and grounding line retreat could occur due to thinning from enhanced calving or more basal melting. The speed with which the processes would act were of course not known. But the processes were described by these two researchers 20 years ago.

  20. 70
    Gary Tucker says:

    Has nobody ever heard of the Thermal Haline Circulation function of Global weather? This is a long term feed-back mechanism of global climate control…..also………….All official temp. readings have been taken for about 110 years in the US……….and in the last 100 years what has happened to those locations, that is more than enought to account for the less than 1 degree warming? Ever hear of Thermal Mass….???? as in paving….buildings…etc………I live in the country……about 20 miles from the city…..I have a very accurate weather gathering station… temp. are at least always 3 to 8 degrees cooler………DUH,,,,, I am located in a setting much like the weather gathering stations of the past…before they became so “Built Around” ……Global warming freeks use the excuse…..I repeat “Excuse” that there is a “FORMULA” for adjusting for that fact…..LOL….Give me a break…………with a diff. of less that 1 degree in the last 100 years……that is a joke !!…………WHY DOSE NO SCIENTIST EVER BRING UP THE FACTS???????????????????????

  21. 71
    Ike Solem says:

    Thanks for the historical background. I suppose the correct interpretation is that climate models have a difficult time dealing with the many complex processes involved in the cryosphere, and so did not take into account processes that had indeed already been described by glaciologists.

  22. 72
    Chuck Booth says:

    Re # 70 Perhaps because all those smarty-pants climatalogists and oceanographers with their lofty university degrees just don’t know as much as you?

  23. 73

    Re #70 — Please note that global warming is also taking place in sea surface temperatures. It’s very difficult to attribute that to an urban heat island effect, because there are so few cities on the surface of the sea.

    The UHI accounts for, at most, 15% or so of measured global warming, and that’s being extremely generous.

  24. 74
    ajg says:

    RE: #70.

    Not to be rude, but who exactly do you think analyzes and describes how the ocean functions in the climate system? Perhaps the very same group of scientists you criticize? Do you really honestly think no one accounts for that, and that you have uncovered some ground-breaking piece of unforeseen evidence? I suppose if climate scientists simply looked at the thermometer nailed to the birdfeeder in your backyard, they’d be forced to recant all of their work. Or perhaps you should check out the link titled “Oceans” on the right-hand side bar. May I also suggest the use of the spell-checker function on your computer?

  25. 75
    ray ladbury says:

    #70 Gary, I can only assume that your isolation has kept you from reading relevant journals on climat and oceanography. Otherwise I could not excuse your ignorance of the fact that such effects are in fact discussed seriously by scientists every day.

  26. 76
    Hank Roberts says:

    Please don’t feed trolls. Let the topic be for Dr. Pelto to explain his work.

  27. 77
    ajg says:

    Do the alpine glaciers in the Cascades show an ELA change with latitude or longitude? I’m a little more familiar with the systematic ELA variations in the Sierra, but I’ve always wondered about the North Cascades and Olympics. Any thoughts?

  28. 78
    mauri pelto says:

    These small glaciers do not have a true equilibrium line altitude. The accumulation zone is not typically a single contiguous area. If you look at the elevation of the mean glacier elevation you can see that change within the North Cascades. The bigger change is from west to east across the ranges precipitation divide. If you compare the ELA from Mount Rainier to Mount Baker you can see the latitudinal change. Mount Rainier ELA’s typically have been 2100 m, compared to 1800 m on Mount Baker 2 degree latitude north. The Olympics have similar ELA to the North Cascades.

  29. 79
    Brooks says:

    With such a short time span for your data. Do you think about what if this is cyclical. And we dont know enough to be making determinations of what is really happening ? Could it be as simple as the earth is 5 miles or 500 miles closer to the sun on average? Maybe the earth climate is on an up shift for temp. and will come back down in 50 years? The sky is falling is hard to swallowwhen this started out with a volcanic rock. Since then we have had how many ice ages ? In school we were taught another was coming. The problem here is we as humans are usually very short sighted and every little change in a very complex pond. We have 500 people saying why. I think if we focus on what is simple we will find something more likely. We are a spec in time on this planet. Lets not be to hasty in our speculations. Im not saying your wrong. Im saying at least leave room for something else. I am undecided. But talking in circles … lol…I would find it easier to believe the planet is a little farther or closer to the sun in its orbit than to blame a few degrees on us.

    Thanks for your time

  30. 80
    tamino says:

    Re: #79 (Brooks)

    I would find it easier to believe the planet is a little farther or closer to the sun in its orbit than to blame a few degrees on us.

    Changes in earth’s orbit can indeed have a profound effect on climate. But earth is not “a little farther or closer to the sun…” The average distance earth-to-sun can be measured with amazing precision, and the changes of orbits under the perturbing influences of the other planets have been well understood for about 2 centuries.

    Changes in the eccentricity (degree of flattening) of earth’s orbit do effect the total energy we receive from the sun, but the change in average insolation is generally less than one half of one percent, and the cycle of eccentricity changes is extremely slow (about 100,000 years). Changes in the direction of earth’s spin axis, and the tilt of that axis, don’t effect the total sunlight we receive, but do effect the distribution of that energy; in particular, they effect how much goes to the polar (as opposed to equatorial) areas, and how solar energy is distributed throughout the year. This can effect the growth or decay of polar ice, and the “signature” of these cycles (about 41,000 years for the tilt angle cycle and 22,000 years for the axis direction cycle) are plainly visible in records of ice growth and decay during glaciations and deglaciations.

    The effect of changes in earth’s orbit are thoroughly well studied; the cycles which effect climate are often referred to as “Milankovitch cycles” (after the mathematician who argued strongly for their importance in driving ice ages).

  31. 81
    sidd says:

    i have been reading “Climate Change and Trace Gases”, Hansen et al., Phil. Trans. R. Soc. A (2007) v365, pp 1925-1954. I have not yet digested it fully, but two points struck me as relevant to this discussion
    1) the paleo record reflects the timescale of the forcings rather than the internal timescale of the icesheet melt. the rate of melting is not bounded above by the paleo record, the icesheets could melt much faster than the paleo indicates.
    2)albedo flip: Fig: 2 seems rather convincing. A model with albedo change included matches the Vostok record well…but with a sensitivity of 1.5C/(Wm^-2) rather than the 3/4 number, which does not include changes in surface properties.

    Hansen goes on to indicate that albedo flip is under way, as seen by the increasing summer melt in Greenland and Antarctica.

    A question i have is: what is the internal timescale for ice sheet melt ? i have heard that Prof. Alley is working on a model, but i have not seen any publication yet.


  32. 82
    Timothy Chase says:

    sidd (#81

    Hansen goes on to indicate that albedo flip is under way, as seen by the increasing summer melt in Greenland and Antarctica.

    I tend to put a fair amount of stock in what he says. Not that my opinion means that much, though.

    A question i have is: what is the internal timescale for ice sheet melt ? i have heard that Prof. Alley is working on a model, but i have not seen any publication yet.

    It is probable that they have gotten a little further than this might suggest:

    Report of the Workshop on Ice Sheet Modeling
    at the NOAA Geophysical Fluid Dynamics Laboratory
    8 January 2007

    … but there are undoubtedly a number of climatologists working in this area.