Comments 1 to 15:

1. Riduna at 15:58 PM on 17 December, 2011
   It would be interesting to know how deterioration of glaciers is effecting human populations and their agriculture. We know that contraction of glaciers on the Sierra Nevada is causing competition between urban populations and Central Valley agriculture in California. Is this evident elsewhere?
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2. oneiota at 17:11 PM on 17 December, 2011
   @1 What are the specific issues arising in California? The "flow-on" effect (forgive the pun) of glacier retreat whilst not particularly on the topic is an interesting one. I guess it is where science will inform policy. I'm aware that one of the howling points raised by the doubters has been the IPCC's assessment of numbers of people who could be affected by loss of melt water from glaciers. There is a recent paper: Contribution potential of glaciers to water availability in different climate regimes that attempts to correlate dependence on melt water with population density: "Although reliable figures are often missing, considerable detrimental changes due to shrinking glaciers are universally expected for water availability in river systems under the influence of ongoing global climate change. We estimate the contribution potential of seasonally delayed glacier melt water to total water availability in large river systems. We find that the seasonally delayed glacier contribution is largest where rivers enter seasonally arid regions and negligible in the lowlands of river basins governed by monsoon climates. By comparing monthly glacier melt contributions with population densities in different altitude bands within each river basin, we demonstrate that strong human dependence on glacier melt is not collocated with highest population densities in most basins." This sort of study will hopefully help the IPCC sharpen its pencil in this area.
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3. John Russell at 20:50 PM on 17 December, 2011
   There is more info and a more up to date picture of the Chacaltaya glacier here. Last time I was in Chile, the issue of reducing glacial run off from the Andes was becoming very important for agriculture as it accounts for most of the irrigation required on the coastal plain to the west. I would guess it has similar effects to the east where glaciers feed the headwaters of some of South America's major rivers. A good summary here.
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4. mspelto at 23:06 PM on 17 December, 2011
   Shrinking is not quite the right word for the annual mass balance losses that are the focus of this bulletin. Shrinking will occur with ongoing mass balance losses and this makes 22 straight years of global mass balance loss, and we thought Chrysler and Greece needed a bailout. The loss of a glacier is just no longer unusual. It could be the Swiss, Cavagnoli Glacier or the Milk Lake Glacier in Washington today we have had to publish a model for forecasting glacier survival (Pelto, 2010)
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5. Steve Case at 01:57 AM on 18 December, 2011
   http://www.skepticalscience.com/glacier-update-0809.html#70066 "Although reliable figures are often missing, considerable detrimental changes due to shrinking glaciers are universally expected for water availability in river systems under the influence of ongoing global climate change. We estimate the contribution potential of seasonally delayed glacier melt water to total water availability in large river systems. We find that the seasonally delayed glacier contribution is largest where rivers enter seasonally arid regions and negligible in the lowlands of river basins governed by monsoon climates. By comparing monthly glacier melt contributions with population densities in different altitude bands within each river basin, we demonstrate that strong human dependence on glacier melt is not collocated with highest population densities in most basins." Most needs to be defined, but that says that most people don't rely on glacier melt water. And where it does if it does, it all boils down the difference in melt rate between ice and snow.
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   Moderator Response: [muon] 'Most' does not need to be defined here. Please do not attempt to bog a discussion down in such pedantics (defined as 'excessive concern with formalism and precision').
6. John Russell at 04:06 AM on 18 December, 2011
   Re:#5 It's worth pointing out that in the area I mentioned at #3 -- the coastal region between the foot of the Andes and the Pacific, which is over 2,000 miles long and ~100 miles wide (and the reason Chile is called 'El Spaghetti' by the locals) -- meltwater from the Andes is vital for agriculture in the warmer central and northern areas. Without this water the area would suffer major drought, as in some places the annual rainfall is only 120mm. The rivers flowing off the Andes are dammed and the meltwater is fed through a series of reservoirs, canals and channels to irrigate the entire plain. Though the plain is not densely populated in places, the food grown there is vital both for feeding the population in the cities and for growing produce for export. The reduction in the size of glaciers is greatly concerning the South American countries that rely on this source of water.
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7. Steve Case at 05:49 AM on 18 December, 2011
   It's also worth pointing out that as long as the glaciers continue to recede, the total amount of water in the various watersheds will exceed the amount of precipitation. If the glaciers no longer recede or disappear, the water in the watersheds will reflect the amount of precipitation. Should the glaciers grow again, the amount of water building up in those growing glaciers won't be flowing in the rivers. In other words, taking ground water and evaporation into account, the water available to be dammed up and fed through the various reservoirs, canals & channels for irrigation is the sum of total precipitation in the watershed plus or minus the glacier melt or ice build-up. Countries that rely on water from watersheds that contain glaciers as a source will enjoy more water than supplied by precipitation as long as the glaciers continue to melt. They can't continue to melt forever, the recession will either stop or the glaciers will disappear, either way the volume of water currently available will decrease.
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8. mspelto at 06:04 AM on 18 December, 2011
   Glacier runoff does not generally increase or decrease the total annual runoff. It does alter the timing of the runoff and hence the seasonal distribution. As the glacier shrinks in area the amount of water it releases via melting declines during the melt season, which is typically the dry season. Runoff will generally increase in winter and during the spring melt when streamflows are generally high. The glacier acts as a natural reservoir. The smaller the frozen reservoir the less the melt it releases. This is widely observed already today, during retreat, that glacier runoff in the melt seasons declines and is not offset by the increased melt rate once significant area loss has occurred (Pelto, 2008). The loss for example has enhanced the increasing low summer flows in many rivers including the Skykomish River (Pelto, 2011). Now picture yourself as the water manager involved with the Artesonfaju Glacier where is the extra water? In this case since you are losing your melt source, and evaporation of precipitation is high, net annual runoff is already declining.
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9. Kiwiiano at 06:12 AM on 18 December, 2011
   An aspect that isn't made clear here is that if the retreats are due to reduced snowfall in the icefields, eventually the river flows will diminish. If they are due to a change from snow to rain, even with no change in annual volume, an additional problem arises of the rain pouring down the mountains in a rush, boosted by melted ice, leading to flood pulses that may overwhelm storage or protection systems.
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10. Steve Case at 06:34 AM on 18 December, 2011
    http://www.skepticalscience.com/glacier-update-0809.html#70120 An aspect that isn't made clear here is that if the retreats are due to reduced snowfall ... The IPCC tells us: For a future warmer climate ... Globally averaged mean water vapour, evaporation and precipitation are projected to increase.
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    Moderator Response: [muon] fixed closing italics tag.
    Note the use of the word future in your IPCC quote. Note also the words globally averaged mean water vapour. Clearly glaciers are not a global mean occurrence.
11. John Russell at 07:23 AM on 18 December, 2011
    10# Yes, but -- as Kiwiiano writes -- what farmers need is steady flow in rivers throughout the year -- not seasonal floods and droughts.
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12. Steve Case at 08:15 AM on 18 December, 2011
    http://www.skepticalscience.com/glacier-update-0809.html#70126 ...what farmers need is steady flow in rivers throughout the year -- not seasonal floods and droughts. This discussion if allowed to proceed will wind up comparing the difference in melt rates between snow and ice.
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13. MarkR at 13:15 PM on 18 December, 2011
    10 Steve Case: Yes, that's true. Total precipitation is expected to increase. But as temperatures rise, regions currently near 0C will go above 0C, so see less snow. Also, the Hadley cells are expected to expand, meaning an extension of the try zones above and below the equator. So it depends on where you are!
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14. John Russell at 20:29 PM on 18 December, 2011
    #12 If you read the first link I provided in #3 you will see a photo of the mountainside with the Chacaltaya glacier and snow almost completely gone. There's then this unequivocal quote... “Here you have precipitation only part of the year,” said French glaciologist Patrick Ginot as he stood at 16,500 feet next to Zongo glacier last year. “But it’s stored on the glacier and then melting throughout the year, and so you have water throughout the year. If you lose the glacier, you have no more storage.”
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15. Dart. at 23:12 PM on 12 December, 2012
    Kiwiiano asks about reduced snowfall. The critical factor in the demise of Chac' was cloud cover reduction since about 1980. No cloud means no snow replacement during the short snow season but crucially it means no shade from the intense tropical sunlight which, at that altitude drives very rapid sublimation of ice to water vapour. Sunlight warming peripheral rocks of the glacier would also have caused more melting than in earlier times. There isn't a weather station in the Chac' valley but near-by monitoring stations showed only margin warming and certainly not enough to melt ice significantly. One has to be careful to read widely and take into account all the variables and even then it is hard to draw solid conclusions. Sorry I've lost the link to this information now but it was from Bolivia,-The Monday Morning Newsletter or something like that and mostly written in Spanish.
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