Threats to Water Supplies in the Tropical Andes

Raymond Bradley has a paper in tomorrow’s Science[1] laying out the significant threats to water supplies in the Andes as a result of global climate change. This is the sort of thing Roger Pielke Sr. wisely argues that we need – a better understanding of climate change effects on a regional scale “focused on regional and local societal and environmental resources of importance.”

Bradley and colleagues argue that melting glaciers threaten water supplies in places like Quito and La Paz, where water flowing off of glaciers serves as a critical dry season buffer:

Although an increase in glacier melting initially increases runoff, the disappearance of glaciers will cause very abrupt changes in stream-flow, because of the lack of a glacial buffer during the dry season. This will affect the availability of drinking water, and of water for agriculture and hydropower production.

Adaptation will be required, they say. Apparently no way to mitigate this on any reasonable time scale:

It is in the tropical Andes that climate change, glaciers, water resources, and a dense (largely poor) population meet in a critical nexus. Some glaciers have already reached the threshold at which they are destined to disappear completely; for many more, this threshold may be reached within the next 10 to 20 years. Therefore, governments must plan without delay to avoid large-scale disruption to the people and economy of those regions.

[1] Threats to Water Supplies in the Tropical Andes, Raymond S. Bradley, Mathias Vuille, Henry F. Diaz, Walter Vergara, Science 23 June 2006: Vol. 312. no. 5781, pp. 1755 – 1756, DOI: 10.1126/science.1128087


  1. This sounds very important, but I continue to wonder why the same thing hasn’t been done for Tibet, where it seems to me that the scale of the problem is vastly greater. Maybe I’ll be so bold as to email Ray and ask him.

    Will you maybe be doing a story on this? One could imagine that refugees will be trying to come north, so there’s your local angle.

  2. Sorry, won’t be doing a story. I’ve stretched darn far looking for New Mexico angles, but I’d be hard pressed to make that case to my editor with a straight face. 🙂

    But I do think it’s sad that all yesterday’s media coverage of the NAS hockey stick report so completely dwarfed this, which in the long run is the more imporant story.

  3. I didn’t stick a smiley on that suggestion, but I kind of figured it would be a stretch.

    I’ve believed for a couple of years now, ever since I started looking into the details of AGW, that the loss of the non-polar ice caps would be a huge deal. The fact that there’s no real argument over the science or the timing has probably resulted in it getting much less attention. The exception to that is of course the glacial canary in the coal mine, Kilimanjaro, where skeptical arguments about causes are more plausible since it’s such an isolated case.

    The AR4 ought to make a *huge* deal out of this issue, which is far and away the biggest impact we will see in the near future (although ocean acidification is a bit of a question mark in that regard). Hurricanes are relative chicken feed IMHO. Glacier loss is unfortunately a close relative of drought from a PR standpoint, although at least those before and after glacier pictures can be pretty effective. Unfortunately the before pictures tend to be unavailable for many of the big glaciers, in Tibet especially. (Side note: Three of the countries that will experience major and potentially politically destabilizing impacts from the loss of the Tibetan glaciers have nuclear weapons. And we’re all so worried about North Korea.)

    Maybe we should give people names to the big glaciers? E.g., “Bob is melting! Madge has retreated 80 feet this year!” Of course the most prominent one would have to be named Elphaba. What do you say to green dye on Kilimanjaro? 🙂 (Yeah, I know reducing the albedo would be a Bad Thing. Is there such a thing as reflective green dye?)

  4. “Although an increase in glacier melting initially increases runoff, the disappearance of glaciers will cause very abrupt changes in stream-flow…”

    This is the cruel irony: that the initial runoff increases can last for years or decades. Meanwhile, people living along the banks of these rivers build their livelihoods based on a fundamentally unsustainable water resource. Capacity building on a quick-sand foundation…

    Steve, I don’t think that snow-water on the Tibetan Plateau is projected to be quite as vulnerable as on the Andes, at least in the next century. I base this on a couple poster presentations by Steve Ghan — who has developed a subgrid parameterization scheme for downscaling GCM output — and I seem to recall that his prelim results point to this conclusion.

    You’ll find a couple papers on this in:
    Journal of Climate: Vol. 19, No. 9, pp. 1589–1604.
    (subscription required)

    …or, if you are going to “be bold” and e-mail anyone on this subject, you might try Dr. Ghan. 😉


  5. Steve,
    I found a paper with the figure that I had in mind :
    PDF: see Figure 3.

    There are a few important caveats… but the basic findings are consistent with my comment above.

  6. Thanks for that, James. I’ll look that stuff over in detail and then maybe drop him a line.

    This is an example of the sort of thing I was referring to: . I haven’t seen the underlying paper, but there have been a number of similar articles out in the last couple of years, at least one of them predicting loss of the ice within fifty years. A possible reason for the apparent discrepancy between the article and Ghan’s paper is that Ghan studied the Himalayas whereas the Chinese rivers have their source elsewhere in the Tibetan region. The paper didn’t include exact bounds for the regions studied, so it’s hard to know. I’m extremely interested in this, so I’ll try to tie up the loose ends and will email John with whatever I find.

  7. Pingback: jfleck at inkstain » Blog Archive » On Hockey Sticks and Glaciers

  8. I haven’t read the above linked articles yet, but wouldn’t the height of the glaciers have an important role? Thus the larger, higher caps be less vulnerable?

    Steve, I think it was you I replied to on James Annan’s blog with a reference to all the glacier fed rivers that are/could be under threat and the populations of their catchment areas? This could give a large population vulnerable to effects of glacier loss. E.g. China, India, Pakistan, Afghanistan, parts of Russia, the Alps, the Andes and possibly parts of the Middle East?

    One of my thoughts is that in many areas where the precipitation is seasonal, the glaciers will regulate the flow, where the high precipitation season grows the glacier, which reduces the downstream flow; while in the low precipitation season the increased meltwater increases the downstream flow.

    So what this could lead to is a(n increased) flood/drought cycle which would cause problems in densely populated areas, or those of high agricultural yield.

    What should always be taken into account by those who may not be interested, as they are not directly affected is that these sorts of problems (as well as increased chances of conflict) are the increased migration flows that will be created.

  9. The difference with height sounds right, but remember that the Kilimanjaro glaciers are thought to be disappearing as a result of reduced precipitation that allows ablation to dominate (although I believe it’s the case that there is still some melting involved).

    Your description of the consequences with regard to seasonal river flow is exactly right, Adam. It will be very interesting to see how the new paper discusses all of this with respect to the Andes. Here in California, it is expected that we will have to spend many billions of dollars adapting out current water infrastructure to the changed flow regime (actually just the result of a seasonal snow pack loss since the Sierras aren’t high enough to have much in the way of glaciers). Ultimately California can afford that, but so many others won’t be able to.

    Regarding your rivers reference, I’m afraid I missed it. Do you still have it?

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