RealClimate logo

Amazonian drought

Filed under: — gavin @ 4 August 2006 - (Slovenčina)

There has been a flurry of recent commentary concerning Amazon drought – some of it good, some of it not so good. The good stuff has revolved around a recently-completed interesting field experiment that was run out of the Woods Hole Research Center (not to be confused with the Woods Hole Oceanographic Institution), where they have been examining rainforest responses to drought – basically by using a very large rainproof tent to divert precipitation at ground level (the trees don’t get covered up). As one might expect, a rainforest without rain does not do well! But exactly what happens when and how the biosphere responds are poorly understood. This 6 year long field experiment may provide a lot of good new data on plant strategies for dealing with drought which will be used to improve the models and our understanding of the system.

The not-so-good part comes when this experiment is linked too directly to the ongoing drought in the southern Amazon. In the experiment, older tree mortality increased markedly after the third year of no rain at all (with around 1 in 10 trees dying). Since parts of the Amazon are now entering a second year of drought (possibly related to a persistent northward excursion of the ITCZ), the assumption in the Independent story (with the headline ‘One year to save the Amazon’) was that trees will start dying forest-wide next year should the drought continue.

This is incorrect for a number of reasons. Firstly, drought conditions are not the same as no rain at all – the rainfall deficit in the middle of the Amazon is significant, but not close to 100%! Secondly, the rainfall deficits are quite regionally variable, so a forest-wide response is highly unlikely. Also, the trees won’t all die in just one more year and could recover, depending on yearly variation in climate.

While this particular article is exaggerated, there are, however, some issues that should provoke genuine concern. Worries about the effects of the prolonged drought (and other natural and human-related disturbances) in the Amazon are indeed widespread and are partly related to the idea that there may be a ‘tipping point’ for the rainforest (see this recent article for some background). This idea is exemplified in a study last year (Hutrya et al, 2005) which looked at the sharp transition between forest and savannah and related that to the coupling of drought incidence and wild fires with the forest ecosystem. Modelling work has suggested that the Amazon may have two vegetation/regional climate equilibria due to vegetation and climate tending to reinforce each other if one is pushed in a particular direction (Oyama and Nobre, 2003). The two alternative states could be one rainforested and wet like today, the other mainly savannah and dry in the Eastern Amazon. Thus there is a fear that too much drought or disturbance could flip parts of the forest into a more savannah-like state. However, there is a great deal of uncertainty in where these thresholds may lie and how likely they are to be crossed, and the rate at which change will occur. Models go from predicting severe and rapid change (Cox et al, 2004), to relatively mild changes (Friedlingstein et al (2003)). Locally these responses can be dramatic, but of course, these changes also have big implications for total carbon cycle feedback and so have global consequences as well.

Part of that uncertainty is related to the very responses that are being monitored in the WHRC experiment and so while I would hesitate to make a direct link, indirectly these results may have big consequences for what we think may happen to the Amazon in the future.

Special thanks to Nancy Kiang for taking the time to discuss this with me.

Update: WHRC comments on the articles below.

128 Responses to “Amazonian drought”

  1. 101

    Re #85 and ” I have a much stronger affiliation for the social markets of northern europe than for the religion of free markets that entralls the US.”

    Good for you. I myself remain a free-market freak. Europe’s three big attempts to remove the evils of the free market have included two huge failures (Naziism and Communism) and one semi-failure (modern mixed economies with sky-high unemployment).

    The alternative to private ownership of resources is public ownership — either the community owns stuff and decides what to do with it, or, more realistically, the state does. You’d think after Hitler and Stalin more Europeans would be wary of giving the state too much power, but they don’t seem to learn very well.

    [Response:I think that this caricature of Europe is very wrong. By the way, modern mixed economies such as those in the Nordic countries (Iceland, Denmark, Sweden, Finland & Norway, the latter which admittedly has an abundance of natural resources) rank at the top of the lists of countries with the best standard of living. Neither is the unembloyment high either (Finland had high unemployment for a brief in the 1990s though). -rasmus]

  2. 102
    Fernando Magyar says:

    Re: 101
    As a product of three cultures; European, North and South American, I currently tend to think that a form of non ideological anarchism, the kind that limits the influence of centralized systems of power may be the new paradigm to pursue. A concrete and simple example of this would be individuals becoming self suficient by generating their own energy and being free of the electric utility monopolies. The same concept could be applied to most centralized concentrations of power.

    Donella Meadows has written this paper: Leverage Points – Places To Intervene In A System

    I think she gives a very good of explanation why paradigm shifts of this nature are so difficult to bring about at the societal level.

  3. 103

    Re #100 which asks “So where has all the water not now falling as rain gone?”

    Here’s one place.
    See “India floods close energy plants”

    [Response:Eastern Europe also have got some rain… You can also check out the WMO site: . -rasmus]

  4. 104
    Liam says:

    “Thus there is a fear that too much drought or disturbance could flip parts of the forest into a more savannah-like state. However, there is a great deal of uncertainty in where these thresholds may lie and how likely they are to be crossed, and the rate at which change will occur”

    I had a relook at the Maslin and Burns (2000), and it made me wonder about just how uncertain we are…

    they suggest that at the time of the younger dryas, there was a 60% decrease in river flow. They attribute this reduced precipitation to a more zonal formation of the ITCZ.

    I’d like to get my hands on current up to date temperature records for the area, for one of the characteristics of the time was the reduction in temperature, which would have reduced plant water requirements, even as the athmosphere grew dryer. Weather here is extremely variable, and we routinely have 3 to 5 day “friaje” events (cold fronts) that reduce ambient temperature by 50%. I imagine these friajes occurring in the dry season have a significant limiting effect on drought. Though it must be noted that these fronts are more local effecting mostly the southwest as far as the Ucayali.

    Further adding to the variability is the metabolic gradient that runs from west to east with the eastern forests, slower growing and longer living, therefore with a much slower turnover rate.

    Gavin, I don’t know if you’ve seen the paper, but if you have, does it say if this dieback of larger trees was across all species, or species specific?

    Plus i’m having difficulty figuring out how a transformation to savannah would occur away from the margins. What sort of dispersal mechanisms would be applied, etc.

    In their press release they make reference to plants in the understory halting woody growth due to drought. However this is a common mechanism for plants here which have adopted a kind of ‘gambler’ strategy of sitting and waiting for a tree to fall and a gap to form in more mature stable forests. A number of tree species employ this strategy here with growth rates highly variable depending on habitat, and therefore light variability. Reproductive strategies are similarly effected. A good example of this Bertholletia excelsa which will begin flowering and fruiting at less than 10 years old in open conditions, but can take up to 100 years in mature forest according to matrix models.

  5. 105
    Pete Best says:

    The World is geared up for Fossil Fuel. 100+ years of providing infrastructures, prospecting, drilling and harnesing Oil Gas and Coal has made it cheap and easy to move around great distances. Humankind has almost cheated nature in many ways by the discovery and use of these fuels. These reasons also tell us why no true easy to turn on alternatives have ever been discovered. Climate Scientists speak of tipping points whereby the climate cannnot be reversed for many years after the point has been reached. The same goes for economies and civilisations, fossil fuels will require at least 50 years to replace in their entirity even if we have a real alternative available within the next 5 years (which I doubt) at full production level.

    Their is a massive latency (time) in something being discovered and it being all pervasive like fossil fuels are. 50 years at least I would suggest.

  6. 106
    Brian Gordon says:

    Re: Free markets versus Centrally-planned:

    The US does not have a free market or capitalism. You have something new, perhaps called corporatism; consumers or individuals are not making the primary decisions, though they think they are (and could, if they woke up). Your forebears were very right to fear slick-talking Washington lawyers.

    Not sure what this has to do with climate change science…except, I suppose that these things are preventing the truth about same, and about practical solutions, from coming to light.

  7. 107
    Kieran Morgan says:

    After finding out this week that Tony Blair is only now planning on having eco bulbs put in 10 Downing Street I don’t have much faith in any major policy changes by the UK government re. the Amazon. Politicians just don’t seem willing to grasp the nettle.
    This past week I read that China has plans for a huge scheme to bring water from the Qinghai/Tibet plateau to more arid regions to provide supplies that will “last a 1000 years” , building to begin 2010.
    If the glaciers there are melting at the rate of 7% a year there’s not going to be much water there at all in a few decades. Is this right or have I got my facts wrong?
    If it is right then there isn’t much communication or sensible thought going on in the Chinese government.

  8. 108
    Jeffrey Davis says:

    re: 105

    50 years? Consider how quickly the country adopted the electric light and the telephone. What needs transformation is the way people think.

  9. 109
    Gar Lipow says:

    >fossil fuels will require at least 50 years to replace in their entirity even if we have a real alternative available within the next 5 years (which I doubt) at full production level

    1) We have alternatives now – efficiency means, wind. Many of them are at full production levels. Others could be brought to that level very quickly.

    2) In the U.S. at least most of our energy consuming infrastructure is projected to last less than 30 years. For example the average lifespan of a U.S. car (from assembly line to car-crusher) is aroud 12 years. Commercial buildings generally need a full rehab every 25 years. So there is no reason we cannot replace our energy consuming infrastructure AS IT WEARS OUT over the course of 30 years to reduce energy consumption, and also phase in renewable sources, and thus completely phase out fossil fuels if we wish.

  10. 110
    Steve Bloom says:

    Re #107: As I understand it, the concern about Tibetan region glacier melt is that without the glaciers there will be a major interruption in water supply each year since the glaciers serve to “even out” water flow off the plateau. We have something similar with the Sierra snowpack here in California. If warming causes winter precipitation to fall as rain rather than snow, it will run off immediately rather than be stored as snow that will melt and supply water well into the summer dry season. The key point is that warming can create serious water supply problems in such areas even with no change in precipitation.

  11. 111
    Pete Best says:

    Re #109. You have no alternatives to replace then more lip service to fossil fuels. Nothing at the present time can, look into it seriously, Wind and othe renewables without massive investment and uptake can do what fossil fuels can do at the present time.

    I would imagine that if the western world embraced all methods of fuel conservation, inclusing smaller cars, micro wind and massive wind and decided to cover huge swathes of land and sea in solar panels then yer we may have a chance of mitigating climate change but not eliminate it utterly. Fridges need more efficient technology as do all electrical equipment but it all takes time and lots of money to achieve.

    Unfortunately Aircraft have no alternative fuel that they can use, cars might have a chance with Ethenol but unless new techniques are brought in to produce it then there is not chance of it mitigating GAS/Petrol for many years to come, around 50 probably.

    Then there is the politics, free market capatalism will prevail and hence we await a natural solution rather than a state induced one

  12. 112
    Phillip Shaw says:

    Re #111: In America, we already have “huge swathes” of land set aside that could be used for solar energy. I’m referring to the millions of acres of highway right-of-ways, airports, country roads, railroad easements, parking lots, urban and suburban roads, rooftops and so forth. Use of this land will require only refinements of existing technologies rather than technological beakthroughs. And the electronics industry has a superb trackrecord of refining manufacturing processes and bringing down costs in response to market opportunities. Remember the good old days when a calculator cost what an entire computer costs today?

  13. 113
    Pete Best says:

    Re #112 I do not doubt it that the USA could if it wanted to (which is seemingly does not at the present time) could make add to the mix of current energy needs solar, pv, wind, geothermal, tidal and wave etc, however at the present time its fossil fuels all the way due to the politics of energy a well as the enourmous costs involved in switching infrastructures.

  14. 114
    liam says:

    Hi Gavin, Rasmus.

    I know you have for good reason ruled out posts on politics, though posts about the role of the media do come close. However given that there seems to be a huge amount of interest in finding a viable solution, which tends to bring out some argumentative comments, some very valid i might ad, would you consider doing a post that looks at the possible solutions, from either an economic, technological or societal point of view? An example is an evaluation of the Kyoto Protocol and its different aspects.

    Ultimately it doesn’t have to be political or promote any one system over another, but just a space where people can trash out these ideas. It might help to keep such conversations from filtering into every thread. It appears also that there is huge interest, judging from the amount of posts this thread received.

  15. 115
    Ike Solem says:

    By the way, it’s not just the Amazon and tropical forests that are changing under the influence of AGW; Northern Boreal Forest could also start releasing CO2 at a greater rate. There is a good Woods Hole site on this as well:

    There is more carbon in the soils of boreal forests then in tropical forests, and warming at the poles is expected to be higher then the global average warming. Release of CO2 from northern forests and permafrost is an additional concern.

  16. 116
    Joseph O'Sullivan says:

    There have been other papers on anthropogenic effects on forests and tipping points. Both of these are about cloud forests and how man-made changes reduce forests which in turn reduces cloud formation, therefore there are less cloud forests.

    Climatic Impact of Tropical Lowland Deforestation on Nearby Montane Cloud Forests

    Forest on the edge: Seasonal cloud forest in Oman creates its own ecological niche

  17. 117
    Gar Lipow says:

    >Wind and othe renewables without massive investment and uptake can [not?] do what fossil fuels can do at the present time.

    Without massive investment – no. But our infrastruture requires massive investment anyway. Put 300 billion a year into efficiency and renewables, and you could completely phase out fossil fuels over thirty years. Further, by picking “low hanging fruit” first, you could front load the phaseout. What we are lacking is not techical capability or resources to implement the capability, but political will.

  18. 118
    Gar Lipow says:

    Re 114 – I can think of several scientific issues related to alternative energy that would be interesting to cover:

    For example.

    “Evaluation of global wind power” by Cristina L. Archer & Mark Z. Jacobson. Their conclusion was 72 terawatts, many times total world power consumption. Now of course even there really is this much wind power available,without some sort of storage or backup you can’t rely totally on variable pwoer such as wind. But still it is important if true. However I’ve heard some criticism of this study on grounds they are using point source extrapolation. Basically the criticism says, that there isn’t really 72 terawatts available in 100% of the wind passing over land (as opposed to water) at the height they checked wind speed at. That if you started tapping a significant portion of the winds power, there would be less available elsewhere – that in short they are simply measuring the same power many times. And I don’t know if that critique is crackpot or if the authors of the paper made a really basic mistake. I’m not qualified to judge. But it really is a scientific question; how correct is this study, and also even if it is correct, what are the full implications? Without storage the variable nature of wind limits how much of our grid we can safely supply from it at a reasonable cost – no matter how much is available. Anyway I think this might be an interesting scientific issue to do a post on. And, you know, based on a scientific paper published in (I believe) a peer reviewed journal.

    If you like this one, I can probably come up with others.

  19. 119
    Gar Lipow says:

    Rather than just the abstract – here is a link to the entire paper

  20. 120
    Kieran Morgan says:

    Re No.110: Thanks for your comment Steve. Filled in a few gaps in my knowledge.
    Here’s a link regarding the Chinese scheme which also raises doubts about it’s long term viability,

  21. 121
    Mike Atkinson says:

    Field Ecological research like Woods Hole Research Center are trying to do is very difficult, and so results must be treated with caution.

    They are only performing the experiment at one site, Amazonia is vast with great diversity. I would hope to see similar experiments in other areas. Replicated results are always preferrable and I would judge in this case essential.

    Soil depth seems very important to drought resistance, from Forest impacts of our artificial drought:

    “By the end of the five-year period of exclusion, many trees in the forest were drawing water from more than 40 feet deep in the soil.”

    “Once the moisture that is stored in deep soil is depleted, the largest trees – towering 130 to 150 feet above the ground and basking in full sunlight – begin to falter and die. The death of such large trees that may take centuries to reach the top of the forest canopy, increased from about one percent per year to nine percent in the fourth year of the experiment, when soil water was depleted.”

    I was under the impression that Amazonian soils were ofter shallow, so other parts may not fare as well.

    Lastly, I’ve tried to find published results of this work but have failed, Publications of the Woods Hole Research Center does not seem to have any. Have I missed something

  22. 122
    Robin Johnson says:

    While I am dismayed as anyone at the destruction of rain forests in the Amazon, I think we need to get our facts straight. It does NOT take centuries for trees to reach the top of the canopy in the rain forest (or any other forest for that matter). When a gap in a forest canopy appears, seedlings and younger stunted trees grow like mad to reach mature height. In the rain forest, this takes longer because the canopy is higher [30 – 50 meters] compared to most forests [20 – 25 meters]. Depending on species, it takes 30 to 80 years for trees to reach mature height in rain forests (only 10 to 40 years in other forests) which isn’t exactly zippy but its NOT centuries. Most trees live for a dang long time after reaching mature height – but they have to reach mature height QUICK or competitor trees will beat them to the sky and shade them out. So evolution has driven deep forest trees to reach mature height really fast. Rain forest trees have a higher mature height – because they can given the nature of abundant water supplies.

    A recent article in the Smithsonian about the Serengeti was instructive because it outlined that the only reason the Serengeti remains a savannah is the PRESENCE of the massive herds of wilderbeests. When the wilderbeests died back due to an accidentally introduced cattle virus, the savannah rapidly grew brush and trees. The general evidence is that any region that gets more than 35+ inches of rain per year is gonna be forested in the absence of other pressure (humans or grazing animals removing the mature trees and preventing seedlings from growing). In my native Georgia, US, any field left unmowed for more than three or four years will be teeming with pine trees and completed forested within 10 – 20 years.

  23. 123

    I am making a TV series (in the Community Television sector) titled MEDIA AND GLOBAL WARMING
    and found the Amazon is Dying item.
    I am interested in making contact with people who might like to contribute in some small way. Text can be quote from screens with approval.
    I have also asked here in Australia if the Bush War on Terrorism is a distraction to the more serious question of Climate Crisis.

  24. 124
    Mike Atkinson says:

    There are obvious questions about this study.

    1. is it typical of the amazonian forest as a whole in terms of tree species soil type and depth, etc.?

    2. Even if it is typical how many non-typical areas are there which might respond in a different manner to drought?

    3. How much rain reached the soil? How does that compare with the current drought?

    4. How much water seeped in from the sides (under the trench)?

    5. In a drought the relative humidity of the air would be less, how does this affect tree growth and mortality?

    6. The study area is within a reserve where presumably human interference is minimal, how does drought react with other human activities (e.g. selective logging)?

  25. 125
    liam says:


    1. is it typical of the amazonian forest as a whole in terms of tree species soil type and depth, etc.?

    This is one limitation in this type of study. Whilst there are typical soil types, and tree species throughout the region, there are many variables relating to those very factors.

    2. Even if it is typical how many non-typical areas are there which might respond in a different manner to drought?

    Locally, the Amazon is extremely diverse, though it has a broad degree of homogeneity on a regional scale, once you back up to genus/family level.

    Things to consider however are the regional gradients in climate and thus adaptations to climate. An example is the increased rainfall heading from south to north. The physiology of within tree genera can be quite distinct based on these factors. Further, there is a gradient of fertility running roughly east to west with the west most fertile. For example the forests of Manaus have a much lower fertility than those around Iquitos (Peru). This gradient also coincides with an increased metabolic requirement in western forests, with shorter life spans and much higher turnover. Western forests are much more dynamic, and as a result more biodiverse.

    6. The study area is within a reserve where presumably human interference is minimal, how does drought react with other human activities (e.g. selective logging)?

    The big one there is slash and burn, as aerosols from the fires have a direct impact on cloud formation. Selective logging on the other hand is usually targetted towards species such as mahogany and spanish cedar (most valuable), though other hard woods are sold for domestic markets and/or charcoal. Larger species are usually targetted. Another factor relating to human activities is along the rivers. Most of the diversity in species is in the varzea (floodplain forests) as they are the most fertile with the highest rate of disturbance. These also would have a very different reaction to drought than terra firme forests.

    Regarding any papers about the project I can point you only to one which i provided above but is useful only for learning about their methodology as it is about imaging spectroscopy.

  26. 126

    Re #121 Where Mike wrote “Lastly, I’ve tried to find published results of this work but have failed, Publications of the Woods Hole Research Center does not seem to have any. Have I missed something.”

    There is an piece about the Amazon drought in tomorrow’s issue of Nature. It says that a paper by Nepstad of WHRC is under review at the jounal Ecology. It continues “Nepstand is now lighting test fires to assess whether one fire makes the forest more vulnerable to others.”

  27. 127
    David says:

    danger: soja in Amazonia!!


  28. 128
    Mike Atkinson says:

    Liam, Alastair, Thanks! The Asner et al. paper answers some of my questions.

    It only seems to have used data from 2000 & 2001, in 2001 the covering reduced the 1920mm rainfall in the control area to about 600mm in the dry-down area – which is what the area would recieve during a drought. Locally droughts are caused by ENSO events.