Science: Global warming is killing U.S. trees, a dangerous carbon-cycle feedback

Figure 1Contrary to the popular notion that increases in carbon dioxide emissions increase vegetation, a “stunningly important paper,” in Science finds the reverse has been true.

Led by scientists from the US Geological Survey and USDA, “Widespread Increase of Tree Mortality Rates in the Western United States,” finds:

Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions.

After examining a variety of potential causes, the study concludes:

Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.

And this is only the tip of the (disintegrating) iceberg — the planet is on an emissions path to warm 10 times as much in the coming century as we warmed during the period examined in this study.

As a co-author warned the BBC, this is another potential major amplifying feedback that could itself accelerate global warming in the coming years and decades:

“We may only be talking about an annual tree mortality rate changing from 1% a year to 2%, but over time a lot of small numbers add up,” said co-author Professor Mark Harmon from Oregon State University.

He feared that the die-back was the first sign of a “feedback loop” developing.

As regional warming caused an increased number of trees to die, there would be less living trees to absorb carbon dioxide from the atmosphere.

Yet there would be an increased proportion of decaying trees, releasing the carbon that had been locked away inside the trees’ wood.

And indeed, as I’ve previously reported (see “Something else for the deniers to deny: The ocean is absorbing less carbon dioxide“), the Global Carbon Project analysis of the “natural land and ocean CO2 sinks” finds:

… the efficiency of these sinks in removing CO2 has decreased by 5% over the last 50 years, and will continue to do so in the future. That is, 50 years ago, for every ton of CO2 emitted to the atmosphere, natural sinks removed 600 kg. Currently, the sinks are removing only 550 kg for every ton of CO2 emitted, and this amount is falling.

Of course, no discussion of the impact of global warming on Western tree mortality would be complete without a mention of climate-driven pest infestations:

Warmer temperatures might also increase the number and prevalence of insects and diseases that attack trees, the team added.

They used the example of recent outbreaks of tree-killing bark beetles in the US, which have been linked to a rise in temperatures.

Glad to see the major British media outlet get this important part of the story right (see “Even conservative San Diego Union knows climate change is killing Western forests” but “The New York Times blows the bark beetle story“).

Science‘s news story on the piece, “Western U.S. Forests Suffer Death by Degrees” (subs. req’d) notes:

“This is a stunningly important paper,” says David Breshears, an ecologist at the University of Arizona, Tucson. For years, he and others have lamented massive diebacks that occur when fungal and insect pests ravage stands of trees. “What’s harder to detect,” he explains, is any subtle but significant shift in the trees’ background death rate. “They have done a very thorough job” of documenting it….

Ultimately, “the finger seems to be pointed to warming.” says Breshears. Temperatures in the [Western] United States have risen about 0.4°C per decade in the past 40 years. Snowpack of the regions examined diminished over the time period they studied and is melting earlier, effectively lengthening the summer drought. Warmer air also leads to more evaporative loss, exacerbating the effect.

Julio Betancourt of USGS adds “Models suggest that most of this change was due to the buildup of greenhouse gases.”

The ultimate danger is that what we are seeing so far is just the beginning or ‘linear’ stages of what will ultimately be a nonlinear catastrophe. After all, the planet has warmed only about 0.6°C in the past 40 years. We are on track to warm close to 10 times as much this century (see “Hadley Center: Catastrophic 5-7°C warming by 2100 on current emissions path“). What would that mean?

Another member of the team, Dr Nate Stephenson, said increasing tree deaths could indicate a forest that was vulnerable to sudden, widespread die-back.

“That may be our biggest concern,” he warned.

“Is the trend we’re seeing a prelude to bigger, more abrupt changes to our forests.

So add this study to the list of key amplifying feedbacks that the major climate models are missing:

The time to act is now.


28 Responses to Science: Global warming is killing U.S. trees, a dangerous carbon-cycle feedback

  1. Gail says:

    Why isn’t anyone doing the equivalent study on the east coast? It’s happening at least as fast, if not faster.

    Oh wait…where’s my koolaid? I need another sip!

  2. paulm says:

    Interesting article on China…good environment links….

    Sustainability in China
    Dr. Lin Jiabin, the deputy director of the department of social development research at the state council, on China’s environmental policies and current Chinese efforts to address global warming. From WorldChanging, part of the Guardian Environment Network

  3. Anthony says:

    It is way past the time to act.

    Add to your missing feedbacks; mecanical failure of the ice. We are watching the defrosting freezer counting the drips convincing ourselves it will take weeks to defrost.

  4. Vernon says:

    Just a question but the oceans ability to absorb CO2 should have increased now that the PDO has entered its cold phase which should last for the next 20-30 years. The cold PDO is linked to lower annual rain fall in the South West and West.

    While everyone agrees there is warming in the US, the actual warming for the last century for CONUS peaked in the 1930,s dropped and when warming resumed still has not reached that level of warming within CONUS.

    Has there been any studies that show how much of the tree death is due to replacement of the current species with trees that better suited for the changing climate?

  5. Gail says:

    Vernon, I don’t know where you live but I live in rural western New Jersey. I am not a scientist so I haven’t done any studies and I can’t explain the technicalities. But I’ve got two eyes and I can walk outside and see that every specimen of every variety of tree is on the verge of death if not dead already around here. Individual trees have been dying here and there the past 30 years but since last summer, when the leaves were brown, scorched, and falling off early, I can’t find one that isn’t dropping limbs, covered with lichens and a black fungus, with peeling bark. Since October the pines have been shedding their needles at a breathtakingly accerating rate and many are now completely bare. Just the past two weeks, the evergreen shrubs, like boxwood, started to turn yellow. Once that happens they don’t come back.

    I can’t predict what might replace the species we have now. The problems I see are that 1. The soils have been stripped of nutrients by acid rain and 2. The varieties suited to much drier climates won’t survive cold snaps where gets down to 7 degrees (yesterday morning).

    As far as I’m concerned you can quibble about statistics and there is a place for that. But the evidence for catastrophic climate change (wildfires, mass extinction, water shortages) is right outside my window.

  6. John Mashey says:

    To be even clearer about the various bark beetles:

    1) Some of them generate their own antifreeze, so they are killed off by coldspells in autumn (before the antifreeze develops) or even deeper coldspells in winter. Of course, one of the signatures of AGW is that it warms faster at night than at day, during winter rather than summer, and nearer the poles. I.e., perfect for the beetles.

    2) The beetles are having a good time here in British Columbia. Satellites show the problem, and they have an extensive website, just on pine beetles (among others). See the animated map at the end of this page.

    2) In Colorado, they say

    “Every large, mature lodgepole pine forest in Colorado and southern Wyoming will be dead within three to five years, killed in a mountain pine beetle infestation unprecedented in the state.”

    I thought I would see other opinions, so I went to Science Policy, Colorado and Prometheus, and searched for “beetle”, but for some reason, this topic doesn’t get much coverage. Well, Prometheus doesn’t have *any* I could find.

  7. Vernon says:

    About your points:

    1. Acid rain has been non-existent within the US for about 20 years.
    2. I have to disagree, cold and dry is a common environment. Hot and dry largely exists in rain shadows (the American SW) or at +/- 30 degrees lat due to global wind patterns.

    I agree pollution is bad. I agree, introducing foreign species into bio spheres is bad.

    But I just cannot see blaming everything on CO2. If you want to correlate temperature to anything, then try the PDO and solar activity. Between them for the 20th century they have correlation of .88 vs. the .44 for CO2. Basically the first PDO warming in the 20th century gave the US the dustbowl and a Mississippi that nearly dried up. When the PDO switched to cooling phase it gave us cooling from the 40s to the 70’s. Then when the PDO switched to warming, we had warming till it switch just switched cooling, which we have now (global temps have been trending down for the last 8 years). Though out all of that, CO2 did nothing but go up but the temperature did not follow it. Do not take my word for it, research it your self.

    (As a side note, Mona Loa has shown the rate of CO2 has been increasing has be slowing since the PDO cold phase started and the oceans can now absorb more CO2)

    Oh, and yes I think we are affecting the climate but only slightly via CO2. I think that land use and miss use is doing much more to change climate than anything else man can do to change it.

    So while I agree with the effect, I question the causal relation ship to CO2.

  8. John McCormick says:


    You are stepping out of your league.

    You said:

    [1. Acid rain has been non-existent within the US for about 20 years.]

    That is a false and devious statement.

    Acid rain is the product of NOx and SO2 emissions from fossil-fired boilers in the case of both and, in the case of internal combustion engines, NOx.

    Congress did enact an acid rain contol program that required power plant SO2 emissions be reduced by 10 million tons. CORRECT ME IF I AM WRONG, but that leaves half of the baseline — approx 22 million tons of SO2 emissions still going up into the Eastern North American atmosphere to mix with rain and fall as acidic rain or as dry sulfate.

    NOx rains down as nitric acid or nitrates. Lots of NOx going into the atmosphere.

    You can look up the data yourself on EPA website or take my word for it; acid rain continues but not in the media.

    John McCormick

  9. Gail says:

    I’m not at all certain what is leading to such a sudden destruction although it has been gradually creeping up. I don’t know if it’s warming or pollution or some combination, or some other factor. I do know it is rapidly accelerating.

    As far as the acidification of the soils goes, please check this link:

    One of the (many) things that concerns me about the notion of a sudden tipping point leading to mass extinction has more directly to do with humans, specifically:

    1. I fear a sudden and across the board attack of cancers; and

    2. Sperm counts have been declining dramatically, which is so well documented you can easily google many studies all over the world. What if that decline gets even more dramatic?

    These sorts of things are of particular interest precisely because no one seems to understand the exact mechanism leading to a sudden mass mortality.

    As the study mentioned with the trees on the west coast – and I see the same here – every age, in every environment, of every species is suffering equally. So it can’t be the air, or any particular bug or blight.

    That’s why I suspect it is from acid rain, which pretty much spreads all over, and drying and warming.

  10. Max says:

    Let’s not forget that heat can also have big impacts on agriculture. See e.g.:

    Science 9 January 2009:
    Vol. 323. no. 5911, pp. 240 – 244
    DOI: 10.1126/science.1164363

    Prev | Table of Contents | Next
    Historical Warnings of Future Food Insecurity with Unprecedented Seasonal Heat
    David. S. Battisti1 and Rosamond L. Naylor2

    Higher growing season temperatures can have dramatic impacts on agricultural productivity, farm incomes, and food security. We used observational data and output from 23 global climate models to show a high probability (>90%) that growing season temperatures in the tropics and subtropics by the end of the 21st century will exceed the most extreme seasonal temperatures recorded from 1900 to 2006. In temperate regions, the hottest seasons on record will represent the future norm in many locations. We used historical examples to illustrate the magnitude of damage to food systems caused by extreme seasonal heat and show that these short-run events could become long-term trends without sufficient investments in adaptation.

    1 Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195–1640, USA. E-mail:
    2 Program on Food Security and the Environment, Stanford University, Stanford, CA 94305–6055, USA. E-mail:

    Read the Full Text
    The editors suggest the following Related Resources on Science sites:
    In Science Magazine

    CLIMATE CHANGE: Higher Temperatures Seen Reducing Global Harvests
    Constance Holden (9 January 2009)
    Science 323 (5911), 193. [DOI: 10.1126/science.323.5911.193]
    | Summary » | Full Text » | PDF »

  11. Vernon says:

    I was wrong on the acid rain.

    I know it is no excuse but I did not realize that the source and the impact site were different. Since there was not a national mean, I looked at the network at USGS since they are tasked to monitor acid rain. Did not see much in the states that emitted sulfates and nitrates (Midwest) but did not think to look at EPA.

    So I was wrong but I learned something new.

  12. Gail says:

    We are all here to learn, Vernon. (Except troll deniers.)

  13. DavidONE says:

    Additions to the list of feedback mechanisms (or are these currently included in the models?):

    1. loss of albedo effect due to ice-free Arctic ocean and absorption by darker water – + +

    2. methane clathrate release –

    3. oceans releasing CO2 as they warm and therefore making a net contribution instead of reduction – not sure about this one, it’s from memory and can’t find source

    And all of this is happening faster than predicted:

    “This is typical of so many climate studies—almost without exception things are turning out to be worse than we originally thought.” – +


    > …how much of the tree death is due to replacement of the current species with trees that better suited for the changing climate?

    Last time I checked, trees don’t migrate.

  14. Aaron d says:

    OK, I’m running on a tangent here but, we all talk about CO2 levels effects on the environment but I feel human health is kind of overlooked to some degree (though I’m way more interested in the environment :) ). The effects of 400-500ppm CO2 may not be that noticeable. But getting closer to 800ppm and people get headaches/dizziness, all the way to 1000ppm and its enough to kill you. Funny how many of the earth’s past extinctions (not KT) had CO2 levels at 1000ppm or above. I realize this point may be mute because humans may not survive an iceless planet which will come before 800-1000, but if most other organisms are effected in some way by increased temp and CO2 its possible its already having an effect on our own biology.

  15. Gail says:


    In theory trees could migrate but there are many reasons they won’t. For one thing, change is happening too fast. Also, their mobility (and that of other species as well) is restricted by human development – roads, cities, subdivisions; and furthermore, they cannot keep up with the invasive species that are more suited to dominate in sunny areas absent the canopy of a forest. In my area, the deer eat every single shoot. There are no trees under the age of around 15 years outside of fenced landscapes.

  16. Steve Bloom says:

    Gail, surely there are tree ecologists working for the state and/or universities in NJ. Have you talked to any of them?

  17. Steve Bloom says:

    Joe, this other new paper adds a further unfortunate wrinkle. Per this study we need the big trees for effective sequestration, but per the one you posted on the big trees are dying. Not good. Title/abstract:

    Fuel treatment effects on tree-based forest carbon storage and emissions under modeled wildfire scenarios

    Forests are viewed as a potential sink for carbon (C) that might otherwise contribute to climate change. It is unclear, however, how to manage forests with frequent fire regimes to maximize C storage while reducing C emissions from prescribed burns or wildfire. We modeled the effects of eight different fuel treatments on tree-based C storage and release over a century, with and without wildfire. Model runs show that, after a century of growth without wildfire, the control stored the most C. However, when wildfire was included in the model, the control had the largest total C emission and largest reduction in live-tree-based C stocks. In model runs including wildfire, the final amount of tree-based C sequestered was most affected by the stand structure initially produced by the different fuel treatments. In wildfire-prone forests, tree-based C stocks were best protected by fuel treatments that produced a low-density stand structure dominated by large, fire resistant pines.

  18. Mick says:

    If I’m not mistaken jungles covered the earth during the dinasaur period, because the earth was much warmer and had much higher levels of carbon dioxide. Indeed, past incidences of warmer climate are all marked by higher levels of vegetation and larger animal life (those cute long necks had a large appetite).

  19. llewelly says:

    If I’m not mistaken jungles covered the earth during the dinasaur period, because the earth was much warmer and had much higher levels of carbon dioxide. Indeed, past incidences of warmer climate are all marked by higher levels of vegetation and larger animal life (those cute long necks had a large appetite).

    Very good point. I can’t imagine why everyone is so upset about having to wait a few million years for plant and animal life adapted to the new environments to evolve.

    (Actually, ‘jungle’ is something of a misnomer – most of the mid-latitudes consisted of dry forested areas, akin to modern New Mexico. And there were also some areas that had little or no vegetation.)

  20. Barry says:

    A few more key points on this study:

    1) The washington post article stated “”It is the largest research project ever done on old-growth forests in North America.” This isn’t a “maybe” result.

    2) The excellent Globe & Mail article quotes the scientists as saying that once a mature tree dies and releases its carbon: “there’s no way that you can make up for that, that you can recover that by either creating wood products or by growing a young stand of trees.”

    3) The study says the number of new baby trees has stayed the same…but more older trees are dying. The trend will result in forests with only have the average age as now. That means lots less carbon stored.

    What is hinted at but not clearly pointed out in my opinion is the brutal fact that forest ecosystems take far too long to fully mature to hope to co-exist with the rapidly shifting climate bands. Long before a forest can hit old growth maturity the climate will have moved on.

    We aren’t just killing current trees…we are destroying the very possibility of mature northern forests.

    And as Joe points out, mature forests hold far more carbon than young ones.

    I know two former executive directors of large forest-focused environmental groups who have switched to focusing full time on stopping climate change for this very reason. Even pristine old-growth forests will die unless we get climate change under control soon.

    We all have to choose whether we want fossil fueled luxury for a few more years OR old-growth forests. We can’t have BOTH.

  21. I was interested to find the Climate Progress blog. 20 years ago I had a book published on different economic concepts to point the way to a sustainable world economy. Someone who liked the book contacted me this year to suggest that I update and re-publish it as a blog. She set up the blog, and the book is now complete on the blog in a series of postings. There are now also additional pieces on global warming and other subjects. Here is the link:

    With all good wishes,
    Charles Pierce

  22. Maarten says:

    If (since) climate bands are wandering to the poles much more quickly than most tree seeds can migrate, wouldn’t it be advisable to collect trees seeds (before those trees died in their current geographical band) and nurture young trees to plant them out 200 to 1000 km more to the north? It would take about 2 to 3 millions acorns/seeds per square kilometer, and in the USA and Canada there are about 6 millions km2 of forested areas so we need roughly seedlings. OK, some species travel on their own (such as birches), but oaks, maples, beeches and many coniferes could definitely need some help. At a (conservatively) estimated cost of 2 $ per seedling this would be about ….

    And this expenditure would even grow our economy, because ecological services that we used to receive for free would finally become part of the financial circulation. (This paragraph is ironic.)

    Oh, I forgot that the climate is shifting so rapidly, that we probably can’t mature forests in one location, so we have to do it twice, much better in economic terms!

    Why do some mainstream economists systematically overestimate the costs of mitigation, and severely underestimate the costs of adaptation? Must be a cult of reductive, unsystemic thinking (I am trained as an economist, so I know what I am talking about).

  23. Russ says:

    Why do some mainstream economists systematically overestimate the costs of mitigation, and severely underestimate the costs of adaptation? Must be a cult of reductive, unsystemic thinking (I am trained as an economist, so I know what I am talking about).

    So you think your colleagues for the most part truly believe in these estimates, and that it’s not a calculated delayer tactic, the way it is for most other professional deniers?

  24. Gail says:

    Steve Bloom, should you check back.

    I have spent many hours these past few months researching and writing every public service forester, conservation group, and university researcher I can find – I am making quite a pest of myself! Many (most) of them do not, I believe, fully acknowledge the scope of the problem. The NJ DEP wrote me back about drought, for instance. The ONLY way they measure it is reservoir levels. So if we get the same amount of rain, but in shorter, heavier bursts, and this leads to drier soils, the DEP has no way of monitoring that. They obviously don’t want to.

    In the case of groups such as NJ Conservation Society, the Audubon Society etc., the livelihood of the staff depends on the existence of forests. Who is going to donate money to charitable organizations to plant trees and protect habitat if those groups openly admit that the trees and habitats are not salvageable? It’s perfectly human of them. They will release reports on particular species but never give the big picture.

    And then there are some who recognize explicitly that we are in the midst of a mass extinction, but they tend to feel, that there isn’t much that can be done. There is a group in England that is collecting seeds and preserving them, but only from arid climates because they don’t have the technology for seeds from temperate climates that will not germinate after the drying and freezing process they use.

    One of the reasons I was given that there is relatively little focus on the Eastern forests is that conservationists, several decades ago, made the strategic decision to focus their efforts on “hot-spots”, areas with the greatest biodiversity, such as Madagasgar and Costa Rica. The Eastern US is, in comparison, not very diverse. It has occurred to me that some of that has to do with the fact that the entire Eastern seaboard has been clearcut, sometimes more than once. At any rate, I believe this is behind the general lack of interest and although I understand the importance of biodiversity, this concentration of research capability on exotic, remote species has contributed to a complacency allowing people in the US to ignore the impacts of climate change.

    Once people realize that the trees in their own back yards and parks are going to fall over or burn, and they face extended power outages and expense, perhaps they will find remediating CO2 emissions less onerous. Unfortunately by then it will be too late for the plants and animals now living.

    Frankly at this point I think if we don’t kill each other in wars over water, we will have to get busy building giant domes with filtered air and water to keep species alive until some day when they can be planted outside again. So far, the environmental movement has been too fragmented.

    Here are some links that may be of interest:

  25. DavidONE says:


    > In theory trees could migrate…

    Yes, over hundreds or thousands of years, but they cannot ‘migrate’ (what’s the correct scientific term for climate-driven movement of flora?) quickly enough to cope with current rapid warming. I should’ve made clear that’s what I meant initially.

    [JR: Yes, plus there are cities and farms and other man-made developments in the way of such migration, now. More likely is that you would split apart ecosystems and the fast-moving more adaptable species might survive, while everything else dies or gets eaten by invasive pests.]

  26. Russ says:


    The NJ DEP wrote me back about drought, for instance. The ONLY way they measure it is reservoir levels. So if we get the same amount of rain, but in shorter, heavier bursts, and this leads to drier soils, the DEP has no way of monitoring that. They obviously don’t want to.

    Since one of the flaws in the Highlands RMP is its failure to take climate change into account, I’m not surprised that the Council and the DEP would measure aggregate rainfall only without taking into account the projected (lesser)frequency and (greater, acute) severity of rainfall events, though I hadn’t thought about this angle before.

  27. Sherrill says:

    I still believe Carrie Prejean should have won the Miss USA 2009, and not Kirsten Dalton.

    Carrie was looking stunning in her swimsuit