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How do trees change the climate?

Filed under: — group @ 27 October 2014 - (Deutsch)

Guest commentary from Abby Swann (U. Washington)

This past month, an op-ed by Nadine Unger appeared in the New York Times with the headline “To save the climate, don’t plant trees”.  The author’s main argument is that UN programs to address climate change by planting trees or preserving existing forests are “high risk” and a “bad bet”. [Ed. There is more background on the op-ed here]

However, I don’t think that these conclusions are supported by the science.  The author connects unrelated issues about trees, conflates what we know about trees from different latitudes, and fails to convey the main point: tropical trees keep climate cool locally, help keep rainfall rates high, and have innumerable non-climate benefits including maintaining habitat and supporting biodiversity.

Numerous scientists have already replied to the original op-ed, highlighting the points above and adding others.  But some of those responses made confusing arguments too, muddying things further.

So what is going on?  Why is it so complicated to say scientifically what trees do to climate?  The answer lies in the fact that trees have multiple pathways for influencing climate, and the relative importance of these pathways varies depending on where we look on the globe.

First lets talk about how plants can influence climate—directly through carbon, directly through energy fluxes, and indirectly through various channels.  Then I’ll cover how these factors combine in different places across the globe.

1. Carbon

Carbon dioxide (CO2) in the atmosphere is a greenhouse gas. All plants take in CO2 when they photosynthesize, fixing it into sugar and ultimately into the plant tissue itself.  At the same time, soils release CO2 through decomposition and plants respire it in order to maintain tissues.  The two numbers – CO2 leaving the atmosphere through photosynthesis and CO2 entering it through respiration – are both large and close in magnitude, with slightly more leaving the air than entering.  Changes in this delicate balance are one way that the land surface influences global temperature.  Planting new trees tips the balance further toward carbon going into the land, at least for a while.  When the forest matures, growth and decomposition come back into balance.

CO2 gets mixed quickly by the atmosphere. So after 1-2 years, a release or uptake of CO2 anywhere affects CO2 everywhere.  Right now, plants on average take C02 out of the air (helping keep Earth cool), and it doesn’t really matter where those plants live.

2. Energy

Energy is received by the Earth’s surface directly from the sun and also via long-wave radiation from the atmosphere.  The land surface has to balance this incoming energy by radiating energy away and through evaporation and conduction of heat.  The hotter the surface, the more it radiates and conducts.  So if there is a change in evaporation or in the incoming energy, the only way for the surface to balance the budget is to change temperature. There are three main ways that plants influence the amount and type of energy exchanged between the land and the atmosphere:

Albedo, or plant color:

Plants have different colors, which cause them to absorb different amounts of energy from the sun (the term “albedo” refers to the fraction of light reflected, i.e. how “shiny” something is).  In general, trees are darker than grasses, which are darker than bare soil.  If we start growing trees where there was once grass or bare ground, the surface will absorb more incoming energy from the sun. **All else being equal**, the surface would have a higher temperature.

Water fluxes:

Plants also release water through their leaves (called transpiration) while photosynthesizing.  Just as your skin cools when your sweat evaporates, transpiration cools leaves. The amount a plant transpires is directly related to how much it photosynthesizes. Plants that photosynthesize a lot also pump out a lot of water, and with it a lot of energy.  If we replace less productive plants or bare soil with highly productive plants,  **all else being equal**, the surface will have a cooler temperature.

Surface roughness:

Trees and plants affect how ‘rough’ the surface feels to the winds. The rougher the surface, the more it slows the winds down, and so trees in particular can affect near surface winds, and this affect leads to a small differences in local stability which can have varying (but small) impacts on fluxes of water and energy from the land surface.

3. Indirect links

Atmospheric circulation

While changes in temperature caused by changes in CO2 occur evenly across the globe, the same is not true for those driven by changes in albedo or water fluxes. These are local affects, concentrated near the place where the change in tree cover has occurred.  But if we plant trees all in one area, and that causes a region of heating or cooling, the atmosphere can respond by changing it’s circulation – and that might have further climate effects in places far away from where the trees were planted.  This can even happen on a global scale.  Significant changes in tree cover in Eurasia could cause an energy imbalance between the Northern and Southern Hemispheres, shifting the entire global circulation of the atmosphere, including the location of rainfall in the tropics.   And changes in where it rains in the tropics would impact plants there, and how they are controlling their local energy balance.

Plant-emitted volatiles

The recent op-ed by also discusses how plants can influence climate by altering chemical reactions in the atmosphere.  Plants emit volatile compounds, which are tropospheric ozone and aerosol precursors. According to the op-ed author’s calculations, more trees lead to more particles and more ozone, and the combination **generally** makes things warmer because warming from the heat-trapping ozone is bigger than the slight (and very uncertain) cooling from the particles.  This piece of the puzzle is still fairly new and is a great topic for further scientific study and discussion. The exact magnitude of these effects is not yet robustly known.

4. How does this all play out at different latitudes?

In high latitudes, trees are dark and absorb a lot of sunlight, especially in the snowy season when they are much darker than the bright ground below. Also, most high-latitude trees don’t photosynthesize quickly (it’s a tough life up there!). So the water losses and energy fluxes associated with photosynthesis are low.  Taken together, this means that Arctic forests help keep the surface warm.  As a slight variant on this, some deciduous trees that grow at high latitudes are not quite so dark and have bigger water fluxes.  Some of my work suggests that these trees also warm the surface, both because they make the land surface darker and because the water they release acts locally as a heat trapping gas. Trees (and shrubs) also reduce the impact of snow on albedo by standing taller than the snow cover.

In the tropics, trees photosynthesize at very high rates (growing conditions are great!).  These trees shed a lot of water during photosynthesis, with associated high energy losses.  And tropical trees are not much darker than the pasture grasses that replace them after deforestation.  This means that tropical forests help keep the surface cool and wet.

In the mid-latitudes, both of these effects (albedo and water) are competing. Forests are both darker and have higher water losses than grasses that are likely to replace them.  That makes it much harder to say what will happen, on balance, if we were to plant large areas of new mid-latitude trees. The net outcome depends on exactly how much each factor changes, as well as on the amount of moisture in the soil.

Bottom Line

So should we try to slow global warming by planting trees?  On a 20-40 year time scale, there is no question that planting trees will transfer carbon from the atmosphere into the trees, slowing the growth of CO2 in the atmosphere and thereby slowing global warming.  On a 100-year time scale, I would say that we cannot plant our way out of the problem.  However, we know that tropical forests keep carbon out of the atmosphere, keep the land surface cool, and play a critical role in providing habitat, maintaining biodiversity, and other good stuff for people.  These things are hugely important and it is a no-brainer that we need to fight to keep tropical forests as intact as possible.  Maintaining tropical forest does lots of great things, and also helps to slow global warming. But we probably shouldn’t expect to combat global warming in the long term by planting trees in other latitudes.

It might not totally save the planet, but we should do everything we can to maintain the tropical forest.

49 Responses to “How do trees change the climate?”

  1. 1

    This is an informative summary of the effect of land use/land cover change on climate. The text

    “…if we plant trees all in one area, and that causes a region of heating or cooling, the atmosphere can respond by changing it’s circulation – and that might have further climate effects in places far away from where the trees were planted. This can even happen on a global scale. Significant changes in tree cover in Eurasia could cause an energy imbalance between the Northern and Southern Hemispheres, shifting the entire global circulation of the atmosphere, including the location of rainfall in the tropics.”

    applies to inadvertent land use/land cover change as well.

    On the subject of this post, see also

    Mahmood, R., R.A. Pielke Sr., K. Hubbard, D. Niyogi, P. Dirmeyer, C. McAlpine, A. Carleton, R. Hale, S. Gameda, A. Beltrán-Przekurat, B. Baker, R. McNider, D. Legates, J. Shepherd, J. Du, P. Blanken, O. Frauenfeld, U. Nair, S. Fall, 2013: Land cover changes and their biogeophysical effects on climate. Int. J. Climatol., DOI: 10.1002/joc.3736.

    Pielke Sr., R.A., A. Pitman, D. Niyogi, R. Mahmood, C. McAlpine, F. Hossain, K. Goldewijk, U. Nair, R. Betts, S. Fall, M. Reichstein, P. Kabat, and N. de Noblet-Ducoudré, 2011: Land use/land cover changes and climate: Modeling analysis and observational evidence. WIREs Clim Change 2011, 2:828–850. doi: 10.1002/wcc.144.

    Pielke Sr., R.A., 2001: Influence of the spatial distribution of vegetation and soils on the prediction of cumulus convective rainfall. Rev. Geophys., 39, 151-177.

    Pielke, R.A. and R. Avissar, 1990: Influence of landscape structure on local and regional climate. Landscape Ecology, 4, 133-155.

    Roger Sr.

  2. 2
    Nichol Brummer says:

    There’s another obvious fact: it takes more time for trees to grow, than to chop them down. If you plant trees now, and we find out later that they were too many or in the wrong place, it is easy to chop them down. If we don’t plant trees now, and we find out later that it would have been better to have them .. we will be planting them later.

    So I don’t really understand the ‘high risk’ verdict. The bet is asymmetric, but safely so, because chopping down trees is quicker than waiting for them to grow. And chopping down trees usually has some short term profit.

  3. 3
    Tim Joslin says:

    Thanks for this review of the issue. I have some outstanding questions, though:

    1. Water vapour is lighter than air. Doesn’t this have an effect on the loss of heat from sunlight absorbed by leaves? i.e. the water vapour will tend to carry heat (in the form of warmed air and latent heat) higher in the atmosphere, reducing surface warming.

    2. What exactly is meant by “surface” in this post? You write:

    “The hotter the surface, the more it radiates and conducts.”

    But heat absorbed by leaves 10m off the ground (say) may be transferred to the air but is not going to warm the ground itself by conduction, whereas if the ground is warmed directly heat will be conducted away. E.g. buildings in cities, stone etc surely stores more heat when exposed to direct sunlight.

    Does it make a difference that trees prevent some light reaching the ground?

    3. I’m concerned that there are dangers in a focus on the average surface temperature. Surely, for example, it’s a no-brainer to maximise foliage in mid-latitude cities (e.g. London) to keep the surface cool in summer, through the provision of shade and evaporative cooling (transpiration). In particular, reducing the heat absorbed during the day would be highly desirable.

    4. Won’t the fate of arboreal carbon depend on climate and hence to a large degree on latitude? In the tropics carbon is returned to the atmosphere within a few decades, whereas at high latitudes won’t it last much longer and even simply build up in the soil indefinitely?

  4. 4
    Don Strong says:

    Not mentioned but hugely important is that tropical forests are the keepers of a large fraction of biological diversity.
    Lawrence et al. 2012. Averting biodiversity collapse in tropical forest protected areas. Nature doi:10.1038/nature11318

  5. 5
    Don Strong says:

    Yet another value of forests, also on Yale Environment, “Forest top soils capture and stabilize nitrogen pollution very quickly but release it slowly, according to new research published in the journal Ecology, Lewis, D. B et al 2014. Forest succession, soil carbon accumulation, and rapid nitrogen storage in poorly remineralized soil organic matter. Ecology 95:2687–2693.

    Read More:

  6. 6

    Hmmm…. Trees as both object and metaphor.
    Short-term gain has often been exchanged for long-term disaster.

    Until science can explain how we might prevail, I listen carefully for guidance as to how we might extend the time we have left.

    Thanks for this important discussion/

  7. 7
    John Mashey says:

    Can you add something on the climate effects of bark beetles in the Canadian boreal forest?

  8. 8
    Doug Heiken says:

    Thank you for shedding some light on Unger’s confusing opinion piece.

    It is abundantly clear that forest conservation can play a helpful role in mitigating climate change. And that logging forests will make things worse. Here is a potentially relevant slideshow – Myths & Facts on Forest, Carbon and Global Warming slide show clarifying many misconceptions about forests, logging, and carbon:

  9. 9

    Paraphrasing the text in the post, aerosols that are input into the atmosphere, due to their spatial heterogeneity, also cause regions of heating or cooling that the atmosphere can respond to by changing its circulation – and that might have further climate effects in places far away from where the aerosols are input. This can even happen on a global scale.

    Thus it is not only land use/land cover changes that can cause this effect.

    We discuss this in our paper

    Matsui, T., and R.A. Pielke Sr., 2006: Measurement-based estimation of the spatial gradient of aerosol radiative forcing. Geophys. Res. Letts., 33, L11813, doi:10.1029/2006GL025974.

    where our abstract starts with the text

    “This paper diagnoses the spatial mean and the spatial gradient of the aerosol radiative forcing in comparison with those of well-mixed green-house gases (GHG). Unlike GHG, aerosols have much greater spatial heterogeneity in their radiative forcing. The heterogeneous diabatic heating can modulate the gradient in horizontal pressure field and atmospheric circulations, thus altering the regional climate…”

    The importance of heterogeneous human climate forcings does not diminish the important of added greenhouse gases, but does indicate that more attention needs to be given to these other human climate forcings, including how they can modify atmospheric and ocean circulation features.

    Roger Sr.

  10. 10
    Lance Olsen says:

    Unger’s cherry-picking of the data is similar to the cherry-picking employed in analyses of forests’ role in the hydrologic cycle. Those studies assert, quite correctly, that trees use water, and can readily cite studies demonstrating that removal of trees can result in (at least temporary) increase of local water table and streamflow. But the water a forest uses is partly there because, first, the canopy intercepted it and prevented its rapid return to the seas, and because, second, the shade cast by the canopy prevents rapid evaporation from soils. The analyses of these processes is too frequently excluded from the narrow analysis of forest demand on water. Possibly worse, though, studies that focus entirely on a forest’s demand on water supplies neglect that even this superficial selfishness results in transpiration that effectively irrigates the atmosphere, providing an important supply of water for downwind rainfall: e.g., Hornberger et al say “Evapotranspiration represents a dominant outflow of water from most catchments and accounts for approximately two-thirds of precipitation over most continental land masses.”

    Hornberger, George M.; Raffensperger, Jeffrey P.; Wiberg, Patricia L. and Eshleman, Keith N. Elements of Physical Hydrology, The Johns Hopkins University Press

  11. 11
    J4Zonian says:

    Among the many, many beneficial effects of trees:

    While different species of trees have different natural lifespans, from 20 to at least 5,000 years, in terms of absolute mass, trees actually tend to speed up growth as they get older, and they sequester more carbon per unit of time as they age.* Using the process of succession, as for example, permaculture does, we can maximize carbon sequestration while meeting multiple other needs–by planting a combination of trees and other plants. We can mix faster growing shorter-lived tree species, either with or followed by longer-lived species, as well as selecting for pioneer species, food, fodder, lumber and other products, usually more than one from each tree during its life cycle. We can also consider diversity and wildlife value, etc.. Further, if we use a tree for construction lumber or turn it into charcoal as in tera preta horticulture, its carbon can be safely stored for centuries.

    A major result of climate catastrophe is harmful reduction of soil moisture especially long-term. Annual crops tend to make that worse. Perennial herbs, grasses and shrubs are somewhat better, and trees in a diverse forest or more savannah-like edible forest garden, with trees, shrubs, herbs, vines and ground covers (all of which can provide food and materials as well as benefit each other)** are even better at preserving soil moisture while also encouraging diverse wildlife and providing high yields of multiple food crops, lumber and other materials. Forest cover helps preserve and even out rainfall and moisture over a large area and decreases both droughts and floods, which can mutually reinforce each other to bad effect.(Both cause plant cover to decrease which causes soil moisture decreases often making storms less frequent, less regular, less predictable and more intense, further decreasing plant cover, reducing or killing crops, eroding soil (leading to decreased fertility, water pollution, etc.) and spiraling down into desertification and the creation of wasteland from what had been a fertile, verdant landscape.

    In hot climates trees reduce local temperatures, providing cool refuge, decreasing violence*** and reducing air conditioning costs and GHG emissions–while providing food, lumber, vertical playgrounds, etc.. In colder climates deciduous trees can do the same while allowing winter sunlight in, reducing heating costs and emissions.

    Trees provide green–a psychologically beneficial background–and what I call “flock movement” of leaves, movement that is complexly, even wickedly, ‘coordinated’ but non-regimented (non-identical). Symphonic movement, one might say. This is a psycho-physiologically significant alternative to sterile walls and the movements humans tend to create through machinery; and as is becoming more clear (see works on Nature Deficit Disorder, for example*+) is crucial to human development and psychological health. (Diverse prairie systems also provide this as well as bird movements and other natural phenomenon but trees and forests, the natural habitat where most humans live, are unparalleled in this regard.


    **see Toby Hemenway, Gaia’s Garden: A Guide to Home-Scale Permaculture or especially the mind-blowing 2 volume Edible Forest Gardens by Dave Jacke and Eric Toensmeier as well as Bill Mollison’s Introduction to Permaculture and Permaculture: A Designer’s Manual

    *** and many other studies confirm this.

    *+ Also see Paul Shepard, Nature and Madness.

  12. 12
  13. 13

    One thing that seems to be overlooked completely here. Trees do not simply absorb energy and get warmer. The albedo effect is more relevant with inorganic surfaces that living matter. The solar energy is used to combine H2O and CO2 into hydrocarbons, an endothermic reaction thus storing large amount of energy that can released later, by burning the wood for example. It seems to me that young trees grow more quickly that older trees, thereby absorbing more energy per plant. Exactly the opposite that is stated in the article. A 3 to 5 year planting cycle is much more efficient than waiting for the 20 years suggested. The plants could be used to make paper and cardboard for much more Eco-friendly packing material than current plastic products.

    E. Lombardi

  14. 14
    Abby Swann says:

    Re: John Mashey

    Check out this paper on the climate impacts of bark beetles in Canada:

    H. Maness, P. Kushner, and I. Fung. Summertime climate response to mountain pine beetle disturbance in british columbia. Nature Geoscience, 6(1):65–70, 2012.

  15. 15
    Russell says:

    The central question at issue is the magnitude of the albedo forcing from changes in human land use.

    The impact of tree planting in previously unforested areas can be paradoxical, as it depends both on seasonal changes in surface albedo, eg, from snowfall and sun angle a and shadow area effects.

    In circunstances where these factors combine – planting trees as carbon offsets above the lcal tree line at high latitudes, modeling indicates that the radiative forcing impact can outwigh the benefits of carbon sequestration= cf

    Bala et al., Combined climate and carbon-cycle effects of large-scale deforestation, Proc. Natl. Acad. Sci., 104, 6550-6555, 2007.

  16. 16
    Mack says:

    Maybe planting trees is meaningless. Seems that the “lungs” of the world is not the Amazon forests after all, but the oceans. Oh dear, how sad, there there, never mind.

  17. 17
    Bart Declercq says:

    @13 E.Lombardi – you state “it seems to me young trees grow more quickly than older trees” – this is only true proportionally – in absolute terms, an older tree grows more “quickly” – a 20% increase in a small tree weighing 100kg = 20kg of additional mass, a 5% increase in a 1 tonne tree (“slower growth”) = 50kg of additional mass.

    So the bigger tree will be absorbing more energy in the absolute, even if its proportional growth is less.

    You might argue that you can plan 3 small trees in the space needed by the bigger tree and the advantage tilts back to the smaller tree, but one would need to measure and test this before making such a statement (for example, an old, large tree could easily weigh 100+ tonnes, grow at 2% (2000kg) per year yet take up less space than the 100 small trees required to store the same amount of mass.

  18. 18
    Hank Roberts says:

    Caldeira’s 2007 discussion, which Dr. Unger cites in her linked summary of support for her piece, is quite clear:

    Those who haven’t read that are missing the points that need to be understood.

  19. 19
  20. 20
  21. 21
    Hank Roberts says:

    > It seems to me that young trees grow more quickly than older trees

    Everyone has an opinion; facts are harder to come by and site-specific.
    Scholar can be helpful if you’d like facts.

  22. 22
    Hank Roberts says:

    Thank you Kevin McKinney.
    I’m reposting your link to catch the attention of those who won’t bother to click on it:

    Rate of tree carbon accumulation increases continuously with tree size
    Nature 507, 90–93 (06 March 2014)

    Editor’s summary:

    Old forests and their leaves fix less carbon than do new forests, but does this apply at the individual tree level? This study uses a global analysis of more that 400 tree species to show that it does not. Rather, larger and older trees accumulate carbon more rapidly than do younger, smaller ones. This can be reconciled with the effects at other levels by taking into account increases in leaf number and reductions in forest density with age. The rapid growth of large trees means that, relative to their numbers, they could have a disproportionately important role in forest feedbacks to the terrestrial carbon cycle and global climate system.

  23. 23
    Abby Swann says:

    Re: Hank Roberts about Ken Caldeira’s 2007 opinion piece

    The Caldeira piece is consistent with the science I lay out above.

    Caldeira says “preservation and restoration of forests *outside the tropics* will do little or nothing to help slow climate change”
    (emphasis mine)

    “In contrast, tropical forests appear to be doubly valuable to the earth’s climate system. Not only do they store copious amounts of carbon, the roots of tropical trees reach down deep, drawing up water that they evaporate through their leaves.”

    So I concur with Caldeira – trees in mid and high latitudes have uncertain or warming effects on climate, but trees in the tropics keep things cool.

  24. 24
    Jim Bullis says:

    This all leaves me scratching my head in disbelief.

    In the rush to punish coal with actions that have debilitating effects on our economy some effort to find alternative actions seemed worthwhile. The chemical composition of wood fiber, including roots and the entire vegetative mass is roughly the same, ton for ton, as coal from the Powder River Basin. I made that point here.

    In that context, I found and reported here that China was asserting in their State of the Nation report that they were acting to combat CO2 by planting forests. This was in conjunction with their massive water projects.

    This all went to my not well received suggestion that we take constructive action to create standing forests in the vast areas of under-used land in the USA, preferably on a whole North American continental basis. This would involve a large scale water project modeled after the California Aquaduct system. Yes, that would be costly, but it could easily be paid for out of agricultural proceeds that could be an associated benefit of general irrigation.

    We could actually do even more if we actually established a universal irrigation project on a scope much less than our national highway system.

    Then I find that there are famous persons going about proclaiming a huge water shortage. My answer to that is that there is only a water distribution problem that we know well how to solve, as did the Romans and the Babylonians some time ago.

    And then I discovered the great Dam removal project terrorist acts. Ah yes, as shown on youtube and, for example, the Carmel Valley absurd actions on behalf of some salmon. Salmon need help, but lets get a grip.

    Oh well, it seems there is huge enthusiasm for bashing coal and making salmon happy, but not much for serious constructive action.

    And now we have an assertion that the CO2 effect of trees is approximately balanced between photosynthesis and aspiration. Huh? If we assume that trees are all burned down eventually, maybe so. It is not balanced if the trees stand and forest products are long preserved.

    If there is a massive aggregate of standing vegetative mass that was not there before, then there could be a serious mechanism for capturing and storing CO2.

  25. 25

    Here are two more papers on this subject that you might find useful.

    McAlpine, C.A., J.G. Ryan, L. Seabrook, S. Thomas, P.J. Dargusch, J.I. Syktus, R.A. Pielke Sr. A.E. Etter, P.M. Fearnside, and W.F. Laurance, 2010: More than CO2: A broader picture for managing climate change and variability to avoid ecosystem collapse. Current Opinion in Environmental Sustainability, 2:334-336, DOI10.1016/j.cosust.2010.10.001.

    Marland, G., R.A. Pielke, Sr., M. Apps, R. Avissar, R.A. Betts, K.J. Davis, P.C. Frumhoff, S.T. Jackson, L. Joyce, P. Kauppi, J. Katzenberger, K.G. MacDicken, R. Neilson, J.O. Niles, D. dutta S. Niyogi, R.J. Norby, N. Pena, N. Sampson, and Y. Xue, 2003: The climatic impacts of land surface change and carbon management, and the implications for climate-change mitigation policy. Climate Policy, 3, 149-157.

    The abstract of the second paper reads

    “Strategies to mitigate anthropogenic climate change recognize that carbon sequestration in the terrestrial biosphere can reduce the build-up of carbon dioxide in the Earth’s atmosphere. However, climate mitigation policies do not generally incorporate the effects of these changes in the land surface on the surface albedo, the fluxes of sensible and latent heat to the atmosphere, and the distribution of energy within the climate system. Changes in these components of the surface energy budget can affect the local, regional, and global climate. Given the goal of mitigating climate change, it is important to consider all of the effects of changes in terrestrial vegetation and to work toward a better understanding of the full climate system. Acknowledging the importance of land surface change as a component of climate change makes it more challenging to create a system of credits and debits wherein emission or sequestration of carbon in the biosphere is equated with emission of carbon from fossil fuels. Recognition of the complexity of human-caused changes in climate does not, however, weaken the importance of actions that would seek to minimize our disturbance of the Earth’s environmental system and that would reduce societal and ecological vulnerability to environmental change and variability.”

    The importance of land use/land cover change in the context of policy responses to the human involvement in the climate system has not been adequately recognized in the IPCC and other climate assessments.

    Roger Sr.

  26. 26
    SecularAnimist says:

    Jim Bullis wrote: “In the rush to punish coal with actions that have debilitating effects on our economy …”


    First, there is no such “rush to punish coal”.

    Second, even the fastest possible phase-out of ALL coal use would have NO “debilitating effects on our economy”, and would in fact have enormous economic BENEFITS — even apart from reducing carbon pollution.

  27. 27
    Michael Hauber says:

    What impact do trees have on wet bulb temperature? The issue being that at a wet bulb temperature of 35 degrees C it is impossible to keep a human from dieing from heat by evaporation, wind or shade. As the wet bulb temperature is effectively the lowest possible temperature that evaporative cooling can reach I would guess that the evaporative cooling effect of trees would have no impact on wet bulb temperature (but simply cash in some of the maximum possible evaporative cooling potential). If so that would leave the albedo effect to push the wet bulb temperature up, and the carbon storage effect to provide long term cooling. The impact on wet bulb temperature may not be so relevant now with nowhere on earth close to the 35 degree limit. But worst case scenarios suggest this limit may be reached by the end of the century.

  28. 28
    Radge Havers says:

    Effects of Climate Variability and Accelerated Forest Thinning on Watershed-Scale Runoff in Southwestern USA Ponderosa Pine Forests

    “Results of this study and others suggest that accelerated forest thinning at large scales could improve the water balance and resilience of forests and sustain the ecosystem services they provide.”

    Setting priorities. Effects of thinning on climate minimal, negative or beneficial in this context? Comments?

  29. 29

    De nada, Hank–and thanks for quoting. Of course, it is only one study, and it presents a somewhat surprising result, but it looks to my amateur eye as if they did a pretty substantial analysis. I’d think the result ought to receive some weight, provisionally at least.

  30. 30
    Jim Bullis says:

    RE 26 by Secular,

    If you have been following the actions and planned actions of the EPA which seem to be putting unreasonable demands on new coal plants, you might not make the same pronouncement. You might also ask a power company, though some tend to be more PR oriented and might not be truthful in the face of pandering to the local utility commission.

    This is not to say that true pollution that actually damages air quality should not be seriously suppressed. Practical systems that would reduce the use of coal are desirable. But the responsibility for CO2 should not be as heavily carried by low cost coal power plants.

    As to the economic “BENEFITS” you assert, I suggest the opposite. As I see it, low cost electric power is key to our industrial economy, which you might have noticed is not doing so well these days. Yes, that is hard to measure but quite frequently it seems those who plan expansions say that planning is hard in the present regulatory environment.

  31. 31
    Jim Eaton says:

    The obvious move is to protect all old-growth forests for their carbon sequestration and resistance to wildfires, especially as climate change is increasing drought conditions in some large areas of forests.

    A major problem with extensive plantings of young trees is their susceptibility to fire, especially those planted as monocultures in rows for future logging operations. Even plantings after salvage logging in a burned area often burn the hottest. “Reburn severity in managed and unmanaged vegetation in a large wildfire,” Jonathan R. Thompson, Thomas A. Spies , and Lisa M. Ganio, PNAS June 19, 2007.

    In addition, at least in coniferous forests, by far the most successful replantings are from tree stock genetically adapted to a particular area. This requires collection of seed from a diverse cross section of the forest before it was burned or logged.

    And the idea of re-plumbing North America to create forests in “under-used land” is ludicrous. There are reasons forests don’t flourish in the lowlands of the Great Basin, Mojave Desert, or Great Plains, and the absence of water is not it. It would be far easier to reestablish the vast hardwood forests of the eastern half of the United States that existed before their conversion by European settlers. And water diversions would not be needed.

  32. 32
    SecularAnimist says:

    Jim Bullis wrote: “the actions and planned actions of the EPA which seem to be putting unreasonable demands on new coal plants”

    I have been following the EPA’s proposed power plant emission regulations very closely, and there is nothing remotely “unreasonable” about the proposed regulations for new coal-fired power plants. (I note that you have not provided any specifics as to what you consider “unreasonable”.)

    Indeed, given the severity and urgency of the global warming problem, an immediate, outright ban on new coal-fired power plants would not be “unreasonable”.

    Jim Bullis wrote: “low cost electric power is key to our industrial economy”

    I agree — electricity is the life blood of modern civilization.

    However, natural gas and wind are both already cheaper than new coal-fired power plants, even without the new EPA regulations. Electricity from distributed solar photovoltaics is projected (by the CEO of Duke Energy, one of America’s largest utilities) to cost less than the retail price of grid power in 25 states within a couple of years.

    And when the full costs of coal are taken into account, there is nothing remotely “cheap” about coal.

    Jim Bullis wrote: “the responsibility for CO2 should not be as heavily carried by low cost coal power plants”

    As noted above, coal is not “low cost”.

    As for “the responsibility for CO2”, according to the EPA coal combustion represents about 39 percent of electricity generated in the USA, but accounts for 75 percent of the CO2 emissions from electricity generation, which accounts for 32 percent of total US emissions.

    Moreover, according to the US Energy Information Administration, CO2 emissions from coal increased more than 4 percent from 2012 to 2013, and were up almost 12 percent in January-February 2014 from January-February 2013.

    In short, coal-fired electricity generation produces a grossly disproportionate (and increasing) part of the USA’s total emissions relative to any “benefits”, and as such it is completely logical to target coal for rapid, steep emission reductions.

    We already have better and cheaper ways to produce plenty of electricity.

  33. 33
    Magnus W says:

    One argument for planting trees up north in Sweden has been that it might increase wood as building material instead of concrete… and that would lower co2 emissions… any one tried to make that point in this debate?

  34. 34
    John Mashey says:

    re #14 on bark beetles
    Thanks Abby.

    We ski in B.C. every year, and can see the beetle damage when we fly over to Kelowna, but it’s nice to see the climate effects quantified.

  35. 35
    Russell says:

    John, how much of a carbon credit do I get for not heli skiing ?

  36. 36
    John Mashey says:

    Magnus: try

    “March 22nd saw the topping-out ceremony of the tallest contemporary wood building in the world, and the first that might be considered a true skyscraper, the 30m Wood Innovation and Design Centre (pictured) in Prince George, British Columbia. Its designer, Michael Green, says, “Frankly, we aren’t breaking a sweat. It’s only public perception and emotion trumping science that stalls us moving higher.” He hopes to start work on a 20-storey vertical food-farm in Vancouver later this “year.

  37. 37
    mitch says:

    Jim Bullis, who wrote ” But the responsibility for CO2 should not be as heavily carried by low cost coal power plants….”

    You should be aware that back in the 70’s coal power plants were exempted from many of the pollution regulations (grandfathered) with the idea that these plans would be replaced over the next 40 years. Instead, many operators upgraded the plants so that they could keep the exemption and would not have to deal with clean air regulations. Many of these are the “low cost” coal plants of today. They made a business decision, assuming that they could continue to externalize the cost of cleaning up.

    They’ve had around 40 years to clean up their act, so I cannot feel sympathetic that they didn’t use the time to actually try to comply with the spirit of the law.

  38. 38
    JustinWonder says:

    Great article, a joy to read!

  39. 39
    Matthew R Marler says:

    That was a good short article.

    Quick note on afforestation in California: CA used the irrigation provided by its water project to sustain large groves of fruit and nut trees (whose product had large commercial value); subsequently, many of these trees were sacrificed to enhance the natural environment of the Delta Snail Darter (which has no commercial value), which has resulted in large stands of dead or moribund trees. It would be nice to have a review of the “experiment’s” (my word) two phases from the point of view of regional and global (I’m guessing small) climate effects.

    San Diego County has large stands of citrus fruits and avocados where there used to be only chaparral. A review of the climate effects of those stands might also be informative. Previous generations of CA sacrificed some of the natural water ways to produce these trees and other crops; there is now somewhat of a movement to sacrifice the trees and restore the natural water ways. I think a review of the CO2-harvesting effects of these trees (and contrariwise, of their removal) would be worthwhile, in case anyone can cite or link to a review.

  40. 40
    Hank Roberts says:

    > Matthew R Marler
    > … to enhance the natural environment of the Delta Snail Darter

    You’re rebunking an old talking point long ago “generated by right-wing strategist and marketer Richard Viguerie” and recently being rebunked at Fox.

    It’s a way of attacking protections — minimal ones — for the environment in which that fish is an indicator species.

    It’s a peculiarity of our system in the US that we consider and sometimes provide protection for only one type of rivet at a time, each new type debated at great length, although we understand that the ongoing loss of a few rivet here, a few rivets there, ends up — eventually, unpredictably — in a crash; the point of protection is to preserve the integrity of the whole, not the individually charismatic pieces.

  41. 41
    Jim Eaton says:

    RE 38 by Matthew Marler,

    There has been a loss of citrus groves in California, but it largely was due to houses being built on agricultural lands in Orange County, other Southern California locations, and the Silicon Valley. The citrus industry responded by moving their orchards to the San Joaquin Valley.

    Your information is backwards on the sacrificing of fruit and nut trees due to the lack of water. One of the major criticisms of farmers during the current California drought is that so much acreage is being converted to fruit and nut trees (particularly almonds for export), crops that much be irrigated, even during a drought when annual crops can be fallowed.

    There is no such fish as the “Delta Snail Darter.” Perhaps your mean the delta smelt (Hypomesus transpacificus), an endangered species. Under the federal and state endangered species acts it does not matter if a species “has no commercial value;” indeed, most of the world’s species have “no commercial value” yet are critical to the functioning ecosystems on our planet.

    And replacing native chaparral with citrus and avocado orchards is not a good thing in terms of carbon sequestration. Chaparral does not require irrigation from distant water sources, and these species do a good job of capturing carbon dioxide.

  42. 42
    peter azlac says:

    An interesting and balanced article but it lacks at least one important fact on the response of trees and other crops to increases in atmospheric carbon dioxide – that is the change in root to stem ratio to favour the former. The result is better capture of water through improved soil structure – due primarily to the conversion of some of the captured carbon in the decaying roots and leaves into humic acids that coat soil particles giving greater stability and pore size – hence the ability to accept and retain water. This has a direct effect on the minimum and maximum surface temperatures through retained heat from a higher thermal mass and surface cooling from increased evaporation. Of course things are not as simple as this with many other factors such as nutrient supply, surface solar insolation and wind speed coming into play as can be seen in the model studies of Herb et al. (Journal of Hydrology (2008) 356, 327–343). This effect of increased CO2 on root to stem ratio has been discussed by Freeman Dyson as a major factor in mitigating temperature change:
    There is data available from several studies covering tropical to northern latitudes, e.g.:

  43. 43
    Hank Roberts says:

    Peter, the string “Delta Snail Darter” is a dog-whistle for the denial crowd, they use it to organize their blogging complaints about ecosystem conservation.

    Anything they can’t take to the bank gets called a snail darter.

    Many of them believe it actually exists, despite how easily they could look it up ….

  44. 44
    James says:

    I remember reading the original op-ed in the NYTimes and being confused by several of her points. Your response has clarified a lot for me.

    I have one concern however, that if policymakers use trees as part of a strategy to reduce C02, that they would not then make the necessary reduction emissions in the 20-40 year timespan. So, essentially, we would have more carbon entering the atmosphere in the 100 year timespan than otherwise.

    This assumes that officials would claim that planting trees would replace the need to reduce emissions, but judging by how officials have behaved thus far, I think this is a reasonable thing to expect.

    Could you comment on this scenario and its likelihood?


  45. 45
    Rafael Molina Navas, Madrid says:

    #18 Hank Roberts, #23 Abby Swann (and others?)
    Caldeira says:
    “the roots of tropical trees reach down deep, drawing up water that they evaporate through their leaves. In the atmosphere, this water may form clouds that reflect sunlight back to space, helping to cool the earth”.
    It´s far more complex than that.
    – Water vapour, until forming clouds, acts as a strong GHG.
    – On the other hand, water evaporation on leaves cool them, but vapour takes the heat with itself … until cloud formation: a complex energy cycle.
    – Depending on how high clouds are formed, their net effect may be different.
    In any case, my “poorly educated guess” also points to a net cooling effect. Mainly due to the withdrawal of CO2 from the atmosphere, without any significant reduction of albedo.
    At least on humankind time scale. If after millions of years, converted that carbon into fossil fuel, “somebody” comes and burns it … But that would be another “story”!

  46. 46

    “Cannot plant our way out of the problem …” Well, first and foremost we must stop losing trees on such gigantic scales. And we must stop top soil loss. The deposits in the Gulf of Mexico have an uncanny resemblance to those deposits found in estuaries and sea beds close to the Mycenaean and Roman civilizations, temporally close to their collapse. From what I see, all tree planting ideas combined are not a fraction of the current losses. Other than that the utility doesn’t stop here: forests and brush are the reservoirs of (land-based) biodiversity.

  47. 47
    Mike Hart says:

    Ungers proposition is that trees are a bad bet as a GHG mitigator and solution and cites a variety of studies that show tress do other things and are in mass in some areas a closed loop system, therefore the benefits are for the trees and stay local. Unger also states that there is insufficient evidence to show whether trees v grasses are better C02 absorbers and whether removal of trees has contributed to warming etc.

    The significant loss of forests and trees world wide has been ongoing for centuries as measured by deforestation rates and land mass changes, it may be that as a singular functional unit a trees leaves does not do anything better or less than the equivalent area of grass leaves as far as basic atmospheric chemistry is concerned but a tree has a very significant biological, ecological and existential value that goes beyond a simple argument about chemical activity. Trees use a lot of water, yes, but they store a lot of water and in so doing they keep the ground at a deep level of saturation that cannot be done by grasses with shallow root systems. They are very effective at holding soil in place and changing the simple mineral composition of soil sufficient to allow for widespread microbiological and microbial activity. The majority of birds rely on trees for habitat. For the entire anthropocene, trees have provided the basic materials for the sustenance, comfort, transportation and habitat of humans and still do. Human and animal congregation is simply not possible on any meaningful scale in a non arborial environment so to argue that trees are not an answer to climate change strategy is simply nonsense.

  48. 48
    Hank Roberts says:

    > officials would claim

    That would be officials — and businesses — located in the big industrial countries. Those are the places where the profits for big timber operations would be made under the false claim.

    Heck they ARE being made and HAVE BEEN.

    California fell for that scam for years and lots of trees were cut. Sanity was regained to some extent on that recently.

    The loggers are looking at Alaska now, and Canada, and Siberia of course.

    Her point is that more trees in the tropics are needed — that’s where reforestation would help and existing remnant forests should be protected.

    She’s not arguing for cutting what’s left outside the tropics, she’s saying the climate change money could be diverted outside the tropics by timber industry deception.

    My words, not hers.

    I think that’s the sound part of her piece, hidden in the verbiage, and it could and should be pulled out and stated explicitly

  49. 49
    Hank Roberts says:

    And here, explicitly, the documentation — clearcutting tropical forests, which is counterproductive for climate change, although obviously profitable in the short term:

    … Exporters in Peru and importers in the United States and around the world are currently integral parts of a systematic flow of timber from the Peruvian Amazon. Sometimes intentionally, sometimes through sheer negligence, each of the actors and agencies involved in this system are working as gears in a well-oiled machine that is ransacking Peru’s forests and undermining the livelihoods and rights of the people that depend on them.
    … this report … documents the systematic export and import of illegal wood from Peru to the US….”

    Hat tip to

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