Study: trees in cities actually make pollution worse during heat waves

From the AMERICAN CHEMICAL SOCIETY comes this counter-intuitive finding:

During heat waves, urban trees can increase ground-level ozone

Planting trees is a popular strategy to help make cities “greener,” both literally and figuratively. But scientists have found a counterintuitive effect of urban vegetation: During heat waves, it can increase air pollution levels and the formation of ozone. Their study appears in ACS’ journal Environmental Science & Technology.

Previous research has shown that planting trees in cities can have multiple benefits, including storing carbon, controlling storm water and cooling areas off by providing shade. This has spurred efforts in cities across the U.S. and Europe to encourage the practice. However, it’s also known that trees and other plants release volatile organic compounds, or VOCs, that can interact with other substances and contribute to air pollution. And when it’s hot, plants release higher levels of VOCs. Galina Churkina and colleagues wanted to investigate what effects heat waves and urban vegetation might have on air pollution.

The researchers compared computer models of air pollutant concentrations in the Berlin-Brandenburg metropolitan area in Germany in the summer of 2006, when there was a heat wave, and the summer of 2014, which had more typical seasonal temperatures. The simulation showed that during the summer of 2006, VOCs from urban greenery contributed to about 6 to 20 percent of the ozone formation, and that during the heat wave period, the contribution spiked to up to 60 percent. The researchers suggest that in addition to tree-planting campaigns, efforts to improve cities’ environments should include other measures such as reducing vehicular traffic, a major source of nitrogen oxides that can react with VOCs and form ozone.


The authors acknowledge funding from E.U. COST’s GreenInUrbs project.

The paper’s abstract is here:

135 thoughts on “Study: trees in cities actually make pollution worse during heat waves

  1. I would love to see the equations they use for all these computer model studies, and how they derive the coefficients.

    • POOMA

      [The mods must ask if that POOMA has a greater, or less, heating value per cubic cc than POOPMA?
      Which has greater heating value per gm? .mod]

      • Obviously you are referring to the well-known guesstimation acronym, “Preliminary Order of Magnitude Approximation.”

      • seaice1 wrote, “Read the papers.”
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    • A quick online search shows that the models are available free. There appears to be copious documentation.

  2. Since trees also create VOCs when they are NOT in cities, we better rid ourselves of all of them….. No Wait ! They store carbon, so keep them. …. No Wait ! A recent “scientific” study also shows they do not store ENOUGH carbon to really help us. So what good are they?
    Bye.. Bye Trees.
    BURN them all for BIOFUELS.
    /sarc, sarc, and sarc.

  3. Compared computer models?????? WTF! They are in a large metropolitan city. Can’t they get off their behinds and actually take measurements of VOC’s during these times, then use a computer to tabulate their findings? And maybe even verify their models?

      • As soon as I saw the word “simulation” I knew it was bogus. How hard would it be to set up a few sensors to monitor the air? Or maybe they are afraid reality would not duplicate their simulations! Cities have had trees for millennia, don’t these people ever study?

    • ..Have you seen the size of some of those “behinds” ?? I think they grown permanently fixed into those chairs !!

    • rbabcock

    • Your methods are outdated and overrated. Only true science by computer can produce research that shows that, inter alia, trees in cities provide shade.

  4. The result of the simulation seems to be a hypothesis about VOC’s. Don’t they now have to prove it by observation?

  5. Trees also add water vapor through transpiration which makes the air more humid and lowers wind speeds below their canopies. Biological stress from heat Increases with dry bulb T, RH, and decreases with wind speed (at light and moderate speeds).

    • Wyguy – you are behind the times. In climate science, model output IS (are) data! It is then available for use as input parameters in other models, and so on and so on. Real data? Come on, guy, that’s SOOOOO… twentieth century. I mean, really, how can you expect them to get the results that show “it’s worse then we thought” and/or “we’re all going to die” if they’re at the mercy of messy, unpredictable real-world data?
      Better add /sarc for the humourless cohort.

  6. They simply needed an excuse to cut them trees up and burn them as mandatory biofeed in the nearest coal plant. All this is humanities fault. So fleece them to the bone.

  7. Aren’t those some of the same VOC’s emitted from fossil fuels (the ancient residue of vegetation) when they are refined or incompletely combusted? And aren’t trees and their pollen a very real and proven cause of asthma attacks (unlike the tenuous associations made with small particulates by combining poorly done epidemiological studies with linear no-threshold models of toxicity)? Not to mention the epidemic of trauma when children climb trees and fall. And surely many citizens must complain the trees obstruct the view of wind turbines and solar arrays. And the leaves – what a chore! We need a global agreement to limit trees before the environment is so stuffed with them that there is nowhere left to have our climate meetings.

    • “We need a global agreement to limit trees before the environment is so stuffed with them that there is nowhere left to have our climate meetings.”
      Absurd absolute.

  8. Volatile organic compounds produced by trees do not cause pollution. Photo-chemical ozone pollution is created when vehicle and power plant pollution is broken down by strong sunlight in the presence of any number of volatile organic compounds. Photo-chemical smog is only ever created in areas that receive large amounts of pollution from vehicles and power plants. Without the input of large amounts of man-made nitrogen oxides by vehicles and power plants, the VOCs released by forests on a summers day have nothing to react with and they do not create any pollution.

    • In layman’s terms… The concentrations of humans breathing, vehicular and industrial pollutants are being trapped because the blocking effects of the Flora by density, doesn’t allow enough wind to remove those pollutants from the vicinity. Kind of like the Amazon Rain Forest without the dense population of people.

    • You a wrong to say “Volatile organic compounds produced by trees do not cause pollution.” To use just one example, natural volatile organic compounds (VOC) produce NOx (mainly NO and NO2) by reaction with atmospheric nitrogen. This usually occurs when organic radicals (mainly charged H, HC, CH2 radicals) react with atmospheric nitrogen (N2) to produce nitrogen compounds like HN, HCN, and H2CN. These nitrogen compounds then oxidize in to produce NOx that, in turn, react with VOC’s to produce ozone. ( in fairness similar nitrogen compounds are produced as the result man-made combustion, but, are considered minor sources of anthropogenic NOx). VOCs also react with volcanic sourced SO2 to create “vog”… one type of natures many natural smog’s.
      You are also wrong to say “Without the input of large amounts of man-made nitrogen oxides by vehicles and power plants, the VOCs released by forests on a summers day have nothing to react with and they do not create any pollution.” I sorry, but your statement suggests a serious lack of understanding of the complexity of atmospheric chemistry. One can point to, as examples, the direct detrimental health impacts created by natural aldehydes, natural oxygenated VOCs, natural NOx, natural SO2, and natural peroxyacyl nitrates… all components of photo-chemical smog.
      For example, a considerable amount of NOx is produced by nature via natural combustion (forest fires, volcanoes, and lighting. NOx is commonly produced any time you have a combustion processes in with nitrogen (N2) and water present… and man isn’t the only source of high temperature heat. Lighting alone produces and estimate 8.6 million tonnes/yr of NOx… compared to an estimated 28.5 million tonnes from fossil fuels. (According to Wikipedia under the heading “NOx”)
      Moreover, NOx is continually produce biologically during the nitrogen fixation process both as a result of natural fixation and during the breakdown of nitrogen fertilizers. I don’t have estimates of the amount of NOx produced biologically, but, I’m willing to bet the amount of natural NOx produced exceeds the amount of manmade NOx alone.
      As for the study above, we’ve know for decades that cities surrounded by forested areas have more problems with natural “smog” issues than cities surrounded by barren areas. The study simply illustrates how even relatively small amounts of vegetation within a city can impact “smog” issues.

      • Get real! Yeah pollution can be a problem, but only an idjit would argue trees are a problem. We evolved in the forest and I’m only a geologist and engineer, but vocs are normal and no matter their reactions to produce NOx which probably winds up as a fertilizer in the natural enviro. If it makes ozone buddy, it’s therefore good for you. Almonds and other fruit seeds and nuts contain cyanide, but a lethal dose of milk will kill you dead with 100% certainty.
        I’m a year away from 80 and am still working and I used to play with mercury as did my friends, the family used mercurichrome for antiseptic, gargled with iodine in water for a sore throat. I found an old lead weight used for anchoring a delivery horse lead while the milk man, ice man or junk man made a delivery or pick up. I used to melt it in the house furnace and cast all kinds of things for fun. Once along the railway tracks, I found a large blob of tar that you could break pieces off and chew like gum. I sold pieces to the local kids for a penny. OK, these things were not a good idea, but it does show that our fears are orders of magnitude larger than our dangers. Don’t be a wimp.

      • Dear Mr Pearse @ May 18, 2017 at 7:16 pm
        Regarding you statement that I “argue trees are a problem”. I have not done so and did not make such a claim in my comment.
        I was merely pointing out the Sasha’s claim that “Volatile organic compounds produced by trees do not cause pollution” is simply not true. Nor was his claim that “Without the input of large amounts of man-made nitrogen oxides by vehicles and power plants, the VOCs released by forests on a summers day have nothing to react with and they do not create any pollution.” More, specifically I was challenging Sasha’s suggestion that air pollution, NOx related or otherwise, would disappear if one were to stop the “input of large amounts of man-made nitrogen oxides by vehicles and power plants”.
        My point was, and remains, that a certain level of “air pollution”, as defined by environmental groups, is produced by nature. Even if the U.S. were to produce power solely via renewable and/or nuclear power we would still experience “air pollution” health events in the U.S. under the “air standards” they typically propose.
        In my view, environmental activist expend a considerable effort to obscure the existence of “natural pollution” from the general public in an attempt to scare the public from using of fossil fuels. Activist’s efforts to obscure the prevalence of nature sourced mercury in the environment provides another example.
        I prefer a more balanced approach that recognizes that we can’t eliminate all sources of “air pollution” and would be better off setting realistic “air standards” while expending the bulk of our efforts on more pressing issues. An example of a more pressing issue would be to address the rise of childhood asthma due to issues apparently unrelated to “air pollution”. (Most medical experts I’ve talked to believe the growing prevalence of asthma in the U.S, is linked to “hygiene hypothesis”. Where the “asthma hygiene hypothesis” suggests the rise in asthma to compromised immune system development in infants due to excessive hygiene).
        To paraphrase your words, I’m suggesting environmentalist quit being wimps. A first step in insistenting they quit being “wimps” being confronting them with the hard fact that sweet old “Mother Nature” spends a good deal of time trying to kill them… and explaining to them just how she’s doing it. Most of us understand that, while we like “Mother Nature”, she’s also something of a B_tch.

    • The entire southern United States (mid-Florida north to the VA-TN-KY border, from the Atlanta coast to the Mississippi River) would politely disagree with you … Actually, we would LOVE to be able agree with you, except as we are required to buy excessively expensive gasoline every summer and fall BECAUSE the natural pine-tree VOC/ozone levels ARE exceeding the EPA’s limits for city air pollution. In the countryside 150 miles from cars.
      Ever hear of the “Blue Ridge Mountains” … Named back in Jefferson’s day for the haze and hydrocarbons emitted from ten trillion pines.

      • RACookPE1978: This exactly.
        All others: It is of historical interest (to some, perhaps) that when Gen Lee used the mountains of western Virginia as cover for his advance into Pennsylvania, it was not only the hills themselves that blocked many of the potential long-range sight lines of Federal scouts, but also the thick forests those mountains host including the related summer haze.
        Vancouver, BC, suffers similarly during inversions despite its relatively small population of people, cars & industry compared to other urban areas of North America, & it is precisely because of its proximity to heavily forested areas, as well as its location in what amounts to a mountain-walled depression bordering the ocean. Much like L.A., this results in the easy entrapment of lower air levels during temperature inversions, but in this case the primary source of “pollutants” is not human activity, but largely due to the huge stands of nearby forest. This phenomenon has been seen in that area since the 1800s, & the largely plant-based nature of those “pollutants” tested/validated repeatedly in the scientific literature through actual observation.
        Incidentally, I don’t believe anyone here is saying that this haze is a huge public health risk — just because someone suffers from asthma (which CAN be made worse by trees & other vegetation) doesn’t mean trees aren’t a net public/environmental benefit. I think what we’re mostly saying is that the ozone/smog phenomena been around a lot longer than the automobile, that the risks are a lot smaller than many concerned citizens often make it out to be, & that the “solutions” may be far more complicated — in both implementation AND consequence — than simply heading back to the days of the horse & buggy (e.g.).

    • You forgot to mention that thunderstorm lightning also produce natural NOx. and when mixed with natural VOCs produced by plants (scents, fragances, mainly monoterpenes: myrcene, pinene, camphene, menthol, Carvacrol, linalol, tymol, and many more … ) they produce ozone, ground level ozone.
      Natural ozone, which is as bad as the car produced one. The only difference is that you cannot sue nature. And we know that the green movement is all about money.

  9. It would be interesting to know what the Green Radicals think is NOT a pollutant (if anything).

    • Oh, pot** is not a pollutant. (eye roll) It’s “natural.” <– That makes it okay. Still clean and sober! (not)
      If you want to find Enviro-Pharisees, go to Seattle. Try to order a famous dessert at a popular waterfront restaurant: "Sorry, not on the menu anymore. Transfats. No transfats in Seattle." Walk down Alaskan Avenue a few blocks. "What's all that stench?" The annual "Hempfest." Don't try to smoke a regular cigarette there, though… (eye roll).
      Alcohol is still okay, too. It's "sort of" natural.
      ** Note: While I think pot is disgusting, I am NOT condemning its use (out of the public square —
      since merely to use it is to be “drunk”). Alcohol for some, pot for others. Whatever.
      What I am condemning is: hypocrisy.

  10., all along it’s been the trees..second hand ozone
    Trees have been around for a long time… with it

  11. This is such old news. I remember when Reagan claimed that trees cause pollution. Boy, was he condemned.* But, he was right. Terpenes was the name for the pollutant then. The EPA, in assessing pollution in a city, allowed higher levels of pollution in cities with a large number of trees since the pollution from trees was natural.
    Terpenes make the smoky mountains smoky.
    *Then, as now, the R’s took the vituperation in silence. Instead of ridiculing the D’s, they just took their licks in the court of public opinion. I suspect they did that to avoid alienating the great majority of the voters, who of course are ignorant.

    • Indeed, very old news. In fact, it’s so old (40-plus years) that I would have thought these “experts” had already done a good estimate of the contribution by trees to VOC emissions was pretty well understood.

  12. This isn’t about trees. It’s about the continuing efforts of the left to get us to abandon automobiles. What we should abandon are the diesel buses used for fixed-route transit in most cities, if they can’t attract more ridership than they have, which makes them less fuel-efficient than private automobiles.

  13. ” trees and other plants release volatile organic compounds, or VOCs, ” –
    the main producers of condensation germs in the concrete and asphalt deserts of cities – let it rain !

  14. Back when I was young in Los Angeles County, they were proposing new emissions rules for cars including volatile emissions reductions. But even back then people noted that 50% of all hydrocarbon emissions were from plant life. Therefore, smog in LA can never be controlled completely unless you remove all plants.
    Much like the “Smoky Mountains”, terpenes cause air pollution.

      • San Francisco was Yerba Buena or ‘good herb’. 1792 log of HMS Discovery recorded anchoring ‘about a league below the Presidio in a place they called Yerba Buena’. (Theme Time Radio Hour and Wiki)

  15. We just need to borrow 1 quadrillion dollars from China so that we can have them build a gigantic air purifier and we’ll all be safe from trees. Now, just where to put it…

  16. There was a story put out by a paint contractors’ trade publication that some California restrictions on VOC’s were fully met by vegetation emissions in the LA basin, so no industrial emissions were allowable without the LA basin being in noncompliance.

  17. Since UHI is greater than the unlikely 2° C from CO2, shouldn’t cities be banned?

  18. “The researchers compared computer models”
    Stop right there. This not science. It is mathematical onanism. If they actually went out and measured something somewhere and somewhen, it might be science. But, running models does not produce science anymore than sketching pictures produces science.

    • I suspect this is an example of science for millennials. Just my impression, but it seems that even measured data must be passed through someone’s SimEarth, otherwise the youngsters won’t take it seriously. A computer model and a connection to climate change are the new basics of science. You need them just to sneak in the empirical data.

    • I suspect this is an example of science for millennials. Just my impression, but it seems that even measured data must be passed through someone’s SimEarth, otherwise the youngsters won’t take it seriously. A computer model and a connection to climate change are the new basics of science. You need them just to sneak in the empirical data.

    • You shouldn´t. have stopped.
      It is OK to compare a computer model to reality. It is part of the scientific method, which is:
      1. make a guess. Build a hypothesis.
      2. Calculate the consequences of your hypothesis. In the old days it was done with paper and pencil, now computers are used. They are quicker and less error prone.
      3. Set up an experiment to obtain empirical data.
      4. Compare the empirical data to the results of your calculations.
      5. If the result of your calculations, or the output of your model is different from the empirical data, then trash your hypothesis.
      6. If the experimental data agrees with your computer model, you can keep your computer model (until there is another set of experiments that refute your model).
      The problem with CAGW is that the output of their computer models is different from the empirical data, but they refuse to get rid of the models.

    • Walter,
      “mathematical onanism”
      My compliments, Sir! A clever turn of phrase. Although my spell checker is not happy with it, but what does it know! With your kind permission, I think that I may appropriate it on occasion.

  19. I produce considerable VOCs when I eat too much onion or cabbage. Sounds like my days are numbered.
    What the heck, they are numbered anyway.
    Please pass the fried onions this way.

  20. So according to this article all plants should be removed from any areas that might be subjected to warm temperatures because plants may contribute to polution. So that includes most of the Earth. But without plants, most animals will end up starving to death. This article seems to support the idea than all life should be eliminated to “save the planet”,

  21. Ive mentioned on an earlier ridiculous thread on planting trees not being effective strategy in reducing CO2. The recent papers on rapid greening with 14% increase in forest area on the planet have spooked climate doomsters into worrying that the planet has much higher negative feedbacks and sinks and ‘carbon’ appears to have mainly beneficial effects. This ridiculous paper is part of the same campaign and I predict the team uses centralized response planning and their will be a flood of “anti-greening” papers. They want to prepare their unquestioning clones to fear what is happening with greening. I’m sure we will be seeing legions of trolls doing battle for their masters.

  22. From the abstract linked above:
    “We use the Weather Research and Forecasting Model with atmospheric chemistry (WRF-Chem) with emissions of volatile organic compounds (VOCs) from vegetation simulated with MEGAN to quantify some of these feedbacks in Berlin, Germany, during the heat wave in 2006.”
    It looks like a case of models upon models. I don’t contest the theory, but I need at least some real-world observations before accepting their quantification of urban trees influence, and even moreso before accepting policies based on this theory.

  23. IMO they use ozone as a distraction from really noxious disease-causing pollutants so those don’t have to be dealt with. Ozone is actually a necessary component of air as it cleans (oxidizes) a lot of crap out of the air. It’s no more of a pollutant than CO2 is. That sweet smell during a thunderstorm is ozone. Trees emit terpenes which UV acts upon to produce ozone. All of it perfectly natural.

  24. So I guess the reason for the ozone hole over the south pole is because of the lack of trees in Antarctica. A logical deduction – no?

    • Right on RoHa
      Just what the song said.
      Whispering Grass
      Don’t you tell it to the breeze
      She will tell the birds and bees
      And everyone will know
      Because you told the blabbering trees
      Yes, you told them once before
      It’s no secret anymore

  25. Ground-level ozone does not affect weather nor climate. It affects health, but only in rare instances. Ozone is second only to H2-F, also known as Hydro-Fluoric Acid, as an oxidizer, which can really mess up the chemistry in the lungs of any air-breather, killing at a very low Parts Per Billion level. Has NOTHING to do with climate, yes I am SHOUTING. Why is this post on WattsUpWithThat?
    Ozone at the Top Of Atmosphere is Created by Ultra-Violet Radiation from the Sun, and is largely responsible for Life on Earth, as without it all animals and plants would be subjected to Ionizing, equals Fatal, radiation at all times. Thank God for Ozone…

    • Michael – “Ozone is second only to H2-F, also known as Hydro-Fluoric Acid, as an oxidizer, which can really mess up the chemistry in the lungs of any air-breather, killing at a very low Parts Per Billion level.”
      That conclusion is probably not valid (or not as valid as implied). There are numerous studies that show a very minor (albiet statistically significant) increase in premature deaths when there is an increase in ground level ozone (many using a 10ppb increases). Each of the studies have numerous errors such as data collection lack of control group, etc, One well known study was based on 100 north american cities. This particular study is one of the most often cited, but it also suffered from numerous short comings, lack of control, accuracy of data collection. A several of the cities with low levels of ground level ozone had much higher rates of premature deaths, than cities with high levels of ozone. All the cities had high correlation of premature deaths with increases in heat and while the correlation of increases in premature deaths to increases in ground level ozone varied widely between cities.
      In summary, ground level ozone may be a health hazard, but the studies making those claims are not supported by the underlying data used in those studies.

    • Difficult to know what to make of an assertion which displays an impossible chemical formula for hydrofluoric acid, HF.

  26. The good folks at Branscomb AFB, the progenitors of MODTRAN, figured this out a long time back. Mid-latitude heat waves are temporary episodes of the tropics pole ward of their normal bounds. In MODTRAN there is an explicit factor: Tropical Ozone (ppb).
    In the tropics, CO2 radiation to space tracks with the tropopause; and ozone tracks with the surface.
    Ozone will form from O2 wherever sufficient energy exists. We call it smog at the surface, and we laud its protective effects in the lower stratosphere.

  27. Whatever may be the arguments on VOC, Asthma, etc, etc,— the basic reason why environmental groups encouraged urban tree plantations are to minimise the pollution in that zone.
    For the last several decades, Zoo park with thick green trees in Hyderabad/India continuously presented lower pollution levels over other parts of the city with fewer trees and concrete jungle showing higher pollution levels and thus at higher levels [vertical] also in the night showing higher temperatures and thus increasing power consumption. The Zoo area creates a temperature gradient and thus pollutants disperse with the wind associated with the temperature gradient path. Quite coincidently todays news reports states: that Pollution Control Board suddenly changed and saying high pollution index at Zoo Park.
    Dr. S. Jeevananda Reddy

  28. If you take someplace like Phoenix and plant trees not natural to the area you might be able to bump up the humidity of an already hot place with the extra water needed to keep them alive. However I do not believe the presence of the trees makes Phoenix hot:)

  29. So the trees make VOCs, these make photo-chemical smog containing ozone and O3 is known to damage trees.
    (There is a feedback loop in there, can these bozos see it?)
    Yet the trees are thriving, they love it in the cities. Victorians planted parks (with trees) in London (notably Victoria Park in E London) to alleviate smog and bad air. This made the people healthier and hence= more productive workers. The economy boomed with trains, canals, bridges, sewers, cotton/wool industry etc
    Don’t tell me, the Victorians burned vast amounts of coal, started the industrial revolution and that made/makes them evil. sigh
    This is just more noise. They have discovered some miniscule & trivial drawback to ‘swimming’ while blind to the huge benefits swimming confers on, say, fish.
    Proud, self confident and clear headed people making up our society & its leadership would have the strength to ignore these folks and they would simply then, Go Away. Maybe even Get A Life for themselves.
    Do we have any ‘strong’ people (left) like that?

  30. “The researchers compared computer models of …”
    Does this research hold up in a tropical rain forest ?

  31. Why did they have to use computer models rather than empirical research, and how were they able to establish that it was the trees that created the additional ozone? It seems to me that they are trying to justify, rather crudely, their research grant.

  32. My God, you can always find something. Maybe you will find soon GreenInUrbs is a Green-pups. Ozone is only half the problem, or not at all, if you behave sensibly during a heat wave and do not perform any major physical exertion. If, someone should mean to have to do at 14.00 clock at the Timesquare at noon his 14th July Marathon then of course this looks different with the ozone. This is not, however, by ozone, but by this idiot. The bigger problem is the merciless heat in the cities through heated building and road surfaces. Trees can bring a lot of relief. Even in a forest, trees are releasing more ozone from a certain temperature. Nevertheless, during a heat wave under a tree, one feels more comfortable than in the open field. Because trees produce more ozone during heat waves, one should now stop the greening of cities, is a great bullshit. The study belongs to the category: Mega bullshit.

  33. You will never meet anyone in your entire life that is allergic to CO2. Countless people are allergic to pollen and get hayfever. Mother Nature is by far the greatest “polluter.” She also creates the most CO2.

    • Mother Nature still produces thousands of other substances against which ozone and NOX feels like milk and honey. The only problem with the whole thing is the stupidity of the people whose infinity Albert Einstein has already established.

  34. Heat causes trees to emit more terpenes. UV light breaks down terpenes and other VOCs in the air to produce ozone. It’s all basic chemistry.

  35. I actually do not see a problem with their research. They are simply recognizing that man is not solely responsible for the ozone pollution, which can be a big problem in cities and affects not only cities but entire regions during heat waves, and in fact, much of it is natural. No one is suggesting getting rid of, or not planting more trees; their benefits are well-known. There are no easy solutions, but we can only try to limit our own contributions. It is actually a relief to hear about actual pollution instead of fake pollution. I used to see the summer haze over our White Mountains here in New Hampshire, and felt somewhat ashamed as a human, thinking that mankind was solely responsible. Then I did read about the Smokey Mountains, and how they acquired that name.

    • Their results are from a computer model and presents a possibility that may or may not show reality. However, it would seem to this simple red neck that it would be easy to test that possibility if only they got out of the air conditioning and into the field.

    • Ozone is not the problem during heat waves.
      Heat is the problem during heat waves.
      The increase in premature deaths during heat waves is almost identical in low ozone areas as in high ozone areas.

  36. Thank you, oh, thank you, you have just solved my winter heating problems, I will not need to use transport to heat my house, ( I live opposite the park ) so I can save the planet.
    Anybody got a good used chainsaw for sale ??

  37. Chop ’em all, and chip ’em quick!
    Urban forests in Brit cities could keep Drax running for how long? Think of the savings in terms of marine-shipping-costs, concomitant fuel-oil emissions; reduced asthma deaths; increased Urban Heat Island effects (to the extent that they will offset the coming ice-age in N. Europe– thanks to global Warming — thus less freezing Grannies in blacked-out hovels, and even the Greeniacs will be happy with a treeless urban-scape.

  38. Their “models” indicate that trees produced 6-2-% of the ozone and this went to 60% in heatwaves. But nowhere in this is any indication of the actual level and whether or not this poses any threat. You would have to go to the paper for that….
    Typical scary headline with a tiny fact pulled from the paper to make a story and get some clicks. This is pure spin and doesn’t deserve any more attention, but the headline will get repeated endlessly as more “proof” that climate change is bad and we have to stop using energy now!

  39. Yep, I saw the phrase, “computer models”, and immediately my red flags came a flyin’.
    I would like to see phrases like, “strategically place monitors in such a such fashion”, “ran multiple trials of measuring at such and such times of day”, “figured averages”, etc., …. and, yeah, use the computer to tabulate the MEASURED, ACTUAL data, … and THEN tell me what you find.
    Oh, and do this for areas with large numbers of trees, compared to smaller numbers of trees.
    Needless to say, I find it hard to trust the conclusions, based on the methods, … and I’m not even a scientist !

    • “Yep, I saw the phrase, “computer models”, and immediately my red flags came a flyin’.”
      Computer models
      Skeptical Science has a recent post of several studies that conclude that since actual temps are less than the models, that the natural variations in temps are negative by the amount the models are overstated.

    • So you completely missed the part where they said that they used DATA from two events. They used the models to compare the actual events.

      • … two events
        Is that sarcasm ? What I see is that computer models are used to estimate, rather than actually measure, certain quantities. And ONLY two events would seem to make this MODELING even less convincing.

    • Two events, each lasting all summer. They could not, cannot, measure in isolation the discharge from foliage. They do however have good estimates and measurements of emissions from human activity. They put the known values​ into the model and see what value is needed to tune the models to get values matching the many in-situ measurements that they have from various locations and at various times throughout the summer, given also the measured weather conditions.
      They are using the models to calculate a mass balance.

  40. I believe this is a recycled study. It seems that when it comes to climate rent seeking everything old is new again. So trees gather CO2, H2O and sunlight and emit various metabolites. President Reagan was ridiculed by the enviro extremists of the day for pointing this out. Greens are in it for the green: power that is.

    • Yes, I’m old enough to remember them ragging Reagan for saying that Smog was actually made by trees. It was one of their proofs he was not smart at all, even though he was quoting a study by one of the Cal universities.
      But then, according to the same set, John Effing Kerry and AlGore were so much smarter than GWB who had higher grade point averages and could fly a jet from Texas to Florida with the canopy painted white so you could not see out.

  41. The VOCs released depend on the plants involved and the local conditions. This should be a consideration when planning plantings.

  42. When I started my university studies, scientists did actual measurements. They were measuring things not only in labs but also out in the real world. Now it seems science is all computer models. Put your numbers (from another model?) into your model get the result and publish them. This kind of results means absolutely nothing until confirmed with on the spot measurements and some convincing reasoning as to why this ozone has to come from the trees.

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