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Climate Hustle

Turbines in Texas mix up nighttime heat

Posted on 10 May 2012 by MarkR

Satellite measurements of temperatures near wind farms in Texas from 2003-2011 have suggested that wind turbines have mixed up the nighttime atmosphere, bringing warmer air down to the ground (Zhou et al, 2012). When looking at the physics it turns out that this suggests the chance of a (very, very small) global cooling effect.

The satellites measure that downwind from wind farms the surface is warming more than other places nearby, but only at night. In the windier Texan summer the night warming has been 0.73°C per decade, but the calmer winter months have only gone up at 0.46°C per decade.

The authors blame wind farms because the warming happened where turbines were built, as shown in Figure 1 below. The patterns also match expectations from physics: the effect is stronger when it's windier, downwind from turbines and at night.

Figure 1 - map of changes in temperature across Texas in degrees Celsius. Crosses represent places where there are wind turbines, and the prevailing wind is from the south. The area average temperature has been subtracted from each point, so a blue area doesn't necessarily mean it cooled, just that it warmed less quickly than the turbine areas.

At night the Earth's surface cools quickly as it radiates efficiently to space and the cold dense air right next to it can't rise. The air above is warmer and less dense so it floats on top and heat becomes 'trapped' (rather like the alcohol content in the layers of a B-52 cocktail) unless something helps mix it up - and this is where the chopping blades of wind turbines come in (stirring a B-52 does the same job).

During the day the Sun quickly warms the surface and the lower air heats up and rises. The extra chopping of the turbines doesn't make a difference because the atmosphere is already well mixed (to try this at home, try pouring a B-52 the wrong way round and too quickly, then see how much difference stirring it makes!).

What does this mean for climate change?

Lead scientist Liming Zhou from the University of Albany released a Q&A to explain that the effect is local, and that

Very likely, the wind turbines do not create a net warming of the air and instead only redistribute the air’s heat near the surface...fundamentally different from the large-scale warming effect caused by increasing atmospheric concentrations of greenhouse gases due to the burning of fossil fuels.

However, newspaper headline writers and political commentators disagree with physics, measurement and scientific experts and think that the results mean "Wind Farms Cause Global Warming". As Professor Zhou explained, this just ain't the case.

Tiny and local effect, but physics suggests wind farms cause small global cooling!

It's possible that the global effect of this would be a (very small) cooling. It works like this: the Earth's surface can emit in the atmospheric 'windows' where heat leaks easily to space. This is why the Earth's surface cools down faster at night than the atmosphere.

Typically the upper air stores this heat at night, but if it's mixed up by wind turbines, it loses it to the ground and the ground then leaks this heat efficiently into space. Like the radiator fins on a car: the radiator warms up, but it helps keep to cool the whole engine.

When contacted, Professor Zhou responded that "Your explanation is interesting and physically correct to me but the warming-induced emission is very small." He also commented that there might be other effects related to the efficiency of Earth's heat loss which could work in the opposite direction and that more research is needed. Regardless, the effect on global temperatures will be too tiny to measure.

Effect important locally, and a good reminder to stay skeptical

The nighttime warming measured by satellite is quite large compared with the global warming signal of ~0.2°C per decade. But it's only in a very small region and doesn't have much effect on global warming.

As Professor Zhou says, more research is needed. Is this effect widespread in other places? Does it explain some of the night warming measured by near surface thermometers? The area of atmosphere affected is too low for satellites, but maybe nearby weather balloons could be used to check these results.

The media confusion shows us how important it is to stay skeptical and try and think of the whole picture. The turbines are just mixing up heat that was already there and by only measuring where the heat is moving to you can easily get confused and draw the wrong conclusions.

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Comments 1 to 32:

  1. I'm fairly incredulous of the idea that such pundits actually /believe/ such arguments - given the obvious ways to disprove it (e.g. the Arctic sea warms sans windmills, etc...). However, it does appear that any straw that can be is being grasped in an attempt to keep the mouthpieces afloat.

    It does appear that willful blindness is heady draught for some. Over at Roy Spencer's site the april UAH anomaly has been announced at ~ .3 of a degree, complete with the graph of monthly values going back over the last 3 decades or so. One commentator immediately wrote a very convoluted post arguing that the yearly anomaly will only be ~ .05, based on the first three (la nina) months averaged with the most recent value.

    If he'd bothered to look at the trend graph he was commenting on, he'd have noticed the short term nature of la nina - and that such predictions would be highly unlikely - to say the least. However noisy data seems to attract noisy posters, looking for the starship in the sunshine, so to speak.

    ^^ I know it's way off topic - but it's pretty reflective of how obvious science is mangled atm.
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  2. MarkR,

    Thanks for this post.

    It is ludicrous (but not surprising) how the "skeptics" are trying to spin this-- they are once again grasping at straws, doing their best to fabricate doubt and create a new meme.

    As 70rn noted @1, warming is occuring over regions with no wind farms. In fact, the strongest warming is occuring over the Arctic with nary a wind turbine in sight.

    Additionally, orchard farmers have been using turbines for a very long time to reduce the chance of freezing during the winter. It is well known that the turbulence from the blades causes mixing in the vertical and that at nighttime the turbulence mixes down relatively warmer air from aloft (from the temperature nocturnal inversion to be specific). But this works only at a very local scale and only at night.
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  3. MarkR,

    That is a very interesting hypothesis that you present!
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  4. This particular myth dovetails two of the denialists' favorites passtimes - attacking renewable energy, and attacking climate science. So it's not at all surprising that they jumped all over this myth.
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  5. "At night the Earth's surface cools quickly"

    That statement needs modification when talking about Texas.

    Sweetwater, TX is in the midst of the wind belt (and is within the geographic rectangle of Figure 1). In August, peak daytime temperature (~ 33 C on average) is reached in the late afternoon (5pm) and overnight lows, at about 6am, average 22C. That's a change of 11C in 13 hours. In January, the maximum is also at 5pm and the nightime low at 5am, but the daily temperature range is -1 - 15C, yielding a slightly higher cooling rate of 14C/12 hours. It cools more rapidly in the winter because the overnight humidity is lower - and it did that before there were any wind farms.

    Another important observation is that these wind turbines are on flat-topped ridges with a local topographic relief of 100 ft relative to the valleys below. The hilltops are bare pasture; the low ground tends to be covered with brush, suggesting different warming and cooling rates to begin with. I would take issue with the 'downwind' claim, as wind directions can vary considerably - especially around topography.
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  6. As detailed in the following article, the ultra-conservative spin machine in the US has embarked on a massive campaign to discredit clean energy, especially the use of wind turbines.

    “Conservative think tanks step up attacks against Obama's clean energy strategy”, The Guardian (UK), May 8, 2012
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  7. Everyone who grows trees -- as I do -- knows that large structures create microclimates around them. I can take you to areas in my woods where trees planted ten year ago are twice the height of those in other areas, even though they were all planted at the same time. The difference is that the first trees were planted in the shadow of a group of much taller, mature trees and thus they've been exposed to warmer temperatures and less wind than those in the open. The same also happens when buildings create a wind shadow.

    The fact is, I'd have been astonished if it had been shown that tall structures like a wind farm didn't take energy out of the air and create microclimates.

    More on micro-climates here.
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  8. I don't think Zhou (as reported) does have the fluid dynamics right. There's nothing to "bring warm air down to the ground". In an uninterrupted flow over a flat surface you get a fairly thin boundary layer with a more uniform velocity above. The boundary layer has a large velocity gradient and transfers heat very well; the uniform layer transfers heat poorly.

    Making the flow more turbulent enhances heat transfer in the uniform layer. But it thickens the boundary layer, which diminishes heat transfer there.

    A lot of the thinking about inversion layers (as here) is misplaced. It relates to a calm atmosphere. But then the turbines aren't doing anything.

    My view is that the effect is heat generated from turbulence - the part ofd the wind KE that was not converted to electricity. The scale matches. A wind farm generates enough power for a small town, and that power can create a UHI. The turbines are probably less than 50% efficient overall, so equivalent heat is also released at the wundfarm. You're seeing a UHI.

    But it isn't nett warming. The KE of the wind was always going to be converted to heat somewhere. The turbines move the location of the conversion.
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  9. Like 70rn I find it hard to credit that many of those spinning this are doing much more than going through the motions. (There's a hell of a lot of whistling in the dark going on among the contrarians at the moment, and, to further mangle some metaphor, any straw must be not only clutched, it must be brandished aloft and proclaimed as a mighty oaken bough!)

    But who's really fooled? There was even a higher-than-usual count of 'hang on a minute' responses when this came up over at WUWT. You'd really have to be crazy-like-Fox to swallow the 'windfarms cause warming' tosh.

    But don't they just hate the windfarms? With a passion? The level of hysteria is palpable, and more than a little unsettling; the fervour required is, after all, inversely proportional to the inherent inoffensiveness of the technology.

    Why? Because the turbines represent the possibility of a workable future that simply must not be, lest the 'skeptic' be revealed as an irrelevant anachronism, and their world-view be blown to the winds.

    Spend some time at WUWT and Bishop Hill, and discover that, for some, any success of any renewable technology is an existential threat, so much so that they'd rather that the future be impossible.
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  10. It seems this story started a few months ago:

    How the 'windfarms increase climate change' myth was born

    The article that really gave this idea a push online was published on Sunday evening [Feb 5] on the Daily Mail's website. It was delivered with the headline: "Wind farms can actually INCREASE climate change by raising temperatures and causing downpours, warn academics."

    The original study is Roy and Traiteur 2010:

    Data from a meteorological field campaign show that such wind farms can significantly affect near-surface air temperatures. These effects result from enhanced vertical mixing due to turbulence generated by wind turbine rotors. The impacts of wind farms on local weather can be minimized by changing rotor design or by siting wind farms in regions with high natural turbulence. -- emphasis added

    So this 'windfarm heat island' is weather, not climate. I guess we can't expect the denialosphere to know the difference. The Guardian goes on to describe how the original author's work was misused to suit the Daily Mail's preconceived notions.

    "I am already getting emails on this. I will have to categorically say that the headline is not an accurate representation of my work. But I guess there is little I can do now."

    Sounds like the recent 'ikaite in Antarctica means MCA/MWP is global' flap. A pattern emerges that is disturbingly familiar, if you know your Ingsoc.
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  11. Nick: That doesn't seem right to me. My understanding of the UHI (from Wikipedia) is that it is primarily associated with changes in albedo and heat retaining properties of surfaces, rather than heat emissions from human activities.

    We might be able to check this:

    Sunlight ~ 100W/m2
    High city population density = 10^4/km2 = 10^-2/m2
    Domestic energy consumption ~ 10^3 W/capita

    Thus human energy emissions ~0.1*sunlight
    Suburbs probably 10 times lower.

    How about wind farms? A 5MW turbine sits in a space of about 5x10^5m2, generating an average of 1-2MW. Again about 2-4W/m2. A bit lower than dense city, but higher than suburb.

    OK, it's not clear cut.
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  12. I think you need to clarify your point; re - 'no such thing as local warming'. Sure - an energy input will disperse over a wide area - and the amount of joules will be fairly fixed - but the resultant temperature change is spatially dependent.

    If I introduce heating from a gas burner in my house - the temperature change is greater near the source. Similarly with figure 1 above, it's very much a localized effect.

    But this is moot point regardless. Night time does create a thermal stratification of a given air column - with warmer above and cooler below. If you mix up that air column - you end up bringing warmer air down towards the surface - and thus the temperature at the surface goes up.

    You havent created any new warmth - there's been no enrgy input to do so - you've just moved what's already there around.

    Amazingly, there's a marvellous example of this mixing process creating spatially localised temperature fluctuations, with out changing the earth's global heat budget as a whole, on a much larger scale. It's called weather.
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    Moderator Response: TC: Edited to remove all caps. Compliance with the comments policy is not optional. Please ensure you comply in future, as failure to do so may result in deletion of your post, which requires much less effort from moderators.
  13. The overall efficiency of a wind turbine is limited mainly by the amount of kinetic energy from the wind it is able to convert into blades movement. It does not produce waste heat, the energy remains as kinetic energy of the wind. Friction and efficiency of conversion to electricity produce heat; they are are much smaller, though not negligible.
    I don't have any reliable number but I don't think that the waste heat from wind turbines has any significant effect. Thermoelectric power plants probably produce much more heat per unit power output.
    The boundary layer mixing explanation isn't very convincing too. As it happens, wind turbines are often placed on top of a (windy) slope, where vertical mixing should already be significant.
    An intriguing result, though clearly irrelevant from an energy and climate point of view.
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  14. This is why I lean towards the mixing argument - I cannot see how the blades' motion will translate into heat capable of a measurable temperature change for kilometres, or tens of kilometres about the farms.

    I do concede that the windy area would be reasonably mixed as well. All i can suggest is that (possibly) the resultant turbulence directs air flow into colder hollows and valleys down-slope that are other wise relatively sheltered, displacing the cooler air and giving a warmer than average surface reading. Obviously I present this tentively - pure speculation.
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  15. Wait,wasn't it the AGW 'skeptics' that also made the claim that:
    "Humans are too insignificant to affect global climate"?
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  16. tmac57 - Agreed; I find the contradiction between "we are too small to affect climate" and the "windmills are dangerous influences on climate" nonsenses terribly ironic...
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  17. It wasn't that long ago that the concern in the pseudo-skeptic world was wind turbines would slow the wind and even the earth's rotation.

    For some much needed relief, see Climate Crocks.
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  18. 70m
    Night time does create a thermal stratification of a given air column - with warmer above and cooler below.
    Yes, on a calm night. But then how can a turbine mix? I think any wind strong enough to turn the blades would have already mixed the air.

    It does not produce waste heat, the energy remains as kinetic energy of the wind.
    I believe it does. The drag on any aerofoil is the result of linear momentum being converted to angular and shed as vortices. Those vortices (turbulence) decay through viscous effects, releasing heat.

    My understanding is that direct heat release is a big part of UHI. But I don't currently have a source for that.
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  19. 18, Nick,

    I think you are grossly over-guesstimating the amount of heat released. You have broken the model into two components... wind energy converted to power, and the remnant converted to heat. Clearly this is grossly over-simplified, but more importantly it is completely unquantified.

    Your argument amounts to "it seems to me." The Internet is rife with such "thought experiments" which put together a few simple concepts, gloss over all of the details, and arrive at "it's obvious that..."

    I think that unless you are able to properly, mathematically estimate the amount of heat actually generated by each turbine, your suppositions are merely that... suppositions.

    Personally, the idea of a temperature inversion drawing down warmth from higher altitudes is substantially more likely than such a large amount of excess heat being generated through inefficiency.

    And even if it is... it's still not actual warming. The energy was already in the system. It's not a net change, just a redistribution. The whole question of the source of the heat is rather pointless (except to an engineer who is concerned with mitigating its local effects).

    This study says zero about climate change, or the impact of wind farms on climate change.
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  20. @ Sphaerica. You may find yourself having to make that point rather regularly over coming months, I suspect.
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  21. Sphaerica @19, I would not be so immediately dismissive of Nick's idea. Turbulence does result in reduced air velocity, and the energy of that reduced air velocity must be preserved as heat. Where Nick's hypothesis is not plausible is in assuming that if 50% of the wind's energy is captured by the turbine, the remainder of its energy will be released as heat as the result of turbulence. I suspect that a 50% efficient wind turbine captures 50% of the wind energy in the area of its blades - not the swept area of its blades meaning that most of winds energy is neither captured nor lost to turbulence. Given that, it is possible that an amount is lost to turbulence equivalent to the amount captured.

    Having said that, I agree that hand waving arguments are irrelevant. They need to be placed into a mathematical form to render them more precise. Should Nick desire to do so (and I know he has the mathematical ability), the relevant equations are discussed by Science of Doom, Wikipedia, and in the linked UCLA lecture notes.
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  22. I do not understand the discussion of wind generators heating up the atmosphere. Wind is motion of molecules and is part of the atmospheres heat budget. Wind generators do not add any waste energy to the system, they remove energy from the system. Any heat in the generator came from the atmosphere in the first place and does not heat the air when it returns. They affect the atmosphere locally by converting kinetic energy into electricity that they export. Since energy is exported the local system is cooled. According to the OP they heat the surface by mixing hot air downwards during the night. Overall the atmosphere would be cooler since energy is removed. That energy would be returned to the atmosphere as waste heat when the electricity is used. The reduction of the airs' velocity is just conversion of one form of energy in the atmosphere into another. The root mean square speed of nitrogen at 25C is 1150 miles/hour (515 m/s). Adding a little bit of wind speed would make little difference in the temperature. Where have I made a mistake?
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  23. Tom and Sphaerica,
    You can quantify the process. The way any aerofoil generates force is by deflecting a flow. The force is proportional to the mass rate of deflection and the velocity change. The inevitable byproduct is the generation of turbulence, which decays, converting the energy to heat.

    Wiki has an article on Betz' Law. This says that the best a turbine can do is extract 59.3% of the kinetic energy from the wind. But the key is that in that state, the axial component of the velocity has been reduced to zero. It has been entirely converted to swirl, and can't go anywhere much. Viscosity will convert its KE to heat.

    Of course total removal of axial velocity is unrealistic, so practical efficiency is less. But the ratio of swirling flow energy to output energy will remain about the same, 59:41.
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  24. Or, I should say, 41:59.
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  25. Nick,
    The temperature of the air is related to the square of its root mean square velocity which is 1150 miles/hr for nitrogen at 25C. The kinetic energy of the wind is related to the square of its speed which is say 30 mph on a windy day (the power in the wind is related to the cube of the speed). It seems to me that if all the energy in the wind was converted into heat it would not raise the kinetic energy of the molecules significantly.

    Possibly a calculation of the heat capacity of the air would be more useful. The specific heat of air is about 1.0 kj/kg. 1 kg of wind at 30 mph has about 80 joules of energy in it. That comes to about .08C increase in temp if 100% of the energy is converted into heat. That means a maximum of 0.03 C temp increase if the wind generator removes 59% the energy. In addition, a wind generator only converts the wind that passes over its blades into energy, since a much larger vertical slice of the atmosphere is moving, most of the energy stays as wind.

    Can you provide a calculation as to how much you think converting all the wind energy into heat would raise the temperature of the atmosphere? A peer-reviewed citation would be better.

    Wind generators remove energy from the local atmosphere. They do not provide any energy and cannot heat the atmosphere. The wind motion is part of the measure of temperature.
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  26. Michael,
    Yes, I think your second calculation is more convincing. It does say that the temp rise would be less that what is observed.

    But I think my claim that the energy liberated would be comparable to the elec output is still OK. It sounds like a variant of the problem with the inversion layers. When the turbines are working, there's too much wind for the waste energy to create a temp rise.

    So for my part, I now don't know what could explain the temp rise.
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  27. Nick,
    Please provide a reference that shows the wind speed is not part of the temperature. In the College chemistry textbook that I teach from, the temperature of a gas is proportional to its total kinetic energy. To me that would include the wind kinetic energy. I think you separating the wind speed from the temperature is incorrect. There is no energy liberated to convert into heat. Wind generators remove energy from the air. That cannot heat the air. A small fraction of the wind kinetic energy from the atmosphere is turned into heat which is returned to the atmosphere as molecular kinetic energy. In the absence of the generator friction would convert the wind into heat somewhere else when the wind dissipated.

    My calculation indicates that maximum temperature increase is 0.03C. I doubt real increase could be anywhere near 0.01C for the wind that goes through the generator (although I know little about turbulence), the rest is unaffected. Obviously most of the energy is not trapped or the wind farm would cause the wind to stop in its vicinity. The generator definitely exports 59% of the energy.

    The OP states that mixing of the atmosphere causes the temperature increase. Mixing can provide all the needed energy, converting wind into heat cannot.
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  28. Betz's law says that the horizontal wind velocity can not be zero downwind and that there is a theoretical maximum efficiency of 60%; hence 40% of the "input" energy is still kinetic energy of the wind. If the real efficiency is for example 50%, i.e. it's electricity to be used elsewhere, the remaining 10% goes into heat due to electricity production losses, friction and viscosity due to the increased turbulence. The latter can then be just a few percent.
    Even that 10%, though, could be locally significant if sustained. But once it gets diluted by natural mixing over relatively large volumes I don't see how it could produce significant warming.
    I too think, with Nick, that we do not have a clear explanation of the tempemperature rise.
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  29. Michael,
    one could say, why not add in the rotational velocity of the Earth as well? Or is air in a plane hotter after take-off?

    KE for this purpose would be calculated with mean square velocity - ie square of deviations about the fluid mean velocity. Otherwise it depends on frame of reference.

    Definition of temperature requires local thermodynamic equilibrium in a volume. That volume would have to be moving with the fluid. If there is flow through the volume, then there is advection - no equilibrium.

    I don't believe the generator exports 59% of the wind energy - that's a very theoretical max.

    I agree that the 40% refers to emerging KE. My point is that it's no longer avial flow - it's azimuthal or turbulent eddies, and it can't go anywhere much. When it decays, as it must, it will be converted to heat. But MS's calc says it won't raise the temperature much.
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  30. Nick
    I think I see what you're saying but the derivation of the Betz's law you quoted assumes a purely laminar flow along the turbine axis, no turbulence whatsoever. If that is the ideal case, heat dissipation due to turbulence damping has to be part of the about 10% overall losses. Then, muoncounter's number is largely overestimted.
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  31. As Scott Mandia said -- Moving money from one bank account to another won't make you richer, so windmills just move air around, they don't make it warmer.

    Blogged here...
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  32. Riccardo,
    I have to agree - when I look at the derivation of Betz, it is odd. It assumes total downstream KE is derived from the axial flow component, which ignores swirl. But the flow exerts a torque on the turbine; the turbine must exert a torque on the air.

    I guess that's one more layer of mystery. Anyway, I'm convinced by SM's second argument about the smallness of temperature change.
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