Half truths

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(Nederlandse versie hier)

 

There are many “half truths”, which in principle are correct (and are also known to the climate scientists, and are taken into account in climate models), but which are often wrongly interpreted:

 

-     Climate has changed in the past as well (and there were no humans to blame. So right now it’s probably a natural process as well.) The fact that climate has changed in the past due to natural forcings (no scientist would deny that), is no evidence whatsoever that the same is happening now. Natural factors (eg solar activity) are not suddenly turned off now, and they are included in climate models, but they can only explain a small fraction of the observed recent climate change. This is probably the most popular “skeptic” talking point, even though the logic of the argument is flawed. The fact that forest fires occur naturally, is not a valid alibi for Johnny who just caused a fire by throwing away a burning cigarette. This example also illustrates another logical fallacy: That we, insignificant human beings, could never influence something so large as the Earth’s climate. (As if we’re committing hubris by pointing out that CO2 traps infrared radiation.) While a burning cigarette can cause quite a forest fire. And in the history of the Earth, micro-organisms (still a lot smaller than humans) have had quite an influence on the changing climate.

-     Nature emits more CO2 than humans (so therefore the human contribution to atmospheric CO2 -and thus to climate change- is negligible.) The first part is correct (and it happens to be part of my current research area), but the conclusions that are often drawn from it are way off the mark. Nature is in balance (emission and uptake of CO2 by the biosphere are roughly equal over multi-year timescales, except in times of climate change). An increase in CO2 emissions from another source, even though small compared to biogenic emissions, can indeed increase the atmospheric concentration. Indeed, it has been proven that the increase in CO2 is due to human emissions, using the fact that fossil (anthropogenic) CO2 has a different isotopic composition that (current) biogenic CO2. The funny thing in this “skeptical” line of thinking is that it implicitly assumes that CO2 affects climate.

-     Water vapor is the most important greenhouse gas (dwarfing the effect of CO2.) Again, even though the first statement is in principle correct, the second part is dead wrong. Water vapor doesn’t change due to increased emissions, but it changes in response to the increase in temperature: warm air can hold more water vapor. Therefore water vapor works as a positive feedback: If the climate warms, it causes even more warming, and vice versa. Water vapor is in a fast equilibrium with the biosphere, and it is not a climate forcing (cause), but rather a feedback (reinforcing effect). If we wouldn’t have changed the climate, water vapor would not have changed either. The effect of water vapor is definitely included in climate models: A large part of the warming from CO2 occurs indirectly via water vapor as a positive feedback. See also here and here.

-     At the end of the ice ages, temperature rose first, followed by CO2 ~800 years later (and thus CO2 is an effect of temperature rise rather than a cause.) However, both temperature and CO2 continued to rise for another ~4000 years. Thus, the initial temperature increase at the end of an ice age is not caused by CO2, but CO2 did contribute to the remaining temperature increase in the final ~4000 years. It acted as a positive feedback, similar as water vapor. Another major feedback mechanism at the end of the ice ages is due to the changing albedo (reflectivity) when ice sheets disappear and the darker land (or water) surface is exposed again. Temperature and CO2 influence each other in both directions; it is a bit of a chicken-egg discussion. However, the current situation is clearly different from that at the end of the ice ages, since now we know that the extra CO2 is brought into the atmosphere by human activity. Currently, CO2 is not increasing in response to the warming, but rather due to human emissions, and as such it is now one of the driving forces of the warming. You may have noticed that Al Gore doesn’t discuss the full complexity of the ice ages in “An inconvenient truth”. See also here.

-     The greenhouse effect is natural (and thus humans have nothing to do with it.) First part is absolutely true, otherwise the average temperature on Earth would be well below zero degrees C, and life as we know it would not have been possible. But we are strengthening the natural greenhouse effect by increasing the amount of greenhouse gases in the atmosphere. This statement is actually a very strong argument that greenhouse gases indeed influence the temperature. And since it’s certain that we are increasing the greenhouse gases in the atmosphere, it is therefore clear that current climate change is indeed due to human emissions.

Most other often heard “criticisms” of climate science are not even half true, but just plain wrong. Many of them are debunked here and here.

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16 Responses to “Half truths”

  1. Jason Says:

    Unfortunately, the climate models are a long way from even being retroactively accurate. They are junk. Period.

  2. Bart Says:

    It escapes me what exactly you are responding to, and your final statement is not very conducive to a discussion.

    That said, global temperature change over the past 100 years is very well reproduced by models (see eg http://www.ipcc.ch/graphics/graphics/syr/spm4.jpg), and so are smaller scale events such as volcanic eruptions. Predictions made 20 years ago also match the measured trends since then fairly well.

    Models are not perfect, but they are a useful interpretive and -within limits- predictive tool.

    Our understanding of climate change rests on historical climate changes, current observations, and climate models. And thus, as eg McIntyre also acknowledges, “it is possible to be concerned over CO2 impact even if you are not sold on GCMs.”

  3. AllPunsIntended Says:

    I’m trying to get my head around this, perhaps you can clarify

    “Thus, the initial temperature increase at the end of an ice age is not caused by CO2, but CO2 did contribute to the remaining temperature increase in the final ~4000 years.” -Bart
    -High resolution temporal ice-core proxies do show the 800-year CO2 lag from onset of temperature rise, to onset of decline. I find it hard to believe that CO2 is a significant feedback given the fact that the warmings stopped and reversed while still in the presence of highest CO2 levels, or in today’s day and age, that it was “possibly temporarily overwhelmed by things like aerosols, the details of which we do not fully understand” (not your quote, sorry) as is the argument for mid-century cooling.

    “… fossil (anthropogenic) CO2 has a different isotopic composition that (current) biogenic CO2.” – Bart
    -There seem to be few graphs floating around with different concentration vs year of isotopes of carbon. Could you show which one you use to come to conclusion that current CO2 concentrations are anthropogenic? I’m not contesting that fact, just that if you are referring to the declining C-14 content of the atmosphere as proof (Ref. http://en.wikipedia.org/wiki/File:Radiocarbon_bomb_spike.svg), it looks like it is simply returning to the equilibrium after the nuclear testing.

    “But we are strengthening the natural greenhouse effect by increasing the amount of greenhouse gases in the atmosphere.” – Bart
    -Sure, we’re increasing the GHG, but to what degree does that effect the feedbacks? I know that’s not your argument, but consider the long-wave absorption spectrum (Ref. http://en.wikipedia.org/wiki/File:Atmospheric_Transmission.png ) where H20 dominates the far-IR, where CO2 is opaque in only 3 narrow peaks which are already saturated! What does that say about the role of CO2?

    Thanks

  4. Bart Says:

    API,

    Towards the ending of an ice age, the temperature continued increasing after the CO2 started increasing, and the total amplitude of the temperature rise cannot be explained without the warming effect of CO2. See this nice historical overview on how the science developed: http://www.aip.org/history/climate/cycles.htm. In trying to unravel the cycles of ice ages, the warming effect of CO2 seemed a necessary factor to include. Much more evidence about its significance has been added since.

    There were –and are- multiple forcings and feedbacks active at the same time, so the existence of periods where the correlation between temperature and CO2 apparently stops is no evidence for a lack of a causal relationship. A cooling effect of aerosols does not prove that greenhouse gases cannot have a warming effect. In the global mean, they work against each other, and can thus cancel out if they are of a similar magnitude (as happened mid 20th century).

    Regarding the evidence for the rise in CO2 being anthropogenic in origin, see e.g. this essay with some useful graphs:
    http://www.ferdinand-engelbeen.be/klimaat/co2_measurements.html#The_mass_balance
    Or this, without graphs,
    http://www.realclimate.org/index.php/archives/2004/12/how-do-we-know-that-recent-cosub2sub-increases-are-due-to-human-activities-updated/langswitch_lang/de
    Good paper on the carbon cycle:
    http://www.atmos.ucla.edu/~gruber/publication/abstracts/Sarmiento_Gruber_pt_02_fig.htm.

    The CO2 absorption spectrum is not completely saturated, rather it will widen at the wings in response to a higher concentration. Moreover, a higher degree of saturation at the surface causes the IR to be radiated into space from a higher altitude. Since the energy content of radiation depends on temperature, radiation leaving the atmosphere from higher, colder regions, contains less energy. Therefore, more energy stays within the Earth’s atmosphere, which will warm as a result.
    See also http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument-part-ii/langswitch_lang/de

  5. AllPunsIntended Says:

    –CO2 lag–

    The first link you gave me was “tl;dr” with too many fanciful embellishments, so I’ll give you the benefit of the doubt with regards to whatever it claims. The main point though, is that the argument about ice-core CO2-Temp dependance is almost always of the form: the warming effect of CO2 must be significant because temperature rise cannot be explained without the warming effect of CO2, sometimes with a caveat like: ..according to our current understanding of climate and State-of-the-Art CGMs. Whatever I did read from that article was no exception. Fair enough. However significant it was, it was still driven by presumably weak solar cycles. It had all the signs of being a system response. I don’t find it puzzling that CO2 rise lagged rising temperature, even that it contributed through feed-back, but I do find puzzling that the CO2 was still increasing when temperature started decreasing, and that the temperatures continued to fall when CO2 was at it’s peak. I think that’s a genuine concern

    Now forward to modern time, when we’ve upset the equilibrium, and the high CO2 levels have become the “primary forcing”, though there are also “multiple forcings and feedbacks active at the same time” which may at times overwhelm the primary forcing that’s CO2.. I’m not too au fait in aerosol literature (which is becoming vast), but seems very ‘convenient’ that that’s what happened, given that majority of aerosols are natural, and increasing (literature that I have)

    –Anthro CO2–

    With regards to anthropogenic CO2, thanks for the links. I was wondering how it is concluded without the carbon-balance stuff. Let’s be honest, the carbon mass balance is a very crude estimate (if we can call it that) of what goes on, at best. We need something we can actually measure. 13C/12C ratio is good enough for me, even if it comes from weird sponge proxies. The 14C wasn’t.

    –IR–

    I was a bit underwhelmed by the RC article on CO2-IR, they seem to have a lot of soft points and clever wordsmithing. I was expecting some kind of saturation chart, like I posted, but for different altitudes. They make a point that CO2 spectrum isn’t saturated at the wings (which are small in relation to the saturated bands, and effects of which increase logarithmically) and that the saturation density is not linear with altitude due to pressure broadening. The latter point seems irrelevant to me because the net radiation heat transfer only cares about total absorption – from ground to free space. I’ll have to investigate this further. Perhaps you can give me an analogy to electrical circuits (or radar theory) where this point is made clear.

    You said: “Moreover, a higher degree of saturation at the surface causes the IR to be radiated into space from a higher altitude” — this statement seems contradictory. If radiation occurs at the ground surface, it would be logical that a saturated band near the surface absorbs and re-emits the longwave back to space (in wavelength which is now transparent to gases) or to ground which is again re-emitted to space.

    Further: “Since the energy content of radiation depends on temperature, radiation leaving the atmosphere from higher, colder regions, contains less energy. Therefore, more energy stays within the Earth’s atmosphere, which will warm as a result.” This is the basis of an argument that overall increases in stratospheric CO2 (spectrum of which is less saturated due to the combo platter of pressure broadening and no water) are significant (which is what RC is presenting, I think). This would make sense if the stratosphere was warming. Is the stratosphere warming?

    Thanks

  6. Bart Says:

    API,

    It’s hard to address your question about ice ages without knowing the specifics: What time period and measurements are you referring to? How reliable are the measurements? However, the question is perhaps better posed to scientists active in that area (you could try RC). My guess is that the answer has to do with the different timescales of CO2 mixing into (and out of) the deep ocean versus atmospheric temperature change, as well as the fact that different forcings act on the system simultaneously.

    I’ll ignore your remark about aerosol forcing being convenient; it is has the smell of conspiracy to it. From what I’ve read, the increase in aerosol forcing since pre-industrial time is predominantly anthropogenic in origin, not natural.

    On the CO2 saturation effect:
    The amount of absorbed radiation depends on the product of concentration and pathlenght (Lambert-Beer law). If the atmosphere as a whole (interpreted as a pathlength from groundlevel to the top of the atmospere) were saturated with respect to radiation in the CO2-absorbed wavelength, then shortening the pathlength (increasing the mean altitude of re-emission) will at some point lead to it not being saturated anymore. There will always be an altitude above which so few CO2 molecules remain that the radiation can escape to outer space. The re-emission takes place at a wavelength characteristic for the object’s temperature (black body radiation), and thus less energy leaves the system as the mean altitude of emission shifts to higher, colder layers of the atmosphere.

    I think most absorption takes place well below the stratosphere, however. A stronger greenhouse effect causes more IR to be absorbed (below the stratosphere), so less reaches the stratosphere, which cools as a result. Climate models predict that the stratosphere will cool in response to an increasing GHG abundance. (As a consequence of radiative transfer the vertical temperature gradient increases.) See eg http://www.atmosphere.mpg.de/enid/20c.html for more explanation. The stratosphere has indeed cooled. (Note that if solar forcing was responsible for the surface warming, the stratosphere should have warmed as well.)

    Some other explanations on the saturation effect:
    Chris Colose (in an RC comment): “CO2 is a greenhouse gas, and thus absorbs/emits infrared radiation. At current, terrestrial conditions it makes the planet completely opaque to infrared in the 14-16 micron window (and some distance away from those edges as well). Increasing its concentration allows the mean altitude of emission to be shifted to colder layers of the atmosphere where the influx of energy then exceeds the output, and the planet must warm.”

    http://www.realclimate.org/index.php/archives/2006/04/lessons-from-venus/langswitch_lang/sp:
    “Despite the fact that Venus has vastly more CO2 in its atmosphere than Earth, the same basic principles govern the operation of the greenhouse effect for both planets: the fact that air cools by expansion as it rises means that the upper parts of the atmosphere are colder than the surface, while the opacity of greenhouse gases to infrared means that infrared radiation can only escape from the upper portions of the atmosphere. Since the rate of radiation goes down with temperature, the net effect allows the planet to lose energy at a rate much lower than it would if the radiation from the surface escaped directly to space.”

  7. AllPunsIntended Says:

    Bart, thanks for reply. With regards to ice-core data, I was referring to the latest high-resolution data from Dome C (Something like Luthe et al, 2008), I don’t have it on hand, I’ll have to source it for you. It’s the interglacial period

    You may ignore my aerosol comment if you like, you should question however, why IPCC Fourth Assessment does not mention natural aerosols or their significance in their forcings ( http://ipcc-wg1.ucar.edu/wg1/wg1-report.html Vol. 2) while Encyclopedia of Atmospheric Sciences claims that they dwarf all tropospheric anthro sources and all have increased since the 70′s. Not a single mention of natural mineral, sea-salt, and gas-to-particle aerosols with regards to their forcings/feedbacks. What conspiracy?

    As far as IR spectra, again, you seem to be contradicting yourself. I’ll attempt to break down your reply piece by piece in case I’m missing something.
    You say: “If the atmosphere as a whole (…) were saturated with respect to radiation in the CO2-absorbed wavelength, then shortening the pathlength (increasing the mean altitude of re-emission) will at some point lead to it not being saturated anymore”
    -How is shortening the pathlength increasing the altitude of re-emission? Let’s say that before your pathlength was from ground to 100km, and it was saturated by 100km. Now you add more gas, and under the assumption that CO2 is well mixed (constant ppm at all altitudes, what RC suggests), let’s say that it now becomes saturated by 50km, right? Is there another mechanism at work here?
    You go on: “The re-emission takes place at a wavelength characteristic for the object’s temperature (black body radiation), and thus less energy leaves the system as the mean altitude of emission shifts to higher, colder layers of the atmosphere.”
    -Which then would require that the stratosphere (higher, colder layers) be warming. If it is absorbing more radiation, it is heating up. You and I both agreed that the stratosphere has cooled.
    Further still, you say: “I think most absorption takes place well below the stratosphere, however. A stronger greenhouse effect causes more IR to be absorbed (below the stratosphere), so less reaches the stratosphere, which cools as a result.”
    -In a discussion which argues the importance of high-altitude increase of concentration of CO2 (no water + pressure broadening) and its lower state of black-body radiative potential, you then say that most of the absorption takes place far below the stratosphere (where majority of CO2 absorption already exists due to density and is competing for spectrum with H2O), and therefore at lower altitude and higher BB-temperature for radiating to space.

    The RealClimate posts and comments fall apart at the same place: “…to be shifted to colder layers of the atmosphere where the influx of energy then exceeds the output, and the planet must warm” — must warm in the stratosphere.

  8. Bart Says:

    The Luthi 2008 paper you refer to is probably this one:
    http://www.nature.com/nature/journal/v453/n7193/full/nature06949.html
    I don’t see evidence in that paper that “CO2 was still increasing when temperature started decreasing” (your words). Instead, I read in the paper that “During these 800 kyr, CO2 is strongly coupled with the Antarctic temperature”, which is consistent with evidence from other ice cores. The high resolution figures in their paper (fig 3 and fig S4) do not show a clear departure from this coupling at all. However, specific issues are probably best communicated with the authors of the paper; I’m not a paleo-climate expert.

    On CO2 saturation, the issue (as I see it) is that increasing the altitude of re-emission is shortening the effective pathlenght, so there inevitably will be an altitude above which the amount of CO2 is no longer saturated (in the centrepoint of its absorption bands; the wings can move so to speak, so there is no 100% saturation to begin with). Adding more CO2 to the atmosphere increases the level of the altitude.

    I did some more searching on reasons why the stratosphere cools as a result of more GHG forcing, and it appears to be a result of applying full radiative transfer codes, without an easy to grasp explanation, except perhaps for this one I referred you to before: http://www.atmosphere.mpg.de/enid/20c.html It probably still leaves some questions though. Digging through radiative transfer literature and codes, or consulting an expert in that specific area may be the best way to go to.

    Some other attempts, which I don’t feel I can improve on:

    William Connolley:
    http://mustelid.blogspot.com/2005/03/why-does-stratosphere-cool-under-gw.html:

    “… the stratosphere is considerably warmer than it would be under just longwave (LW, or IR) forcing; and CO2 is only effective in LW frequencies;

    hence, increasing CO2 *increases* the stratospheres ability to radiate in the LW, but doesn’t substantially increase its ability to gain heat, because most of that comes from the SW;

    hence it cools.”

    And perhaps the simplest one, similar to what I was trying to express:
    http://climate-change.suite101.com/article.cfm/climate_change_basics:

    “Since the heat gets trapped here [presumably the troposphere – red.] by greenhouse gases that absorb it, less heat travels outward to the stratosphere. It’s as if radiated heat on the Earth is a dense collection of balloons—if there are more balloons near the surface, there will be fewer balloons for higher up.”

    Article on upper atmospheric trends:
    http://www.ann-geophys.net/26/1255/2008/angeo-26-1255-2008.pdf:

    “In the upper atmosphere, the radiative effects of greenhouse gases, particularly CO2, become more pronounced and produce a cooling, rather than a warming effect. At these altitudes, CO2 is optically thin and is unable to contain outgoing infrared radiation; thermal energy is transferred by collisions with ambient gas to the excited states of CO2 molecules and then lost to space via its infrared radiation.”

  9. AllPunsIntended Says:

    Bart, I thank you for your patience with me. Hopefully this discussion is furthering your understanding of the subject as much as myself.

    Let me abandon my ice-core argument for a sec. I must have the wrong study. Somewhere I have seen a graph where it clearly showed temps going down while CO2 was increasing, now that I think back it had the 800yr lag as well so it couldn’t be this study. Anyhow, that point was not contested, it was somehow danced around with the “there are multiple highly nonlinear forcings/feedbacks going on in a climate system” argument. I’ll give it to you for now.

    On with the radiation. I think I understand your confusion with saturated/unsaturated business. Let me make an analogy. I have a monochromatic source of light (let’s say at 10 micron wavelength) and five discrete layers of semi-transparent glass blocks. By itself, the first block filters 70% of the monochromatic light, second – 30%, third – 10%, fourth – 5%, fifth – 2%. While it’s true that the fourth and fifth blocks are not saturated (actually, none are in my example), since the light originates at the first block, it is completely blocked by the end of the second block (troposphere). Increasing the opaqueness at the last blocks (stratosphere) has no significance. You are implying that increasing the concentration of last blocks somehow shifts the “center of gravity” of absorbtion to higher altitude — but that presumption is incorrect and still would require more light (heat) to stay in those higher blocks.

    The links you’ve provided (thanks for digging them up) mostly fall into two categories:
    1. The stratosphere is cooling due to increased GHGs in troposphere, hence less radiation getting to statosphere.
    If you’d like to hitch your argument on that ride – you have no objections from me. However, you then have to answer to my saturation argument, namely, that CO2 IR spectrum is already saturated at that altitude (competing with itself and H2O) and adding more GHGs has no noticeable effect. I’ll have more on this later

    2. Alternate theories.
    a) Here we have William Connelley. Basically what he says, ‘well, if stratospheric GHG’s weren’t increasing, the stratosphere would be even cooler!’. Incredible argument. This starts losing it’s comic value once one realizes that “in a former life I was a climate modeller at BAS” as written on his Wikipedia page

    b) The last link you provided (J. Lastovicka et al.). “In the upper atmosphere, the radiative effects of greenhouse gases, particularly CO2, become more pronounced and produce a cooling, rather than a warming effect”. This one takes the cake. Are we encouraged to assume that CO2 has reverse greenhouse effect, a negative absorption spectrum at that altitude?

    The first link you provided talks about the first category, but also contains a nice colourful graph of stratospheric cooling rates (in milliKelvin/(day*cm-1). Incidentally, this is referenced in many blogs (even RC). This graph makes more sense if it is actually showing atmospheric heating rate (i.e. positive = heating). You will notice in this pic ( http://en.wikipedia.org/wiki/File:Atmospheric_Transmission.png ) that O3 has a smaller peak in the 9-10um range (wavenumber ~1000 cm-1) and CO2 has a bigger peak at 15um range (~650 cm-1). Ok, that makes sense. So then in the 50km altitude we should be seeing a combined heating effect of CO2 and O3 (by eye-integrating over the wavelengths, it’s all positive), which is not what we see in the graphs just below that article.

    We’re back to contradicting arguments.

  10. Bart Says:

    The spectrum is not already saturated, not even at low altitude. The absorption wings broaden upon adding more CO2. See eg http://members.casema.nl/errenwijlens/co2/co205124.gif

    That means that with increasing CO2, more IR is absorbed in lower layers of the atmosphere, so less reaches the stratosphere, which must therefore cool. Perhaps not the most elegant explanation around, but I think that’s the hunch of it. I may take my own advice and consult people more knowledgeable than myself on this topic though.

  11. AllPunsIntended Says:

    Why can’t I post here yooo

  12. AllPunsIntended Says:

    The chart you provided considers CO2 only (!) and focuses on a very tight band of IR, this is not a real representation of the atmosphere, at any altitude.

    A better model is here:
    [http://geosci.uchicago.edu/~archer/cgimodels/radiation.html]
    Not to worry, the author is a contributor of RealClimate.
    The important inputs are Sensor Altitude and Direction (0, and looking up), and all the other parameters you can tweak.

    I’ve uploaded a pic of 4 runs of CO2 at 0ppm, 375ppm, 750ppm, 3750ppm
    Take a look
    [http://s3.supload.com/free/absorption.jpg/view/]

    I may have found my answer

  13. Bart Says:

    and…?

  14. AllPunsIntended Says:

    Oh sorry, I thought it was evident
    Going from 375ppm to 750ppm (100% increase) we get an increase in 0.03% in IR absorption. I’d say that’s saturated
    There is a higher increase in subarctic winter scenario (I haven’t been able to run the numbers, server was down) but I still can’t find how IPCC came up with the 1.6W/m2 forcing for the +15ppm/year rate that we have

    In any case, due to near-saturation, the change in absorption is far far less than logarithmic

  15. Andrew Fong Says:

    Regarding CO2 effects, I hear a lot of hand waving going on, and it clearly is a very complex process. I have often wondered whether folks are including pressure broadening variability in their models. At high altitude one would expect the absorption spectrum of the CO2 band in question to be a series of sharp(er) spikes. How does that change things?

    The true test is experimental evidence. I am not an atmospheric scientist. Are there satellite data of the IR emissions of the earth? How well do the real data fit with the various theories.

    As for the assertion that there should be a positive feedback because of water vapor, that assumes that increased water vapor in the atmosphere is not cancelled out by increased cloud formation (albedo effects), and I don’t think there is a great understanding of the myriad types of cloud formations and their interactions with one another.

    I also wonder whether desertification can cause a negative feedback. Large, dry areas should radiate much more efficiently back out to space, cooling the earth.

  16. Hans Says:

    There are more half-truths about climate change:

    Reduction of airpollution resulted in global brightening since 1975. This can partly explain higher temperatures in Europe and the US in the last decades.

    Urbanization can explain part of the “global warming” in the last 40 years.

    During the medieval warm period the climate was even warmer than today. There was a permanent settlement of Vikings on Greenland, maybe it was hot enough for agriculture.

    After the Little Ice Age, the climate got warmer without the help of human CO2 emissions.

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