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How do we know more CO2 is causing warming?

What the science says...

Select a level... Basic Intermediate Advanced

An enhanced greenhouse effect from CO2 has been confirmed by multiple lines of empirical evidence. Satellite measurements of infrared spectra over the past 40 years observe less energy escaping to space at the wavelengths associated with CO2. Surface measurements find more downward infrared radiation warming the planet's surface. This provides a direct, empirical causal link between CO2 and global warming.

Climate Myth...

Increasing CO2 has little to no effect
"While major green house gas H2O substantially warms the Earth, minor green house gases such as CO2 have little effect.... The 6-fold increase in hydrocarbon use since 1940 has had no noticeable effect on atmospheric temperature ... " (Environmental Effects of Increased Atmospheric Carbon Dioxide)

The greenhouse gas qualities of carbon dioxide have been known for over a century. In 1861, John Tyndal published laboratory results identifying carbon dioxide as a greenhouse gas that absorbed heat rays (longwave radiation). Since then, the absorptive qualities of carbon dioxide have been more precisely quantified by decades of laboratory measurements (Herzberg 1953, Burch 1962, Burch 1970, etc).

The greenhouse effect occurs because greenhouse gases let sunlight (shortwave radiation) pass through the atmosphere. The earth absorbs sunlight, warms then reradiates heat (infrared or longwave radiation). The outgoing longwave radiation is absorbed by greenhouse gases in the atmosphere. This heats the atmosphere which in turn re-radiates longwave radiation in all directions. Some of it makes its way back to the surface of the earth. So with more carbon dioxide in the atmosphere, we expect to see less longwave radiation escaping to space at the wavelengths that carbon dioxide absorb. We also expect to see more infrared radiation returning back to Earth at these same wavelengths.

 

Satellite measurements of outgoing longwave radiation

In 1970, NASA launched the IRIS satellite that measured infrared spectra between 400 cm-1 to 1600 cm-1. In 1996, the Japanese Space Agency launched the IMG satellite which recorded similar observations. Both sets of data were compared to discern any changes in outgoing radiation over the 26 year period (Harries 2001). The resultant change in outgoing radiation was as follows:


Figure 1: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature (Harries 2001).

What they found was a drop in outgoing radiation at the wavelength bands that greenhouse gases such as carbon dioxide (CO2) and methane (CH4) absorb energy. The change in outgoing radiation is consistent with theoretical expectations. Thus the paper found "direct experimental evidence for a significant increase in the Earth's greenhouse effect".

This result has been confirmed by subsequent papers using more recent satellite data. The 1970 and 1997 spectra were compared with additional satellite data from the NASA AIRS satellite launched in 2003 (Griggs 2004). This analysis was extended to 2006 using data from the AURA satellite launched in 2004 (Chen 2007). Both papers found the observed differences in CO2 bands matching the expected changes from rising carbon dioxide levels. Thus we have empirical evidence that increased CO2 is causing an enhanced greenhouse effect.

Surface measurements of downward longwave radiation

A compilation of surface measurements of downward longwave radiation from 1973 to 2008 find an increasing trend of more longwave radiation returning to earth, attributed to increases in air temperature, humidity and atmospheric carbon dioxide (Wang 2009). More regional studies such as an examination of downward longwave radiation over the central Alps find that downward longwave radiation is increasing due to an enhanced greenhouse effect (Philipona 2004).

Taking this a step further, an analysis of high resolution spectral data allows scientists to quantitatively attribute the increase in downward radiation to each of several greenhouse gases (Evans 2006). The results lead the authors to conclude that "this experimental data should effectively end the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming."


Figure 2: Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapor is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

Conservation of Energy

Huber and Knutti (2011) published a paper in Nature Geoscience, Anthropogenic and natural warming inferred from changes in Earth’s energy balance.  They take an approach in this study which utilizes the principle of conservation of energy for the global energy budget using the measurements discussed above, and summarize their methodology:

"We use a massive ensemble of the Bern2.5D climate model of intermediate complexity, driven by bottom-up estimates of historic radiative forcing F, and constrained by a set of observations of the surface warming T since 1850 and heat uptake Q since the 1950s....Between 1850 and 2010, the climate system accumulated a total net forcing energy of 140 x 1022 J with a 5-95% uncertainty range of 95-197 x 1022 J, corresponding to an average net radiative forcing of roughly 0.54 (0.36-0.76)Wm-2."

Essentially, Huber and Knutti take the estimated global heat content increase since 1850, calculate how much of the increase is due to various estimated radiative forcings, and partition the increase between increasing ocean heat content and outgoing longwave radiation.  The authors note that more than 85% of the global heat uptake (Q) has gone into the oceans, including increasing the heat content of the deeper oceans, although their model only accounts for the upper 700 meters.

Figure 3 is a similar graphic to that presented in Meehl et al. (2004), comparing the average global surface warming simulated by the model using natural forcings only (blue), anthropogenic forcings only (red), and the combination of the two (gray).

knutti attribution

Figure 3: Time series of anthropogenic and natural forcings contributions to total simulated and observed global temperature change. The coloured shadings denote the 5-95% uncertainty range.

In Figure 4, Huber and Knutti break down the anthropogenic and natural forcings into their individual components to quantify the amount of warming caused by each since the 1850s (Figure 4b), 1950s (4c), and projected from 2000 to 2050 using the IPCC SRES A2 emissions scenario as business-as-usual (4d).

knutti breakdown

Figure 4: Contributions of individual forcing agents to the total decadal temperature change for three time periods. Error bars denote the 5–95% uncertainty range. The grey shading shows the estimated 5–95% range for internal variability based on the CMIP3 climate models. Observations are shown as dashed lines.

As expected, Huber and Knutti find that greenhouse gases contributed to substantial warming since 1850, and aerosols had a significant cooling effect:

"Greenhouse gases contributed 1.31°C (0.85-1.76°C) to the increase, that is 159% (106-212%) of the total warming. The cooling effect of the direct and indirect aerosol forcing is about -0.85°C (-1.48 to -0.30°C). The warming induced by tropospheric ozone and solar variability are of similar size (roughly 0.2°C). The contributions of stratospheric water vapour and ozone, volcanic eruptions, and organic and black carbon are small."

Since 1950, the authors find that greenhouse gases contributed 166% (120-215%) of the observed surface warming (0.85°C of 0.51°C estimated surface warming).  The percentage is greater than 100% because aerosols offset approximately 44% (0.45°C) of that warming.

"It is thus extremely likely (>95% probability) that the greenhouse gas induced warming since the mid-twentieth century was larger than the observed rise in global average temperatures, and extremely likely that anthropogenic forcings were by far the dominant cause of warming. The natural forcing contribution since 1950 is near zero."

Conclusion

There are multiple lines of empirical evidence that increasing carbon dioxide causes an enhanced greenhouse effect. Laboratory tests show carbon dioxide absorbs longwave radiation. Satellite measurements confirm less longwave radiation is escaping to space at carbon dioxide absorptive wavelengths. Surface measurements find more longwave radiation returning back to Earth at these same wavelengths. The result of this energy imbalance is the accumulation of heat over the last 40 years.

Last updated on 10 December 2011 by dana1981. View Archives

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Further reading

A good summation of the physics of radiative forcing can be found in V. Ramanathan's Trace-Gas Greenhouse Effect and Global Warming.

Comments

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Comments 1 to 50 out of 158:

  1. The graph you show is not "The resultant change in outgoing radiation was as follows" as you state.

    What it is the data that has been manipulated to highlight the drops in radiance in the regions of the spectra that are absorbed by CO2 and CH4 (and other trace gases). The fact that no area of the graph goes above the 0 point (dotted line).

    The real observed data shows a large region of the spectra above the 0 line. You can see this in the original Harris 2001 paper (unfortunately subscription) or Fig 3 in the Chen paper (free access). What this says is there are other parts of the spectra which are letting more energy escape from the planet in 2006 than in 1970.

    if you take the CO2 part of the spectra in isolation this would suggest greater energy retention (global warming). If you highlighted just the positive areas (say spectra from 800-1000) you would conclude greater energy radiance (global cooling). If you took the whole of the spectra I'm not sure whether you'd conclude greater or less radiance in more recent years.

    Why don't people look at the whole of the spectra and what would be the explanation for greater radiance at other wavelengths? I accept that this analysis might be way of highlighting the CO2 'signature' in the spectra I don't see how you can conclude global warming without analysis of the whole wavelength spectra.
    Response: Harries 2001 does look at the full infrared spectrum except for wavelengths less than 700nm (which happens to be where a large portion of the CO2 absorption occurs). The observed changes in the spectrum from 1970 to 2006 are consistent with theoretical expectations. As the atmosphere warms, more infrared radiation is radiated to space. However, less infrared radiation escapes at CO2 wavelengths. The net effect is that less total radiation escapes out to space.

    This is independently confirmed by surface measurements which find the net result is more longwave radiation returning back to the Earth's surface (Philipona 2004, Evans 2006). It's also confirmed by ocean heat measurements which find the oceans have been accumulating heat since 1950 (Murphy 2009).
  2. re comment 1 (HumanityRules), referring to apparent increases in radiation in the 800-1000 part of the spectrum in the Harries paper (the full version of which you can find through Google Scholar): on page two, 2nd column, that paper does indeed offer a potential explanation, viz. incompletely-cleared artifacts in the data (due to ice-cloud absorption).

    Considering the study is the first of its kind, its finding of reduced radiation precisely in the wavelengths associated with increased GHG concentrations remains remains highly suggestive, no?
  3. Re #1 (Humanity Rules). My understanding is that Earth emissions are modeled as black body radiation. Given the earths temperature of ~298K that means the IR radiation emitted by the earth peaks at around 600cm-1 and tails off around 1500cm-1.

    There is this on-line model that allows you to play with the earths emissions. This is Climate science 101 so it may be hopelessly naive !
  4. Re #2 (Hugh)

    Considering the study is the first of its kind, its finding of reduced radiation precisely in the wavelengths associated with increased GHG concentrations remains remains highly suggestive, no?

    Suggestive that there is a slight increase heat absorbance of CO2 and MH4 - but only if you cherry pick the data. Any atmospheric gas that absorbs infrared is considered a GHG. If you look at the full data you will see that although there is some increase in the heat absorbance of CO2 and MH4, you are seeing more heat escape at wavelengths different from CO2 and MH4. When talking about the causation of global warming, what does that suggest about the "greenhouse effect"?
  5. jabberwockey,
    it is nature that cherry picks wavelength as far as molecular absorption is concerned. On the contrary, thermal emission is broad band and it depends on temperature. Hence what you see in the full spectrum (the background) reflects the increase of surface temperature.
  6. Some quesitons.
    1) Graph one is derrived from the difference between two satelites launched 26 years apart. How are their sensors callibrated so that they would give the same readings on the same day?

    2)Graph 2. Why is there no downward radiation for CFCs, HNO3 NO2 etc, as there is for CO2, when the first graph shows that their energy is being 'trapped' in the atmosphere?


    Additionally. from the picture showing solar light penetrating the atmosphere but terrestrial IR being trapped, what happens to the solar IR?

    The sun produces aproximately 400000 times as much IR at the frequency absorbed by CO2 as the earth. The atmosphere must therefore absorbs and re-radiate half of this back into space.

    Also given the far larger solar IR radiation the CO2 will be saturated. Additional IR from the earth is a tiny amount in comparison.

    Answers appreciated.
  7. 1) different satellites and/or instrumets are always calibrated against one another in a better way than just the reading on a single day.
    2) the first graph is a difference between spectra taken in two different points in time; the second graph is just a point in time. The difference depenss on how much the relative contrbution changed over time.

    IR from the sun is indeed absorbed by CO2. When you calculate an energy balance in a layer of the atmosphere you take both the incoming and outgoing energy into account. The tiny amount of energy (one and something W/m2) taken up by increasing CO2 will not make the earth look like Venus but it's enough to increase the temperature by a couple of degrees, maybe three by the end of the century. That's unfortunately enough to produce a significant change in the biosphere.

    There is no saturation effect to help us. The lifetime of the CO2 excited state is short enough for the CO2 molecules to be ready to absorb more of the incoming photons.
  8. @Ricardo. Point #2. That doesnt answer my question. Graph 1 shows energy being absorbed by all the gasses mentioned. Graph 2 shows enery be emitted by only some of them. Thats my point. How is it selective so that only some of the gasses emmit energy back o the surface which is what the second graph implies?


    Point #3. Energy ballance. So an increase in CO2 will absorb, and re-emit to space more IR from the sun as well as absorb and re-emit to earth more IR from the earth.

    Since the sun produces more IR than the earth how does the extra CO2 cause warming and not cooling?

    Point #4. Excited state duration. If the duration is less than 12 hours then all the energy absorbed by CO2 during the day will be lost at night. How does this generate net warming over a period of days-years?

    I am not being argumentative, I am geneuinely interested in the science behind GH gasses, it is just that logically there seem to be problems with the theory. I am glad you mentioned excited state durtation by the way because I could not find any information on this online and to me it is a critical factor in heat storage.
  9. matt sykes,

    #2
    fig. 1 is a difference spectrum between 1976 and 1990. It shows only the changes during this period of time. If you want to compare the spectrum in fig. 3 (an emission spectrum), you need an absorption spectrum from space looking down. You can play with this using calculated spectra

    #3
    there's much more energy in the visible than IR coming from the sun. It is this that warms the earth, not the IR. As far as CO2 absorption is concerned, it's not true that the IR around the 700 cm-1 band is much more than what the earth emits at the same wavelength. Integrating it over all the IR is wrong.
    Then the earth emits in the IR and this emission happens to peak around a CO2 absorption band; part of it is trapped producing warming. This is the very basics of the greenhouse effect.

    #4
    The lifetime of the excited state is of the order of nano- to -micro- seconds depending on temperature and pressure. The extra energy can be released in two ways, by re-emission of a photon or by thermalization by collision (warming) of the surrounding air molecules; the re-emitted part will be absorbed again by other CO2 molecules and the process repeats itself until the pressure is so low that the photons have a high probability to escape to empty space. This is the way it works, again the basics of the greenhouse effect. If you increase CO2 concentration you slow down the process. You clearly still get cooling at night but not all of the energy absorbed during the day. It is not going to happen even if you do not increase CO2. A good example is the moon which, having no atmosphere, have enormous temperature gradients across the day/night line.

    I would more easily believe in your genuine interest had you not said "that logically there seem to be problems with the theory". Given that dozens of really smart people have worked on this for decades you'd better ask yourself "am i missing something?".
  10. @Ricardo.
    #2 OK, lets ignore thins since no one understands my quesiton.
    #3 According to this, http://en.wikipedia.org/wiki/File:Blackbody-lg.png, the sun emmits far more IR then the earth at the frequencies absorbbed by CO2. Surely all this IR saturates the CO2 in the atmopshere with energy, dwarfing the IR coning form the earth?

    #4 OK, so you say that the CO2 primarially rleases energy as a photon of light at the same frequency it absorbed it at, and that due toi the volume of CO2 it takes more than 12 hours for a photon on average to exit the atmosphere to space.

    Interesting you mention the moon, its daytime high is 105`c. Is it not the case that the reason the earth has a lower daytime high is because gasses in the atmosphere, including CO2, reduce the energy that strikes the surface?

    As for the throry of GH gas warming dont forget the poles are supposed to warm the most, but in fact only one of them is warming, so it seems the planet earth also has a problem with the theory. I am however prepared ot accept that I am in deed missing somehting, hence asking these quesitons.
  11. Sorry, final quesuiton, you say CO2 can release energy as a photon or by direct warming of other molecules. Do you have any figures for the rough percentage of energy released by each meathod?

    Given CO2s absorbiton band is only 8% of the total IR spectrum 92% of any release as a black body would not be re-absorbed by another CO2 molecule and thus exit the atmosphere directly.
  12. matt sykes,

    #3 the effect of CO2 is mainly on the band at about 700 cm^-1 or about 15 microns while the graph you show ends at 3 microns. Also, cosider that the absorption band is relatively narrow, it's really a tiny fraction. It'd be good if you calculate it yourself approximating the incoming radiation with the Plank formula, it will probably be more convincing than my words.

    #4 i did not say that "it takes more than 12 hours for a photon on average to exit the atmosphere to space.". Would it be so long you couldn't have significant day/night temperature variation; just the opposite is true.

    The warming of the poles depends on a lot of things, not least on atmospheric and oceanic circulation. Antarctica is "isolated" both by the Antarctic Circumpolar Current and by the strong westerly winds blowing in the Southern Oceans. No one expect the same warming as in the Arctic.

    I do not any rough figure on the ratio between collisional and radiative de-excitation. It quite complicated and it also depends on density and temperature. You can estimate the overall effect in the atmosphere from the ratio of the energy leaving the atmosphere over energy emitted by the surface.

    You last claim is definitely not true as can be easily seen in the absorption spectra of CO2 from space. Indeed, at the surface level the absorption lenth is pretty short, no way to escape directly to space.
  13. #3 2, 4.3 and 15 microns in fact. The first of which is well inside the graph I linked to. This one shows even more clearly how cooler bodies release less IR than hotter ones. http://quantumfreak.com/wp-content/uploads/2008/09/black-body-radiation-curves.png

    Mind you, this isnt surprising. A piece of metal at 30 `C will be warmer to the hand than one at 20`C becaue it is producing more IR.

    As for calculating the IR of a particular frequency emitted by an object at a particular frequency, I dont know how to, perhaps you could direct me to the relevant formula althogh I think we have establisged that hotter bodies do produce more IR than cooler ones.

    #4. You said "You clearly still get cooling at night but not all of the energy absorbed during the day" I understodd this to mean that the energy absorbed during the day cane be entirely lost at night. If you meant something else then I appologise for misunderstanding you although my understanding seems logical still.


    #5 And there is clearly something else at play in the Arctic too since it is only as warm as it was in the 1930's. But, thats the complexity of climate!

    #6 But you cant fell from space what is happening to the energy absorbed by CO2. It is either re-emmitted at the same frequency and thus bounces around the atmosphere fomr mollecule to mollecule or it is emmitted as broad band radiation in which case all of it except the 8% absorbed by CO2 will end up released to space.

    So from a sensor looking down from space you will never see the CO2 absorbed energy, ie those banmds will be missing from the spectrum, regardless of the re-emmission mathod.

    However, if the re-emission is of broad band, ie black body radiation, than CO2 effectively converts narrow band to broad band radiation. This will increase the levels of non absorbed energy transmitted to space, which is what one of the other respondants above stated had occured in the later sattelite measurement.
  14. @Ricardo. Just found an online Plank law calculator. For wavelenghts betweem 3.9 and 4.1 microns the sun produces 340,000 times as much energy as the earth.
    This is close to my orevious estimate of 400,000.

    So, what does all this SOlar IR energy do in the atmosphere in comparison to terrestrial IR? Is it blocked in the uper atmosphere, does it saturate the CO2? Is it absorbed and re-emmitted into space in tha same way as terrestrial IR is absorbed and re-emmitted to the surface?

    If so, and given that it is 400,000 times stronger the effect of CO2 is to actually reduce the IR at the surface, not increase it.
  15. matt sykes,
    from about 2 to 8 microns IR from the sun is absorbed by water vapour, CO2 has no influence. Also, you still integrate over the whole IR, not just the CO2 bands. At 15 microns, instead, the atmosphere is transparent and the CO2 band is centered at the peak of the thermal emission, the overall effect is then larger.

    Go back to your Plank calculator but this time plugin in the right numbers for a meaningfull comparison. The visible is 0.4-0.8 microns, the CO2 absorbtion band is centered at 15 micron and with a width of 2 microns (14-16 microns, in reality it is much narrower than this). You end up with a ratio of the energy coming from the sun in the CO2 related absorption band and the visible of the order of 10^-4. Negligible.

    #6
    In this very same post you can see absorption from CO2, you must be wrong. Indeed, the radiation abosrbed is re-emitted isotropically and part of it will be converted directly into heat in the atmosphere. At the top of the atmosphere you will see less radiation.

    I'd suggest to read how an idealized model works.
  16. matt sykes,
    i forgot to add that in any case absorption of sunlight is included in the radiative transfer codes.
  17. @Ricardo At 15 microns the sun produces 180 times as much energy as the earth.

    It is irrelevant how much energy the sun produces in the visible, it is the energy emmitted by the earth and absorbed by CO2 which is key.

    Your Siki link states: "Thus heat is easily let in, but is partially trapped by these gasses as it tries to leave. "

    This isnt true. Visible energy is let in, not heat. The heat of the sun is bloocked by the same GH gasses as block the heat going out.

    The difference is that thr sun produces far more heat than the earth. The net effect of GH ghasses is therefore to reduce the maximum temperaturs that would be otherwise acchieved,

    Consider the moon,. Its daytime temperature is 105`C. It gets this hot because it hasnt got an atmosphere.
  18. The net effect of GH ghasses is therefore to reduce the maximum temperaturs that would be otherwise acchieved,

    Consider the moon,. Its daytime temperature is 105`C. It gets this hot because it hasnt got an atmosphere.


    Er, sorry, but no. The moon's daytime temperature may average +105C, but its nighttime temperature is around -150C. Thus, the mean temperature of the moon is around -20C.

    Now, the earth's albedo is higher than that of the moon, so if the atmosphere had no effect (or a cooling effect, as you claim) then the earth should be cooler than the moon. Fortunately for us, water vapor and CO2 in the atmosphere raise the earth's mean temperature via a phenomenon known as the greenhouse effect.
  19. Matt, I think I see where your mistaken understanding is coming from.

    I think you're calculating the spectral radiance from Planck's law (if not, please explain where you get your figure of 180). I get about 150 by my calculations, but that's close enough to demonstrate the problem.

    The thing is that spectral radiance has a rather complicated definition:

    energy per unit time per unit surface area per unit solid angle per unit frequency

    The surface area of the sun is different to the earth - it's about 12000 times greater. 150 times 12000 is actually about 1.8 million. Hence the sun produces about 1.8 million times as much energy at 15 microns. It's hotter and larger, so this shouldn't come as a great surprise.

    But consider the fact that the sun is radiating this energy out in all directions, and that the earth captures only a tiny proportion of that energy because of our small size. Even Jupiter only appears as a tiny dot in the sky without a telescope.

    We can work out exactly how much we capture by dividing the area of a circle the size of the earth by the surface area of a sphere at the radius of the earth's orbit:

    pi*(6400^2)/(4*pi*(150 million)^2) = 0.00000000046

    Taking this, and the 1.8 million value found before into account, the earth would (in the absence of atmospheric absorption) radiate out about 1200 times as much energy at 15 microns as we receive from the sun.

    This isnt true. Visible energy is let in, not heat. The heat of the sun is bloocked by the same GH gasses as block the heat going out.

    Visible energy IS heat. The heat we get from the sun is mostly within the visible and the near infrared, because the sun is hot and has a blackbody curve centred in the visible. When we absorb it, we radiate it back into space according to a much cooler blackbody spectrum, deeper into the infrared.

    This is all fairly basic greenhouse theory, and I don't think you've quite grasped the science behind it.
  20. Matt, the implication of what Stuart just explained in his next-to-last paragraph is that the energy coming from the Sun to the Earth is in the wrong wavelengths to be much absorbed by CO2, but the energy being emitted by the Earth is very much in the right wavelengths to be absorbed by CO2.
  21. Sorry, I meant Chen (2007)

    Here is the working link

    [Source]
    Response: Thanks for pointing that out - I've updated the link.
    [RH] Hotlinked paper in order to fix broken page formatting.
  22. A related question about the basics of the greenhouse effect: I have been sent a "study" by a guy who claims that CO2 cannot be a greenhouse gas because any warming would cause an instant increase in outward radiation due to the increased temperature difference between the atmosphere and open space, which would immediately diminish the warming.

    I tried to discuss some sense into him, but in classic denier fashion, he remained stubborn that nobody so far could give him a "convincing" reason why his objection cannot be true.

    I tried the earth system's heat capacity, speed of propagation of temperature changes in a kilometer-thick atmosphere, dynamic equilibrium, I even quoted the paragraphs from "A History of Global Warming" at aip.org that say that the actual greenhouse effect is caused by a greenhouse gas concentration change at the tropopause, effectively shifting it into higher, cooler layers of the atmosphere, which radiate heat less effectively than warmer layers, which forces the whole of the temperature gradient in the atmosphere to do something like a parallel shift in order to achieve a high enough temperature at the tropopause to force enough radiation out into space to re-establish the equilibrium - all to no avail.

    What would you experts here tell him (assuming he might still be convinced)?

    Any reply very much appreciated!

    Cheers,

    babelsguy
  23. Babelsguy, your friend apparently has not so much a quibble with C02 as a GHG so much as he does with the concept of an atmosphere being able to trap heat. After all, what he believes applies to any GHG.

    How does he explain why Earth's climate is not the same as that of a planet with no atmosphere?
  24. doug_bostrom,

    well, that is the interesting thing: He claims that his "study" would avoid all discussion of other issues but just show that CO2 (and the other GHGs) *cannot* be the culprit due to what he calls the energy balance question...
  25. Well I have now worked out on my own why the bigger temperature difference between atmosphere and space does not cause an energy loss that lets global warming collapse again:

    There is no bigger temperature difference.

    Due to the absorption length of GHGs being much shorter than the height of the atmosphere, the only place that matters for outward radiation is the upmost layer of the atmosphere that radiates at all - minus the absorption length to any reasonably small non-absorbed residue.

    Because the GHG concentration change effectively shifts this outward radiating layer upward into colder heights, and the atmosphere below has an increasing temperature gradient towards the ground, the ground has to heat up "a lot" to let the boundary layer also heat up sufficiently *until is as warm as before*, so it can radiate enough to re-establish equilibrium!
    Q.E.D.

    So the guy's conclusion is wrong because his whole presupposition is wrong. Garbage in - garbage out.
  26. babelsguy, you and I have just written more or less the same post in two different contexts (see here)

    I think that says a lot for the fundamental nature of scientific knowledge ;-)
  27. I'm confused; where is the backscatter represented in this NASA energy budget representation of Earth? http://eosweb.larc.nasa.gov/EDDOCS/images/Erb/components2.gif

    Further to that the whole greenhouse gas theory is based on erroneous assumptions of the Earth system acting as a black body. Incorrect application of the Stephan-Boltzman constant in this case means that the entire theory is based on a basic undergraduate error. NASA discovered this at the time of the Apollo moon landings, when they discovered their assumptions on Moon surface temperatures by day and night were out by up to 60K - because a black body is a THEORETICAL two dimensional surface, there is no such comparison either with the surface of a planet/moon or the 3 dimensional gaseous atmosphere that surrounds it..

    Therefore the entire reason we don't freeze is down to the dynamics of the entire atmosphere that surrounds us, not to trace gases. The charge that the second law of thermodynamics is broken by postulating that heat moves from colder atmosphere to warmer surface has never been satisfactorily answered. NASA know this and put in in plain sight in their energy budget diagram, nothing in that allows for any change in composition of minor components of the atmosphere causing large changes in the planetary heat budget - an increase in heat content just increases the convection and radiation components to maintain balance. The complexity of the climate system is caused by the latent heat content of the oceans, which transfer heat in timescales of hundreds and thousands of years due to the massive difference in their heat capacity.

    And finally the whole system is dependent on the only external energy source, the Sun. It is that, plus the interaction with the oceans on a millennial scale, that controls our planet's climate. For us to pretend we can contol this system by tweaking one trace gas like a thermostat is arrogance of the highest magnitude
  28. The temperature gradient in the atmosphere of 6K/km is mainly due to the adiabatic compression of the air (gravitation) rather than to the mysterious greenhouse effect.
    http://freenet-homepage.de/klima/atmoseff-en.htm
  29. Curious but when showing changes in absorbtion of infrared why is the contribution from H2O always filtered out? It would seem to me that this would become more positive as the atmosphere warms.
  30. TOP - because H2O is FEEDBACK. Any forcing,= that raises the temperature of atmosphere will increase the water content and thus the GHG effect from it. If you want to know about the FORCING due to increase in anthropogenic gases then you need to filter out the feedback.
  31. Stuart, no, you cannot use area. The energy is not absorbed in the intervening area between the sun and the earth, it is absorbed by the first surface it strikes. Using this raito you get an almost 1 to 1 equivalency of IR.
  32. Tom Dayton. Yes, the sun might produce the greater proportion of its energy in the visible, but it produces more IR then the earth. Go back and take a look at black body radiation, youo will see what I mean.
  33. That good friend of AGW, Dr. Roy Spencer, has empirical evidence for the greenhouse effect.

    "Particularly difficult to grasp is the concept of adding a greenhouse gas to a COLD atmosphere, and that causing a temperature increase at the surface of the Earth, which is already WARM. This, of course, is what is expected to happen from adding more carbon dioixde to the atmosphere: “global warming”. ...
    This [Spencer's experiment] shows that the addition of an IR absorber/emitter, even at a cold temperature (the middle level clouds were probably somewhere around 30 deg. F), causes a warm object (the thermopile [in his IR thermometer]) to warm even more! This is the effect that some people claim is impossible.
  34. Poor Roy Spencer doesn't understand that what he is measuring is the temperature of the night air not the absorption of IR from the ground. He (perhaps subconsciously) confuses the phenomenon of emission with reflection. The IR emission of CO2 downwards depends only on the temperature of the air not on how that temperature was achieved; whether though IR absorption, convection, water condensation or sunlight.

    In this above article it says:
    "... Satellite measurements confirm less longwave radiation is escaping to space...Surface measurements find more longwave radiation returning back to Earth at these same wavelengths."
    If the atmosphere is warmer, then it should emit more IR in both the down and up directions. If it's in fact radiating more IR downward but less upward, then this phenomena is reflection not absorption and emission.
  35. Stylo, "more longwave radiation returning back to Earth" means "more than previously." The atmosphere still radiates in all directions, and John actually is referring to absorption/emission rather than reflection.

    Imagine an Earth with no greenhouse gases in its atmosphere. 100% of the thermal radiation leaving the surface and passing through the atmosphere goes upward and out into space.

    Now add a small concentration of greenhouse gases. A small part of that longwave radiation will be absorbed by GHG molecules, which will then emit energy in all directions, including downward. So now somewhat less than 100% of the thermal radiation within the atmosphere is moving in an upwards direction. And as you increase the concentration of GHGs, the downward component becomes correspondingly larger.

    The downward thermal IR flux is indeed caused by emittance, not reflectance.
  36. New summary paper Infrared radiation and planetary temperature in Physics Today. Covers the basic physics, addresses the 'saturation fallacy', compares absorption of CO2 and H2O and ties in AIRS data.

    Adding more greenhouse gas to the atmosphere makes higher, more tenuous, formerly transparent portions of the atmosphere opaque to IR and thus increases the difference between ground temperature and the radiating temperature. The result, once the system has come to equilibrium, is surface warming.
  37. muoncounter - I was just reading that article this afternoon, wanted to put in a link, but couldn't find a publicly accessible version! Thanks for putting it in.

    Excellent paper. I especially appreciated Figure 2d, showing the interleaving of CO2 and H2O spectral lines.
  38. #37: I was surprised to find a way around the paywall. Nice that he also mentions that molecules like N2 are transparent to IR 'in earthlike conditions'. Perhaps some of our deniers are actually speaking about conditions on Titan?
  39. The link to Ramanathan's paper provided under the Further Reading's tab does not work.
    Response: [Daniel Bailey] The free abstract can be found here.
  40. The absorbtion bands of CO2 and water vapor overlap making it difficult to parse out the purely absorbtive greenhouse contribution of incremental CO2.

    How are we so sure water vapor would not absorb the photons if CO2 were removed, and for that matter, sure water vapor is not absorbing the photons INSTEAD of CO2. I realize both signatures are visible from space, but is seems the parsing problem should apply here as well.
  41. Numerous articles about this, but try Schmidt et al, 2010 for serious crack at it.
  42. trunkmonkey@40 The upper atmosphere, where the Earth's energy budget is decided is very cold and hence very dry. There isn't a great deal of water vapour there to absorb the photons.

    There is a good overview of most of this from Spencer Weart and Ray Pierrehumbert at Realclimate here. The summary is here, emphasis mine:

    So, if a skeptical friend hits you with the "saturation argument" against global warming, here’s all you need to say: (a) You’d still get an increase in greenhouse warming even if the atmosphere were saturated, because it’s the absorption in the thin upper atmosphere (which is unsaturated) that counts (b) It’s not even true that the atmosphere is actually saturated with respect to absorption by CO2, (c) Water vapor doesn’t overwhelm the effects of CO2 because there’s little water vapor in the high, cold regions from which infrared escapes, and at the low pressures there water vapor absorption is like a leaky sieve, which would let a lot more radiation through were it not for CO2, and (d) These issues were satisfactorily addressed by physicists 50 years ago, and the necessary physics is included in all climate models.

    There is also a follow up post here where Ray Pierrehumbert goes into the physics in more detail.

    Hopefully those two posts answer your questions, in short, physics has been able to solve this problem for about sixty years.
  43. This is one of the most bizarre things about the AGW 'skeptic' movement. After Arrhenius first suggested the possibility of AGW in 1896 real skeptics countered with arguments like 'the CO2 effect is saturated' and 'oceans would absorb all the extra CO2'... which based on the limited knowledge of the time were compelling enough that the vast majority of scientists rejected AGW. It was only after these objections were disproved by other advancements, around the 1960s, that science started looking at the possibility of AGW again... and found that it was already underway.

    Yet here we are half a century later and the modern 'skeptics' are recycling these ancient arguments as if they were new and valid... rather than long since proven false.
  44. I'm a math major but I haven't even finished my undergraduate degree yet. So, while this stuff isn't a completely foreign language, it's close. I have a couple questions that I haven't seen a straight answer for yet.

    1: I know that the CO2 band is absorbing energy attempting to exit our atmosphere and that the satellite readings prove this. What I've yet to find a straight answer on is this: I read somewhere(I think here?), that despite this absorption from the GHG bands, the actual radiant energy escaping is still greater than going in from the sun. Is this true? Is so, how is this explained? I've tried finding the answer to no avail.

    2. I've seen the graphs showing that since the start of the industrial era temperatures have clearly been on a steady climb and that there's no chance it's solely due to the sun's activity. I've read that temperatures pre-industrial age were also steadily rising. Is this true? If so, is it the rate of increase that is of concern?

    3. Last one. I've read and seen in the graphs that methane can play a much bigger role in the GHG effect than can CO2. What are the real world applications to this? How do they compare to our CO2 production?

    Thanks
  45. learner - as a maths major, you would probably enjoy the treatment of these basic topics over at science of Doom. The questions are really require more treatment than a blog answer can give you. Methane is I think 12 times more powerful than CO2 as a GHG but its concentration is a/ measured in ppb instead of ppm so its effect is less than CO2 and
    b/ eventually oxidized to CO2
  46. Actually Scaddenp, CH4 is 24 times more powerful than a CO2 as a GHG, on a per molecule basis. However, the total amount of CH4 in the atmosphere is nowhere near that of CO2. This has been discussed already and is also treated on RC. Truly interested readers should become proficient at using search engines on both sites.
  47. thepoodlebites - "How do you separate warming from natural climate variability and CO2 rise?"

    Look at the levels of forcings that are currently causing climate change, up in Figure 4 of the 'Advanced' tab of this page. It's really a simple case of attributing cause and effect.
  48. #47 Have there been any updated model runs since Meehl et al. (2004)? It would be interesting to see how the temperature plots (1890-2000) would look including data from the last decade, using the same model assumptions. I'm wondering if all of the natural components have been properly accounted for. Model parameters can be adjusted to match any set of temperature observations. The statement that "late-twentieth-century warming can only be reproduced in the model with anthropogenic forcing" is a bit too strong as a conclusion without including the need for follow-up studies.
  49. "Model parameters can be adjusted to match any set of temperature observations"

    I am tired of this bit of nonsense being repeated. Show me an example of any model parameter in a GCM that is tuned to match a global temperature record. Put up or shut up. Parameterization is done inside the narrow focus of the particular response function. (eg wind speed to temperature gradient).

    If it were possible to "tune" parameters arbitrarily in a physics model then tell me why some skeptic hasn't produced a GCM that make CO2 irrelevant?

    RC regularly updated model-data comparison. Furthermore, the model archives would allow you compare prediction to actual yourself. As for updates - Hansen et al 2011 discussed in "Oceans are cooling" is certainly doing that.
  50. #49 scaddenp So the statement “model parameters can be adjusted to match any set of temperature observations" is not true? I don’t consider “put up or shut up” or “do it yourself” as compelling arguments in a debate on the scientific issues. And for the record, I have never said that CO2 is irrelevant. The question remains climate sensitivity to CO2 rise.

    I was specifically addressing the radiative forcing components that Meehl used. The conclusion that “the negative and positive forcings are roughly equal and cancel each other out, and the natural forcings over the past half century have also been approximately zero” to be extremely convenient. So if “radiative forcing from CO2 alone gives us a good estimate as to how much we expect to see the Earth's surface temperature change,” why not run Meehl’s 2004 model again with the same forcings and include observations from the last decade.

    My guess is that the model temperature predictions based on CO2 rise alone will not match the observed temperature record since 2000. If that is the case then the radiative forcing components that Meehl used in 2004 were not accurate and that forcing from natural climate variability has not been not properly accounted for. Or to put it another way, the temperature record for the last decade can not be reproduced in the model with anthropogenic forcing alone. When you get a chance, can you please point me to RC’s regularly updated model-data comparisons?

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