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How we know we're causing global warming in a single graphic

Posted on 27 July 2011 by John Cook

In 1859, physicist John Tyndall ran an experiment demonstrating the greenhouse effect. Visible sunlight easily passes through our atmosphere to warm the Earth. However, invisible heat rays rising from the Earth’s surface, otherwise known as infrared radiation, don’t easily escape back to space. What Tyndall showed by shining heat rays through tubes filled with different gases is that certain gases like water vapour and carbon dioxide block the heat rays. These became known as greenhouse gases.

Tyndall also made several predictions of what we should expect to see if greenhouse gases were causing warming (Tyndall 1861). In fact, we expect to see a number of distinctive greenhouse patterns in global warming. Observing these patterns strengthens the evidence that humans are causing global warming, as well as eliminates other possible natural causes. Let's have a look at the many human fingerprints on climate change:

How we know we're causing global warming

Humans are raising CO2 levels

The first point to establish is that humans are the cause of the rise in atmospheric CO2 levels. This fact is common sense. The amount of CO2 in the atmosphere is going up by around 15 billion tonnes per year. Humans are emitting around twice that much! On top of this, there are a number of lines of evidence to confirm that we're the cause of rising CO2 levels.

When we measure the type of carbon accumulating in the atmosphere, we observe more of the type of carbon that comes from fossil fuels (Manning 2006). As you burn fossil fuels, you take oxygen out of the atmosphere. Measured oxygen levels are falling in line with the amount of carbon dioxide rising (Manning 2006). There's been a sharp rise in "fossil fuel carbon" in corals (Pelejero 2005) and sea sponges (Swart 2010). Anthropogenic CO2 is penetrating even to the ocean depths (Murata 2010). Measurements of radiocarbon in tree-rings confirms human activity is the cause of rising CO2 (Levin 2000). Even the pages of ancient books trace the rising effects of fossil fuel pollution going back to beginnings of the industrial revolution (Yakir 2011).

So many independent lines of evidence (and common sense) confirm that yes, we are responsible for the recent rise in atmospheric CO2.

The extra CO2 is trapping heat

Our understanding of the greenhouse effect provides a number of verifiable predictions. If carbon dioxide is trapping more heat, we should see less heat escaping to space. Satellites measuring infrared radiation coming from Earth find less heat escaping to space over the last few decades, at those exact wavelengths that carbon dioxide absorbs energy (Harries 2001, Griggs 2004, Chen 2007). The researchers who analysed this data described this as:

"...direct experimental evidence for a significant increase in the Earth's greenhouse effect".
Harries 2001

If less heat is escaping to space, there's only one place it can go - back to the Earth's surface. Scientists check this by measuring infrared heat coming down from the atmosphere. These measurements confirmed the satellite data - more heat is returning to the Earth's surface (Philipona 2004, Evans 2006, Wang 2009). This extra piece of evidence upon the existing body of evidence led scientists 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."
Evans 2006

Unfortunately the scientists underestimated the human capacity to ignore  evidence staring us in the face.

Global warming has a distinct greenhouse signature

As far back as the mid 1800s, Tyndall predicted that greenhouse warming should cause nights to warm faster than days. This is because at night, the Earth's surface cools by radiating heat out to space. Greenhouse gases trap some of this heat, slowing the night-time cooling. It took over 130 years before Tyndall's prediction was confirmed. Over the last few decades, surface measurements have observed nights warming faster than days (Braganza 2004, Alexander 2006, Zhou 2009).

Tyndall made another prediction of what greenhouse warming should look like. Just as greenhouse gases slow down nighttime cooling, they also slow down winter cooling. So Tyndall anticipated winters warming faster than summers. Again, recent analysis of temperature trends over the last few decades bear this out (Braganza et al 2003, Braganza et al 2004). Both thermometers and satellites find winters warming faster than summers.

And the evidence continues to build. Another distinctive greenhouse pattern can be found in the atmosphere. With heat being trapped, we expect to see the lower atmosphere to warm. But with less heat escaping to space and more carbon dioxide in the stratosphere, we also expect to see the upper atmosphere cool. Satellites and weather balloons both observe this curious contrast between upper cooling and lower warming (Jones 2003).

With the lower atmosphere (the troposphere) warming and the upper atmosphere (the stratosphere) cooling, the boundary between the troposphere and stratosphere, otherwise known as the tropopause, should rise as a consequence of greenhouse warming. This has been observed (Santer 2003). An even higher layer of the atmosphere, the ionosphere, is expected to cool and contract in response to greenhouse warming. Satellites measure this effect (Laštovika 2006). We are changing the very structure of our atmosphere.

What's fascinating about all these greenhouse signatures is they also rule out a number of other potential causes of global warming. If the sun was causing global warming, it would cause summers to warm faster than winter, days to warm faster than nights and the upper atmosphere to warm. Observations rule out the sun.

Similarly, the pattern of ocean warming rules out ocean cycles as the driver of global warming. The world’s oceans have been building up heat over the past half century. This isn't a case of heat shifting around due to ocean cycles but the entire global ocean system building up heat. The specific pattern of ocean warming, with heat penetrating from the surface, can only be explained by greenhouse warming (Barnett 2005).

If it walks like a duck and quacks like a duck...

Current global warming shows all the distinctive signatures of greenhouse warming. To be skeptical that humans are causing global warming, you must believe two things. Something unknown is causing warming that happens to mirror the greenhouse effect. And something unknown is somehow suppressing the well understood (and well observed) greenhouse effect. So we can accept what we know to be true (greenhouse warming) or we accept two unknowns.

The saying goes if it walks like a duck and quacks like a duck, then it must be a duck. But climate skeptics are trying to convince us it's some other, undefined animal impersonating a duck that's also mysteriously hiding the real duck.

H/T to James Powell, Scott Mandia and Lou Grinzo whose words inspired this post. The "How we know we're causing global warming" graphic has been added to the Climate Graphics resource and with a Creative Commons Licence, is free to be published elsewhere.

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Comments 1 to 50 out of 55:

  1. If the sun was causing global warming, it would cause summers to warm faster than winter,

    Is this true sufficiently far from the Arctic? My understanding has been that the polar warming will generally tend to be greater in winter than in summer for either CO2 or solar forced-global warming, at least around where sea ice loss is occuring.
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  2. Patrick 027:

    I expect solar-forced warming would not be as effective in the Arctic as a direct heating source in the winter since the Arctic receives so little sunlight.

    The key to the differing warming characteristics from a solar forcing is that the additional energy from a warming Sun arrives at the Earth strictly in those areas actively being basked in sunlight (i.e. during daytime hours); it follows that days would warm faster than nights, and that summers would warm faster than winters on account of the greater amount of sunlight received during the longer summer days.
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  3. Less heat escaping to space? Image

    Harries 2001, Griggs 2004, Chen 2007 are all looking at decreases in the longwave emission over the CO2 band - this does not mean that total loss to space is reduced. The total loss to space is what impacts the energy balance and thus global temperature - looking at the CO2 band in isolation does not tell the whole story. The figure from Harries 2001 gives the (false) impression that OLR has decreased over the period measured:

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    Moderator Response:

    It's true that total radiation should decrease due to GHG blocking, but only instantaneously. You're forgetting the time course of the events. Radiation is decreased by GHGs, which causes temperature increase, which causes increase in attempted radiation, offsetting at least some of the greenhouse gas blocking. It's tricky to estimate how much total successful radiating will be increased or decreased at each of the times in that time course of events. Also tricky to adequately measure total radiation.

  4. There is very little Arctic amplification in the summer because the temperature stays pegged to the freezing point (as the extra energy goes largely into melting and evaporation). Actually the temperature manifestation of the ice-albedo feedback does seem to show up more in the colder months. Mark Serreze has some work on this.

    I don't think the change in the annual cycle with different forcings is a resolved issue. A quick comparison between 2xCO2 and 1.02 solar in the GISS model output that you can run with a one button click from their website didn't show distinct changes between summer and winter. I can't think of a great reference on it actually, but the ones referenced by John Cook, and some follow ups (e.g. Mann and Park, 1996) all suggested it was more complicated, especially as you compared models with observations, take into account internal variability, clouds, other forcings, etc. Some of these issues also apply to the diurnal range as well, but that has decreased, and I'm quite sure that is expected from GHG's primarily (but aerosols modify the picture too).
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  5. Keeping my non-expertise on models in mind, I want to play with the communication aspect of what Chris said vs what composer99 said:

    In summer, the upper temperature limit is restricted by the ice-water phase change to zero centigrade in the presense of massive amounts of ice. The larger (always?) summer energy flux in the arctic shows up as melt water, not air temperature. In winter, the air temperatures are well below zero, and the heat trapped by GHG can raise the air temperature without melting ice. Since some of the winter ice comes out of winter warmer than it formerly did, the volume of summer meltwater will be somewhat higher than if the ice mass had stayed colder during the winter.

    I can't address the question of convective transfer of heat to the polar regions from the equator by either air or water circulation. It's possible to imagine circulation modes that effectively block convective heat transfer, or modes that accelerate it.
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  6. Moderator #1

    "Radiation is decreased by GHGs, which causes temperature increase, which causes increase in attempted radiation, offsetting at least some of the greenhouse gas blocking. It's tricky to estimate how much total successful radiating will be increased or decreased at each of the times in that time course of events. Also tricky to adequately measure total radiation"

    What is 'attempted' radiation. Are we now assuming that radiation makes attempts and succeeds or fails? Sounds more like Mallory 'attempting' Everest.

    A better description of the analagous process might be a heat transfer one. The 'R' or insulation value of the atmosphere is increased, the average temperature differential (T1 - T2) to drive the same heat flux upward to space is increased.

    Heat flux is not 'trapped' - it requires a greater temperature differential to drive it across a better insulator.
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  7. I’m not particularly enamored with the following two sentences of John’s essay:

    “If the sun was causing global warming, it would cause summers to warm faster than winter, days to warm faster than nights and the upper atmosphere to warm. Observations rule out the sun.”

    Although the statements are correct in science-speak shorthand, they may not make sense to the average person reading the article. After all, the ultimate source of the infra-red radiation that is reflected back to Earth via the greenhouse effect is the sun.

    Perhaps John should tweak these two sentences.
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  8. I have made the comment quite a few times at SKS that I do not think there is adequate evidence that DTR changes are due to an enhanced greenhouse effect. Here are my main points:

    1) I have looked quite a bit and I have never found a peer reviewed article that attributes the observed change in DTR to an enhanced greenhouse effect alone. All of the references above either do not attempt to attribute the change in DTR to anything or they attribute it to cloud/aerosol suppression of daytime warming (while both tmin and tmax are presumed to be warming do to an enhanced greenhouse effect). This makes a change in DTR an anthropogenic fingerprint but that is very different than an enhanced greenhouse effect fingerprint.

    2) the IPCC AR4 Summary for Policy Makers says the following:

    " A decrease in diurnal temperature range (DTR) was reported in the TAR, but the data available then extended only from 1950 to 1993. Updated observations reveal that DTR has not changed from 1979 to 2004 as both day- and night-time temperature have risen at about the same rate. The trends are highly variable from one region to another. {3.2}"

    They are saying that DTR has not changed during the time period when greenhouse warming is thought to have been largest. This certainly does not seem to support DTR changes as a signature of an enhanced greenhouse effect.

    Not to mention, if it really was a signature of an enhanced greenhouse effect wouldn't that have made it into the report?

    3) The following conceptual model seems over simplified:

    " This is because at night, the Earth's surface cools by radiating heat out to space. Greenhouse gases trap some of this heat, slowing the night-time cooling."

    For one thing the earth is always radiating heat to space and it actually should radiate more heat to space during the day when its warmer. Therefore it does not seem obvious why an enhanced greenhouse effect should cause more warming at night than during the day. I believe the argument goes something like this:

    "relatively speaking, nighttime temperatures are effected more by the greenhouse effect because daytime temperatures are a product of solar radiation + back radiation whereas nighttime temperatures are dominated by back radiation."

    My main issue with this is that it seems to imply that Tmin and Tmax are independent. They are not. The greenhouse effect suppresses cooling at night but then when the sun comes up it will cause the temperature to begin rising from wherever nighttime cooling left off. Therefore any suppressed cooling at night should feed back into daytime warming and thus effect the tmax as much as tmin.
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  9. 6, ptbrown31,
    ...I do not think...


    Your post is riddled with errors. You seem to have a very, very weak grasp of how the greenhouse effect works, and as a result you draw many invalid conclusions from invalid premises and illogical arguments.

    I started to detail them for you, but really, it just became too much. I would strongly suggest that you put your cynicism aside and spend a lot of time studying and learning what is involved, and then make a judgment on this issue when you are actually qualified to do so.
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  10. ptbrown31 - actually there's far better case to be made that human-caused aerosol pollution explains the DTR trend, moreso than the increased Greenhouse Effect. It would also explain the patchy distribution of DTR trends. As aerosol pollution is short-lived in the atmosphere, local effects depend upon where the pollution came from and the seasonal atmospheric circulations (weather patterns).

    The massive increase in industrialization and growth after the 2nd World War pumped huge amounts of aerosols into the air. This would have served to decrease solar radiation reaching the Earth's surface - ergo daytime cooling. At the same time, the increased Greenhouse Effect would have kept nights warmer. After the clean air acts of the 70's were introduced, and sulfates particularly were dramatically reduced, days would have warmed up because of the increase in solar radiation now heating the Earth's surface. What we'd expect to see is an increase in the DTR range, and indeed that is what happened. The timing matches well.

    I'm covering all this in some posts on aerosols and 'Global Dimming/Global Brightening'.

    The DTR range as a GHG 'fingerprint' is not compelling IMO.
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  11. 7, Sphaerica -

    Please enlighten me of the errors that I have made rather than simply being dismissive and condescending.
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  12. Sphaerica,

    Actually, there's nothing in ptbrowns post that indicates he doesn't know what he is talking about. Indeed, I believe he is mostly right in describing the complexity of the variations in the diurnal temperature range, and how it depends on cloud cover, vegetation, and in some studies, aerosol effects-- many of these forcings (or feedbacks) are of anthropogenic origin, but it's not obvious that increasing CO2 alone is a primary factor here, or that it is a significant attribution tool (see e.g.,Dai et al., 1999, J. Climate; Stone and Weaver, 2003, Climate Dynamics; Makowski et al., 2008, Atmos Chem Phys; Zhou et al., 2009, Climate Dynamics).
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  13. Rob -

    Yes, that is exactly how I understand it as well. It is also what the literature indicates. This is why it bothers me that SKS continues to imply that the DTR changes are a fingerprint of an enhanced greenhouse effect. It undermines the credibility of the site and I am a big fan of the site in general.
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  14. ptbrown31 - let me refine my comment. The DTR is weak evidence for the increased Greenhouse Effect, but insofar as a human 'fingerprint' on climate, that seems to be on far safer ground. This is evident in the 'weekend effect' in heavily industrialized areas for instance. The DTR undergoes change over the weekend when industrial sources of pollution abruptly drop.
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  15. Would not another signature of anthropogenic warming by greenhouse gases be that high latitudes are warming faster than areas near the equator ?

    This is the opposite of what one would expect from increased insolation, and is only explicable by a mechanism that increases the insulating properties of the atmosphere.
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  16. ptbrown-- There's a lot of examples of erroneous but popularly held notions that many people have :-) Don't blame the site too much, hopefully that will correct itself.

    Keep in mind that something not being a 'fingerprint' doesn't necessarily imply it is consistent with everything either. There's no indication that the trends in DTR are consistent with a pure natural forcing.

    Chemware-- No, a reduction in the pole-to-equator temperature gradient is one of the most robust responses to any global warming situation.
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  17. DTR looked better than a weak fingerprint on this prior thread.

    We are currently experiencing global warming. If an increased greenhouse effect is a significant part of this warming, we would expect to see nights warming faster than days. There have been a number of studies into this effect, which confirm that this is indeed the case. One study looked at extreme temperatures in night and day. They observed the number of cold nights was decreasing faster than the number of cold days. Similarly, the number of warm nights was increasing faster than the increase in warm days (Alexander 2006).
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  18. 10, Chris Colose

    Thanks for the support. By the way, I have seen your posts around the climate blogosphere before and I noticed you are at Wisconsin. I got my undergrad from the AOS dept in '08. Go Badgers!
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  19. muon-- Hopefully we'll gear future posts away from that meme, or at least clarify the many nuances.

    ptbrown-- Thanks! I got my B.S. there too, but am doing graduate work elsewhere. Room 1411 will be missed.
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  20. I'm not that fond of any single phenomena being called a "fingerprint of anthropogenic warming". I think it would be better to think of the fingerprint as being a set of phenomena observed together. This is much more discriminatory power for comparing anthropogenic warming to natural forcing. Ideally you run the model with anthropogenic forcings, and then same model but with different natural forcings, all of the same strength and compare outputs. There is some data for that here and perhaps in the Hansen et al 2005 paper
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  21. Nothing about retreating glaciers?
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  22. My two cents worth on the DTR. In my humble opinion (and understanding) the reduction of DTR is theoretically a (subtle) symptom of an enhanced "greenhouse" effect. The problem with the DTR is finding a suitable dataset (and long enough) that has not been affected by other factors that might influence trends in DTR. I think that Braganza is one record saying that because of the myriad of factors that can affect DTR in the real world, it is not the most robust fingerprint of AGW-- note that does not equate to saying that it is not a fingerprint, or that the enhanced "greenhouse" effect has been overturned. If I recall correctly, Braganza considers the seasonal change in warming to be a far better and unambiguous fingerprint of an enhanced "greenhouse" effect.

    I suggest reading this informative post by Dr. John Nielsen-Gammon, as well as the recent paper by Zhou et al. (2010).

    At the end of the day, when we consider the body of evidence, a coherent and robust picture of an enhanced "greenhouse" effect on account of an increase of CO2 levels from humans burning fossil fuels emerges. As to which of the many indicators is the best or more robust, well that is open for discussion.
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  23. ptbrown @31, I think there is a crucial logical point that is often missed in popular discussion of the fingreprint of the greenhouse effect. That point is that no individual "fingerprint" is unique to the enhanced greenhouse effect. What is unique is the combined pattern of effects. It is also important to realise that the water vapour feedback, being itself an enhanced greenhouse effect will contribute some of the finger print of an enhanced greenhouse effect to any warming. It will, however, do so by counteracting the primary effect of many warmings, so while it means we cannot be simplistic in our analysis, it does not prevent fingerprinting analysis.

    With that in mind:

    1) An increase in albedo due to increased aerosol optical depth will decrease the Diurnal Temperature Range, as will a decrease in Total Solar Irradiance. However, both of these phenomena will also cool the lower troposphere overall. Cooling the lower troposphere will also reduce average specific humidity, which will tend to increase the diurnal temperature range. So, to the extent that increased aerosol optical depth (or reduction in TSI) is responsible for the reduction in DTR, it is only because some other factor is increasing tropospheric temperatures more than they are tending to decrease them.

    2) As it happens, global sulfur dioxide emissions have reduced since 1975. Therefore, to the extent that they do influence the Diurnal Temperature range (which is substantial) we would expect the DTR to have increased over that period. The fact that it has not suggests some other factor is causing the DTR to decrease. Thus we could divide the later part of the 20th century into to intervals. During the period 1950 and 1975 the two factors influencing DTR acted in concert to decrease it, while after 1975 one factor continued to decrease it, while the influence of aerosols was to increase it, with no net effect. This is in effect the reverse of the temperature trend. Between 1950 and 1975, increasing aerosols tended to cool the planet, counter acting some other factor that was warming it with a resulting very small change in global temperatures; while after 1975 both the reduction in aerosols and the other factor have tended to warm the troposphere, resulting in a sharp rise in temperatures. This other factor, which both reduces the DTR and warms is therefore an enhanced greenhouse effect.

    3) If your argument at this point were valid, then it would also apply to the effect of aerosols. Hence in that you apparently accept the influence of aerosols on DTR, you are contradicting yourself.
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  24. Tenney-- The sun can melt ice too :-)
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  25. Chris Colose @17, are you saying that increased CO2 will not decrease the DTR, or that the decrease is small compared to other factors, ie, changes in aerosol load, and or changes in specific humidity?

    Further, do you have any comment on my points 1) and 2) @21. Am I getting the nuances right?
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  26. Anybody got some thoughts on Chemware's question posed @13?

    "Would not another signature of anthropogenic warming by greenhouse gases be that high latitudes are warming faster than areas near the equator?"

    Chris?
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  27. I would have thought not quite so simple. Any warming invokes feedbacks, including water vapour though CO2 effect should be more prominent in cold, dry air, but at the poles there are other factors (especially Antarctica) to muddy the waters (eg ozone).
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  28. Tom,

    If you take a look at the studies I cited, CO2 itself (directly) plays a very minor role relative to cloud cover development, precipitation, water vapor feedback, aerosol effects, albedo, and other factors like irrigation and soil moisture as you approach the regional level (see Engelhart and Douglas, 2005 for a discussion on Mexico's DTR for example). The shortwave component of the energy flux seems to exert a much stronger control through changes in aerosol burden (Wild et al., 2007; Makowski et al 2008). I think I can buy that large increases in sunlight would tend to enhance the DTR itself, in the opposite direction of the long term trend.

    One thing I haven't seen in many of these DTR studies is a consideration of the top of the atmospheric energy balance, as opposed to just the surface forcing. This seems, to me, to be a very incomplete view and I'm not convinced the authors in some of these works appreciate this. When the atmosphere is well-mixed vertically, radiative forcing at TOA has a greater influence upon the surface air temperature than forcing at the surface, since the atmosphere itself adjusting its OLR is a primary way that equilibrium is re-established in response to forcing. Much of this re-adjustment occurs in the high atmosphere, since the bulk of the boundary layer is optically thick due to CO2 and water vapor.
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  29. Chris Colose @26, thank you for your response.

    I notice that @14 you say:

    " No, a reduction in the pole-to-equator temperature gradient is one of the most robust responses to any global warming situation."


    I have a problem with this in that, if you ignore feed backs than in increase in CO2 will decrease the temperature gradient, while an increase in insolation will increase it. That being the case, even if feedbacks result in a reduction of the gradient regardless, there should still be a difference in the change of the gradient in the two cases. Is that difference really not detectable in the models?
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  30. Actually the TOA radiative forcing for both the sun and CO2 (as well as the potency of the water vapor feedback) are maximized in the tropics...note the spatial structure of the radiative forcing has a complex pattern even for a well-mixed gas. So it's not immediately obvious from simple conceptual thoughts of the forcing that any polar amplification should occur at all

    The climate sensitivity (for the same forcing) would be higher in the poles for a bare blackbody rock, based on the Stefan-Boltzmann law, where the hotter body must respond more sluggishly to the same input of energy.

    In reality, the polar amplification (for any forcing) depends on complex dynamical transport processes which "export" sensitivity from low to high latitudes, as well as thermodynamics (radiation, ice-albedo feedback, etc) which tends to produce polar amplification for a wide range of idealized cases and forcing. In the upper atmosphere, the tropics are amplified relative to the poles. See for example,

    Pierrehumbert RT (2007), The Hydrologic Cycle in Deep Time Climate Problems. Nature 419,191-198

    Peter L. Langen & Vladimir A. Alexeev (2007), Polar amplification as a preferred response in an idealized aquaplanet GCM, Climate Dynamics DOI 10.1007/s00382-006-0221-x

    Ming Cai & Jianhua Lu (2007), Dynamical greenhouse-plus feedback and polar warming amplification. Part II: meridional and vertical asymmetries of the global warming, Climate Dynamics DOI : 10.1007/s00382-007-0238-9

    Jianhua Lu & Ming Cai (2010), Quantifying contributions to polar warming amplification in an idealized coupled general circulation model, Climate Dynamics DOI 10.1007/s00382-009-0673-x

    Also the GISS output shows the patterns I described above as well for solar or CO2 forcing
    http://data.giss.nasa.gov/efficacy/
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  31. While the TOA radiative forcing is maximized in the tropics for both CO2 and solar forcings, the relative strength of polar forcings for CO2 is greater than that for solar forcings. From the gisstemp efficacy page we have the following plots:

    Adjusted forcing for doubled CO2:


    Adjusted forcing for 1.02 times solar radiation:


    The adjusted forcing is the initial forcing as adjusted after the stratosphere reaches equilibrium, ie, the radiative forcing as per the IPCC definition.

    As you can see, for solar forcing the forcing at the tropics is over four times greater than that at the North Pole, and over six times greater than that at the South Pole. In contrast for CO2 forcing of similar strength, the forcing in the tropics is only 1.6 times greater than that at the North Pole, and only 2.4 times greater than that at the South Pole. So even though the greatest forcing is in the tropics for both forcings, the polar forcing is relatively greater for CO2 than for solar forcings as I had deduced.

    It still seems likely to me that this difference will result in some temperature differential between poles and tropic after all feedbacks for two types of forcings. Of course, the feedbacks for both forcings are very similar, so the stronger the feedback (and hence the higher the climate sensitivity) the less difference in temperatures will survive. I see that the GISS model E predicts very similar final temperature outcomes:





    As you can see, the model shows the CO2 forcing results in the world being 0.4 degrees warmer at the poles than for solar forcing, but 0.2 degrees cooler in the tropics. That 0.6 degree differential in terms of polar amplification set against an increase in mean global temperature of around 2 degrees C, which would surely be detectable. Of course, with the actual forcing todate, the difference may be two small to use polar amplification for attribution purposes (which I guess is what you would argue). You might also argue that model results are, as yet, insufficiently accurate to place much weight on so small a difference for the two forcings.

    Never-the-less, it seems to me that:

    1) Polar amplification is currently much stronger than expected, which sits more comfortably with a green house rather than a solar primary forcing;

    2) Any "skeptic" who denies that there is a substantial difference to the response to CO2 and solar forcings at different latitudes is thereby committed to a large climate sensitivity, and given the known CO2 forcing from anthropogenic sources, is also logically committed to anthropogenic greenhouse gases being the primary agent in the current global warming.

    The second point means that the rapid warming in the Arctic is indeed not just evidence of warming, but evidence of anthropogenic warming.
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  32. 7, ptbrown31,

    Apologies if I came off harshly, but your post hit a few of my buttons. I also have not read the long trail of posts that have followed... I am in my second week of the sore-throat-cold-from-hell right now, and sort of faded for a day there.

    First, for the record, I myself have gotten flack here at SkS for disagreeing with both the DTR and Winter Temperature Range arguments. The bottom line is that any warming or cooling involves strong GHG feedbacks (including H2O and eventually CO2), while even CO2 warming involves strong albedo feedbacks and others.

    The net result is that any such distinction is going to be very small.

    The first item of your post that set me off was any requirement that DTR not merely be expected and detected, but that some level of attribution be made. To me, the latter is a ridiculous and unattainable requirement at this point in time, or in the near future. As it is, anthropogenic warming to date is only 0.6C at best (and there are arguments, good and bad, that at least some of that warming may be from other causes).

    0.6C of 288.6K is only 0.2%. I don't know what the change in DTR is as a result of that, but it's obviously going to be small. Given all of the spacial, temporal and observational complexities involved, I can't imagine how anyone could ever perform a proper attribution until warming has reached seriously dangerous proportions.

    As such... your contention, to me, is very similar to that of other posters, by setting an unnecessary requirement (not merely detection, but attribution of a secondary component of warming) that can never be fulfilled.

    In particular, I was bothered when you said "Not to mention, if it really was a signature of an enhanced greenhouse effect wouldn't that have made it into the report?" AR4 wasn't released until 2007. The three papers merely confirming some observation of the expected DTR change were Braganza 2004, Alexander 2006 (March), Zhou 2009. At best two, and possibly only one, of those three studies were available in time for inclusion in a 3,000 page report that was released in 2007. Any extensive statement in AR4 about DTR would probably have been ripped apart by skeptics.

    As far as your actual grasp of the factors involved, I was bothered by the "simple model" approach of your logic. This very, very often fails. It fails to include all of the variables, as well as to properly quantify those variables. The latter is very often a problem.

    Specifically:

    You state that the earth "should radiate more heat to space during the day," and then conclude that "it does not seem obvious why an enhanced greenhouse effect should cause more warming at night."

    I see absolutely no tie between these two points, or why they should appear together in an argument. They seem to completely miss the actual reason why a GHE would expand the DTR... the simple fact that during the day, solar radiation dominates temperatures, while at night, only the GHE has an influence.

    I'm also unsure myself if your initial premise is even correct. When the sun is up, certainly the earth receives more radiation. I've never seen a study that compares sun-side to night-side TOA radiation. But reflected radiation doesn't count, and since outbound radiation (due to GHGs) comes from the TOA, not from the surface or lower troposphere, then the fact that those areas have warmed substantially in the day time, due to solar radiation, does not translate directly to the idea that the earth as a closed system (or, more specifically, the sun-side) will radiate more during that time. One might actually guess (again, using what is probably too simplistic an approach) that the most radiation comes from the evening-night-side quarter of the earth, versus the sun-side half or morning-night-side quarter.

    Also, I am unsure that your simple tie between TMin and TMax applies. TMax may not depend so much on the starting point (TMin), as it will on the overall level of solar insolation (which in turn is affected by TSI, ground albedo, clouds, aerosols, etc.) and GHG effect. The end result is not that your premise is wrong, but rather that the system is clearly more complex than this, so it is a dangerous premise to make without considering far more factors, and actually running the numbers or doing observations.

    To me, making this your "main issue" put an exclamation point on the sort of incomplete and shoddy "thought modeling" that I think gets a lot of people into trouble (myself included).
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  33. Sphaerica, very sorry to hear about your ill health. I hope you are better soon.
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  34. 30, Sphaerica -

    The first item of your post that set me off was any requirement that DTR not merely be expected and detected, but that some level of attribution be made. To me, the latter is a ridiculous and unattainable requirement at this point in time, or in the near future.

    I don't really understand your complaint here. I am not the one who said that the DTR change needs to be attributed to anything. I am criticizing the fact that the OP attributed it to an enhanced greenhouse effect when the papers that are cited do not corroborate that conclusion.

    As far as your actual grasp of the factors involved, I was bothered by the "simple model" approach of your logic. This very, very often fails. It fails to include all of the variables, as well as to properly quantify those variables. The latter is very often a problem.

    I agree. All I was doing was giving a counter example to the conceptual model that SkS presents to argue for DTR changes being due to an enhanced greenhouse effect. Both of our models are ultimately wrong on some level.


    You state that the earth "should radiate more heat to space during the day," and then conclude that "it does not seem obvious why an enhanced greenhouse effect should cause more warming at night." I see absolutely no tie between these two points, or why they should appear together in an argument.

    My only point here is that the greenhouse effect suppresses radiative cooling at all times, not just at night. When the earth is warmer (during the day) it is radiating more (to the 4th power of temperature) and therefore there is more radiative cooling to suppress.

    They seem to completely miss the actual reason why a GHE would expand the DTR... the simple fact that during the day, solar radiation dominates temperatures, while at night, only the GHE has an influence.

    I understand SkS's conceptual model. That point is exactly what I articulated in the quotes.

    The end result is not that your premise is wrong, but rather that the system is clearly more complex than this, so it is a dangerous premise to make without considering far more factors, and actually running the numbers or doing observations.

    I totally agree. I was not trying to say "SkS's conceptual model is wrong and my conceptual model is right". Instead I was trying to say that it is not obvious that SkS's model is correct. Therefore, it is not obvious that an enhanced greenhouse effect should decrease DTR. This would be fine if SkS had cited articles that went into a rigorous treatment of whether or not we would "expect" to see a decrees in DTR with an enhanced greenhouse effect. But the articles cited to not do this. None of them say anything to the effect of "an enhanced greenhouse should decrease DTR because....".

    This bothers me slightly because I would prefer if SkS's missions was to DEFEND the established science against bad skeptical arguments not make up its own physical arguments and cite sources that don't back up those arguments.
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  35. 32, ptbrown,

    Why do you keep referring to SkS's model? What do you think here is conjured by the people behind SkS, instead of part of the actual science? The OP is riddled with references to papers, and it's long been part of both the literature and established theory. It's admittedly an area, compared to others, that's a little thin... but what makes you think that it was somehow invented by SkS?
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  36. I am referring to it as SkS's model because:

    1) They make the same claim in many posts

    2) It is NOT a part of both literature and established theory. That is the whole point. Again, the references do not support the claim being made (that we would expect DTR to decrease as the greenhouse effect is enhanced).
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  37. 34, ptbrown31,

    I have no idea why you'd think such a thing.

    From this post:

    As far back as the mid 1800s, Tyndall predicted that greenhouse warming should cause nights to warm faster than days.


    From google scholar, searching for papers that reference "Diurnal Temperature Range" and "Climate Change":

    Google Scholar... 74,200 papers

    Including:

    Diurnal temperature range as an index of global climate change during the twentieth century
    -- Braganza, DJ Karoly… - Geophys. Res. Lett, 2004


    Global warming: Evidence for asymmetric diurnal temperature change
    TR Karl, G Kukla, VN Razuvayev… - Geophysical Research …, 1991 - agu.org


    Biological response to climate change on a tropical mountain -- JA Pounds, MPL Fogden… - Nature, 1999 - cct.or.cr

    An improved method of constructing a database of monthly climate observations and associated high‐resolution grids -- TD Mitchell… - International journal of climatology, 2005 - Wiley Online Library

    Adapting stochastic weather generation algorithms for climate change studies -- DS Wilks - Climatic Change, 1992 - Springer

    And a bazillion others.
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  38. 35, Sphaerica

    You are only proving my point. All you have to do is read some of these papers to see that none of them back up the claim being made about DTR.

    Comparison of observed and anthropogenic-forced model changes in DTR over the last 50 years show much less reduction in DTR in the model simulations due to greater warming of maximum temperatures in the models than observed. This difference is likely attributed to increases in cloud cover that are observed over the same period and areabsent in model simulations.
    -- Braganza, DJ Karoly… - Geophys. Res. Lett, 2004

    The cause(s) of the asymmetric diurnal changes are uncertain, but there is some evidence to suggest that changes in cloud cover plays a direct role.
    -TR Karl, G Kukla, VN Razuvayev… - Geophysical Research …, 1991 - agu.org

    Because the daily minimum has increased relative to the daily maximum, the diurnal temperature range (DTR, the difference between the two) has declined (Fig. 2a). The negative trend implies increasing cloudiness, which lowers daytime temperatures by blocking solar radiation and raises night-time temperatures by reducing radiative heat losses
    -- JA Pounds, MPL Fogden… - Nature, 1999 - cct.or.cr

    The other references you cite don't really address the question. But as you can see, none of the above papers say that the change in DTR is an outcome of an anthropogenic enhancement of the greenhouse effect.
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  39. ptbrown31, it seems to be that you are engaged in cherry-picking the Braganza et al 2004 paper.

    First, you claim that "none of the above papers say that the change in DTR is an outcome of an anthropogenic enhancement of the greenhouse effect."

    Yet, the abstract of Braganza et al states:

    The usefulness of global-average diurnal temperature range (DTR) as an index of climate change and variability is evaluated using observations and climate model simulations representing unforced climate variability and anthropogenic climate change. On decadal timescales, modelled and observed intrinsic variability of DTR compare well and are independent of variations in global mean temperature. Observed reductions in DTR over the last century are large and unlikely to be due to natural variability alone. [Emphasis mine.]


    Assuming Braganza et al take climate change to mean anthropogenic climate change, I do not see how it can be seen as a useful index of change & variability unless the authors expect a reduction in DTR to be a result of anthropogenic greenhouse warming.

    From the conclusion, we find:


    Diurnal temperature range appears to be a suitable index of climate variability and change, in the context of similar simple global indices outlined by Braganza et al. [2003]. While changes in maximum and minimum temperature are strongly associated with changes in global mean temperature, DTR provides additional information for the attribution of recent observed climate change. [Emphasis mine.]


    Again, unless Braganza et al attribute global warming to non-anthropogenic sources, their suggestion that the change in DTR is a useful index of climate change certainly implies that it is indeed an outcome of the human acceleration of emissions.

    The statement you cite, from the abstract, suggests a qualification of the model runs, indicating that DTR diverged more in the models than in empirical observations because of a factor that was not included in the models used. They are pointing out a quibble with the climate models, which seems to me to be a rather different thing than what you claim they are saying.
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  40. I should clarify my final paragraph by noting that Braganza et al indicate that maximum temperatures increased faster in the model runs than in the empirical observations.
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  41. 36, ptbrown31,

    You are arguing two different things. I'm not arguing about whether or not the case is proven. I'm arguing that you said:

    1) They make the same claim in many posts

    2) It is NOT a part of both literature and established theory.


    I have categorically proven your second claim to be false, and so the first claim to be moot.

    Do you accept that the predicted reduction in DTR is, in fact, a long established and well recognized aspect of GHG theory?
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  42. 37, Composer99

    This is from my 1st post (#6):

    1) I have looked quite a bit and I have never found a peer reviewed article that attributes the observed change in DTR to an enhanced greenhouse effect alone. All of the references above either do not attempt to attribute the change in DTR to anything or they attribute it to cloud/aerosol suppression of daytime warming (while both tmin and tmax are presumed to be warming do to an enhanced greenhouse effect). This makes a change in DTR an anthropogenic fingerprint but that is very different than an enhanced greenhouse effect fingerprint.

    And that is why I was clear to say in #37:

    none of the above papers say that the change in DTR is an outcome of an anthropogenic enhancement of the greenhouse effect.

    An anthropogenic signature is not the same thing as an enhanced greenhouse effect signature (which is the claim that the OP makes)
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  43. 36, ptbrown31,
    But as you can see, none of the above papers say that the change in DTR is an outcome of an anthropogenic enhancement of the greenhouse effect.
    You're standing everything on it's head, because no one has written a paper titled "Hey, PTBROWN31, We Think Explicitly That Anthropogenic Climate Change From Greenhouse Gases Will Cause a Noted and Observable Drop in Diurnal Temperature Ranges", I. B. Buttscratch, 1983, Geophysical Research Letters.

    All I did was to point out the very, very frequent references to the concept in the literature. The logic behind it and the nature of it are not fabrications of SkS. That is clear.

    Whether it is proven or not is another issue, and one that I have already said I don't think is possible until temperatures increase considerably, and even then, possibly not.

    But your initial claim was not that it was unproven, but rather that what changes were observed could not be attributed to the GHE, and my answer to that is that you are demanding that someone show you the far side of the moon to prove to you that it exists, because you can't see it from here.

    It's a fools errand, and not one that anyone is going to bother to undertake.
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  44. 39, Sphaerica

    I was not saying that the concept of DTR change was not a part of both literature and established theory. I was saying that the claim that DTR should decrease with an enhanced greenhouse effect is not a part of literature and established theory.

    Do you accept that the predicted reduction in DTR is, in fact, a long established and well recognized aspect of GHG theory?

    Again, I have never seen, and you have not shown me any scientific article that says something to the effect of "an enhanced greenhouse effect should decrease DTR and in fact this is what has been observed"

    So I accept that a reduction in DTR has been observed over a certain time period but I do not accept that this is a fingerprint indicating that the warming that we have seen is due to an enhanced greenhouse effect. However, the rest of the original post makes many other valid arguments for recent warming being due to an enhanced greenhouse effect (i.e. I am not an AGW skeptic)
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  45. 41, Sphaerica

    All I did was to point out the very, very frequent references to the concept in the literature. The logic behind it and the nature of it are not fabrications of SkS. That is clear.

    You seem to think that I was arguing that the general concept of DTR change was invented by SkS. That is not what I have been saying. What I do think is that SkS has used DTR change as fingerprint of an enhanced greenhouse effect when the evidence seems to show that the DTR change is actually attributable to the growth of anthropogentric aerosols (and or clouds) suppressing the rate of warmth of Tmax relative to Tmin.

    But your initial claim was not that it was unproven, but rather that what changes were observed could not be attributed to the GHE, and my answer to that is that you are demanding that someone show you the far side of the moon to prove to you that it exists, because you can't see it from here.

    Is it so much to ask for a single sentence in a paper to actually corroborate the original claim* being made?

    *by claim I specifically mean "an enhanced greenhouse effect should warm Tmin more than Tmax and thus DTR should decrease"
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  46. Arrhenius 1896, pg. 265, states "An increase in carbonic acid will of course diminish the difference in temperature between day and night."

    Nothing like the classics...
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  47. 42, ptbrown31,

    I was saying that the claim that DTR should decrease with an enhanced greenhouse effect is not a part of literature and established theory.
    And this was proven to be false. It is in fact a part of the literature and theory.

    The way you keep changing and rewording your stance makes you ineligible for further discussion. It's not worth the effort.

    Suggestion... go read more of the literature before deciding what it does or does not say.

    Here's one that get's pretty close to what you're demanding, but I'm not wasting any more time correcting you after this:

    Daily maximum and minimum temperature trends in a climate model – Stone & Weaver (2003)
    “The recent observed global warming trend over land has been characterised by a faster warming at night, leading to a considerable decrease in the diurnal temperature range (DTR). Analysis of simulations of a climate model including observed increases in greenhouse gases and sulphate aerosols reveals a similar trend in the DTR of −0.2°C per century, albeit of smaller magnitude than the observed −0.8°C per century. This trend in the model simulations is related to changes in cloud cover and soil moisture. These results indicate that the observed decrease in the DTR could be a signal of anthropogenic forcing of recent climate change.


    Emphasis mine.

    FYI... an increase in night time cloudiness, that in turn raises night time warming, is probably also a signature of GHE (I haven't studied the modeled cloud changes in the simulations enough to say, and even if I had, clouds are an area of inaccuracy in past and current models). The fact that the mechanism is or isn't from cloudiness is a mere detail, and as I've already said, I think it is related to all warming. But the expectation that already exists in the literature is what it is, your personal interpretation not withstanding.
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  48. KR, thanks. I kept meaning to go back to that, and lost track.

    Sheesh. I can't wait to get rid of this illness. It's crippling my brain.
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  49. I will point out that this 1896 (!!!) paper by Arrhenius notes multiple fingerprints of AGH (polar amplification, winters warming faster than summers, greater for N. hemisphere than S. due to land mass, faster over land than water, albedo feedback from ice melt) based upon simple radiative physics. Stratospheric cooling is not mentioned; but since the stratosphere was discovered quite some time later, that's not surprising.

    Arrhenius was quite frankly concerned about an ice age, but discovered to his surprise that warming was more of an issue. His work on the subject was excellent, and has held up quite well over the last century.

    I suspect that it's been part of the known literature for so very long that current authors just don't bother to emphasize it.
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  50. Sphaerica - Hang in, this too shall pass.
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