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The human fingerprint in the seasons

Posted on 3 December 2010 by John Cook

In 1896, Svante Arrhenius mentioned that greenhouse warming should cause winters to warm faster than summers (Arrhenius 1896), citing an earlier prediction by John Tyndall (Tyndall 1865). During summer, a region receives more sunlight and warms. During winter, the region receives less sunlight and cools by radiating heat to space. Greenhouse gases stop some of this heat from escaping to space so an increased greenhouse effect slows down the winter cooling. Consequently, if greenhouse gases are causing global warming, we expect to see winters warming faster than summer.

A pair of studies (Braganza et al 2003, Braganza et al 2004) recognise that within the temperature record are a number of climate indices that can tell us more about what's causing temperature change than mere global temperature. The difference in trend between summer and winter, between land and ocean warming, between the equator and the poles - these all hold vital clues into what has caused climate change since the instrumental record began in the 1800s.

They found that winters have been warming faster than summers. What's interesting is how the seasons have changed over time. In the early 20th century, they find the warming is a combination of man-made and natural forcing (eg - from the sun) as well as some internal variability (eg - ocean cycles). In the latter 20th century, man-made forcing accounts for nearly all the observed temperature changes (Braganza et al 2004).

To check this out for myself, I tried plotting the winter vs summer trend using the CRUTemp Northern Hemisphere land temperature record. Robert Way kindly helped out by working out the winter and summer temperature anomalies and plotting them  (here's the Excel file for the curious climate tragic).

 

Figure 1: Yearly temperature anomaly for Northern Hemisphere winter (light blue) and summer (light red) plus five year moving average for winter (thick blue) and summer (thick red). Data comes from CRUTemp, base period is 1961 to 1990.

Not only does the faster warming winter provide evidence for greenhouse warming (on top of many other lines of evidence for man-made global warming), it also provides evidence that the sun isn't the cause of recent global warming. If global warming was driven by the sun, we should see summer warming faster than winter. This is just one of the "solar fingerprints" that we would expect to see from solar warming, that we don't see. Interestingly, many of the solar fingerprints are quite different to the patterns expected from greenhouse warming

For example, greenhouse warming predicts nights should warm faster than days while solar warming is the other way around. Observations are consistent with greenhouse warming. Similarly, if global warming was driven by the sun, we should see the stratosphere warming as well as the troposphere. Greenhouse warming, on the other hand, warms the troposphere but cools the stratosphere. Again, observations match greenhouse warming.

Solar warming should result in the tropics warming faster than the poles. What we observe instead is the poles warming around 3 times faster than the equator. All these pieces of evidence paint a consistent picture - greenhouse gases, not the sun, are driving global warming.

UPDATE 10 Dec 2010: In the original posting of this blog post, I mistakenly posted a graph of global temperature, not northern hemisphere temperature (which is a bit annoying - back when I was preparing this post, Robert and I looked at both NH and global trends then I mistakenly used the wrong Excel file when exporting the final graph). I've updated the post with the NH temps.

UPDATE 11 Dec 2010: Many thanks to muoncounter who went to the trouble to compare Northern Hemisphere winter vs summer temperatures in the satellite record - a handy way to independently confirm the surface measurements (here's the Excel file):

 

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Comments 1 to 50 out of 177:

  1. Whenever I see an article like this, where the signals you are comparing are visually very little different, I wonder about significant figures and signal to noise ratios.

    Chris Shaker
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  2. What illustrates the point even better, is to adjust the anomalies to a baseline of 1850-1890. That really highlights the difference between summer and winter warming over that period - by the 2000s, the difference is about a quarter of a degree C, if I've done the sums correctly.
    Thanks for including the link to the Excel file, too!
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  3. Again I've got the same problem as with the last "human fingerprint" article. Whatever is causing the the recent multi-century warming trend there is going to be a GHG component to that trend in the form of water vapour.

    So identifying a 'GHG fingerprint' in any metric does not mean you have specifically identified a 'human fingerprint'. GHG fingerprints can be natural because of the existence of water vapour.
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  4. Humanity Rules,
    Water Vapor is a feedback to CO2 warming. See Lacis et al. 2010
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  5. #3: "GHG fingerprints can be natural because of the existence of water vapour. "

    I'm fascinated by the play the water vapor argument gets in the skeptic world. If the climate is stable, water vapor should be in a long-term equilibrium. If the climate warms, increased evaporation obviously results in increased atmospheric water vapor content. That acts as a feedback, potentially increasing the warming (due to GHE) or not (due to increased cloud cover.

    Whatever the feedback, increased water vapor is necessarily a response to a changing climate. So how the skeptics claim that water vapor causes the warming in the first place? There must be an external factor to first drive (force) the climate change -- and atmospheric CO2 comes to my mind.
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  6. Thanks for this post. It's interesting to see the relationships from this perspective.

    Regarding water vapour and 'sceptics' - some people have a mental block and can't latch onto the fact that something has to drive the temperature up before more water will evaporate to the atmosphere. Over the past few decades, that's obviously been carbon emissions adding to the greenhouse effect, which is raising the global temperature, which means more water evaporates, which can make the temperature rise even more.

    Some so-called 'sceptics' keep bringing up water vapour to try to confuse the less well-informed.
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  7. Robert Way

    I agree. My point is water vapour is a feedback to whatever is causing warming. Earth is a wet place. The theory goes when it gets warm evapouration increases.

    5 muoncounter

    Again I'm not claiming water vapour is the driver here, I'm cliaming water vapour is a feedback in any scenario that warms the earth. Whether the driver is human or natural.

    The point I'm making is GHG fingerprint does not equal human fingerprint. That's the assumption in John's article again.
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  8. HR, greenhouse gases are increasing. Humans are adding greenhouse gases to the atmosphere which is causing them to increase. The greenhouse gases are causing global warming. Humans are causing global warming.

    What is so hard to follow about that? (Like I say, probably a mental block to the logic of it all. Some people don't want to accept the obvious so they don't :))
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  9. 9 sout

    What is hard to follow is John's suggestion that any GHG warming signal is a fingerprint of warming. I think it's summed up in the over-simplification of the solar-fingerprint. My suggestion is that solar warming will contain a sizable GHG component because of the role of water vapour. Any process that warms the world presumably increases water vapour. This is a GHG. It's not quite the simple all or nothing situation that John sets out.

    Let's use the example you vividly describe i.e. warming from CO2.

    A) The direct effect from the CO2 molecule of doubling CO2 is a temp increase of 1.2oC.

    B) The IPCC estimate for a doubling of CO2 in the earth’s complex system is 2-4.5oC. The increase from A is due to feedback’s (primarily albedo and water vapour)

    C) In simple climate models when clouds, aerosol and ice are held constant (albedo is fixed) then doubling CO2 gives 2-3oC warming. The difference between A and C is essentially the water vapour feedback.

    That means 40-60% of the warming trend from a doubling of CO2 comes from the water vapour feedback. 40-60% of the GHG fingerprint comes from water vapour. The water vapour effect is not specific to CO2 but is a consequence of a warming world. All things being equal, any process that warms the world is going to have this 40-60% water vapour feedback. As water vapour is a GHG then any process human or natural that warms the world will have a sizable GHG component to its fingerprint.

    I don’t think this is sceptical science, it seems mainstream. A study that reports a GHG fingerprint is not reporting a human fingerprint. I understand the significance of that, maybe you don't.

    Finally you really should stick to criticising the points I put forward rather than speculating on my mental processes, it does nothing for your argument.
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  10. Re: HumanityRules (10)
    "My suggestion is that solar warming will contain a sizable GHG component because of the role of water vapour."
    Per this link, there has been no increase in TSI in the past 32 years and thus, no solar-driven warming over that period.

    It's not the sun, yet temps still rising. Water vapor would be a feedback, so it can't drive itself up via driving up temps.

    Ran outta road, HR.

    The Yooper
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  11. There are a couple of problems with this article. There is no time when the Earth is in winter or in summer. By showing only part of the Earth it is certainly possible to miss what is happening as a whole.

    I do agree that the NH is the main driver of the Earth's climate. I have shown that here. The basis is a Jones paper in 1999 that shows that the NH winter and summer drives the global average as shown here.

    The global temperature shows that the anomaly in the Dec-Feb time frame has gone up more than in the June-August time frame. This isn't a surprise as Antarctica has not shown the winter warming that the Arctic region has in its winter.

    But that is a problem. Why isn't the Antarctic warming in its winter in the same manner as the Arctic if the cause is CO2. Both poles should be showing the same behavior, but they are not.

    So while I agree that Dec-Feb are showing more warming than Jun-Aug, the answer is not as simple as CO2. The current orbital obliquity trend should also be favoring warming Dec-Feb as the NH receives slightly more insolation during those months. That would also correlate to cooling Antarctic trend is also observed.

    John Kehr
    The Inconvenient Skeptic
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  12. Both poles should be showing the same behavior"
    Why? I don't believe that at all. The poles are very different, so are the oceanic circulations around them. I think that they actually should not be showing the same behavior.
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  13. 11 Daniel Bailey

    We could argue about solar variation but that would be going OT and is for another thread. The contention in John's argument is

    GHG fingerprint = human fingerprint

    That seems wrong. The suggestion that the solar fingerprint will not contain a GHG component is wrong. The question is whether identifying a GHG fingerprint means you've identified a human fingerprint.

    The problem I have is to call this a fingerprint of human warming it should stand alone, that would be the nature of a fingerprint in this context. As you demonstrate it's only through bring in other arguments that you can make this stand up. This isn't a pedantic arguement. Part of the strength of the AGW arguement is the heaps of independant arguements that point to CO2. I think this and the previous human fingerprint point to a warming trend not a human warming trend.

    Back on the road, DB.
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  14. Re: The Inconvenient Skeptic (12)
    "Why isn't the Antarctic warming in its winter in the same manner as the Arctic if the cause is CO2"
    You present a logical fallacy. Comparing the Arctic, an ice-covered ocean surrounded by landmasses, with the Antarctic, a glaciated continent over 2 miles high: apples and oranges.

    A question to you: Why, exactly, would anyone expect the Antarctic to warm similarly to the Arctic?

    The Yooper
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  15. Re: HumanityRules (14)

    We know CO2 is rising; all other forcings have been flat or declining (source here).

    We know the increasing CO2 comes from us.

    Because increasing levels of atmospheric CO2 drives temperatures up, and the increased CO2 is shown isotopically to be of human origin, ergo: human fingerprint.

    Off road, stuck-in-sand, spinning-wheels.

    The Yooper
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  16. TIS - if climate models work the way you seem to believe they do, then can you find me an example from ANY of the 15 or so models where the Antarctic is behaving like the Arctic? On the other hand, SH is most definitely warming. Why do think 0.25SW/m2/100year (max) has more effect than 3.7W/m2/100 year? Do you believe that energy received at the surface is an inconsequential factor in global climate?
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    Moderator Response: Use italic or bold instead of all-caps, please.
  17. Couple Questions for John and HR,

    Is the theory that a solar (For example) perturbation is what is causing the initial temperature increase which then results in a positive feedback from water vapor causing the GHG signature to be present in the post 1975 warming?

    I don't buy it. I think this graph is very good because it shows that initially summers were warming quicker than winters and that as the effect of CO2 increased so did the rate of winter warming when compared to summer. That makes sense based upon what we already know which is that the early warming of the 20th century was solar induced mostly (I say oceans too but I digress) but that the late century warmth has a large anthropogenic component.

    What we have is a theory, early warming natural, late warming anthropogenic which we can then compare to the data, summers warm more than winters early, winters warm more than summers late. We also know that with increasing CO2 it was predicted that winters warm more than summers. For me its a pretty strong case. There's not really a need to always choose the 2nd or 3rd best theory when the evidence suggests theory #1 is well substantiated.

    Secondly I ask, wouldn't the initial solar warming being most prominent in summer counteract much of the winter warming caused by your GHG feedback?
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  18. I'm Glad to hear that both John and HR acknowledge that water vapor is a positive feedback though. It is good to see when individuals who are skeptical join the scientific consensus.
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  19. Sout @9,

    You are quite right. Hope you don’t mind if I elaborate a bit on what you said.

    We are increasing CO2 (a GHG), as a result the planet is experiencing a net energy imbalance (e.g., Murphy et al.2009) which is causing warming of the oceans and atmosphere. Warming of oceans is increasing WV in the atmosphere (e.g., Dessler et al. 2010), and the air can hold more WV as it warms, one result is an acceleration of the hydrological cycle (e.g., Syed et al. 2010). Another result is that higher WV (a powerful GHG) is causing a positive feedback which is further enhancing the warming that we humans have kicked off.

    To summarize:

    From Braganza et al. (2004):
    “ Observed linear trends over 1950–1999 in all the indices except the hemispheric temperature contrast are significantly larger than simulated changes due to internal variability or natural (solar and volcanic aerosol) forcings and are consistent with simulated changes due to anthropogenic (greenhouse gas and sulfate aerosol) forcing.”

    And
    “It is found that anthropogenic forcing accounts for almost all of the observed changes in surface temperature during 1946–1995”.

    From Dessler et al. (2010):
    “All of the other reanalyses show that decadal warming is accompanied by increases in mid and upper tropospheric specific humidity.”

    Anyway, sadly the point of this post has clearly been lost on some readers. Did they even read the post and references therein? John Cook and Braganza et al. 2003, 2004 have laid out their reasoning very clearly and this final sentence of John’s post sums things up:

    “All these pieces of evidence paint a consistent picture - greenhouse gases, not the sun, are driving global warming”

    Now do any of the resident “skeptics” take issue with that statement? I think not, they would rather argue this WV strawman that they have created.
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  20. Not at all, Albatross :)

    BTW, with regard to water vapour. Some of what goes up must come down. Water comes down more quickly than CO2. And where I live we've been getting a whole heap of it coming down - unseasonally so and in unusual torrents and record breaking amounts. This follows a record long and hot drought. All this weird weather signals a changing climate.
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  21. The final sentence of the article reads … What we observe instead is the poles warming around 3 times faster than the equator.

    I have read that Canadian/Alaskan temps have risen by 5C, a lot more than 3 times the increase at the equator – or does the “3 times” refer to Antarctic temp increase rather than Arctic?

    Can anyone refer me to material which explains difference between average global temps and polar arctic temps please.
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    Moderator Response: [Daniel Bailey] This post by Ned covers much of that. The use of Arctic-only stations relative to the global datasets shows the ongoing polar amplification present in the Arctic.
  22. I like this post showing strong data supporting the AGW argument.

    Do the people arguing WV claim that the sun evaporates more water in the summer and then wind blows it to the other hemisphere so that it is warmer in winter? The WV would rain out on the way. WV would enhance solar warming during the summer but not during the winter. This is the opposite of CO2 warming that is observed. Can the WV people clarify the mechanism whereby WV causes warming similar to CO2 which is always present in the atmosphere. Please provide a link to a detailed argument that supports your claims.
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  23. Daniel Baily #16
    "all other forcings have been flat or declining "

    Correction. Albedo has changed. Human waste heat has only been increasing. Urban heat islands are unfortunately real. Waste heat is real energy, over and above that provided by the sun. (i.e., unlike the flash-in-the-pan diurnal greenhouse effect.)
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    Moderator Response: [Daniel Bailey] Please do not reprise the waste heat discussion here. 356 comments on that topic and counting; if you feel you have something to add that shows waste heat amounts to more than 1% of GHG forcings, go back to that thread and comment on it there. UHI has been thoroughly debunked and rebunked, both here at SkS and by Tamino, to mention but two places. Use the search function to find the most appropriate UHI thread. Waste heat and UHI comments are off-topic on this thread.
  24. 16 Daniel Bailey

    I guess a definition of a 'human fingerprint' would help. Maybe John (or you) could provide one? For me it means evidence that stands alone in identifying a human component and excludes all other possibilities. Without both of these it's just evidence that's consistent with CO2 warming. A discussion of whether there is or isn't a TSI trend will not get us any closer to understanding why this is a fingerprint.

    More back of the envelope calculations. Solar warming would contain a 40-60% GHG component from water vapour. What about IPCCs estimates of CO2 warming? 0-33% comes from albedo and other non-GHG warming forcings and feedbacks and 66-100% from the GHG component (about half that from the water vapour feedback the rest directly from human GHGs).

    Can John's graph differentiate between a 40-60% GHG warming signal and a 66-100% GHG signal? If it can't then it's not a fingerprint of human warming. As I said before, in John's written description of what a solar fingerprint will look like he seems to ignore feedback and focus solely on the direct effect from changes in TSI.

    DB you seem to be taking two steps forward. John introduces the mythical "solar fingerprint' to the the discussion. I'm only going with John's flow here. If there is a problem with considering a solar fingerprint then maybe you should direct your criticism toward him.
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  25. Quoting from top post:
    "Solar warming should result in the tropics warming faster than the poles. What we observe instead is the poles warming around 3 times faster than the equator."

    How come, then, a majority of this year's heat records are from the tropics?
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  26. 20 Albatross

    I haven't had time to fully take in what Braganza is saying but the quote you pick out does highlight a problem I had noticed.

    Let's limit ourselves to just the most recent trend. Braganza seems to investigate two possible scenarios.

    1) The warming is solely from human GHGs.

    or

    2) The warming is solely from solar and volcanics.

    He then tries to see which best fits the observed trends. The problem I have is that those two options aren't the only possibilities.

    2) would demand the position that CO2 increases have no possible role in temperature trends. Some sceptics might hold that position but many more hold a more nuanced position where they accept the role of CO2 as a GHG but are critical of the magnitude of that effect expressed by the IPCC. That would mean that a third possible option exists where the recent warming trend is in response to multiple forcing factors. Simply positing the two extreme positions and asking the question which one is the better fit does not fully investigate the problem. As I said before, but in respect to another issue, this is not an all or nothing problem.

    In fact your quote highlights the problem I have with John's presentation. In Braganza's words this is "consistent" with human GHGs not a fingerprint of it.
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  27. 27 Argus: "How come, then, a majority of this year's heat records are from the tropics?"

    Perhaps because annual events are not climate trends.
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  28. HR, I think the disconnect here is that you aren't considering the fact that feedbacks must perforce follow the same 'fingerprints' as the forcings which cause them.

    That is, if a solar forcing were causing increased Summer temperatures that would indeed result in a water vapor feedback effect... during the Summer. When Winter arrived and the impact of the solar forcing declined the water vapor feedback would follow suit. Ditto geography... if a solar forcing were causing accelerated equatorial warming then we would indeed see increased water vapor feedback... near the equator.

    Yes, there is always going to be some overlap and 'flow' of feedbacks, but they must always be most pronounced in the same conditions the forcing causing them is. The law of cause and effect still applies.
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  29. Figure 1 shows green house warming started in 1980. Cool.

    I wouldn't expect the tropics to necessarily warm as much in summer due to solar increases. After all if you look at the globe between the tropics you will see that you are dealing with predominantly water, not land. Greenhouse warming is primarily a land based effect. Oceans absorb almost all the solar radiation they receive and transport that heat via currents to other parts of the globe (very low albedo). Because oceans absorb almost all the heat they receive there is nothing re-radiated for CO2 to absorb. Heating of the air over the oceans is primarily driven by water temperature which is why the temperature record over the oceans is based on sea temperatures. Ocean heating due to solar activity extends to a depth of 50m-100m during the summer. This heat is not all available to heat the atmosphere immediately but is transported via various currents and usually northward and southward.

    So again, Figure 1 points to an effect possibly caused by a shift of some sort in ocean currents or the transport of heat from increased solar activity farther north. England is seeing this effect right now. Most of the big hurricane activity this fall was confined to the North Atlantic taking huge amounts of heat out of the Gulf Stream which warms England and northern Europe and makes it livable.
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  30. Humanity Rules:
    "That means 40-60% of the warming trend from a doubling of CO2 comes from the water vapour feedback. 40-60% of the GHG fingerprint comes from water vapour. The water vapour effect is not specific to CO2 but is a consequence of a warming world. All things being equal, any process that warms the world is going to have this 40-60% water vapour feedback."


    That is a ridiculous over simplification.
    Insolation varies, when you reduce it, you reduce the energy. CO2 is present all the time and is a 'storage' mechanism. The two have different impacts on how they would effect water vapour production and retention.

    It is somewhat duplicitous to accuse John of over simplifying the issue, when you yourself make a faulty assumption.
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  31. #31: "Greenhouse warming is primarily a land based effect. "

    Surely you would include the Arctic as a place where greenhouse warming is significant... that is the Arctic Ocean?
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  32. Minor remark:

    The link to the data on the graph subtitles points to CRUTEM3v Northern Hemisphere, whereas the graph itself is CRUTEM3 global.
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  33. Re: TOP (31)
    "Figure 1 shows green house warming started in 1980. Cool. "
    Why in the world would you make that conclusion? Are you just graph-mining? Or do you actually have a source for that opinion?

    I call a balk.

    The Yooper
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  34. I have to agree with HumanityRules' logic that any warming will increase water vapor, providing a strong, positive H2O GHG feedback in all cases, for all forcings which will give any warming of any sort a GHG signature. For this reason, it is difficult to apply the argument made by this post.

    Of course, to accept this logic, one must also accept net positive feedbacks and a higher climate sensitivity. You can't argue for this, and also argue that clouds will cause a negative feedback that will hold temperatures (and therefore H2O content) relatively constant. You also can't argue the line that atmospheric H2O content simply won't change with warming.

    But the correct statement should be that "solar forcing without any positive GHG feedbacks" would warm days more than nights and summers more than winters. This is of course a purely abstract statement, since any warming will introduce a positive H2O GHG feedback, and eventually a positive CO2 GHG feedback as well.

    It would also, I think, warm the troposphere but not the stratosphere (since the warming would not necessarily increase H2O content in the stratosphere, although that's now a "known question mark", as highlighted by Solomon 2010). But it is still a difference between CO2 and solar warming, in that CO2 will reach and actively cool the stratosphere (at least until solar warming introduces its own carbon feedbacks). So that argument stands as a distinction between CO2 and solar (or other) warming.

    So, in the end, one is again left with the question of what has caused the warming? AGW proponents say CO2 (which, for the record, is my stance). AGW deniers say that it is the sun, or cosmic rays, or seasonal Eurasian Leprechaun Farts, or just that we don't know, but it must be something other than CO2.
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  35. John, can you show what the summer's max temps are doing? What is the trend of the hottest days of the year?

    What is the prediction of AGW theory about the number of extreme hot days in the summer?
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  36. Sphaerica #36

    I think you spotted one more conflicting contrarian argument.

    And of course the "it´s the sun" reasoning lacks one key fact: sun´s irradiance has not been increasing for several decades, now.

    On the other hand, the increasing direct CO2 forcing can be calculated with established, century-old science.
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  37. First, let me reiterate Sphaerica's very important point that if you accept a 40-60% water vapour feedback from increases in temperature, then you are logically committed to accepting that feedback as a result of the approx 1.2 degrees C per doubling of CO2. Given that, and given the degree of warming in the 20th century, accepting a strong water vapour feedback commits you to CO2 being the main driver of that warming.

    Second, if the watervapour feedback was a consequence of solar heating, it would not exhibit the same pattern as that resulting from greenhouse warming. In particular, direct solar heating would make days hotter than night, summers hotter than winter, and tropics hotter than the poles, while to a first approximation, the water vapour feedback would have the reverse effect. Because the initial forcing and feedback have effects opposite in sign, they would cancel each other out (to a first approximation), thus resulting in no signal. In contrast, a water vapour feedback to a green house forcing would reinforce the signal, consistent with the strong signal observed.

    Third, all though a water vapour feedback is identical to a first approximation, more detailed analysis reveals important differences. In particular, direct solar heating would result in a significant increase in humidity in the tropics, but not the poles. So the water vapour feedback induced from solar warming would also be stronger in the tropics than at the poles. In contrast, water vapour feedback from greenhouse forcing would be more evenly distributed, tending to reinforce the signal. So, stronger warming in the poles than tropics is a clear signal of an initial greenhouse forcing, irrespective of the additional signal from water vapour.

    Fourth, water vapour is largely confined to the troposphere, so the water vapour feedback will not result in stratospheric cooling. Consequently, for a solar forcing we would expect a warming stratosphere, and this signal would not be masked by a water vapour feedback. From a greenhouse forcing, we would expect a cooling stratosphere, with the signal not reinforced by water vapour feedback. The cooling stratosphere is the death knell to any theory that solar radiation has driven the twentieth century warming.

    Similar considerations show that the warming is not due to a reduction in aerosol albedo, nor due to a reduction in cloud albedo, nor due to variations in ocean heat content (ENSO, PDO, AMO).
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  38. I agree with CBDunkerson: The feedbacks will follow the forcings.
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  39. In recent decades, could the upward trend in the arctic oscillation index be the cause of enhanced winter warming in the mid latitudes because a cooler stratosphere strengthens the polar vortex and the westerlies?
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  40. Thank you, CBDunkerson and Tom Curtis, for your explanations.
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  41. Argus @27,

    "How come, then, a majority of this year's heat records are from the tropics? "

    Could you provide a source for this please? Assuming it is correct, I'm going to hazard a guess that the records this year in the tropics were most likely because of the fairly strong El Nino event.
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  42. Hi HR,

    Re @26&28,

    "I guess a definition of a 'human fingerprint' would help".

    I think this is why we are speaking past each other HR, because we have different understandings of what is meant by "finger print". If I recall correctly Santer et al. coined the term "fingerprint" and have applied it to the increase in the height of the tropopause. So your objection would probably apply tho that work too?

    HR "The problem I have is that those two options aren't the only possibilities."

    Could you please elaborate or present a conceptual model (preferably with links to reputable sources) which explains the observed pattern (both spatially and in the vertical) and magnitude of the observed long-term warming trend? Braganza designed an experiment which excludes solar and volcanoes. Tom's excellent post @39 leaves you even fewer options, if any at all.
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  43. @Daniel Bailey

    Why I would say that Figure 1 shows global warming starts in 1980 is based on the premise of the author's argument, that winter warming increasing faster than summer warming is a fingerprint of AGW. The winter warming shift occurred in the 1980s. Prior to that there was a summer shift, again according to the author's premise, showing primarily solar warming. If AGW had started in the mid 1800s then this shift should have been there all along to some extent.

    @muoncounter

    The contribution of the Arctic and Antarctic to warming is minor compared to areas nearer the equator. For 6 months out of the year the solar radiation input and hence the effect of any GHG on blocking radiation to space there is minimal to non-existent. The heat causing melting of polar ice has to come from elsewhere. Sea ice formation is from under the ice.

    It melts because there is heat input. The source of the heat doesn't have to be in the arctic areas. It can be transported there.
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  44. TOP @45,

    You may have a point regarding 1980. Tamino has undertaken some analysis and concluded that the anthro warming signal emerged circa 1975. See here [the title is a little confusing, but the text clarifies everything] and here.
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  45. #45: "The heat causing melting of polar ice has to come from elsewhere. ... The source of the heat doesn't have to be in the arctic areas."

    As long as you agree that the greenhouse-trapped heat is melting Arctic ice, what difference does it make where the heat comes from? I guess I misunderstood your prior 'greenhouse warming is primarily a land-based effect'.

    "Sea ice formation is from under the ice." How do you figure that? From wikipedia:
    In calm water, the first sea ice to form on the surface is a skim of separate crystals which initially are in the form of tiny discs, floating flat on the surface ... In rough water, fresh sea ice is formed by the cooling of the ocean as heat is lost into the atmosphere.

    Are you referring to 'anchor ice'?
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  46. ATTN: muoncounter at 13:22 PM on 3 December, 2010

    The wind is by far the most important mechanism that transports surface water into the atmosphere. Evaporation of water in still air is a slow process.

    Set out a shallow dish with water and determine how long it takes for the water to evaporate. Refill the dish to the same level and have a fan blow a gentle current of air over the water. The water level will decrease much faster in the presence of the wind from the fan.

    Clouds are another source of water vapor. Since they can move about freely and quickly, they can alter the local humidty --like to 100% of they bring rain or snow.
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  47. 39 (Tom Curtis),

    When you say:

    direct solar heating would make days hotter than night, summers hotter than winter, and tropics hotter than the poles, while to a first approximation, the water vapour feedback would have the reverse effect. Because the initial forcing and feedback have effects opposite in sign, they would cancel each other out (to a first approximation), thus resulting in no signal.


    I'm not entirely sure that this would be true. It would be a question of degree (i.e. is one strong than the other), and I think the H2O forcing would stand out, although not to the same degree as with solar versus CO2.

    To be a little more specific, if we assume a 3C total warming from a doubling of CO2, which itself contributes 1C to the 3C, then feedbacks (primarily H2O, or follow-on CO2, but also including changes in albedo due to ice retreat) are responsible for roughly 2C of warming.

    This implies that a change in insolation which causes 1C of warming will in turn cause 2C of feedback (primarily in the form of GHG).

    As a side note, as the topic of different forcings has come up often of late, I'm finding myself annoyed at the previously convenient definition of climate sensitivity in terms of doubling of CO2 -- it implies that CO2 is necessary, and provides no yardstick for other forcings. A definition of "X degrees of change per Y degrees from external forcing" would be more workable. Oh, well...

    So the GHG signal would be stronger than the insolation signal, and should stand out as described in the original post, just not to the same degree as pure GHG forcing would (by a factor of roughly 3:1). But there would be no way (without a trial run of each) to be able to tell whether what we are seeing now is or is not a result of insolation forcing (1C) + GHG feedback (2C), or GHG forcing (1C) + GHG feedback (3C).

    Except, of course, for the differences in stratospheric cooling, and numerous other methods not mentioned in or relevant to the original post (Dan listed a number of them), such a the need to explain why the globe somehow isn't warming from CO2 emissions if another cause were found.

    In particular, direct solar heating would result in a significant increase in humidity in the tropics, but not the poles.


    I'm not sure that this is true, either. Transport of moisture from the tropics to the poles is an important part of both climate and weather, and changes in such transport are one of the biggest dangers of a warming world (expansion of the arid subtropic zones), so I don't think a blanket statement could be made in this regard.
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  48. A few things I've noticed:

    #12 TIS says: "There is no time when the Earth is in winter or in summer."

    This is a valid concern. It would be pretty easy to aggregate summer as NH June-July-August and SH December-January-February, and vice versa for winter. However, I can't tell if this has been done within the spreadsheet since I don't see a column for hemisphere.

    TIS's comment about orbital obliquity is of dubious merit; it just doesn't change much over the course of decades. That effect operates over millenia.


    #39 Tom Curtis:
    "Fourth, water vapour is largely confined to the troposphere, so the water vapour feedback will not result in stratospheric cooling. "

    I agree with other points, but I'm not sure I'm following this. Most of the atmosphere in general, including CO2, is within the troposphere. The average height of emission of a photon to space is around 5-6K, and the tropopause is mostly above that. My understanding is that any GHG effect results in tropospheric warming and stratospheric cooling. The atmosphere is heated from above by whatever SW radiation is absorbed and it is heated from below by whatever LW radiation is absorbed, plus conduction. The tropopause exists where these balance each other.

    #36 Sphaerica,
    An excellent point regarding water vapor and climate sensitivity. You can't have it both ways.

    #45 TOP:
    "For 6 months out of the year the solar radiation input and hence the effect of any GHG on blocking radiation to space there is minimal to non-existent."
    This is wrong at the conceptual level. The surface of the earth is above 0K at all times and at all locations; therefore, it is always emitting radiative energy at all times and at all locations. See Planck's Law.


    General comment regarding comments on water vapor versus CO2 effects:
    The phase state diagram of H20 is different from CO2. Humidity varies a lot by both altitude and latitude because the temperature and pressure where it precipitates is common within our atmosphere. CO2 precipitates at temperatures and pressures not common within our atmosphere; so, it varies a lot less with altitude and latitude. I'm pretty sure one could define different warming, 3D spatial signatures to be expected dependent on which was changing more. However, since the two are linked via the physical properties of the system, it is not clear to me how the data could be unobfuscated. (Hmm, on another read, I see that #4,22 Mike has already made this point, and asked the same question I'm leading to.)
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  49. Doh, regarding my first point, I now see that the data are for NH only. So, my concern is moot. However, I think it would make for a stronger case if SH temps were included as well.
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  50. To this end, I played with some graphs at Wood for Trees. I got this far before running out of time. Four plots of January,July versus Southern, Northern hemispheres.

    Ran out of time before figuring out how (or if it is possible) to aggregate between series, but the test is basically: "Are red and purple rising faster than blue and green?" I believe the answer is "yes".

    I smoothed over best-guess of the time interval for 6 solar cycles.
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