Comments 1 to 26:

1. johnd at 12:09 PM on 21 October, 2010
   The article seems to overlook the relative importance of solar radiation and wind as being the two main drivers of evaporation, translating as the skin temperature of the evaporating surface rather than ambient temperature, and the airflow over it, which in the case of solar radiation would make water vapour more of a forcing than a feedback. This paper details the calculations and the various inputs that are involved BUREAU OF METEOROLOGY REFERENCE EVAPOTRANSPIRATION CALCULATIONS
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2. Ned at 12:51 PM on 21 October, 2010
   No, johnd. If increasing solar radiation were warming the planet, then solar radiation would be the forcing and increasing humidity would be one among several feedbacks. Likewise, if increasing CO2 leads to an increase in humidity, CO2 is the forcing and water vapor is the feedback. On Earth, water vapor is basically never seen acting as a forcing.
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3. GFW at 13:22 PM on 21 October, 2010
   Does he have a point about wind though? If warming temperatures caused less mixing, the absolute humidity at the surface could rise, while falling at greater altitudes, thus introducing a negative feedback. I suspect that warming temperature _won't_ cause less mixing, and there's probably already data on whether it will/does, but at least it seems like there's a scientific possibility in there. (Not that anything really defeats the paleo evidence for a sensitivity around 3, so any discovery of previously overlooked feedbacks is like showing your work when the answer is known.)
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4. johnd at 13:39 PM on 21 October, 2010
   Ned, for solar radiation to manifest itself as a forcing it must first be absorbed. In this case I am referring to that portion of the solar energy that is absorbed at the skin of the water and is immediately transferred to the water vapour so transformed and becomes part of the atmosphere. The solar radiation that is not absorbed at the skin, but progressively at further depths then goes on to manifest itself, and be measured, in different ways.
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5. Ned at 13:57 PM on 21 October, 2010
   This doesn't seem like a particularly relevant or useful start to the discussion of this topic. John's done some nice work looking at humidity trends wrt the water vapor feedback, and it would be a shame to divert the discussion right from the start into a lot of wrangling over minutia.
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6. johnd at 14:06 PM on 21 October, 2010
   Ned, I feel it is both relevant and important enough to clarify given the statement in the article "Water vapor provides the most powerful feedback in the climate system. When surface temperature warms, this leads to an increase in atmospheric humidity." I feel that is not conveying a sense of the correct drivers that are most relevant to how water vapour enters the atmosphere in the first place. There is a need to be sure that the foundations any discussion is built upon are fully understood and solid.
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7. Stephen Baines at 14:14 PM on 21 October, 2010
   Johnd, are you suggesting that the most solar radiation is absorbed by the skin of the ocean, rather than by layers beneathe the surface? The citation you refer to is for calculating evapotranspiration on land, where light does not penetrate beneathe the "skin", at least not far. Water is actually fairly transparent to light so the very thin "skin" accounts for little of the absorbance, although eventually most incoming light is absorbed at depth. The skin temperature of the ocean (where the vast majority of evaporation on earth happens) is largely a function of mixed water column temperature as a whole, which reflects the balance between inputs (solar radiation, incoming IR radiation) and outputs (outgoing IR radiation, evaporation, convection, mixing)of heat energy. As the earth's temperature increases that heat balance results in higher mixed layer temps, which leads to high skin temps and greater evaporation.
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8. scaddenp at 14:43 PM on 21 October, 2010
   JohnD - how can wind be a forcing?
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9. Stephen Baines at 14:47 PM on 21 October, 2010
   BTW. This is a nice summary by John. I learn something again. I also want to agree with Ned. This discussion of insolation and skin temperatures is a distraction. All other things being equal (insolation included), evaporation and water vapor should increase if the earth and atmosphere warm.
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10. Ari Jokimäki at 15:50 PM on 21 October, 2010
    By the way, there's also a new paper out from Dai et al. who make an effort to homogenize the radiosonde humidity records: http://journals.ametsoc.org/doi/abs/10.1175/2010JCLI3816.1 (I haven't found full text, sorry.) Their conclusion: "The DPD adjustment yields a different pattern of change in humidity parameters compared to the apparent trends from the raw data. The adjusted estimates show an increase in tropospheric water vapor globally."
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11. Bern at 18:35 PM on 21 October, 2010
    Thanks for the new article, John. It's an interesting read. I caught that Q and A show earlier in the week, too, and was surprised by Marohasy's statement about humidity - it appeared Tim Flannery was too, although it was almost amusing to watch him twitch at some of the more outrageous statements she was making. He displayed admirable restraint, though - keeping it polite even when giving her a dressing down for interrupting! I note she also quoted Roy Spencer, but more than a few of her talking points might have been easily rebutted by a reference to this website...
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12. Eric (skeptic) at 22:09 PM on 21 October, 2010
    Since the article is about upper atmosphere humidity, shouldn't it talk about convection? Surface moisture ought to increase with CO2 warming (C-C relationship), but then that moisture has to rise. Does convection change on average based on CO2 warming? Mostly I am interested in the nature of the convection, concentrated or not, as well as the overall amount.
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13. Lassesson at 22:37 PM on 21 October, 2010
    I recently saw a similar cherry picking event here in Sweden. A couple of professors emeriti (in chemistry and solid states physics) had read a new article by Spencer and Braswell and concluded from that article that the climatet sensitivity must be at (exactly) 0.6 degrees C for a doubling of CO2 and therefore there was no rush in reducing our GHG emissions, since it wouldn't make any significant difference anyway. Spencer and Braswell, however, did not say that in their article. They said that they had found a short term climate sensitivity that seemed to be around 0.6 degrees C. What "short term" means in this subject I'm not sure, but I believe they were talking about days or weeks. At least it seemed like that when I looked at their data. Spencer and Braswell pointed out on several occasions in their article that they had no intention of pointing out what the long term climate sensitivity would be, but that was just what our cherry picking professors emeriti did. They drew a conclusion from one single article, ignoring all the rest, despite the fact that the writers of the article themselves pointed out that this conclusion could not be drawn from their results. Maybe I should have written this in the "climate sensitivity is low" argument instead, but i thought the cherry picking was too similar. I just had to tell it here.
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14. Scrooge at 00:30 AM on 22 October, 2010
    I just have to wonder what role the hadley cells and subsidence play in the tropics. Does it not go along with the desert areas expanding? Since we are talking about the upper trop I could be wrong but it does go along with the feast or famine rainfall amounts we see in different parts of the world.
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15. johnd at 03:20 AM on 22 October, 2010
    Stephen Baines at 14:14 PM, this chart gives an indication of the absorption of solar radiation in water, about 13% of the total having been absorbed in the first 1mm.
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16. t_p_hamilton at 06:40 AM on 22 October, 2010
    JohnD is confusing radiation, radiation forcing and feedback. Solar radiation is not a forcing. Change in solar radiation is a forcing. In other words, if the average solar radiation was the same year after year (it is easy to understand it is location and season dependent) the climate would not change. Forcings by definition are changes not derived from the climate system. Feedbacks by definition are changes derived from the climate system. CO2 increase today is not due to climate, but burning carbon, hence is a forcing. The average increase in water comes about from the changed climate.
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17. protestant at 20:12 PM on 22 October, 2010
    And John, do you have ANY evidence at all that water wapor never acts as a forcing? It seems evident, that the earths temperature anomalies match quite a bit with the tropospheric humidity. This correlation also includes the flattening on the GW trend during the last decade. No net warming since 1998 and also no net humidity increase since 1998. Which is the cause and which is the effect? Also one crucial point about water wapur feedback is the increase in the upper troposphere. Some evidence sugggest, that there is no increase. Also according to the latest McKitrick et al and Christy et al, no tropospheric hotspot is also to be found, on any of the datasets. You also point out, that humidity increases during El Nino events. It also means, during La ninas it will decrease. And according to history, the next 30 years or so will be the times of La Nina. That could also mean decreasing in humidity for the same period. The most critical question is... where do trade winds come from. As far as I understand, clouds drive them through condensation (models do not take in accoutn the pressure loss through condensation, this was pointed out by Jeff Id lately), and solar activity with planetary rotating parameters is driving the clouds. If you just compare the detrended global thermometer data and compare it to PDO and AMO indexes you will see that all of the 30 year variation must be driven by those events. This also suggest cooling for the next 30 years (and 30yrs of warming after that), thus a total of 1C increase temperatures for the next 90 years.
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18. johnd at 20:13 PM on 22 October, 2010
    t_p_hamilton at 06:40 AM, the point I was making is that the energy that is transferred directly from sunlight to water vapour through the evaporation process is no different in terms of the effect it exerts once in the atmosphere, to that energy that is absorbed directly into the atmosphere from incoming solar radiation. On the point of confusion, a forcing is a mechanism that can exert change. It does not cease being a forcing agent because at some point in time it has a value of zero. What perhaps does confuse the issue is that the values that are subscribed to the various forcings are anomalies, with the base year I believe being 1750. This is all very well in the case of CO2, but I feel, given the poor understanding of the state of the climate at that time and the relatively inadequate measurements of all the other factors driving it, anomalies may not allow the weight of each forcing mechanism to be fully appreciated when being worked into models. Would it not be better if absolute values were to be used instead?
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19. t_p_hamilton at 02:48 AM on 23 October, 2010
    johnd said:"On the point of confusion, a forcing is a mechanism that can exert change. It does not cease being a forcing agent because at some point in time it has a value of zero." This is so vague (confused) as to not even be wrong. Forcing in climate is something that changes the level of radiation energy on average. That is why they are expressed in Watts per square meter. Something vague like "change" could be the difference between day and night, windy or not windy, as you specifically mention as being important. That is just weather, so your point is irrelevant without any consideration of whether these important factors change on average, which would be climate changes. This article is only about climate observations (total amount of water in the air, which influences radiation at the surface), not an explanation of the basics of evaporation as a weather phenomenon. Until you realize climate changes are changes in average weather, not changes in weather, you will remain confused.
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20. Stephen Baines at 03:10 AM on 23 October, 2010
    JohnD, the effect of solar radiation on evaporation is not "direct." Evaporation (and water vapor content) increases when the surface waters and wet soils warm, when the air warms and when wind allows the air near air/water interfaces to be replaced quickly before it saturates with water vapor. So the effect of temperature is indirect, through it's effect on surface and air temperatures. Since climate models are built on the fact that solar radiation is the primary source of heat energy into the system, variations in solar energy are built into predictions of atmospheric water vapor. They can't be causing the apparent increase in water vapor because there has been no recent increasing trend in solar inputs. In any case the existence of the water vapor feedback is not related to whether sun or GHGs are driving the temperature change. If the sun were causing the current warming, there should still be a water vapor feedback for that warming, just like there is for warming induced by icreases in GHGs. If there were no water vapor feedback, the climate system much more insensitive as a whole than it apparently is. It would be very hard to explain past climate variations, including those that have been caused by changes in solar radiation. As for the ocean discussion, we can quibble about how deep the skin is (I think of it as much thinner than a millimeter) and what consitutes a fraction that is large (13% still seems low to me). But there are several points your graphs bring up. First, whereas on land all the visible light waverlengths would be absorbed/reflected near the surface, visible light penetrates pretty deeply into the water column. That's the point I was trying to make about the difference between the ocean and land. Second, most of the solar radiation absorbed near the ocean surface is in the infrared range. If you were to extend that x-axis out, you'd find that the contribution of downwelling thermal IR to the near surface radiation budget is actually larger than that of downwelling solar IR. That downwelling thermal IR is of course a function of air temperature not solar radiation. Finally, the point of where the radiation is absorbed is moot in the ocean because that heat gets dispersed through the water column pretty quickly, as evidenced by the fact that you don't see strong persistent gradients in temperature near the surface. Sometimes this forum makes me feel like I'm reliving my prelims all over again!
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21. Stephen Baines at 04:36 AM on 23 October, 2010
    Obviously, when I wrote "So the effect of temperature is indirect..." I meant "the effect of solar radiation." Basically the discussion boils down to this, so to speak. You should get more water vapor as earth's temperature warms. If that didn't happen, it would be really really interesting scientifically - a potential Nature or Science paper and fame for anyone who could prove it. However, the strength of the physics behind the expectation, the fact that most data collected to date clearly shows a response of water vapor to atmospheric temperatures, and the difficulty in accomodating for past climate change in the absence of such a relationship suggests we should look at countervailing evidence with a very critical eye, especially when we know how easy it is to misinterpretation radiosonde data.
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22. Marcus at 11:59 AM on 23 October, 2010
    Ah, I see protestant is repeating the "no net warming since 1998" myth. He seems to be blissfully unaware that 1998 was a *very* anomalous year-consisting of both the strongest El Nino on record *and* the peak of the last solar cycle. 9/10ths of the 1990's had temperature anomalies of between +0.1 to +0.40 degrees C. By contrast, No year of the last decade has had temperatures *below* +0.3 degrees above the mean, & has seen temperature anomalies of between +0.35 to +0.60 degrees C-in spite of being dominated by a solar minimum.
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23. Kooiti Masuda at 12:49 PM on 23 October, 2010
    Paltridge et al. (2009) did suggest that water vapor feedback would be negative, and the paper by Dessler and Davis can be considered as an activity to rebut the claim. The paper by Dessler and Davis (2010), however, concentrated on discussion whether we can say, based on scientific evidence, that water vapor has decreased in the upper troposphere during the late 20th century. It did not involve discussions of the mechanism of changes in humidity or other climatic elements. Both Paltridge et al. and Dessler and Davis made use of "reanalysis" data sets. Simply speaking, a reanalysis is an attempt to incorporate historical meteorological observations into a weather forecast model to produce a physically consistent data set describing the actual atmosphere. There are several multi-decadal reanalyses of the atmosphere, and NCEP/NCAR is the earliest achievement among them. Concerning humidity, NCEP/NCAR Reanalysis gave large weights on observations by radiosondes (baloons equipped with sensors and radio wave transmitters) conducted routinely by meteorological services of many countries. Later reanalyses put more weight on records retrieved from satellite observations. Radiosonde observations are valuable sources of information about climate change, especially because we can go back with them before the start of satellite observations. We must be careful to discuss trends based on them, however. Specific humidity (concentration of water vapor in the air) has order-of-magnitude difference between lower and upper parts in the troposphere. A radiosonde usually conducts observations while rising from the moister lower part to the drier upper part. Some sensors had long response time, or had difficulty in keeping accuracy at low humidity after having observed high humidity. Changes of sensors are likely to be improvements from longer response time to shorter one, likely to cause apparent decrease of humidity in the upper troposphere. In addition, sensors receive solar radiation in daytime and have temperature considerably different from that of the surrounding air. This difference causes a bias in records of humidity. It is a difficult issue to account for the bias, because it is dependent on many factors, such as darkness of the equipment, mechanism of the sensor, choice of expression for initial recording of humidity information, and attempt to compensate for the bias before official reporting. Teams that produce reanalyses, such as those of European Centre for Medium-Range Weather Forecasts (ECMWF) and of Japan Meteorological Agency (JMA), try to find and to correct biases of radiosonde observations which they incorporate. They cannot achieve perfection, but they achive gradual improvements.
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24. Rob Painting at 20:18 PM on 23 October, 2010
    Thanks for the clarification Kooiti. Readers might want to view a video of Andrew Dessler addressing the humidity issue & climate sensitivity at Deltoid
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25. scaddenp at 20:03 PM on 24 October, 2010
    "And John, do you have ANY evidence at all that water wapor never acts as a forcing?" How do you get water vapour to change without something else (eg temperature rise) first causing it? The cause of the temperature rise is the forcing not the water vapour.
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26. Rovinpiper at 08:11 AM on 13 January, 2011
    Can the amounts of dust in different layers of an ice core be used as a proxy for humidity? I was reading in Mark Bowen's book Censoring Science about the Vostok ice core. It says, "Methane, like carbon dioxide, rose and fell with temperature, whereas dust tended to go the other way. This made sense, as methane and carbon dioxide levels fell and the air cooled, it would have lost water vapor through feedback and become drier and more dusty." Yet, I also recall reading in a US Army Southwest Asia (Middle East) manual that as the temperature increases in summer the soil dries. Without water to hold it down dust goes into the air. So in that case warmer air is dustier. How can we be sure that increased levels of dust in layers of an ice core indicate lower humidity?
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