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Explaining how the water vapor greenhouse effect works

What the science says...

Select a level... Basic Intermediate

Increased CO2 makes more water vapor, a greenhouse gas which amplifies warming

Climate Myth...

Water vapor is the most powerful greenhouse gas

“Water vapour is the most important greenhouse gas. This is part of the difficulty with the public and the media in understanding that 95% of greenhouse gases are water vapour. The public understand it, in that if you get a fall evening or spring evening and the sky is clear the heat will escape and the temperature will drop and you get frost. If there is a cloud cover, the heat is trapped by water vapour as a greenhouse gas and the temperature stays quite warm. If you go to In Salah in southern Algeria, they recorded at one point a daytime or noon high of 52 degrees Celsius – by midnight that night it was -3.6 degree Celsius. […] That was caused because there is no, or very little, water vapour in the atmosphere and it is a demonstration of water vapour as the most important greenhouse gas.” (Tim Ball)

At a glance

If you hang a load of wet washing on the line on a warm, sunny day and come back later, you can expect it to be dryer. What has happened? The water has changed its form from a liquid to a gas. It has left your jeans and T-shirts for the air surrounding them. The term for this gas is water vapour.

Water vapour is a common if minor part of the atmosphere. Unlike CO2 though, the amount varies an awful lot from one part of the globe to another and through time. Let's introduce two related terms here: 'non-condensable' and 'condensable'. They set out a critical difference between the two greenhouse gases, CO2 and water vapour.

Carbon dioxide boils at -78.5o C, thankfully an uncommon temperature on Earth. That means it's always present in the air as a gas. Water is in comparison multitalented: it can exist as vapour, liquid and solid. Condensed liquid water forms the tiny droplets that make up clouds at low and mid-levels. At height, where it is colder, the place of liquid droplets is taken by tiny ice-crystals. If either droplets or crystals clump together enough, then rain, snow or hail fall back to the surface. This process is constantly going on all around the planet all of the time. That's because, unlike CO2, water vapour is condensable.

CO2 is non-condensable and that means its concentration is remarkably similar throughout the atmosphere. It has a regular seasonal wobble thanks to photosynthetic plants - and it has an upward slope caused by our emissions, but it doesn't take part in weather as such.

Although water vapour is a greenhouse gas, its influence on temperature varies all the time, because it's always coming and going. That's why deserts get very hot by day thanks to the Sun's heat with a bit of help from the greenhouse effect but can go sub-zero at night. Deserts are dry places, so the water vapour contribution to the greenhouse effect is minimal. Because clear nights are common in dry desert areas, the ground can radiate heat freely to the atmosphere and cool quickly after dark.

On the other hand, the warming oceans are a colossal source of water vapour. You may have heard the term, 'atmospheric river' on the news. Moist air blows in off the ocean like a high altitude conveyor-belt, meets the land and rises over the hills. It's colder at height so the air cools as it rises.

Now for the important bit: for every degree Celsius increase in air temperature, that air can carry another 7% of water vapour. This arrangement works both ways so if air is cooled it sheds moisture as rain. Atmospheric rivers make the news when such moisture-conveyors remain in place for long enough to dump flooding rainfalls. The floods spread down river systems, causing variable havoc on their way back into the sea.

Atmospheric rivers are a good if damaging illustration of how quickly water is cycled in and out of our atmosphere. Carbon dioxide on the other hand just stays up there, inhibiting the flow of heat energy from Earth's surface to space. The more CO2, the stronger that effect.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

When those who deny human-caused global warming use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as potent a greenhouse gas as water vapour, which there's already a lot of, adding a little more CO2 couldn't be that bad, they insist.

What this argument misses is the critical fact that water vapour in air creates what scientists call a 'positive feedback loop'. That means it amplifies temperature increases, making them significantly larger than they would be otherwise.

How does this work? The amount of water vapour in the atmosphere has a direct relation to the temperature in any given region and the availability of water for evaporation. Heard the weather-saying, "it's too cold to snow"? There's more than a grain of truth in that; very cold air has a low capacity for moisture.

But if you increase the temperature of the air, more water is able to evaporate, becoming vapour. There's a formula for this, the figure being 7% more moisture capacity for every degree Celsius of warming. All you then need is a source of water for evaporation and they are widespread - the oceans, for example.

So when something else causes a temperature increase, such as extra CO2 emissions from fossil fuel burning, more water can evaporate. Then, since water vapour is a greenhouse gas, this additional moisture causes the temperature to go up even further. That's the positive feedback loop.

How much does water vapour amplify warming? Studies show that water vapour feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C upward temperature change caused by CO2, the water vapour will cause the temperature to go up another 1°C. When other demonstrable feedback loops are included, and there are quite a few of them, the total warming from a 1°C change caused by CO2 is as much as 3°C.

The other factor to consider is that water evaporates from the land and sea and falls as rain, hail or snow all the time, with run-off or meltwater returning to the sea. Thus the amount of water vapour held in the atmosphere varies greatly in just hours and days. It's constantly cycling in and out through the prevailing weather in any given location. So even though water vapour is the dominant greenhouse gas in terms of quantity, it has what we call a short 'atmospheric residence time' due to that constant cycling in and out.

On the other hand, CO2 doesn't take an active part in the weather. It does hitch a lift on it by being slowly removed from the air as weak solutions of carbonic acid in rainwater. These solutions are key weathering agents, affecting rocks on geological time-scales. Weathering is a key part of the slow carbon cycle, with the emphasis on slow: CO2 thus stays in our atmosphere for years and even centuries. It has a long atmospheric residence time. Even a small additional amount of CO2 thus has a greater long-term effect - and in our case that additional amount is far from small.

To summarize: what deniers are ignoring when they say that water vapour is the dominant greenhouse gas, is that the water vapour feedback loop actually amplifies temperature changes caused by CO2.

When skeptics use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as powerful as water vapor, which there's already a lot of, adding a little more CO2 couldn't be that bad, right? What this argument misses is the fact that water vapor creates what scientists call a 'positive feedback loop' in the atmosphere — making any temperature changes larger than they would be otherwise.

How does this work? The amount of water vapor in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback.

How much does water vapor amplify CO2 warming? Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C. When other feedback loops are included, the total warming from a potential 1°C change caused by CO2 is, in reality, as much as 3°C.

The other factor to consider is that water is evaporated from the land and sea and falls as rain or snow all the time. Thus the amount held in the atmosphere as water vapour varies greatly in just hours and days as result of the prevailing weather in any location. So even though water vapour is the greatest greenhouse gas, it is relatively short-lived. On the other hand, CO2 is removed from the air by natural geological-scale processes and these take a long time to work. Consequently CO2 stays in our atmosphere for years and even centuries. A small additional amount has a much more long-term effect.

So skeptics are right in saying that water vapor is the dominant greenhouse gas. What they don't mention is that the water vapor feedback loop actually makes temperature changes caused by CO2 even bigger.

Last updated on 23 July 2023 by John Mason. View Archives

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Comments 76 to 100 out of 375:

  1. villar, you can demonstrate non-runaway positive feedback in a spreadsheet: Cell A1: 0 Cell A2: 10 Cell A3: =A2+0.5*(A2-A1) Cell A4: =A3+0.5*(A3-A2) Now copy and paste cell A4 into cell A5 and on down the column for about 15 cells. The formula should automatically adjust to each cell, so each cell's value is the previous cell's value plus 50% of the increase that the previous cell had experienced over its predecessor. The feedback is an increase of each increase, not of the total resulting amount.
  2. "The specific humidity has been increasing over the last few decades near the Earth’s surface (as shown by the 1000 mb data), while it has been decreasing in the upper troposphere (as shown by the 300mb data). The increase in specific humidity at the Earth’s surface (1000 mb) is related to surface temperatures. For all except the far southern hemisphere bands, the effect of the 1997/98 El Nino can be seen in the specific humidity graphs. The decreasing specific humidity in the upper troposphere (300 mb) indicates that the warming at the Earth’s surface does not match the CO2 based warming theory. This is especially so in the northern hemisphere, which has experienced most of the warming in recent decades.....This is a major area of contention in the climate science area, since without the water vapor feedback the CO2 based theory loses its significance."
  3. Mizimi, unfortunately who wrote that page failed to notice that using the very same ESR utility it can be easily found that the upper troposphere has indeed warmed. An obvious question then arises as to how can specific humidity drop while temperature rises. It looks more like a convection issue. P.S. The first graph shown in you link has been demonstrated to be wrong quite a while ago. But you know, things keep hanging around forever over the internet ...
  4. Riccardo - thanks for the PS pointer. I recently read a paper ( which I seem to have lost) showing that there has indeed been an unexpected increase in convective turnover in the tropics...must try and relocate it. Just as a side issue... the amount of WV is not just T dependant - you have to have a source. No source in the Sahara so it's dry even tho' T is high. Lots of water in the tropics so it's humid. Also, in general terms, the atmosphere could hold a lot more WV -if the contact surface area between air/water was increased - without an increase in T. Air conditioning spray humidifiers do just that.
  5. Just a thought, are any of you familiar with global dimming. The article states that there has been an increase in WV in the atmosphere which has been recorded since 1988. If this is true then in essence there should be greater cloud cover which acts as both a negative feedback mechanism and a positive feedback. I cant remember where but i read that the suns rays have a greater impact on the evaporation of water than a rise in temperature alone. Something about the excitement of water molecules. So the increase in cloud cover and aerosols act as both a positive and negative feedback mechanism not exactly canceling each other out though. Apparently global dimming is half as strong as global warming. I'm sure that if i have made an error some where in my logic you will correct me.
  6. "Anthropogenic use of water is less than 1% of natural sources of water vapor..." Thus starts a paragraph about water vapor to state somewhat later: "Radiative forcing from anthropogenic sources of tropospheric water vapor is not evaluated here ..." How can some one come to the conclusion we are dealing with science here. By the way these statements are taken from the IPCC's 4th Assessment Report.
  7. If I can comment the discussion of Chris G and Mizimi: It is very interesting and I am amazed by the level of details being analyzed. Nonetheless my impression is that keeping an eye (and mind) focused on details prevents sometimes from noticing more relevant obvious facts which can render the details insignificant. The neglected factor seem to be the stabilising impact of oceans. While it is true that raising temperature of atmosphere increases its capacity to hold more WV thus raising its potential for more efficient GH effect - to fill this capacity requires liquid water reservoirs of elevated temperature. Primary source of globe's VW are oceans, whose heat capacity is several orders of magnitude higher than that of atmosphere and it would take millenia for the air to warm them up. Over the lands water sources are less massive and thus can more readily make use of raising evaporation conditions. However - taking into account high mobility of atmosphere and effective mixing - the warming effect will be diluted (with the probable net effect being enhanced removal of moisture from lands to oceans). As to the C02 - again it is the temperature of the oceans that determines water-air equilibrium concentrations. Antropogenic CO2 will in consequence mostly go to the oceans, and again - taking into account their far greater capacity then that of atmosphere - it should not accumulate to significant long-term levels in atmosphere making its impact on climate transient. How than can be explained the rising concentrations of CO2 in the air - observational fact beyond doubt? Either it is really of antropogenic origin and then it seems to be a small problem (as it is on its way into ocean) or - CO2 is released by the warming ocean (for which I - and perhaps we - miss information), a process that can be of far more dangerous consequences.
  8. lepton writes: Antropogenic CO2 will in consequence mostly go to the oceans, and again - taking into account their far greater capacity then that of atmosphere - it should not accumulate to significant long-term levels in atmosphere making its impact on climate transient. Well, that's true, as long as you can accept a timescale of tens of thousands of years as "transient." Yes, most of the CO2 we add to the atmosphere will end up in the ocean but given the thinness of the mixed layer and the slow rate of movement from the atmosphere to the mixed layer to the deep ocean, that will take a long time to happen. In the meantime, atmospheric CO2 will almost certainly exceed 2X preindustrial levels, if not 3X. How than can be explained the rising concentrations of CO2 in the air - observational fact beyond doubt? Either it is really of antropogenic origin and then it seems to be a small problem (as it is on its way into ocean) or - CO2 is released by the warming ocean (for which I - and perhaps we - miss information), a process that can be of far more dangerous consequences. Actually, it's quite clear that CO2 is going from fossil fuels to the atmosphere to the ocean, not the opposite direction. See, e.g., the following papers: Takahashi et al. 2009. Climatological mean and decadal change in surface ocean pCO2, and net sea–air CO2 flux over the global oceans. Deep Sea Research Part II: Topical Studies in Oceanography, Volume 56, Issues 8-10, April 2009, Pages 554-577. Sabine, et al. 2004. The Oceanic Sink for Anthropogenic CO2. Science, Vol. 305. no. 5682, pp. 367 – 371. Yes, as it warms the ocean can hold less CO2, but CO2 is rising faster in the atmosphere, so there's a net flux from the atmosphere to the ocean. More than anything else, it was studies of the process of CO2 uptake by the oceans that set off alarm bells about global warming in the 1970s and early 1980s. Until then people thought most of the CO2 we emit would end up in the oceans. When it became clear that this would take a long, long time to happen, people began to realize that this is a big problem that we're facing.
  9. to villar at 20:21 PM on 2 December, 2009 Any positive feedback is the source of instability, by definition. I do regret that so many climatologists have so minimal knowledge of process control. Not every positive feedback blows a sytem up. It can also make it oscillate, like in simple systems with phase delay. In climate - an obvious complex set of interwoven processes - a positive feedback like VW induced GH effect - generally will drive conditions to the point when other (up to then dormant) processes will activate, possibly turning on some negative feedback. The very fact that Earth's ecosphere still exist after so many evidences of past climate extremities proves that (regardless of any positive feedback) our globe can always find equilibrium (if only local).
  10. Ned at 13:23 PM on 21 February, 2010 writes: "More than anything else, it was studies of the process of CO2 uptake by the oceans that set off alarm bells about global warming in the 1970s and early 1980s. Until then people thought most of the CO2 we emit would end up in the oceans. When it became clear that this would take a long, long time to happen, people began to realize that this is a big problem that we're facing." Thank you for valuable references. However - as a physicist (not climatologist) - I can hardly understand why the time scale of the process of the CO2 reaching equlibrium between atmosphere and ocean concentrations process might be any different from that of WV evaporation rate due to ocean-air temperature differences. Both processes are based on the same molecular mechanisms. Perhaps the culprit is the ocean surface layer - warmer than the bulk, so evaporating more willingly, at the same time accepting less CO2?. Might it be that the significant factor is the mixing of ocean? Both on surface by waves and in bulk by currents?
  11. A year ago, Mizimi wrote: A few sums ....various sources give our annual energy usage from FF as around 14 terawatts. That's almost certainly the power directly associated with combustion of fossil fuels, not the radiative forcing from the CO2 produced by that combustion. But, more or less coincidentally, the annual increase in radiative forcing is probably pretty close to that 14 terawatts worldwide. (Based on these data from NOAA, it looks like the annual increase is 0.03 watts/m2 which works out to about 1.5E13 watts over the planet as a whole. Mizimi continues: Looking around the Australian Bureau of Statistics gives the following population figures.... People 21 million Horses 400,000 Kangaroos 23 million Camels 400,000 Cattle (dairy and beef) 26 million Sheep 20 million Rabbits 250 million Simple maths - multiplying numbers by the basal metabolic rate at rest of each species gives a daily heat emission of 315 x 10E9 watts or 114 x 10E12 watts per annum. Whoa, do you understand the meaning of the word "watt," Mizimi? A basal metabolism of 1 watt is 1 watt, whether you measure it over a period of days, weeks, years, or millennia. If you're going to convert the metabolic data to watt-years, you need to do the same conversion for the radiative forcing! Let's redo those calculations. I can't find actual data on the metabolic rates of kangaroos etc., but I see that sheep are about 50 watts, cattle about 330, and humans about 78. Using Mizimi's population figures, that works out to a total basal metabolism of 1.1E10 watts, several orders of magnitude less than the figure that Mizimi quotes. I don't think adding in the kangaroos and rabbits would make up the difference :-) The annual increase in radiative forcing from greenhouse gases is 1360 times larger than that. In other words, every 6.4 hours we're adding a radiative forcing to the climate system that's approximately equal to the combined metabolism of all the people, cattle, and sheep in Australia. Two caveats in closing: (1) If that seems like a silly comparison, blame Mizimi not me. (2) Apologies for responding a year late ... but Mizimi's original post was so severely erroneous and had gone uncorrected for so long that I thought it would be good to set the record straight.
  12. Ned, i am interested in your refute of Mizimi's calcualations in post #86. I was wondering if you could answer a question for me. I am no where near the level of discussion on this board. Your first point has to do with the definition of a watt. A watt is equal to one joul/s. The definition of a watt includes a specific amount of energy over a specific period of time. j/s. Why then is it inappropriate for an individual to calculate the total increase in heat or energy over a period of time (a year) based on the length of time given in the definition of a watt? Second, my understanding of radiative forcing is limited. I beleive it is defined as the change in irradiance at the tropopause. It is determined relitave to a base period (usually 1750). The IPCC defines radiative forcing as follows: "Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values are for changes relative to preindustrial conditions defined at 1750 and are expressed in watts per square metre (W/m2).":36 Neither my previous definition nor the IPCC definition refer to change per unit of time. As such how and why would Mizimi have to calculate radiative forcing-years? sure one year may have an increase in radiative forcing, and another a decrease, but they are all relative to 1750 and are not necessarly cumulative. To better explain myself and my understanding of radiative forcing i will use simplified means. Say year XXX1 has a radiative forcing value of +1. this would mean that it is +1(w/m2) over 1750. Assume year XXX2 has a reading of +1 as well, year XXX 3 has a reading of +2, the cumulative radiative forcing is + 2 not +4, right? Love this series of posts, sorry if i am dumbing it down too much.
  13. Okay, first of all I think that evaluating the impact of CO2 by comparison to the metabolism of Australian livestock is not necessarily the optimal way to visualize it. Kind of like weighing yourself and then converting to units of ladybugs based on a sample weight of a ladybug in grams found somewhere on the internet. But, since you asked ... Tadh writes: Why then is it inappropriate for an individual to calculate the total increase in heat or energy over a period of time (a year) based on the length of time given in the definition of a watt? Actually, it's not at all inappropriate in the right circumstances. My monthly electricity bill reports usage in kilowatt-hours. That is appropriate for a quantity that fluctuates over time, like household electrical usage. In this case, it's not necessary, since for both the animal metabolism and climate radiative forcing data we have just an annual average. When Mizimi writes "... a daily heat emission of 315 x 10E9 watts ..." that's not "daily" ... it's 315 x 10E9 watts full stop. When she/he multiplies that by 365 to get "114 x 10E12 watts per annum" that's actually "114 x 10E12 watt-days per annum", though Mizimi doesn't seem to understand the meaning of these units. Now, doing that conversion merely complicates things unnecessarily, since "114 x 10E12 watt-days per annum" is exactly the same as "315 x 10E9 watts" (ignoring leap-years). However, the real problem is that Mizimi converted the metabolic data to "watt-days per annum" but didn't convert the radiative forcing to the same units. In other words, one side of the comparison got multiplied by 365 but the other side didn't. Hope this helps. Let me know if it's still confusing...
  14. Ned, Thanks for the quick reply. You certainly gave me a better understanding of what Mizimi did with the metabolic rate, and how it affects the resulting answer. However I am still a bit lost on the radiative forcing. I think I know where I am going wrong. Radiative forcing would need to be changed in the same way metabolic influence would need to be due to the fact both are expressed in watts? But isn’t the radiatve forcing just relative to a set period, not a predictable increase? In order to do RF-Years wouldn’t we have to apply the change from year to year, and not the actual RF number? Thanks! By the way, apparently I weigh approximately 3,995,933 ladybugs. (wiki says .021 grams per ladybug)
  15. Well, you outweigh me a bit; I'm 3,780,000 ladybugs as of this morning. It's been a long day here and I'm a bit addle-brained right now, but I think you're talking about two separate issues. One is the units involved in comparing radiative forcing to gross Australian livestock metabolism, and the other is how to account for changes in radiative forcing over time.* Is that right? If so, I think we've taken care of the first part (units should be consistent on both sides, either comparing watts to watts or watt-days per year to watt-days per year). Then the other question is about radiative forcing and how to handle changes therein over time. If CO2 and everything else were constant, RF would be constant too (and there'd be no need for this website!) Unfortunately CO2 is increasing, so the RF associated with CO2 is likewise increasing over time. Now, I may have screwed up the calculations somewhere, and I'm too tired to re-check this, but apparently when I wrote that original reply to Mizimi I'd calculated that annual increase in RF as 0.03 W/m2 over the whole earth. (I would not be the least bit surprised to learn that I'd made an error somewhere there, but let's assume it's correct). Now, the RF itself doesn't "accumulate" from year to year, though the energy resulting from it does (until the earth comes back into radiative balance with its exoatmospheric environment). But you can still look at the annual increase in RF caused by the annual increase in CO2, and compare the power of that RF increase with some other thing (like Australian sheep metabolism). Of course, I may be confused about what you're saying, or I might be screwing up the calculations. Or I might be suffering from a case of DK effect. But this is how it seems to me. * This is a sentence that has probably never before been composed in all of human history!
  16. John, The link to Soden 2005 doesn't work. At least for me, it redirects to a search page. Is this the article? One thing would make this web page even more excellent is if you'd list the full references, so that if a link doesn't work, you can try to find it yourself (if not on the web than maybe at a library); at least the abstract. Regards, Simon
    Response: No, that wasn't the article I was linking to although it does look like an interesting paper. The paper I'm refering to is An Assessment of Climate Feedbacks in Coupled Ocean–Atmosphere Models (Soden et al 2005). Thanks for letting me know about the bad link.
  17. Good discussion guys! It appears from the comments in general and reading of various paper quoted innthe Arguements,that the critical longer term measurements behind the IPCC consensus understanding of the CO2 forcing and amplification of the dominant water vapour greenhouse effect and feeed backs are not yet available. So although many suspect or actually believe that CO2 has a strong influence on pushing higher atmospheric temperatures, we dont actually know this to be true as a scientific fact. Arguements were made that increased CO2 has caused a little warming which then causes water vapour increases that sustains and amplifies the warming. But surely as Mizimi has rightly emphasised in many ways, water vapour is the dominant GHG as part of the natural evaporative cycle, so it's always been operating and the anthropogenic CO2 increase has just caused some minor additional warming! How does one differentiate the various contributions to GW? Fundimentally then what is the hard proof that CO2 is the dominant factor in GW?, leaving aside model driven assertions that are by default somewhat suspect because factors chosen or their levels may not be represenative of the real complexities involved. I have read the Carbon isotope proposal showing the increased carbon in CO2 mostly comes from burning fossil fuels-and that seems reasonable, but doesn't in itself prove AGW. Climatologists may be sure that their data tells the story well enough, but so far it does not convince me, though I admit to being worried by the evident belief expressed by many authors. However, asserting that CO2 must be the main cause of AGW because nothing else fits the bill- is not geood enough! The science on this must be more clear cut and definitive than at present, especially when major global policy affecting future world economic growth and energy use is at stake.
  18. Thanks for the comment, Bob Close. First, on the very narrow point of CO2 vs water vapor: As a thought experiment, imagine that you could somehow remove all the water vapor from the atmosphere, and prevent any more water from evaporating into the atmosphere (but you keep CO2 constant). Now, alternatively, imagine that you removed all the CO2 from the atmosphere, but kept water vapor constant. (Yes, both these alternatives are physically impossible on our world, but use your imagination). The world with no water vapor (but normal CO2) would get much colder than the world with no CO2 (but normal water vapor). So in that sense, you could say that "water vapor is more important than CO2 in warming the planet". However, neither of those is a realistic scenario. In the real world, we are doubling CO2 concentration. This has the effect of warming the planet somewhat. Water vapor then acts as a feedback, increasing the warming started by CO2. So we end up with our natural base temperature (t0) plus some new warming from CO2 (dt1) plus some additional new warming from the water vapor feedback (dt2). But in terms of assigning responsibility for that warming ("whodunit?") ... both dt1 and dt2 are a result of the increase in CO2. If we hadn't burned any coal or oil, we would still be at t0. The same would be true if there were some other event that caused warming (or cooling). For example, 65 million years ago a bolide impact on the Yucatan peninsula injected a lot of dust, aerosols, etc. into the stratosphere, which cooled the planet, which in turn reduced the amount of water vapor, which cooled the planet further. None of that cooling would have happened without the bolide impact, so even though some of the cooling involved water vapor, we still would say that all of the cooling was caused by the impact event. That initial cooling was followed by a rapid and extreme warming, caused first by CO and other shorter-lived greenhouse gases, then by CO2 which lasted for many centuries. Much of this new carbon came from the carbonate rocks that were vaporized at the impact site, injecting lots of carbon into the atmosphere. Once again, this warming was amplified by the water vapor feedback, but none of it would have occurred without the presence of CO, CO2, etc. So on both the downswing and the upswing in temperatures, water vapor acts as a feedback amplifying the change in temperature that is created by some other forcing (greenhouse gases, aerosols, solar variations, etc.)
  19. Ned wrote: "As a thought experiment, imagine that you could somehow remove all the water vapor from the atmosphere, and prevent any more water from evaporating into the atmosphere (but you keep CO2 constant)." But that's the rub, you could not prevent water from evaporating, at least not unless you also get rid of the ocean -- nay, all liquid and frozen surface water. Ned: "Now, alternatively, imagine that you removed all the CO2 from the atmosphere, but kept water vapor constant." A far more interesting thought experiment. If you instantaneously removed CO2 you could not keep water vapour constant because removing CO2 would reduce the temperature of the atmosphere, thereby reducing the amount of water vapour the atmosphere could hold, causing a portion to condense and precipitate out. This would in turn cause a further drop in temperature, and so on in an amplifying feedback loop. Temperature would eventually stabilize with substantially more ocean surface being frozen over, possibly even a global snow or slush ball earth state, greatly reducing albedo and cutting off a substantial portion of evaporation such that there would be relatively little water vapour in the atmosphere. In other words, CO2, which does not condense out of earth's atmosphere, makes it possible for there to be appreciable water vapour in the atmosphere, thus CO2 is, in the words of Richard Alley, the 'control knob' of the greenhouse effect.
  20. #94: Interesting! Now imagine that a decrease in water vapor in the atmosphere would cause fewer clouds to form. As a result, the Sun can warm the surface more than when clouds got in the way, which puts more water vapor back into the atmosphere to form clouds. Imagine a planet with no cloud cover. What would the surface temp be? If the answer is "hot", then clouds could rightly be said to have a cooling effect. If not, then the opposite would be the case. Which is it? Clouds have often been said to be where the biggest disagreement remains. Is the net effect of clouds to a positive or a negative feedback? I'm not aware that this has been settled.
  21. daisym, I believe that some types of clouds are understood to have a net cooling effect, and some a net warming effect. The difficulty, I believe, is in figuring out the relative contributions of those types of clouds in any given scenario. Realclimate has a post addressing clouds' role in albedo changes. It has another post on the role of aerosols in triggering cloud formation. And it has another post on the purported role of galactic cosmic rays in triggering clouds.
  22. daisym, there is a good explanation of clouds and climate at the NASA Goddard site.
  23. In a different thread, cruzn246 made this claim relating to Water Vapor concentration (after I stated it represented about 0.4% of the atmosphere): "Actually this is wrong. It ranges from 1 to 4% with the average being between 2 and 3%" Those figure are for surface value. In the entire atmosphere it's 0.4%. "but no one is really sure what that average is on any given day." True, but irrelevant. The water is already part of the weather system, contrary to fossil fuel CO2 which is being added to the atmosphere. "According to NASA, they say the increase in water vapor is probably playing a bigger part in warming now than CO2, but they will not put numbers on either as far as the amount each is contributing." Water Vapor acts as a positive feedback to CO2-caused warming. It has a bigger impact on GW than CO2, but we are not adding new water to the system. We are adding more CO2, which increases the heat, which causes more water to evaporate, which further raises temperature. We all know this. Why do you come here on your high horse and tell us things that we already know as if you had the "killer argument" against AGW? All you're doing is highlighting your own ignorance for all to see. "The difference in an atmosphere with a strong water vapor feedback and one with a weak feedback is enormous," Dessler said." Note the use of the word "feedback." Basically, this articles agrees with the science presented here, and disagrees with you. I know it's common for less experienced deniers to mistakenly provide evidence that goes against their position, but this one's pretty obvious... "I think all you need is a simple climate shift that has nothing to do with CO2 to put more water vapor in the air." Please provide evidence of such an unknown climate shift. Put up or shut up.
  24. "Please provide evidence of such an unknown climate shift. Put up or shut up." The shift of the PDO and NAO at about the same time in the late mid 70s put us into this warm spell. Good enough? There are signs they both may go negative again at the same time and that could put us right back into the type of weather we had in the middle of the century.
  25. @cruzn246: wow, you're really goign through the gamut of debunked arguments, aren't you. What's next, it's because of Solar Irradiance? The PDO is currently *negative*, why would it warm us? It's also a cyclical phenomenon that doesn't show a long-term trend, which we are experiencing. "Good enough?" Not by a long shot. It's Pacific Decadal Oscillation Instead of randomly posting graphs you clearly don't understand, I suggest you actually start learning some science. Again, it's clear you have no idea what you're talking about, and are simply trolling on this site.

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