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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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Further viewing

Further reading

Denial101x video(s)

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Additional video from the MOOC

Expert interview with Steve Sherwood


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Comments 351 to 375 out of 392:

  1. I have some questions if some kindly soul would shed some light on:

    1) What are the limitations on the positive feedback influence of water vapour. What are the physical constraints on run away?

    2) There appears to be much research on the potential impact and technological feasibility of extracting CO2 through reforestation or industrial extraction with everything from algal bioreactors to metal organic frameworks (MOFs). Presuming technological feasibility, in terms of the physical constraints, might some kind of industrial extraction of water vapor be possible on a scale capable of influencing atmospheric temperatures?

    (If not, another side question I have is: does that mean there is no limitation to how much water vapor can be removed from the atmospheric system i.e. if you had a technological non-fossilfuel way to remove the water efficiently at relevant humidity levels (eg to supply water for water scarce areas for drinking, agriculture and dedesertification), could that be done limitlessly (presuming the technological and energetic means to do so). Eg improving and massively scaling up a material and process like this:

    NB This link refers to a scientific study describing an engineered "super moisture‐absorbent gel" that is experimentally shown to be able to remove 20litres of water per kg of material within a 24hour period.)

    Many thanks (from a doctor worried about his kids and trying to understand this science.

  2. Jiminy @351 , I can see that a "super-absorbent gel" could have some uses when deployed in a a confined space.  Rather like silicate crystals currently used for that purpose.

    Large scale :- the Earth has 350 million square Km of seawater surface . . . and so the "engineered" removal of 20 litres (or 20 trillion litres) of water from the air would be a futile exercise.  Within a few days, that mass of water vapor would have been replaced by natural evaporation from the ocean.

    In rough figures, the Earth surface temperature has increased 1 degreeC since pre-industrial times, and as a consequence of that warming, atmospheric water vapor has increased 7% (which has had its own GreenHouse effect, of course).   But the way to reduce that 7% . . . is to lower the temperature (which is achieved by lowering the the level of GreenHouse Gasses of the non-condensing sort i.e. CO2 primarily and to a lesser extent, methane etc.)

    Alternative temperature-lowering methods would be "geo-engineering" such as injection of sulfate particles into the stratosphere (to act as reflective aerosols).  But that is highly problematic, indeed.

    In practical terms, we have to aim at the control of CO2.  Nothing much else would work, in the medium-to-long term.

  3. Jiminy Cricket,

    You have interesting questions.

    Doubling the concentration of Carbon dioxide in the atmosphere would by itself raise global temperatures by about 1C.  Additional feedbacks, of which water vapor is one of the biggest, would then kick in to cause about 2C more increase in temperature.  The exact amount of the total feedbacks are not well known and the accepted range is a total of 2C-4.5 C (including the 1C from carbon dioxide ).  There is a long tail of possiblle larger increases but the 2-4.5C range is reasonably solid.  I will call it 3C for this discussion.  We all hope it is not more than 3C per doubling.

    After the 3C increase per doubling the temperature stabilizes.  Scientists agree that the feedbacks will not continue forever in a runaway temperature increase.  Some feedbacks, like melting of ice sheets, take a long time to reach equilibrium.  On the other hand, the climate changes we see with just 1C of temperature increase, like last weeks heat wave in Europe and California fires, are greater than expected.  Scientists agree that we should keep warming as low as possible.

    Industrial extraction of water from the atmmosphere is really not pratical.  So much water evaporates from the ocean and the land every day that it would be impossible to extract enough water vapor to affect the atmospheric concentration significantly.  Work is being done on ways to extract CO2 which is a gigantic task but smaller than extracting water.

    Your reference talks about extracting water from the atmosphere for drinking in areas where water is very short like in the Sahel in Africa.  This water extraction would be too small to affect the concentration of water in the atmosphere but very valuable to people who live in areas with little drinking water.

  4. Jiminy Cricket @351,

    You ask 1) What are the limitations on the positive feedback influence of water vapour. What are the physical constraints on run away?

    The concept of a "run away" climate isn't always well defined. There can be feedbacks (big & small) that once started will not stop until they reach their limit. These are the climate's tipping points and perhaps the biggest of these would be the idea of a "run away" climate turning the Earth's climate into something like that of Venus. Famously, Jim Hansen, in his 2009 book 'Storms of My Grandchildren' said:-

    "After the ice is gone, would Earth proceed to the Venus syndrome, a runaway greenhouse effect that would destroy all life on the planet, perhaps permanently? While that is difficult to say based on present information, I've come to conclude that if we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty."

    Hansen has since corrected that, saying (here in 2017):-

    "One flaw in my book Storms of My Grandchildren is my inference you can get runaway climate change on a relatively short timescale. You have to get rid of the ocean before you get to a Venus situation, and that requires you getting the water to escape. That took hundreds of millions of years for that to occur on Venus. You could certainly get to a disastrous situation without getting rid of the ocean, but if you want to go to a Venus-type situation, then you have to lose the ocean. Venus did. Hydrogen isotopes on Venus do indicate that it once had a lot of water, but doesn’t now."

    There is a more scientific account by Hansen on this somewhere but at the moment it doesn't come to hand. The mechanism for losing the planet's water is to increase global temperature enough that it wipes out the tropopause allowing water vapour into the high atmosphere and thus a route out into space.

    The reason you need to get rid of the water is that the increasing evaporation/rainfall with rising surface temperature becomes a major cooling mechanism. (It presently constitutes 16% of the upward energy flux at the surface.) A further consideration is that if the primary forcing was due to CO2, that increased rainfall would cause increased rock-weathering and that in turn cause increased CO2 draw-down.

  5. Water vapor is the dominant greenhouse gas, not CO2. The reasons include 1.) water absorbs infrared light much better and across a broader spectrum than CO2 which prevent the Earth from cooling off and 2.) the density (i.e. percentage) of water vapor in the atmosphere is much greater than CO2.  Physics says that the higher the density a chemical in a medium the higher percentage of light it will absorb.  


     Please see the graph from the NASA graph, both H2O and CO2 absorb in the infrared (12 - 14 micrometer) range.  The water vapor window is only slightly affected. Also keep in mind that the concentration of CO2 in the atmosphere is only a fraction of H2O. 

    This is why water vapor is the dominant greenhouse gas. 

  7. Water vapor absorbs more solar energy than CO2 per molecule.  There are many more molecules of H2O in the atmosphere than CO2.  If the amount of absorbed solar energy exceeds the amount of thermal infrared energy that escapes into space, temperatures will rise.

    Burning one molecule of methane produces one molecule of CO2 and two molecules of H2O.  Similar chemical product ratios for burning gasoline, coal, and deisel. 

  8. Wowzee , if you are talking about the "GreenHouse" Effect keeping the Earth's surface warmer than freezing point . . . then certainly the effective strength of H2O's GHEffect is larger than CO2's GHEffect.   This has been known for a very long time.  Yes, a very long time.

    It may be best if you stop thinking in terms of H2O being the "dominant" GHE gas.   It is not.   Or rather, in using dominant , you make a misleading & poor choice of words — if you are trying to mean that it is H2O which dominates or controls the situation.

    A horse is far stronger / heavier / more powerful than its human rider.   But it is the lightweight rider that dominates/controls what the horse does.

    So too, the CO2 controls the climate (along with control by changes in solar output & levels of reflective aerosols, of course).

  9. Please update the link to the article from Santer 2007 -->


    [PS] Thanks very much for that. I have updated the Held 2000 link in the rebuttal as well.

  10. If this were the case, wouldn’t you expect average temperatures in humid parts of the world (like rainforests) to be higher than drier areas (like deserts) at the same latitude?

  11. JamesKL , the answer is more complex, because there are different cases.

    For the classic "hot desert" (e.g. the Sahara, near the Tropic of Cancer) then it's true that the adjacent equatorial forests have a higher temperature at night.   During the daytime, the forests are cooler ~ presumably from the evaporative cooling effect ~ but I stand to be corrected if you have some good official data saying otherwise.  Since the air temperature is measured at 200cm altitude, you get variation according to shading from the forest canopy versus open areas of (moist) grasses/shrubs.   But then we get to the question of day/night averaging & how often in 24 hours the temperature is measured for calculating the average.  And seasonal or summer vs winter average temperatures for desert/forest.

    Then there's the case of a "cold desert" (e.g. the Gobi in Mongolia) compared with adjacent coastal forests having much higher rainfall.   The Gobi is indeed cold at night, and the coastal forests warmer.   But during the daytime . . . do you have any official temperature figures?  I could imagine if you scouted around, you could find some contradictory desert vs forest (or grassland) cases.  

    Difficult enough to find nicely matched cases, of similar latitude / altitude / ocean proximity / or exposure to prevailing or seasonal winds & rainfall.  (Monsoonal rain, or annually well-distributed rain.)

    To boil your question down, and over-simplify : you have to balance daytime evaporation in well-vegetated areas, versus nighttime cooling in dry (deserty) low-humidity areas.   So I am not giving a black-and-white answer to your original question ~ but I hope you can take consideration of the underlying physical principles involved.

  12. Eclectic - The Gobi desert is high in attitude, so will be colder. 

    I have found three places at similar latitudes, with average annual temperatures:

    Midway Atol (Pacific Ocean): 28 °N, 22 °C

    Taipei, Taiwan: 25 °N, 22 °C

    Adrar, Algeria: 27 °N, 24 °C

    Despite Adrar being at a slight altitude (258m), and a very dry climate compared to the other two, its the average temperature is higher! I can not think of anything related to prevaling winds cooling Taipei or Midway Atol which have humid climates.

  13. JamesKL , I am not clear whether you are meaning average temperature for daytime, for nighttime, or for a strict average over the 24 hour day.  Then there are the monthly or seasonal averages (or for "annual average" ~ which is almost a meaningless concept for temperate regions).

    Speaking generally, deserts are "pale" (high albedo = high reflection of sunlight energy) . . . and rainforests are dark, low albedo regions, which absorb more sunlight energy ~ nevertheless much of their temperature difference comes from the cooling effect of evaporation from vegetation. And for deserts at night, the dryness of the land & air means more heat is lost to space.

    Thermometer temperatures are one thing.  But humans' sensation of regional temperature will be perceived according to the extremes of daytime highs and overnight lows, and we tend not to notice those periods when it's "comfortable".  As you know, a high-humidity "hot" day (or night) will be felt as hotter.

    I would imagine that the town of Adrar is quite pleasant, part of each day at least!  Except when the weather produces heat wave conditions

  14. My first post, may it be interesting.  I read these two posts concerning water vapor and GHG. My comment is that for the current level of 409 ppm, there must have been a much higher corresponding amount of water vapor emitted by burning fossil fuels (putatively how the CO2 is formed, eh?).  Now there is a small exception of coal.  However, since a given hydrocarbon has more Hydrogens than Carbon, the worst ratio being Methane 4:1 H:C and thus 2 molecules H2O to 1 molecule CO2 perhaps this will make up for coal then.  So given the increase in water vapor, where is this considered?  To me it seems that some scientists have oversimplified or missed or ignored it.  It is far too easy to follow CO2, but not so easy to follow increased atmospheric water vapor until it enters the water cycle of ecosystem. 

  15. Faerr @364 ,

    it would be an interesting exercise if you made a quick back-of-envelope calculation to give a rough quantification of what you are proposing.

    Use the burning of coal/oil at an annual rate of 10 Gigatons of carbon ~ each carbon atom would liberate about 2 atoms of hydrogen (and hence 1 molecule of water vapor.   Thus (from atomic weights) . . . carbon 12 grams would produce 18 grams of water.

    So, 10 Gigatons of carbon producing 15 Gigatons of water vapor annually (but probably a shade more, allowing for coal impurities and shorter-chain oily compounds).   Let's round up to 20 Gigatons of water injected as vapor into the atmosphere, annually.  You might have to subtract a bit from that (but how much?) to allow for coal/oil burnt in wintry & far northern regions, where you might expect much vapor to condense and "rain out" rather quickly.  And the purer & dryer the coal, the less the water liberated.

    But for simplicity, let's stay with 20 Gigatons water = 20 cubic kilometers of liquid water, annually.

    I can't vouch for Wikipedia's accuracy, but Wiki says the world's annual precipitation is 505,000 cubic kilometers.  In other words, so large that it seems completely justifiable for the scientists to choose to ignore the 20 cubic kilometers.  ( I am without coffee, so please check with your own pen & envelope.)

  16. faerr - you seem to be missing the point that water vapour condenses out. No matter how much water you push up into the atmosphere by any means, the amount of water that the atmosphere will hold is dependent on the temperature. Once saturation is reached, then it just precipitates out. Burning FF does deplete O2 in atmosphere while making that water and I guess it makes an extremely tiny contribution to sealevel rise. (20Gt/y would be about 0.2mm)

  17. Even lower than 0.2mm

    360Gt for 1mm sealevel rise?  Call it close to 0.05mm/year.

  18. Agree Eclectic - shouldnt be trying to do this in a hurry.

  19. I am on a steep learning curve about global warming and would appreciate some responses to my question regarding the impact of water vapor.

    There is a view among many analysts that global warming is a top-down phenomenon.  The heat loss to the universe is determined by the spectrum of infrared at the top of the troposphere.  The temperature at Earth's surface is then determined by the convection and radiation heat transfer needed to establish an adiabatic lapse rate.

    This may seem counterintuitive; like the tail wagging the dog.  But it makes sense to me.  The analogy I make is the way power is often controlled in a nuclear electric plant.  Rather than withdrawing control rods to increase power and thereby produce more steam; what actually is done is to demand more steam, which lowers reactor temperature and produces more power.  The tail wags the dog.

    What this top-down perspective does is reduce our view of the importance of water vapor, because there is much less vapor at the top of the troposphere.  This is illustrated by figure 2.5 on page 23 of Houghton. "Global Warming, the complete briefing."  There is also a good diiscussion at BarrettBellamyClimate. com, "Emissions to Space."

    I would appreciate feedback on my interpretation of this phenomenon.  Thanks in advance.  --RichieB

  20. Ritchieb,

    Your description is similar to my understanding of the greenhouse effect.  

  21. The levels of water vapor has decreased since 1948. See studies of NOAA. This is a serious flaw in the models used by IPCC as they assume that humidity increases when CO2 levels increase. This has not been shown. The tiny part of CO2 can't by itself increase temperature it is all based on an increase in humidity. 


    [DB] In this venue, claims must be accompanied by citations to credible sources.  Please support your claims with those citations.

  22. pbezuk:

    You've managed to bring at least four errors into a four-line paragraph. That's impressive.

    1. NOAA publishes many studies. As the moderator has suggested, you aren't going to be believed unless you can point us to at least one NOAA study that says what you claim. Empty assertions don't count for much here. Try this post to read more.
    2. The 3-dimensional general circulation models that the IPCC summarizes most certainly do not assume that humidity rises in response to CO2. They estimate evaporation into the atmosphere, and they estimate precipitation back to the surface, and they move atmospheric moisture (in all forms) from one geographic location to another (n three dimensions) in response to atmospheric motions. Local (and therefore global) humidity levels are a response to the physics incorprated into the models. Its an output, not an input.
    3. Refering to the "tiny part of CO2..." is a fallacy. Try reading the following page to see the error in your statement: CO2 is a trace gas.
    4. "It is all based on humidity" is also wrong. If models are forced to keep constant humidity (yes, this has been done as far back as the 1960s) then warming still occurs. If humidity is allowed to change (not assumed - allowed), then the temperature rise is roughly doubled due to feedback. Try this post for more information.

    Please place any follow-ups to any of these points on the appropriate pages.

  23. pbezuk: Relative humidity over land has decreased, as predicted by the models; the slower cooling of the oceans has shifted some humidity to the waters. The specific humidity, the total amount of water in vapor form, has on the other hand increased again as predicted, with resulting increases in precipitation and flooding. 

    Your post is simply wrong. 

  24. there are a too many comments on here to review, maybe I misse it, but can anyone explain how irrigation is NOT a significant contributor to greenhouse effect? The evaporative cooling is often cited as a climate cooling effect which is incorrect, as the energy used in evaporation is merely transported elsewhere and released during condensation. The albedo of the otherwise arid lands is changed and the re-radiation to space is diminished As heat sinks into the ground more. So the constant irrigation of millions of ha of land worldwide does not constitute a "short lived" WV effect, it is a constant significant factor. Any land that need to be irrigated at all is contributing. Plus, all the hydrocarbons burned created a continuous stream of WV. 

  25. @374. SkepticalBrian

    Briefly, infrared is lost to space high in the troposphere where there is no water vapour.  Adding CO2 causes this level to rise, where it is cooler, so less energy is radiated, causing Earth to warm. Water vapour certainly warms the surface and the atmosphere but it is CO2 and other non-condensable gases that govern the energy balance. 

    Have a look here too:

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