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Climate Hustle

What is causing the increase in atmospheric CO2?

What the science says...

There are many lines of evidence which clearly show that the atmospheric CO2 increase is caused by humans.  The clearest of these is simple accounting - humans are emitting CO2 at a rate twice as fast as the atmospheric increase (natural sinks are absorbing the other half).  There is no question whatsoever that the CO2 increase is human-caused.  This is settled science.

Climate Myth...

CO2 increase is natural, not human-caused

that atmospheric CO2 increase that we observe is a product of temperature increase, and not the other way around, meaning it is a product of natural may be the Emily Litella moment for climate science and CO2 – “Never mind…” (Anthony Watts)

Simple Accounting

The easiest way to prove that the atmospheric CO2 increase is man-made is through a simple accounting approach (i.e. see Cawley 2011).  The equation for the change in atmospheric CO2 (ΔCatm) is

This says that if we ‘emit’ a ton of carbon by, say, triggering a volcano then the atmosphere will gain a ton. If we ‘absorb’ a ton of carbon by growing a tree, then the atmosphere loses a ton.  We can expand the equation by counting human emissions (HE) and absorption (HA) and natural emissions (NE) and absorption (NA) separately.

This works because carbon is additive. If a volcano emits a ton of carbon and a factory emits a ton then the atmosphere has gained two tons. This is a very simple balance sheet for the carbon cycle and fortunately there are ‘accountants’ who have measured some of these values for us.

Recently the amount of CO2 in the atmosphere has been rising at ~2 parts per million per year, or around 15 billion tons/year. Meanwhile  human emissions excluding land use change (like clearing or planting forests) are 30 billion tons per year. In billions of tons per year we have:

We can rearrange this:

Humans are also clearing rainforests and changing land use, but here we'll assume that human effects on absorption (HA) are not much different from zero, i.e.

So Natural Absorption (NA) must be bigger than Natural Emissions (NE). Nature is absorbing more CO2 than it is emitting. It is not causing atmospheric CO2 to rise at all - in fact it is acting to try and reduce atmospheric CO2, and thus the long term rise is entirely because of humans.

Ocean Acidification

The oceans are the Earth's largest carbon storage medium, so if the atmospheric CO2 increase were "natural", it would likely be coming from the oceans.  But we know the CO2 increase is not coming from the oceans, because the pH of the oceans is dropping (a.k.a. ocean acidification).

When CO2 is absorbed into a solution, it binds with a water molecule to form a molecule of carbonic acid:

CO2 + H2O = H2CO3

H2CO3 has a rather strong acidifying effect in that 95% of it turns into HCO3-.  This loss of an H+ ion causes the ocean pH to decrease (for more details on ocean acidification, see the OA no OK series).

In short, the fact that the pH of the oceans is decreasing tell us that they are absorbing more carbon than they are releasing, not vice-versa.

Oceanic CO2 Rising Fastest at the Surface

If CO2 were being driven into the ocean from the air, the oceanic concentration would rise fastest at the surface.  If CO2 were being expelled from the oceans, we would expect to see the opposite - decreasing concentrations at the surface.

The World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS) has observed that as we expect for CO2 being driven into the oceans, concentrations of CO2 in the oceans are rising fastest at the surface.

Atmospheric O2 is Decreasing

Burning carbon requires oxygen (O2), and when we burn an atom of carbon, the required oxygen becomes part of the CO2 molecule.  So if the CO2 increase is caused by burning carbon (fossil fuels), we would expect atmospheric O2 levels to decrease at the same rate.  And that's indeed what we observe (Figure 1).

Cape Grim O2

Figure 1: Atmospheric Oxygen Concentration observed from Cape Grim, Tasmania

There's no reason to expect that a natural release of CO2 would have any effect on atmospheric O2 levels.  On the other hand, the O2 concentration is changing exactly as we would expect from a fossil-fuel driven CO2 increase.

CO2 Rise is Smoother than Temperature

Some, most recently Murry Salby, have argued that the CO2 rise is in reponse to the temperature rise.  However, the temperature rise has been quite erratic (because there are many factors which impact the average global temperature, especially in the short-term).  If atmospheric CO2 changes were in response to temperature changes, then we would expect to see an erratic rise in CO2 as well.  Instead, the atmospheric CO2 increase is very smooth, similar to the increase in human CO2 emissions.

CO2 emission vs concentration

Figure 2: Human CO2 emissions (blue, left y-axis, Source: IEA) vs. atmospheric CO2 concentration (red, right y-axis, Source: Mauna Loa record)

Isotopic Signature

Carbon is composed of three different isotopes: carbon-12, 13, and 14.  Carbon-12 is by far the most common, while carbon-13 is about 1% of the total, and carbon-14 accounts for only about 1 in 1 trillion carbon atoms in the atmosphere.

CO2 produced from burning fossil fuels or burning forests has a different isotopic composition from CO2 in the atmosphere, because plants have a preference for the lighter isotopes (carbon-12 and 13); thus they have lower carbon-13 to 12 ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same carbon-13 to 12 ratio – about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average carbon-13 to 12 ratio of the atmosphere decreases.

Reconstructions of atmospheric carbon isotope ratios from various proxy sources have determined that at no time in the last 10,000 years are the carbon-13 to 12 ratios in the atmosphere as low as they are today. Furthermore, the carbon-13 to 12 ratios begin to decline dramatically just as the CO2 starts to increase — around 1850 AD. This is exactly what we expect if the increased CO2 is in fact due to fossil fuel burning beginning in the Industrial Revolution.

carbon 13 ratio

Figure 3: Atmospheric carbon-13 ratio observations from Cape Grim, Tasmania

These isotopic observations confirm that the increase in atmospheric CO2 comes from biogenic carbon, not from the oceans or volcanoes.  Some "skeptics" like Murry Salby argue that the carbon-13 ratio isn't unique to fossil fuels.  However, because the carbon-14 ratio has also decreased significantly (Figure 4), we know it's from old (fossil fuel) sources, not modern sources.  This is not new science either, it's something we've known for over half a century (Revelle and Suess 1957), and there  have been many studies confirming these results.  For example, Levin & Hesshaimer (2000):

"It has been erroneously argued that the observed atmospheric CO2 increase since the middle of the 19th century may be due to an ongoing natural perturbation of gross fluxes between the atmosphere, biosphere, and oceans. That the increase is in fact a predominantly anthropogenic disturbance, caused by accelerated release of CO2 from burning of fossil fuels, has been elegantly demonstrated through 14C analyses of tree rings from the last two centuries (Stuiver and Quay 1981; Suess 1955; Tans et al. 1979)."

14C ratio

Figure 4: Temporal change of carbon-14 ratio in tree rings grown at the Pacific coast (Levin & Hesshaimer 2000)

Settled Science

As you can see, there are many lines of evidence showing that the increase in atmospheric CO2 is due to human fossil fuel combustion.  Each one of these lines of evidence is very conclusive on its own, and when all put together, it's abundantly clear that the science is settled on this issue.

Intermediate rebuttal written by dana1981

Update July 2015:

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

Additional videos from the MOOC

Andy Skuce: The CO2 rise is man man-made

Interviews with various experts

Expert interview with Corinne Le Quéré

Last updated on 8 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page


Comments 1 to 29:

  1. {snip}
    Response: If you wish to discuss your egregious misinterpretation of Dr Knorr's article, do so in the thread in which you originally raised the point. There is no need to spam multiple threads. This is a first warning.
  2. Question on the part on "simple accounting":

    I assume if we set HE to 0, then NE-NA would result in a deltaC of 0, representing the pre-industrial equilibrum. This puzzles me: Why is nature (oceans, plants, soils) suddenly able to absorb 15 billion tons more CO2 with an atmosphere with ~400ppm, as opposed to the pre-industrial equilibrium with ~200ppm?

  3. Falkenherz @2.

    If we say there was pre-industrial equilibrium with 280ppm CO2 by volume, NE-NA=0, but the natural carbon cycle is still at work. NE=NA= 770 GtCO2 pa.

    Now we are at 400ppm having pumped something like 2,200 GtCO2 into the atmosphere over the previous century or so and seen a little over half of it get absorbed by the oceans and biosphere, if we stopped emitting tomorrow (HE=0), equilibrium will not be achieved for a millenium or so. The eventual level of atmospheric CO2 would be somewhere near 340ppm.

    Archer et al 2009 is usually seen as a pretty definitive study on the subject.

  4. Falkenherz to add to what MA ROger has already said, if you want a specific mechanism, the transport of carbon dioxide between the atmosphere and surface oceans is proportional to the difference in partial pressure of CO2 between air and ocean. Therefore if we increase the partial pressure of CO2 in the atmosphere (e.g. by burning fossil fuels) then this difference increases, and more CO2 passes from atmosphere to the ocean than in the other direction.  This causes the oceans to take up more CO2 until the partial pressures are in equilibrium again.

    David Archer has written a very good primer on the carbon cycle, which is well worth a read.

  5. Thanks for the answers and sorry for the double-post, no idea how that happened.


    [Dikran Marsupial] no problem, easily fixed.

  6. Oh Dear......



    3) or maybe it is greed:


    Simple Accounting Revisted "It's the Animals"

    The simple accounting demonstration that CO2 increase is manmade is pure crap.
    Animals and plants produce more than 220 GT of CO2 per year. Let’s just change a few words. Let humans be part of the nature term NE (as we are) and let a group of animal species A be the one that produce some extra CO2 by an amount of 30 GT (we can certainly find some species to be blamed). Then we have the same result,
    NE-NA = -15
    but now the added CO2 is blamed on animals, not humans. Why should humans be solely responsible for all CO2 production added to the atmosphere? Is this some sort conspiracy against humans? Why not share the blame among all species, animals and humans included?

    In any way, the NE and NA terms will balance in the future (CO2 will stabilize as it always did in history) and humans and animals will keep on living. As is well known, CO2 levels have been much higher in the history of the planet and life kept growing despite of it. I'm just sick of alarmists and skeptics bashing against those who have another perspective about this whole topic and who, ironically, are also skeptics. Skeptics against skeptics. How uglier can it get?


    [TD] Please do not use all caps, because it is the web equivalent of yelling.  Instead use the bold and italic formatting controls.

  8. Chris, plants take carbon out of the atmosphere.  Animals eat plants.  Animals breathe out CO2.  In general, animals are carbon neutral, just as human breathing is carbon-neutral.  Humans, however, are also digging up and burning billions of tons of carbon that has been stored in the Earth over hundreds of millions of years.  We're adding the carbon of the past to the present (and future).  

  9. Chris636

    You are not doing the accounting correctly.  It's not enough to simply produce CO2. You have to have a net exchange from one reservoir to the other.  

    So, for your example to work, the respiring organisms would have to cause a net loss of plant carbon to the atmosphere.  To match the observed  increase in atmospheric CO2, you would have to move about 250 petagrams of carbon from the terrestrial biosphere to the atmopshere (current - preindustrial atmsopheric CO2 = 850 Pg - 600 Pg).  

    That is 40-60% of current living terrestrial plant biomass (terrestrial plant C ~450 - 650p petagrams C according to the IPCC.)  About 2/3s of that deline in terrestrial biomass would have occured since 1970.  There is large uncertainty around estimates of plant biomass for sure, but you bet we would have noticed such a massive decline over such a short period of time.

    As indicated in the post, the CO2 accumulating in the atmosphere is highly depleted in 14C. We know it is therefore tens of thousands of years old because that is how long it takes for 14C to decay completely.  That age rules out everything except fossil fuels.  

    It is simple accounting in the end.  Really, scientists are not so stupid to miss something so obvious.  If individuals had been, you can bet their competitors would take them to task!

  10. Chris636:  Of course the CO2 level eventually will stabilize--when humans eventually run out of fossil fuels to burn.  The problem is that the ill effects of those high CO2 levels will get much, much worse, for a very, very long time in the time scale of human lives, civilization, and even species.  See RealClimate for a couple of relevant posts.

  11. Chris626 wrote "The simple accounting demonstration that CO2 increase is manmade is pure crap."

    so all of the worlds carbon cycle specialists are wrong, no hubris there then! ;o)

    "Let humans be part of the nature term NE (as we are)"

    This is just silly, if you define humans as part of nature the word "anthropogenic", "artificial" and ultimately "natural" have no meaning. 

    " let a group of animal species A be the one that produce some extra CO2 by an amount of 30 GT "

    The flaw in this argument is obvious.  The carbon dioxide that animals produce through respiration is directly (in the case of herbivores) or indirectly (in the case of carnivores) derived from plant matter, which is contructed from carbon dioxide taken out of the atmosphere.  Thus all animals merely return to atmosphere the carbon that was taken out of it via photosynthesis, and hence are essentially carbon neutral.  Now if you can identify an animal species where this is not the case and is increasing the amount of carbon moving through the carbon cycle, then you might have the beginings of an argument.

    "Why should humans be solely responsible for all CO2 production added to the atmosphere? Is this some sort conspiracy against humans? Why not share the blame among all species, animals and humans included?"

    because humans are the only animals that introduce additional carbon into the carbon cycle by extracting it (in the form of fossil fuels) from the lithosphere and burn it, which puts it in the atmosphere.

    "How uglier can it get?"

    Rather ironic thatyou should ask that, given you started your post by calling the work of eminent scientists "crap".

    It is sad that there are skeptics that can't even accept that the increase in atmospheric CO2 is due to anthropogenic emissions when the evidence is unambiguous and unequivocal.  If the natural environment (including all the animals) were a net carbon source, the atmospheric CO2 would be rising faster than anthropogenic emisssions as both nature and mankind would be net sources.  However we know for sure that this is not the case, the observations rule that out completely.

  12. I dont want to dogpile, but Chris626 is only looking at one of the lines of evidence presented (getting the accounting wrong). If it is "crap" then explain the isotopic evidence and ocean acidification.

    While the planet might have had higher levels in past, (but also different solar input) the problem is rate of change (because adaption takes a long time), not the final temperature.

  13. All: Please do not dogpile any future comments posted by Chris626. Dogpiling is both unnecessary and unseemly. Furthermore, it is prohibited by the SkS Comments Policy.  

  14. chris626: The first comment you posted on this thread was deleted by me because it did not comply with the SkS Comments Policy.

    Please note that posting comments here at SkS is a privilege, not a right. This privilege can be rescinded if the posting individual treats adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Please take the time to review the policy and ensure future comments are in full compliance with it. Thanks for your understanding and compliance in this matter.


  15. Sorry John..being a skeptic is fine but in this case you're incorrect..I'm not saying only mankind is contributing but I'm saying what they manufacture,since the Industrial revolution has contributed could it not?? btw I taught several volunteer FYI college classes at night several years back to mostly adults who had no idea what it was all saying to those who must deny "well I guess we'll just have to wait and see"..yep it's moving along a good deal faster than old mother nature had in mind..have a swell day..glad I found and joined this site..ciao

  16. Hi,

    I agree with the CO2, 13C and 14C could be attributed to fossil fuel burning. But the oxygen isotopes in the CO2 molecule are not mentioned in the explanations.

    Can anyone explain why CO2, 13C and 14C isotopes follow a trend while 18O isotopes of CO2 do not show a clear trend?

    atmospheric CO2 concentration and CO2 isotopes

    Data Sources:


    [RH] Fixed image width. Please keep your images down to 500px.

  17. jlfqam @16, total oxygen content in the atmosphere is showing a trend, as shown in this graph from the 2001 IPCC TAR:

    You will notice that the trend is in parts per million.  The graph you show of the d18O isotope is in parts per mill, ie, parts per thousand.  A reduction of 30 ppm over 10 years (as shown above),ie, three parts per hundred of the scale, will not register on a graph scaled in parts per thousand.

  18. Thanks for the figure of the evolution of atmospheric O2 concentration.

    A comparison of simultaneous variation O2(atm) with CO2(atm) can be seen in the plots from Scripps measurements

    for example.

    I am sorry, I did not explain properly the figures, the fourth plot refers to the Oxygen isotope composition of O atom in the atmospheric CO2 molecule,

    the isotope ratio 18O/16O, in this molecule refers to the ratio of 12C18O16O+12C18O2(less abundant) over 12C16O2, for example.

    According to two possible sources of CO2, biomass/fossil fuel burning or ocean sources: (equations are not stoichiometric)

    In the first case, burning, the oxygen source is atmospheric Oxygen (O2), which is produced by photosynthesis, and ultimately bears the isotope composition of the water used

    by photsynthetic organisms  during the water photolysis reaction: H2O=>O2 + (2H --->sugars (CnH2nO))

    combustion  #CnH(2n+x)+*O2=> #C*O2+H2*O

    #C has the isotope signature of the fuel organic Carbon, and is measured in the 13C and 14C plots in the previous posting.

    *O is the isotope signature of the O2 used in the combustion, and is represented in the 18O plot from Scripps CO2(atm) measurements.

    The second case, the marine source of CO2, has also isotope signatures

    remineralisation of organic matter from ocean bottom organic rich sediments $CnH(2n+x)+^O2=> $C^O2+H2^O

    $C has the isotope signature of the marine biomass remineralised, as it's of biogenic origin is depleted in 13C, and since it's old, it's mostly depleted in 14C, hence in principle it's difficult to distinguish from fossil fuel carbon

    ^O is the isotope signature of ocean waters, either deep or shallow, as bicarbonate or dissolved CO2 rapidly equilibrate with water.


    "C^O·O+H2^O<=>"C^O2+H2·O etc.

    In principle the measurement of the 18O isotope ratio in atmospheric CO2 should tell us which of the two sources is the dominant.

    Is this argument correct?

    Which one of the two sources can explain the plots in the previous posting 16.jlfqam

  19. jlfqam @18, fossil fuels are completely depleted in C14.  That is, there Δ14C = -1000 per mill, as per this chart:


    In contrast, the Δ14C of abyssal oceanic waters averages about -160 per mill, with minimum values of about -240 per mill (See figure 1 here).  Taking those minimum values, you would need an increase in atmospheric CO2 4.2 times greater than has been observed to obtain an equivalent reduction in atmosperic C14 to that which has been observed as a result of the combustion of fossil fuels.  Therefore the C14 evidence by itself is sufficient to show the primary source of the increased CO2 is from the combustion of fossil fuels.

    That being said, it is a bad practise to relly on a single indicator in making these sorts of determinations.  In fact there are at least 10 different lines of evidence that help us determine the source of the increase in atmospheric C14.  Some lines only provide evidence regarding a single potential source, while some provide relevant evidence for all four "major" theories.  Overall, only a fossil fuel source is not contradicted by any line of evidence.  Further, it is strongly supported by five of the ten lines of evidence, and given moderate support by a further two.  This evidence is discussed here, and summarized by this chart:

    I have not come across a discussion of Δ18O in this connection, but given the strength of the evidence from other sources, I would be flabberghasted were it to show anything different.

  20. Thanks Tom,

    I will post the answer in the corresponding forum page. I am sorry I did not find it first.

  21. Tom -

    In your post (29th November 2016, #19 above) you wrote:

    That being said, it is a bad practise to relly on a single indicator in making these sorts of determinations. In fact there are at least 10 different lines of evidence that help us determine the source of the increase in atmospheric C14.

    My understanding was that the levels (or actually ratio) of C14 were decreasing as they come from a fossil source. Am I wrong, or could you possibly have meant CO2?

  22. KalleH @21, you are quite correct that I intended to write "...determine the source of the [decrease] in atmospheric C14".  Thankyou for picking up on my error.

  23. In general, I believe that many things happen naturally. But naturally, cause a very little amount of effect if we compare to human activities. CO2 or carbon dioxide is a colorless gas consisting of carbon and oxygen. It occurs naturally in the atmosphere. Plants use it and animals also produce it in respiration. It is a major greenhouse gas emitted by fossil fuel combustion. Burning fossil fuels is one of the causes that make CO2 increase so we can't say that iCO2 came from natural because human is the one who controls everything even we can control that in next 50 years what we want our world gonna be like. The science researcher says that humans are emitting CO2 at a rate twice as fast as the atmospheric increase (natural sinks are absorbing the other half).Nature is absorbing more CO2 than it is emitting. So, the percent that CO2 increases in our world today caused by human activities whether directly or indirectly way. It has more effect than natural.

  24. MY question is this: Since the total annual amount of CO2 has increased on an annual basis, the amount of CO2 from Earth keeps going up as well. I've seen reports that say "our" Co2 is absorbed to the rate of about 55%. That is consistently evey year. If, as some assume, all of earth's CO2 is absorbed, in ever larger amounts, how come our 45% is always left over?

  25. chromedome49 @24,

    The 45% isn't written in stone. Using the MLO data for atmospheric CO2 and the emissions estimates from Global Carbon Project, the 45% value has remained pretty much the multi-decadal value since 1990. Over the period 1960-90, the Airborne Fraction had been slowly increasing from an early value of 35%.

    There are a lot of waggles along the way. Over the period since 1959, annual Airborne Fractions have varied from 20% all the way up to 80%. The El Nino is one big factor in these waggles, as are big tropical volcanoes. Taking multi-year averages, the percentage is still a bit waggly. After the rise to 1990, there was a short sharp dip caused by the eruption of Pinatubo in the early 1990s, a rise into the 2000s due to the high incidence of El Ninos then a dip into the 2010s due to all the La Ninas.

    Where the Airborne Fraction goes in coming years? If we begin to reduce the acceleration of our emissions, it should start to drop, and drop quicker if our emissions begin to fall. Mind, all the waggles will prevent a clear sight of any such a drop for some time.

  26. I'm having a little trouble getting some math to come out right.  CO2 concentration is rising by 2 ppmv/year.  The net flux is given as 15 gigatons/year.

    For the surface area of the earth I take a radius of 6.4e6 meters to get an area of 171e12 m2.  I convert 14.7 lb/in2 atmospheric pressure at sea level to a metric value of 10.35e3 kg/m2 or 10.35 ton/m2.  When I multiply those together, it comes out to 1.771e15 tons for the total weight of the atmosphere.  One ppm of that would weigh 1.771e9 tons or 1.771 gigatons.

    The atmosphere is primarily molecular nitrogen.  Each molecule has a weight of 28.  The weight for CO2 is 44.  This brings the weight of one ppm by volume up to 2.78 gigatons.  The increase of concentration given in the article, 2ppmv/year would represent a weight of 5.56 gigatons.  This is low by a factor of almost three from the value of 15 given in the article.

    What am I missing?

  27. @26

    Area wrong.  Should be pi times 4 , not pi times 4/3 .

  28. Eclectic

    Thank-you.  It has been a few years.  I was confusing the area and volume formulas.

  29. The arguments presented are helpful and fairly comprehensive, but I was surprised the author, dana1981, did not address what, in my view, is the most important scientific publication on this issue: “The phase relation between atmospheric carbon dioxide and global temperature” by Ole Humlum, Kjell Stordahl and Jan-Erik Solheim in Global and Planetary Change 100: 51-69, 2013. These authors showed, using published temperature time series from multiple sources and global CO2 and anthropogenic CO2 data that, for the years 1980 to 2011:

    1. There was a good temporal correlation between global CO2 and ocean temp, land temp, global temp and lower troposphere temp BUT the global CO2 FOLLOWED the ocean temp, then the land temp, then the lower troposphere temp, in that order, with lags of 9-12 months.

    2. In contrast, there was poor temporal correlation between anthropogenic CO2 emissions and both global CO2 and temperature.

    3. While anthropogenic CO2 was emitted overwhelmingly from the northern hemisphere, the time sequence of ocean temperature variation commenced in the Southern Hemisphere, reasonably close to the equator, then spread north and south to the poles, always preceding the global CO2 time sequence.

    These carefully determined temporal sequences and correlations, based squarely on the published temperature and CO2 data, clearly indicate a causal sequence in which global temperature changes PRECEDE global CO2 changes by 9-12 months, commencing with changes in the ocean surface temperature, then the land temperature, then the lower troposphere temperature. These observations are the complete OPPOSITE of what should be expected if anthropogenic CO2 emissions were driving both the global CO2 levels and then causing a secondary increase in temperatures.

    So, while I appreciate the energy balance and other arguments advanced above, causality requires a demonstrated temporal sequence of changes that the data I describe here simply do not support. I would be very interested in your explanation for these observations.


    [TD] Humlum is wrong. Type "Humlum" into the Search field at the top left of this (or any) SkepticalScience page.

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