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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Empirical evidence that humans are causing global warming

What the science says...

Select a level... Basic Intermediate

Less energy is escaping to space: Carbon dioxide (CO2) acts like a blanket; adding more CO2 makes the 'blanket' thicker, and humans are adding more CO2 all the time.

Climate Myth...

There's no empirical evidence

"There is no actual evidence that carbon dioxide emissions are causing global warming. Note that computer models are just concatenations of calculations you could do on a hand-held calculator, so they are theoretical and cannot be part of any evidence." (David Evans)

The proof that man-made CO2 is causing global warming is like the chain of evidence in a court case. CO2 keeps the Earth warmer than it would be without it. Humans are adding CO2 to the atmosphere, mainly by burning fossil fuels. And there is empirical evidence that the rising temperatures are being caused by the increased CO2.

The Earth is wrapped in an invisible blanket

It is the Earth’s atmosphere that makes most life possible. To understand this, we can look at the moon. On the surface, the moon’s temperature during daytime can reach 100°C (212°F). At night, it can plunge to minus 173°C, or -279.4°F. In comparison, the coldest temperature on Earth was recorded in Antarctica: −89.2°C (−128.6°F). According to the WMO, the hottest was 56.7°C (134°F), measured on 10 July 1913 at Greenland Ranch (Death Valley).

Man could not survive in the temperatures on the moon, even if there was air to breathe. Humans, plants and animals can’t tolerate the extremes of temperature on Earth unless they evolve special ways to deal with the heat or the cold. Nearly all life on Earth lives in areas that are more hospitable, where temperatures are far less extreme.

Yet the Earth and the moon are virtually the same distance from the sun, so why do we experience much less heat and cold than the moon? The answer is because of our atmosphere. The moon doesn’t have one, so it is exposed to the full strength of energy coming from the sun. At night, temperatures plunge because there is no atmosphere to keep the heat in, as there is on Earth.

The laws of physics tell us that without the atmosphere, the Earth would be approximately 33°C (59.4°F) cooler than it actually is.

This would make most of the surface uninhabitable for humans. Agriculture as we know it would be more or less impossible if the average temperature was −18 °C. In other words, it would be freezing cold even at the height of summer.

The reason that the Earth is warm enough to sustain life is because of greenhouse gases in the atmosphere. These gases act like a blanket, keeping the Earth warm by preventing some of the sun’s energy being re-radiated into space. The effect is exactly the same as wrapping yourself in a blanket – it reduces heat loss from your body and keeps you warm.

If we add more greenhouse gases to the atmosphere, the effect is like wrapping yourself in a thicker blanket: even less heat is lost. So how can we tell what effect CO2 is having on temperatures, and if the increase in atmospheric CO2 is really making the planet warmer?

One way of measuring the effect of CO2 is by using satellites to compare how much energy is arriving from the sun, and how much is leaving the Earth. What scientists have seen over the last few decades is a gradual decrease in the amount of energy being re-radiated back into space. In the same period, the amount of energy arriving from the sun has not changed very much at all. This is the first piece of evidence: more energy is remaining in the atmosphere.

 

Total Earth Heat Content from Church et al. (2011)

What can keep the energy in the atmosphere? The answer is greenhouse gases. Science has known about the effect of certain gases for over a century. They ‘capture’ energy, and then emit it in random directions. The primary greenhouse gases – carbon dioxide (CO2), methane (CH4), water vapour, nitrous oxide and ozone – comprise around 1% of the air.

This tiny amount has a very powerful effect, keeping the planet 33°C (59.4°F) warmer than it would be without them. (The main components of the atmosphere – nitrogen and oxygen – are not greenhouse gases, because they are virtually unaffected by long-wave, or infrared, radiation). This is the second piece of evidence: a provable mechanism by which energy can be trapped in the atmosphere.

For our next piece of evidence, we must look at the amount of CO2 in the air. We know from bubbles of air trapped in ice cores that before the industrial revolution, the amount of CO2 in the air was approximately 280 parts per million (ppm). In June 2013, the NOAA Earth System Research Laboratory in Hawaii announced that, for the first time in thousands of years, the amount of CO2 in the air had gone up to 400ppm. That information gives us the next piece of evidence; CO2 has increased by nearly 43% in the last 150 years.

 

Atmospheric CO2 levels (Green is Law Dome ice core, Blue is Mauna Loa, Hawaii) and Cumulative CO2 emissions (DOE Data Explorer). While atmospheric CO2 levels are usually expressed in parts per million, here they are displayed as the amount of CO2 residing in the atmosphere in gigatonnes. CO2 emissions includes fossil fuel emissions, cement production and emissions from gas flaring.

The Smoking Gun

The final piece of evidence is ‘the smoking gun’, the proof that CO2 is causing the increases in temperature. CO2 traps energy at very specific wavelengths, while other greenhouse gases trap different wavelengths.  In physics, these wavelengths can be measured using a technique called spectroscopy. Here’s an example:

Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapor is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

The graph shows different wavelengths of energy, measured at the Earth’s surface. Among the spikes you can see energy being radiated back to Earth by ozone (O3), methane (CH4), and nitrous oxide (N20). But the spike for CO2 on the left dwarfs all the other greenhouse gases, and tells us something very important: most of the energy being trapped in the atmosphere corresponds exactly to the wavelength of energy captured by CO2.

Summing Up

Like a detective story, first you need a victim, in this case the planet Earth: more energy is remaining in the atmosphere.

Then you need a method, and ask how the energy could be made to remain. For that, you need a provable mechanism by which energy can be trapped in the atmosphere, and greenhouse gases provide that mechanism.

Next, you need a ‘motive’. Why has this happened? Because CO2 has increased by nearly 50% in the last 150 years and the increase is from burning fossil fuels.

And finally, the smoking gun, the evidence that proves ‘whodunit’: energy being trapped in the atmosphere corresponds exactly to the wavelengths of energy captured by CO2.

The last point is what places CO2 at the scene of the crime. The investigation by science builds up empirical evidence that proves, step by step, that man-made carbon dioxide is causing the Earth to warm up.

Basic rebuttal written by GPWayne

Addendum: the opening paragraph was added on 24th October 2013 in response to a criticism by Graeme, a participant on the Coursera Climate Literacy course. He pointed out that the rebuttal did not make explicit that it was man-made CO2 causing the warming, which the new paragraph makes clear. The statement "...and humans are adding more CO2 all the time" was also added to the 'what the science says section. 


Update July 2015:

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

Last updated on 12 July 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments 376 to 400 out of 417:

  1. " I sense that few people have more qualifications to discuss the subject than Dr. Michaels."

    This is an astonishing statement and does not give me confidence about your discernment skills. What informs this opinion? Michaels is disinformer who has done no climate science at all. His career has been paid work to try and support the idea that tobacco smoke isnt harmful and climate isnt a problem. Science asks open-ended questions. Michaels is only doing motivated reasoning - ie starting with a conclusion and trying to manipulate the public to believe it. If you are looking for how to find truth, then perhaps brush up on some critical thinking skills.

    As to idea that CO2 isnt from humans. 3 main lines of evidence. 1/ we know how much we have emitted. 2/ Burning carbon reduces oxygen and observed oxygen drops fit. 3/ changing isotopic signature in atmosphere is consistant with fossil origin.

    All this in the summary of peer-reviewed science, known the IPCC WG1 report. Try reading that instead of misinformers.

    Additional question. if you found a source of information was duping you, would you continue to read from that source because you like their conclusions, or would you reject and look for more trustworthy sources?

    Humans are terribly prone to motivated reasoning and very intelligent people are extremely good at doing it. That is the reason why science values peer-review so much. It easy to fool yourself. Your peers, especially those who dont like your conclusions, are more likely to spot your problems. If your sources dont quote peer-reviewed research, then maybe you should ask why. (And watch out, because certain websites quote peer-reviewed research but say that its conclusions are diametrically opposite to what the paper really says, safe in knowledge that their audience of pseudo-skeptics are not going to check).

  2. Oh and glance at that same IPCC report would show you that scientists do indeed consider effects of deforestation etc. It is discussed in forcing as "landuse change", when you can find the peer-reviewed papers that quantify the effects.

  3. Thanks.

  4. Good forest mgt. is NOT deforestation. Deforestation is the destruction of the forest for some other purpose, like a huge solar farm- which causes that landscape to no longer sequester carbon nor produce oxygen. Without good forest mgt. which means some timber cutting based on proper silviculture- you can't live in wood houses with wood furniture and with paper products. And there are "environmentalists" in Massachusetts (who strongly influence others around the USA) who really do want to stop all forest mgt. since they can't grasp the consequences and they can't grasp that good forest  mgt. will NOT have to result in less carbon stored in forests.  Most forests are in poor condition from past bad logging practices. The way to improve them so they can do a better job of carbon sequestering and oxygen production is through silviculture. Look it up. As for peer reviews papers- I've been a professional forester since Nixon was in the White House so they have nothing on me. I find it ironic that this site is supposed to be about having a skeptical attitude but I find that those who swear by the idea that humans are the cause of global warming, while enjoying criticizing anyone who doesn't believe that, have no sense of skepticism of the the IPCC. I'd suggest a true skeptic will be skeptical of just about everyone including scientists, all religeons, all governments, all organizations. I certainly am. So, I'm not a "deniar" but I don't think the IPCC has all the answers either. As for Patrick Michaels, all I see so far is that he is a lobbyist- I see little commentary on what he has actually said. Since he was once the national president of the associaton of state climatologists- I find it odd that some people consider him not to be qualified to offer his thoughts. Instead, you use an "ad hominem" with your "This is an astonishing statement and does not give me confidence about your discernment skills." Personal insults aren't convincing.

    Response:

    [TD] I gave you two links to specific factual rebuttals to Michaels’s claims. Your ignoring of them while repeating your claims qualifies as sloganeering, which is prohibited on this site.

  5. Firstly, let me apologise for the insult, which it was. I still find the statement extraordinary but my response was over the line.

    Your comment about logging might be true but utterly irrelevant. What the science does is study what has been done, good or bad, and then log what its effect on climate parameters are. My point is that climatalogical effects from human land-use activity was accounted for and plainly visible in the reports.

    In my experience of working in science in for 40 year, is that scientists are most skeptical people I know. On this site, we dont swear by some predicate on global warming, we swear by whereever the evidence takes it. Much of my working life has been in coal then oil, and I can assure that evidence disproving a risk from CO2 emissions would be most welcome. My government has effectively cancelled funding of fossil fuels and so my colleagues and I have been reassigned.

    What we do scoff at is garbage pulmagated by disinformation which distort or malign science for generally political and/or ideological goals.  For more detail on what Michaels has said, Try here. Remember too that the IPCC only summarizes what the published science has stated. If you think it has got it wrong, then where is your evidence for that? If it doesnt come from an area where you have specialist domain knowledge, then it had better be peer-reviewed. Science commentary from institutions created to push a political ideology has to be especially suspect.  As I wrote here, liberatians in particular seem have a problem with climate mitigation solutions. The correct response however isnt "I cant see a solution that is compatiable with my ideology, ergo problem cant exist".

  6. "daring to imply that some who work in academia and governement may also have selfish reasons for their expressed opinions"

    Firstly, I generally find this to be an argument by those wishing to cast doubt on science when they have cant find other any other good cause. Secondly, given what I know of how science is funded in USA (at least the NSF), then I find it pretty difficult to see how these selfish reasons work in that framework. As for selfish reasons of government, that is getting into conspiracy theory territory, especially given the antipathy to AGW of current and former governments.

    Real skepticism about climate science has to provide evidence. That is what skepticism is about and that is what this site is about. Real skepticism backed by real data is very welcome.

  7. More links to issues with Michaels.

    https://skepticalscience.com/skeptic_Patrick_Michaels.htm
    LINK

    Response:

    [DB] Shortened URL

  8. Hi everyone!

    I've been away on a vaction and while away I found this news article pop up on my Android.

    HomeWorld News Finnish study finds ‘practically no’ evidence for man-made climate change

    The human caused climate change deniers are jumping all over this!  How can this study be credible? 

  9. Geez!

    Non-peer-reviewed manuscript falsely claims natural cloud changes can explain global warming

    Flawed Reasoning: The authors' argument claims a correlation between cloud cover/relative humidity and global temperature proves that the former caused the latter without investigating whether they have the relationship backwards.

    Inadequate support: The source of their claimed global cloud dataset is not given, and no research on their proposed mechanism for climate change is cited.

    Fails to provide correct physical explanation: The manuscript incorrectly claims that the rise of atmospheric carbon dioxide is caused by release from ocean waters. It also provides no explanation for the claim that an increase in relative humidity causes global cooling.

    KEY TAKE AWAY
    Warming related to human activities is estimated to be around 1°C over the past century. This document claims to overturn decades of scientific findings but provides neither the source of the data it uses nor the physics responsible for the proposed relationship between clouds and global temperature.

  10. What evidence is there to show that the magnitude of the reduction in outgoing longwave radiation has had any measureable effect on World-wide temnperature measurements used to arrive at a global annual mean temperature?

  11. There appears to be no evidence that the reduction in outgoing longwave radiation causes any temperature change that is detectable by the measuring devices used to determine global air temperature.

  12. billev @385 & 386.

    As well as setting out some quite complex questions that could be interpreted in different ways and which could do with being clarified, it would be useful to specifically understand why you say "there appears to be no evidence"?

    It occurs to me that this sounds a bit like a passage in the chat Roy Spencer gave at the Heartland shindig at the end of last month. (His grand assertion was "And as we add CO2 the theory says we've reduced the ability of the earth to cool itself by about one percent. That's according to theory not measurements. None of our satellite measurements of any kind are good enough to measure that. It's a theoretical expectation." Of course, as is usually the case with statements from Roy Spencer, it is wrong.)

    And as the subject is quite a complex one, it might be better to kick-off discussing it with a clear understanding of what you are actually asking about. Thus, could you explain why you say "there appears to be no evidence"?

  13. Billev @385 and @386 ,

    before you ask for evidence, you must first define what you are talking about.

    So . . . what aspect of global air temperature do you mean?  Air temp at the surface (e.g. standard meteorological station thermometers) ? 

    Air temp at 10,000 feet altitude (radiosonde balloon thermometers; or calculated from microwave detection from satellites) ?    Air temp at the tropopause; temp at the lower stratosphere; at the upper stratosphere ?

    What about sea surface temperature ?  Important, surely !!

    How is the "global annual mean temperature" calculated ?

    Is the melting of polar ice counted as a global temperature change?  If not, why not?  (Global ice-melt is occurring at approx 500 cubic kilometers per year.  That's a hefty chunk of ice, by anyone's measure!! )

    Is the ongoing rise in sea level — from the warming of the ocean, as well as ice-melt  — is that taken as evidence of temperature change ?  If not, why not?

    Ain't so simple . . . even before we start measuring whether there is, or isn't, a reduction of outgoing radiation.

  14. billev - According to the IPCC scientific synthesis:

    Scientific evidence for warming of the climate system is unequivocal.

    There's a good and approachable summary of this by NASA that's worth reading, at climate.nasa.gov/evidence/ I'm afraid that the science, the evidence, just don't agree with your claims.

  15. I am talking about air temperature measurements that are used to calculate the global mean air temperature.

  16. billev - I am struggling a bit to understand your issue. The surface air temperature have gone up and that is consistant with the increased irradiation of the surface. While you ask about "reduction in outgoing longwave radiation causes any temperature change", more relevant is measured increase in irradiation of the surface (though one is simply the reflection of the other  assuming conservation of energy). More recent direct measurement of the increased irradiation due to CO2 at the surface is here.  Are you proposing Stephan-Boltzmann law does not apply? If the sun increased its radiation by 4W/m2 and you noticed the temperature increasing as it does today, would you be saying there "was no evidence that increased output from the sun was raising temperatures"? If so, then what kind of evidence are you looking for - or are you engaging in sophistry to support a belief grounded in ideology/identity rather than science?

  17. billev - It's unclear to me what your particular complaints are, but there are more specific Skeptical Science threads for various aspects of your complaints. If you can be more clear, I would suggest taking the discussion to the appropriate thread.

    Now, if you don't have any reasons, supported by evidence, to disagree with those aspects of the global temp. record, it's linkage to greenhouse gas atmospheric changes, and attribution to human causes, your objections IMO come down to an Argument from incredulity, which is a logical fallacy. I'm more than willing to be proven wrong about that, but you're going to have to support your arguments. 

  18. Frankly, I don't think billev is interested in any answers you give him. He's had lots of opportunity to learn from his many ill-posed questions here. What he's really saying is that there will never be any evidence that he will accept. As long as he dismisses any and all evidence, he can conitnue in his beliefs and remain confident of his position.

    Do not feed the troll.

  19. Just reviewed billev posting history on this site. Hmm. I think the response I gave him in 2016 here still applies. What actually is the point of asking the same thing and then running away from the answers? At this point, billev is engaging in evasive repetition and running away when evidence is posted. I am not sure I can find any evidence that he has read single paper that he has been pointed to.

  20. Scaddenp, the measurements of your  referral were claimed to be the result of a 22 per million increase  in CO2.  This would mean that the observed heat increase was caused by an increase of one part of CO2 per each 45,454 parts of atmosphere.  This sounds nonsensical to me. 

    Response:

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

    Moderating this site is a tiresome chore, particularly when commentators repeatedly submit offensive, repetitive (even after being disproven), off-topic posts or intentionally misleading comments and graphics or simply make things up. We really appreciate people's cooperation in abiding by the Comments Policy, which is largely responsible for the quality of this site.
     
    Finally, please understand that moderation policies are not open for discussion.  If you find yourself incapable of abiding by these common set of rules that everyone else observes, then a change of venues is in the offing.

    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, as no further warnings shall be given.

  21. billev - "This sounds nonsensical to me" is a pure Argument from incredulity, an erroneous logical fallacy. As scaddenp points out, you've been repeating this very same fallacious argument for at least 3 years. Clearly you're not learning anything from these discussions. 

    Moderators, might I point out a Comments Policy situation here, namely "Comments should avoid excessive repetition"? Also known as PRATT, a 'point refuted a thousand times'?

  22. So Billev are prepared to actually learn enough physics as to why increasing CO2 to 400ppm from pre-industrial levels can cause such an effect? - (you could see "its just a trace gas" myth but then you have been referred there before without apparently learning anything).  Or do you prefer to stick with your ignorance?

    Can I assume you would happily walk into a room with 400ppm of cyanide gas - you know how could such a tiny amount of gas be a problem?

  23. The  abstract linked @391 which is also the reference supporting the existence of those "measurements of your referral ... claimed to be the result of a 22 per million increase in CO2" which Billev @395 finds so difficult to accept: that abstract is that of Feldman et al (2015) 'Observational determination of surface radiative forcing by CO2 from 2000 to 2010' which can be read in full HERE demonstrating that the effects of the +22ppm increase of atmospheric CO2 2000-10, equal to a climate forcing of +0.26Wm^-2, is directly measureable given the right circumstances

  24. this article begins with an exam of the moon.

    the moon reaches 100 degrees in the day because there are no greenhouse gasses.

    if our Co2 is increasing then why is it not blocking the reducing heat in as is claimed.

    logically if co2 always stopped the earth heating up like the moon and now stops heat escaping then it why does more co2 not prevent more heat coming in?

    please help me understand this.

    many thanks in advance.

  25. 399 ClimateBuddha

    Energy comes in from the sun through the atmosphere at high wavelengths. The atmosphere is largely transparent to these wavelengths, including CO2.

    This energy heats up the planet. This leads to IR (infra-red) radiation in the low wavelengths being emitted by the planet. CO2 is not transparent to IR radiation.

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