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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 151 to 175 out of 417:

  1. I'd like to point out that SoD's The Hoover Incident provides a delightful Reductio ad absurdum argument supporting the greenhouse gas effect - demonstrating that current conditions cannot hold without the greenhouse effect, and that asserting GHG's have no effect results in a contradiction (temperatures that are not currently observed, an average temperature of -18°C; brrrr....).
  2. Tom Dayton at 06:41 AM, unlike the heat temporarily "trapped" in the bicycle tube, heat within our atmosphere has not been physically constrained as the usage of the word "trapped" by some posters seems to indicate. No heat is actually trapped, heat is always being lost, as heat is always being gained. What does change is the rate at which it is either gained or lost by the atmosphere, and it is how that balances out that determines whether the temperature is warming or cooling. This balance, if able to be controlled by man would either have to control the rate of loss, or the rate of gain. Before mankind arrived nature had already put in place processes that must have not only controlled the rate of loss, but the rate of gain. If variations in solar radiation was the sole means by which the rate of gain varied, what were the processes that adjusted the rate of loss?
  3. I take the points of Muoncounter and Tom. Yes the temperature of a compressed gas will equilibrate with the surroundings. Nevertheless we are starting from TOA of Odeg and near 0 pressure....BUT.... that temperature is 0deg,273K, thanks to insolation, otherwise it would be 3K or thereabouts. So that being the case, what must the Temp be at 15lbs/squ in?(at the surface)? I have visited the Science of Doom a few times, but the prejudices of the author are very obvious. Why in this age of computerisation of graphics is this medium so seldom employed? One picture is worth a thousand words,mind you it depends on the quality of the picture.
  4. #152: "nature had already put in place processes that must have not only controlled the rate of loss, but the rate of gain. " And then we came along and starting releasing all that CO2 that was stored away for millions of years... in the last 150+ years. How do you think nature 'reacts' to that? Seems like the answer is right in front of us.
  5. #153: "what must the Temp be at 15lbs/squ in?" Its not a closed system, so the temperature will, as you say, 'equilibrate with surroundings.' Or else we'd only have hot days with high pressure and cold days with low pressure, which would take all the fun out of weather forecasting. This is becoming wildly off-topic.
  6. AWoL - yes Science of Doom prejudices are very obvious. He is extremely in favour of textbook physics. As is John Cook for that matter so why are you here? Would you only accept physics if it is explained by an anti-AGW physicist? Not many around but I would be comfortable say Roy Spencer on this pressure nonsense. His take down of Miskolczi and G&T are fine. Has he talked about it? There is a "information trust" problem that really makes me scratch my head. When you dont have the expertise to make an assessment yourself, then who do believe? Physicists - or Steve Goddard?
  7. Muoncounter. If you think this has gone wildly off topic then you have only yourself to blame. You made the claim that the 33deg C difference between the black body calculation and the actual earth surface temp was due to greenhouse gases. I say no, and that the effect of gravity and the resultant pressure play a bigger role than GHGs in accounting for this discrepancy. That is what my physics textbooks tell me. If I am wrong tell me why. If you can't be bothered, then let's not waste time in discussion, and we'll forget about science and the enlightenment, freedom of speech, and thought, and simply cough up a 5% carbon tax and quiescently submit to the rule of totalitarian scientific technocracy and live happily ever after.
  8. AWoL - sadly you cannot take a high school equation useful within its domain and then use it inappropriately somewhere else. If you want to understand it depth (you do dont you?), then I suggested the Science of Doom article with goes through it in depth. Also this . If this is going to come up often, then perhaps John should get Chris Colose to right up the argument.
  9. #157: "the effect of gravity and the resultant pressure play a bigger role than GHGs in accounting for this discrepancy. That is what my physics textbooks tell me." I'd suggest some new textbooks, although if you have one that mentions greenhouse gases, that's not all bad. My textbooks clearly state that PV=nRT applies to a gas in a closed system. As we said earlier, not the atmosphere. But that was your move to take this off topic. I provided some empirical evidence (Spencer); you have yet to reply to that. As far as a totalitarian scientific technocracy is concerned, I can't wait. For the rest of your rant, see the comments policy.
  10. Tom, The distribution of that energy, and even its form (e.g., sensible heat versus latent heat) is irrelevant to that accumulation that results from that imbalance of in versus out. Do you agree with that? Not a chance in this universe. The distribution of heat is extremely important to the overall heat of the system. And BTW - The imbalance you are talking of gets works out as quickly as nature can muster. 2nd Law. What you seem to be saying is that the imbalance only has a Radiative means to work itself out. Completely true for the entire system, however Temperature by definition is LOCAL. I've often spoke about the dangers of Averages when trying to describe a system. Where temperature is at a particular point and time is why there is such a thing as temperature. Same thing for pressure. If you averaged everything out then there is no need to measure it. Is there much more heat at the equator than at the poles? Yes, but the overall heat of the entire system is the same. Why is the heat at the equator sent mostly to space rather than to the poles? Because space is closer and colder than the poles. The average heat of our solar system is far hotter than it is here on Earth. The distribution is not only relevant, it is more important than any imbalance because it tells you where the imbalance is likely coming from. Since no one can find any flaws with 107-110 I feel safe in continuing with the availability of convection and the elasticity of the atmosphere as a mechanism to deal with the 'Imbalance' locally within the Troposphere.
  11. I've often spoke about the dangers of Averages when trying to describe a system. Where temperature is at a particular point and time is why there is such a thing as temperature. Same thing for pressure. If you averaged everything out then there is no need to measure it.
    I love this paragraph. No idea what it means but I love it because of that. I suggest reading it out loud. The capitalization of Averages is genius
  12. theendisfar, the only part of your reply that addressed my question was "What you seem to be saying is that the imbalance only has a Radiative means to work itself out. Completely true for the entire system." My question was specifically about the entire system--the overall balance of energy, analogous to the overall balance of money in a bank account. That is the point of the section titled "The planet is accumulating heat" in the post at the top of this page. Look at Figure 4. Ignore the breakdown of where that heat is distributed. The top of the curve, showing the total, is what is relevant here. Greenhouse gases cause that accumulation of energy in the entire system by blocking the escape of radiation energy to space, thereby causing that imbalance of energy-in-from-space versus energy-out-to-space. You seem to agree with that, as well. That is the greenhouse gas effect. So you do not really think the greenhouse gas effect is "silly." Your agreement on those points about the entire system, makes me suspect that your disagreement really is only about how that extra accumulated energy is distributed within the system. Am I correct?
  13. Not sure if the post is closed or just taking some time to update, but I believe I have an exercise that will help clear things up. Let's follow the energy. From the Sun to the Earth's surface and back out to space. Toma et al seem to think that where energy is a particular time is irrelevant. Let's follow the energy and see.
  14. No, theendisfar, please do not complicate the discussion. Just answer my most recent question first, with a simple answer.
  15. Energy tracking is discussed here and excellent relevant discussion of the Trenberth & Kiehl diagram here And as for claiming there is no flaws in 107-110. Well we tried to help. I cant even parse what you are trying to say nor could Ian. We've pointed you to a correct interpretation. I give up.
    Response: Yes, theendisfar, if you want to post your version of an energy flow diagram, do it on that thread, not this one.
  16. This may be a rather stupid question given the subject is referred to so often, but I get the impression that at times different people consider it as something different, or perhaps more commonly simply don't know how it is defined and blindly use it. So can someone explain where the Top of the Atmosphere (TOA) is actually located, and why that particular point has been declared as such, and what it would mean if different levels were to be considered the TOA instead.
  17. Johnd @ 152 - "If variations in solar radiation was the sole means by which the rate of gain varied, what were the processes that adjusted the rate of loss?" Just call it an exercise in deductive reasoning
  18. Moderator, If you're going to shut down the ability to post to a thread, can you please notify the audience? Thank you
    Response: There is no feature to shut down posting on any thread. If your comments are not immediately appearing, just check that the number of comments isn't just over the 50 or 100 mark (or some other multiple of 50). Your comment might be appearing on the next page over. Or if it appears then is removed, it would be because its violating the comments policy.
  19. The link to Law Dome Ice Core under Figure 1 points mistankingly to Taylor Dome Data. This is the correct link to Law Dome Ice Core data.
  20. I just wanted to say thank you for this post. The more available and easily readable evidence in public domain the better it is.
  21. How about the pushing of Human Caused Global Warming is just a distraction to postpone doing the necessary things to reduce consumption of oil, reduce consumption of coal, prepare ourselves and the environment for whatever climate changes occur! Pushing the idea that you are effectively know how it all works is a time waster. Plenty good reasons to reduce oil and coal consumption. Its just pushing the all the blame on to industry when individuals consume the electricity and resources.
  22. cloa513 - if we knew everything, then there would be no need to invest in the science. This site is about the science of climate, and in particular pushing against disinformation. I dont like the disinformation industry - telling lies for financial gain is wrong. I think governments should be making the decisions based on best available information, and we will run into trouble with climate before we run out of oil and long before we run out of coal. You are suggesting a PR campaign to sell an idea to an electorate - might work but that would be telling lies too.
  23. Every time I've heard this "empirical evidence" bit the goal posts immediately begin to shift. When I start putting forth the evidence the person's definition of "empirical" and "evidence" start moving all over the place. If you promise to stand still we can show you more evidence than you'll ever be able to process on your own.
  24. 171, cloa513,
    Its just pushing the all the blame on to industry when individuals consume the electricity and resources.
    Yeah, and in the seventies it was pushing all of the blame on the tobacco companies when it was the people doing the smoking. It wasn't the tobacco companies' fault that so many smokers believed their arguments that the science was wrong, and smoking didn't cause cancer. It wasn't until the government enforced packaging and advertising regulations that people wised up. But then, that's their own fault for being so easily fooled, isn't it? There's no reason in the world why we shouldn't expect history to play out differently this time, right?
  25. The latest preliminary estimates of global CO2 emissions for 2009 and 2010 has been released by the CDIAC "These estimates show that 2010 was by far a record year for CO2 emissions from fossil-fuel combustion and cement manufacture. Globally 9,139 Teragrams of oxidized carbon (Tg-C) were emitted from these sources. A teragram is a million metric tons. Converted to carbon dioxide, so as to include the mass of the oxygen molecules, this amounts to over 33.5 billion metric tons of carbon dioxide. The increase alone is about 512 Tg-C, or 5.9%, over the 2009 global estimate. The previous record year was 2008, with 8,749 Tg-C emitted; the 2010 estimate is about 104.5% of that, or 391 Tg-C more." I found this graph interesting at the least for the breakdown of sources:

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