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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 401 to 417 out of 417:

  1. ClimateBuddha @399,

    Out at the distance of the orbit of the Earth, a heat source as strong as the sun can heat a surface up to a temperature of 394K = +120ºC. The temperatures on the moon's equator at mid-day appraoch this value. The primary reason for the Earth's noon-day tropical temperatures never getting close to matching such high levels is because the moon rotates so slowly. During a moon-day, the surface is subject to heating by sunlight for 29½-times longer than during an Earth-day. And through the long moon-night, equatorial temperatures drop to -180ºC, so the heating required to reach those +100ºC moon temperatures is even more impressive.
    The Earth's atmosphere reflects more sunlight than the moon, but this is an average value. On Earth there will be places and times with zero cloud so giving similar reflectiveness to that found on the moon, usually in high desert regions. But with the Earth-day 29½-times shorter than the moon's, there is not the time for the surface to heat up to anything like those +100ºC values. And that's with the Earth's night-time temperatues far higher than those on the moon. The heating through an Earth day, the maximum day-night temperature range is way less than 40ºC compared to the moon's 280ºC.
    Greenhouse gases will not prevent high maximum temperatures. Instead, they prevent the temperature from falling down to those freezing night-time temperatures.

  2. got any links for this John Seers?


    as a kid I remember being told that co2 would stop heat penetrating as it does leaving.

    we were told to expect much colder winters and much hotter summers.

    This seems to have changed.

  3. ClimateBuddha, your childhood memory sounds a bit dodgy.

    What you are asking about, is basic textbook stuff.  Time for you to do some self-educating.  Start with Wikipedia, and go on from there.

  4. ClimateBUddha:

    As a kid, I was told there was a tooth fairy. This seems to have changed.

    Consider three possibilities:

    1. You are not correctly remembering what you were told as a kid.
    2. The person who told you that was lying.
    3. The person who told you that was uninformed.

    You'll need to give a much more concrete example of where you "heard that", and under what context. It does not resemble anything I"ve heard in the 40 years I've been studying climatology.

  5. @ClimateBuddha 402

    Look up "Trenberth Energy Budget". There are many images of this on the internet showing the longwave nature of infrared. You should be able to find more information quite easily. 

    I did not touch on the downward radiation effect but MA Rodger covered this at 401.

  6. 198 topics and none of them directly address the problem of particulate pollution.  Anyone that has been to a number of big cities has noticed the large problem of air pollution by particulates.

    Particles in the atmosphere cause scattering of incoming radiation, so the amount of solar radiation that impact the earth's surface is affected, either incoming or outgoing. This means that higher elevations receive more solar radiation including ultraviolet or UV radiation. So why doesn't greater exposure to solar radiation result in higher air temperatures with elevation? The answer is that very little of the atmosphere is heated directly by absorbing solar radiation.  As any chemist will explain to you, air is mostly vacuum, void.   Instead, most incoming solar radiation is either scattered in the atmosphere or passes through it and is absorbed by the earth. This is why the ground is often warmer than the air surrounding it.

    Therefore, what warms the air, is the direct contact with the ground and liquid surface of the ocean, the interface.   Solar radiation (e.g. light) will pass through the air.  

    Temperatures decrease with distance from the earth's surface. 

    So another factor to be considered is the absorbtion of solar energy by the earth's surface that is clearly changing (in color, material composition, etc) by human activity and natural activity.  


    [BL] One more time: use the Search box to find appropriate threads for discussion. Just because you don't see something here does not mean that it is not discussed elsewhere. Your comments show little evidence that you have actually read and tried to understand anything at this site.

    [TD] In the post about CO2 being the main driver of climate, both aerosols and albedo are addressed. Also use the Search function on those terms. 

  7. Gzzzm2013: You should start with an overview. If you had bothered to read Newcomers Start Here, you would haveseen a recommendation to read Global Warming in a Nutshell.

  8. Gzzzm2013: See A Global Warming Cluedo. Also the post on the new Gillett et al. attribution study.

  9. Gzzzm2013,

    The fact that you feel you can be sentencious and pontificate on such obvious elements as the atmosphere being transparent to solar light reveals that you have not done anywhere near enough reading to form an informed opinion that will hold any value. Everyone who contributes here is well aware of everything you just said. I could be pedant and add that temperature decreases with altitude up until you hit the tropopause, then things get a little more complicated.

    And incidentally, both albedo (which you hint at without naming it) and aerosols have been studied extensively. They are the subject of an entire body of scientific litterature, and are an examined item of denial at SkS: It's aerosols

  10. It is hard to discuss when my comments are deleted entirely.

    So I will leave this echo chamber for now, and hopefully my unanswered questions to the claims made are left for the record. 


    [DB] Moderation complaints snipped.

  11. gzzm2013 @410,

    You complain that your comments are suffering deletion. However, most can still be read by those logged-in and we are not missing much. You do however request a better explanation of the greenhouse effect and also argue that the term 'greenhouse' is not appropriate. I have responded to these comments on a more appropriate thread.

  12. Dayton @407 that is a correlations, no proof of causality.

    Dayton @408 yes the oxidation of carbohydrates produces CO2 gas.  

  13. gzzm2013, here are explanations of greenhouse effect causality:

    1. Benestad at RealClimate
    2. Augmentation of that by And Then There's Physics...
    3. by Chris Colese
    4. Tamino
    5. Earth versus Mars
    6. Falsifiability
    7. Erskine, Demystifying Global Warming
    8. Closer to level of a six year old
    9. Free online book, downloadable, by Schmittner
  14. gzzm2013: In the left margin above the login area, there is a button labeled Interactive History of Climate Science. Click that.


    [DB] Sadly, gzzm2013 has recused themselves from further participation here.

  15. I am looking for the primary source (not secondary or tertiary) for the evidence of anthropogenic-caused climate change.   Just looking for the data and its analysis, as in a peer reviewed scientific paper...looking for the raw data and its mathematical analysis.  Please post link here to the raw data.  Measurements.  Variable definition.  Statistical analysis.  Peer review.  The paper that has it all, as in any PHD level scientific research.  Thanks for the help.  Please post link to such evidence here.  


    [BL] This sort of "impossible expectations" challenge seems to come up frequently.

    Regular viewers: please hold off on responding to this challenge until we have had a chance to see if this new magonz account is another reincarnation of any previously-banned users, which would be a clear violation of our Comments Policy.

    A reminder to all, the Comments Policy includes the following:

    No multiple identities.  Posting comments at Skeptical Science should use only one registered screen name. Use of more than one account will result in all accounts being banned.

  16. Magonz... Are the same person as Gzzzm2013 who was commenting above?


    [DB] Yes

  17. Magonz, you should be more rigorous in your approach . . . a scientifically rigorous bottom-up approach :-

    What is the primary source of evidence of the scientific claim addressing Gravity?   Where is the paper that has it all?   Where is the raw data, unadjusted by falling apples?

    What is the primary source of evidence of the scientific claim addressing electromagnetic Radiation?   Why hasn't it come to light, or at least to infra-red?   We really do need a Red-Blue Team assessment of this.

    What is the primary source of evidence of the scientific claim of the atomic structure of Matter?   Or is Matter simply a natural variation?   Was the landmark scientific paper by Democritus suppressed or canceled for political purposes?   Has the existence of atoms been addressed . . . and what are those quarks really up to?   (Are they on a politically-motivated Charm Offensive?)

    Magonz, these fundamental questions must first be addressed, before you move on to secondary (or tertiary?) issues like Climate.   The true skeptic follows Feynman's motto :- First Things First.


    [DB] The person to whom you are replying is a sock puppet and will not be participating here further.

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