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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 326 to 350 out of 417:

  1. HB @324.

    Addressing your comments in order:

    ☻ - 1- There is no problem with the numbers you present in themselves although using such rough figures are unnecesary. Your use of atm for bar is no great impediment although it is wrong. The problem with both your inexact numbers and inexact descriptions is purely one of clarity and becomes a problem only when combined with your incredibly bizarre assertions.

    ☻ - 2 - See Glenn Tamblyn @325. Note that the 1 bar altitude is set at some 50km on Venus and is roughly the altitude of the top of its troposphere. Above that altitude (65km) are SO2 cloud banks which are more important than any CO2 effect at this altitude. As for the rotation of Venus being slow, the planet's 'day' is some 2,800 hours but with the rotation of the optically thick atmosphere some 50 hours, this length of 'day' is not relevant. It is only relevant way up high in the upper atmosphere, for instance in the mesosphere above the cloud banks  (1 μbar) where the day/night temperature range is 130K.

    ☻ - 3 - You say that you "would choose almost any other explanation than (GHGs being the reason for the high Venusian temeratures). All of them would include heat generation of some sort. Since that is what it takes to heat something up." This is you again arguing that the laws of thermodynamics are wrong. Trust me. Such argument has to be far far stronger than a pantomime.

    I should add a correction to #324. The Venusian temperature at 1 bar (350K) is colder, not hotter, than a pro-rata 1 bar Earth temperature corrected for the elevated Venusian insolation (400K). However, the albedo of the Venusian atmosphere above 1 bar far exceeds that of the total Earthly albedo which makes the pro rata calculation a little unrepresentative.

  2. HB #324:
    "...we can assume that a gas that is not a heat source, is the cause of high temperature?
    I would choose almost any other explanation than that. All of them would include heat generation of some sort. Since that is what it takes to heat something up."

    The temperature of a system (a planet, a house or whatever you like) can be raised just as much by decreasing the heat loss as increasing the heat gain. That’s exactly what thick clothes, fur, feathers and house insulation do. None of these are able to generate heat by themselves, but they can still raise the temperature by preventing heat loss from the system, provided that the system has a heat source to begin with, like the sun.

    Let’s suppose that the high temperature on Venus really was caused by some kind of heat generation, and not atmospheric insulation as the standard greenhouse theory claims. What would we observe if this was true?
    A surface temperature of 735 K corresponds to an IR flux of ~16,500 W/m2. All this radiation should escape to space and be very easy to detect, especially by space probes like the Venera series, Magellan, Venus Express and so on.
    Here’s a spectrum of Venus obtained by Venera 15 (bottom). You find the same figure on page 4 here.

    IR spectrum of Earth, Mars, Venus

    I did a rough estimate of the "area under the curve" for Venus, and came up with about 55 W/m2. Note that the flux is given as W/m2 per steradian (sr), so we have to multiply this by pi, which gives us ~173 W/m2.
    This is in good agreement with the amount of solar energy absorbed by Venus when distance, spherical shape and albedo are accounted for (assuming an albedo of 75%).

    So, where is the rest of those 16,500 Watts that you claim is generated on Venus?
    Have they just disappeared in thin air, violating the first law of thermodynamics in that process?
    Or is your "theory" about Venus completely wrong, and the high temperature actually caused by an insulating atmosphere that allows only about 1% of the surface radiation to escape to space?
    Your "theory" has a huge hole in it!

  3. Isn't CO2 plant food? The more CO2, the greener the planet. It's very basic science...


    [DB] Anyone wishing to respond to this user please do so at this thread

    Galen, please read the linked thread and the comments there in their entirety to edify yourself.  If, after doing so, you still wish to pursue this line of discussion, please do so there, and not here. 

    Just remember to bring credible evidence in the form of links to the primary research published in credible, peer-reviewed journals that seem to support your contentions.  Thanks!

  4. Galenpsmith @328 , you have posted in the wrong thread.  Please choose a more appropriate thread for discussing the good and bad effects of CO2 / hotter climate in relation to greater greenness (good for goats and insects) and reduced food production for humans.


    [DB] Thank you.

  5. A lot of the links to sources on the intermediate section are dead links or just go to a top level domain. I've found new links for a few if you want to update them. It's probably not all of them. Just the ones I was looking for anyway. 2004) (Wang, 2009)

    LINK (Chen, 2007), 2004)
    LINK (Harries, 2001)


    [PS] Sadly, an endless problem. Thanks for those. I have converted to links.

    [RH] Shortened links.

  6. The basic notion of greenhouse warming, and that man is adding CO2, are both widely recognised.    The real question is surely How Much How Soon ?

     The article says a creeping energy  imbalance has been noted,  but is this just a relative measurement, or have we actually measured, in absolute terms, the energy imbalance?  And if so, what does that amount of added energy mean in terms of degrees warming per unit of time ?

  7. Gail @331 , you will have to explain what you mean by a "relative measurement".   That's a very odd term !

    In absolute terms, the energy imbalance [i.e. rate of energy gain by the planet] is in the region of 2.0 - 3.0 watts per square meter (if I recall the figure correctly! ).  That is, gain averaging "per square meter" planet-wide 24 hours per day 365 days per year.  Doesn't sound much — equal to a small LED bulb — but do your math and multiply by 510 million square kilometers planet-wide year after year . . . and you can see why hundreds of cubic miles of ice are melting, the sea level is rising on an accelerating path, plants & animals are changing behavior and location, and the ocean is warming (also causing a rapid temperature rise in the thin planetary layer we call "the surface").

    The speed of warming is something you should educate yourself about (you will best look at more appropriate threads & articles on this SkepticalScience website).   As a heads-up, the ballpark figure for warming to an equilibrium . . . is that a doubling of atmospheric CO2 produces eventually around 3.0 more likely 3.5 degreesC surface temperature rise.   How much of that we get now depends on how quickly we stop adding CO2 to the air.

    Of course, for your own lifetime and the lifetimes of your grandchildren, you will more immediately be concerned with the "transient" [= short-term] speed of planetary response.   Already we have had around a 1 degreeC rise in little more than a century — so it's going on at a galloping speed.

    But do please pursue the matter on more appropriate threads.  This particular thread is more about the mechanism of global warming & its connection/causation by human industrial activity — and both those points have been determined beyond all doubt (though there's always a "Flat-Earther" who likes to argue against all the evidence  ;-)  

  8. Gail @331,

    I think I should correct the comment @332. The climate forcing is of the order of 2 to 3 watts per square metre. The imbalance will be less as the planetary warming means more radiation to space and thus a smaller imbalance, roughly 1 watt per sq metre.

    The imbalance can be measured by satellites but while the satellite measurements are good at measuring the wobbles in the incoming & outgoing radiation, they are very poor (so far) at giving accurate net values or good long-term trends, this due to calibration problems. However, we do have good measures of Ocean Heat Content which is where most of the energy ends up and OHC measurements are the exact opposite accuracy-wise.

    So if we say the change in Ocean Heat Content is very roughly 0.77 watt per metre squared (as quoted for Ocean Heat Content in the Intermediate version of the OP) and say that a quantity of energy very roughly about 5% of the ΔOHC would be used to warm the atmosphere, we can calculate that the atmosphere would be warming at about 0.19ºC per decade. This is a little above the actual measured rate of warming of 0.175ºC per decade which has remained remarkably constant over the last few decades (as this graph demonstrates - usually 2 clicks to 'download your attachment')

  9. MAR @333

    I'm assuming planetary warming necessarily means an energy imbalance.  Correct ?

    And you seem to be confirming what I have read elsewhere, namely that current satellite measuremen technology cannot yet tell us what the imbalance actually is in absolute units per unit time.  They seem confident only that the outgoing heat is decreasing,  but can't quantify in absolute terms,  only the relative wobbles you mention.


    [TD] See Trenberth's energy budget diagram, which appears in many places such as this post. Wikipedia has a good article. More technical is a rebuttal to the misinterpretation of a Trenberth quote; read the Intermediate tabbed pane, then watch the video, then read the Advanced tabbed pane.

  10. Gail, the most recent account of measured global energy (im)balance that I know of is Trenberth, Fasullo, von Schuckmann, and Cheng, Insights Into Earth's Energy Imbalance from Multiple Sources, 2016.

  11. TD,  thanks a most useful article. Trenberth there says "...the planetary imbalance at TOA is too small to measure directly from satellite".  Which directly answers my question.

  12. Gail @336 , the essential point is that the planet's ice is melting, the sea level is rising, the ocean and atmosphere are warming.

    So it is plainly obvious the imbalance exists.  Which is at the heart of your question, is it not?

  13. eclectic @337
    But all of that circumstantial evidence can be questioned.   Antarctic ice expanding, sea level been rising since long before CO2 did,  ocean heat only superficially known, no significant change in surface temps for nearly two decades.     Whereas a direct measurent of an imbalance would be unanswerable.   

  14. Gail @338 , where on Earth (or some alternative planet?) have you been getting your Fake Scientific News from?  WUWT? Or somewhere even crazier than WUWT?

    "No significant change in surface temps for nearly two decades"??!!  (Actually there's one wacko site which has claimed no significant warming for about 50 years!)   On real planet Earth, the ice is still melting at hundreds of cubic miles per year, and the sea level rise is accelerating, plants and animals are changing location & habits . . . and the surface temperature has presented us with the four hottest years on record ( 2014 / 2015 / 2016 /2017 ) .

    No significant sanity at WUWT . . . only total "imbalance"  ;-)

  15. Gail @338,

    Echoing the message @339, you are certainly setting out some very bold assertions. Evidence can always be questioned but not ad nauseam and evidence certainly it should not be subjected to defenestration.

    ♥ "Antarctic ice expanding, sea level been rising since long before CO2 did,"

    Levels of antarctic ice (even Sea Ice Extent remains effectively unknown for almost all the period over which CO2 levels have been rising, increasing CO2 levels having been significant from about 1800.

    Keelig cirve 1700-to-date

    Global sea level  is considered essentially flat over the previous three millennia following the final vestiges of Ice Age melt and evidence of the wobbly beginnings of the present rise are evident from about 1800.

    SLR 1700 to 2002

    "ocean heat only superficially known, no significant change in surface temps for nearly two decades"

    The OHC over recent decades is understood far more than "superficially" and even if the "surface" mentioned continues to refer to oceans, there is a significant rise in SSTs over the last decade or two. (The addition of the adjective "statistical" would yield a different answer, but I would not be guided by the answer given by Jones to Harrabin in 2010, which was a very poor answer in my opinion. Answer B should not be "Yes,..." but "What a very stupid question.")

    Global SST 1997-2017

  16. Perhaps some who are more knowledgeable than me on radiatve processes can answer this but would we not expect the energy imbalance to be too small to measure until equilibrium has been reached? The Harries studies show the TOA radiation changes that would be expected with the changes in trace gas concentrations.

  17. Phillippe, some of the papers in the reference list in this paper by Trenberth et al. might provide that answer, if you feel like digging through them.

  18. To inject an analogy into all this earth energy balance discussion:

    Let's assume I keep all my money in a piggy bank. Everything I receive gets placed in the piggy bank, and all my expense are paid out of the piggy bank.

    I want to know if I am gaining or losing money over time. There are two ways to figure this out:

    1. Track every transaction that adds money to or takes money out of the piggy bank, and add them up to see what the ultimate change is.
    2. Count the contents of the piggy bank at the start and end of the month, and see if the total is different.

    If accurate, both methods will come up with the same result. If either method is error-prone, differences will occur within the margins of error.

    In the climate system, the earth-atmosphere as a whole is the energy piggy bank. The ocean is the largest energy store. The only way to add or remove energy from the earth-atmosphere piggy bank is through radiative transfer with space. The sun provides the input, and the earth emits IR radiation.

    We can track the imblance in two ways:

    1. Measure the radiation input and output, using satellites, and add everything up over time to determine the imbalance.
    2. Pick a start and end time, and measure the temperature changes and heat content of the oceans and atmosphere.

    Climatologists are smart. They are doing both. The first shows fingerprints of increased CO2 in the spectral distribution. Error sources in the absolute readings make an exact determination of the imbalance very difficult. The second unequivolcally shows warming/heating. The imbalance must be a net positive addition of energy.

    The two methods agree within the margins of error. The combined evidence leaves little room for doubt that increased CO2 has led to a radiative imbalance that is warming the planet.

  19. Recommended supplemtal reading about ocean warming...

    The oceans hold the story of a planet warming as fossil fuels are burned. Here is what scientists have discovered, in four charts.

    The Most Powerful Evidence Climate Scientists Have of Global Warming by Sabrina Shankman & Paul Horn, Inside Climate News, Oct 13, 2017

  20. Bob Loblaw @ 343

    Everyone agrees that additional levels of CO2 are causing global warming.  The question always comes down to how much warming.

    So does this not again bring us back to how good the GCMs are at predicting what impact this increased CO2 will have on future temperatures, even assuming that all of the observed warming since 1970 is attributable to CO2 emissions? 

    The models predict that we are not going to make a 2C limit for the period from 1870 to 2100 without serious cuts in CO2 well before that time.   

    But if you look at the "ballpark" .8C increase in the last 150 years that works out to around .05C per decade.  At this rate, 100 years from now (not 1870) we will add a further .5C representing a total of 1.3C since 1870.

    So we have to go back to the climate models to get to the 4-6C increases which are in the IPCC Fifth Assessment.  I may have these numbers wrong but you get my point.

    Perhaps someone can provide me with a calculation of the observed decadal increase from 1950 onwards, or, for that matter, even from 1975 to date.  


    [TD] It is very clear that warming has accelerated over the last 150 years. Right off the bat, you must account for the pause in warming between about 1940 and 1970, which was due to huge dumping of reflective aerosols into the atmosphere. Ignoring that by averaging from 1850 onward seriously reduces the temperature increase rate estimate so it is unrepresentative of warming before that dumping and since that dumping was drastically reduced.

  21. Moderator

    That was why I was asking for some estimate of warming from two periods, 1950 and 1975. Perhaps the two periods should be from 1950 and 1970.

    I agree that the rate from 1950 at least would be more relevant.  It would be interesting to look at 1970 but we then get into shorter periods which themselves have problems.

    I appreciate my question on models might be getting "off topic" but these questions on the rate of temperature increase certainly relate to "empirical evidence".

    Perhaps this is just impossible to do because of other factors such as the aerosols, volcanoes, ENSO etc.  I think I read somewhere on this website that ocean temperature increases are a better measure.  Surely there are some papers on this because it is such an obvious question.

    Are there studies to show accelerating ocean temperatures?


    [TD] See the Nuccitelli 2012 graph to see that ocean heat content dwarfs that of air, land, and ice. Then for a more up to date graph of ocean heat content, and one in which the acceleration is obvious, see John Abraham's post, and for details read the article that John's post summarizes.

  22. Gail: The bank balance accounting that Bob Loblaw explained to you has been done for planetary energy imbalance, using the types of evidence that MA Rodger described to you. But instead of your subjective, reference-free and incorrect assertions of the bank balance, climatologists have done the accounting quantitatively, with references to source data and methods,  including uncertainties, and from multiple types of data, data sources, and methods. The most recent I've found is a reference I already pointed you to, which you seem to have totally ignored: Trenberth et al. (2016). If the body of the paper is too technical for you to understand, at least read the Abstract, Introduction, and Conclusions.

  23. NorrisM: You ask:

    Are there studies to show accelerating ocean temperatures?

    Recommend that you peruse the comments previously posted on this OP and the related lecture-video from Denial101x that is appended to the OP. 

  24. NorrisM: "Everyone agrees that additional levels of CO2 are causing global warming."

    Everyone? I constantly see people deny that there is any warming at all, and constantly see people deny that CO2 can cause any warming.

    How much of the current warming is due to rising CO2 is an attribution question, and the IPCC has stated that the best estimate is all of it.

    As for how much warming is predicted in the future, under different scenarios, and how models fit into the question, please take that to the Models are Unreliable thread,

    ...and the future predictions are not a linear extrapolation of recent trends. That is not a good way of doing the analysis.

  25. Recommended supplemental reading::

    Laffoley, D. & Baxter, J. M. (editors). 2016. Explaining
    ocean warming: Causes, scale, effects and consequences

    Full report. Gland, Switzerland: IUCN. 456 pp.

    IUCN = International Union for Conservation of Nature and Natural Resources

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