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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 101 to 125 out of 160:

  1. cherrypicked at 11:46 AM, the qualification "or" will apply if you consider how conditions can change given IR radiation is always from the warmer body to the cooler. The relative differences in state between the surface and the atmosphere can be that the surface can be warmer than the atmosphere, OR both the surface and the atmosphere are in equilibrium, OR the surface is cooler than the atmosphere. It is always going to be one OR the other depending on all the variables that combine to produce the climate that exists on this planet.
  2. cherrypicked at 11:46 AM, just to add to my earlier post to help get it into perspective and hopefully not further confuse the matter, temperature drops about 7oC for every 1000m increase in altitude above sea level, that is about 1oC for every 140m increase in height, all other things being equal.
  3. Riccardo #98 You wrote: "it's not clear to me what you mean by "the assumptions in your version of the heat balance equation". Which assumptions did "I" make? Did you find something wrong?" I used the term "your version" to be simply descriptive, not pejorative. Strictly we have covered three versions of the heat equation:- Schwartz version:- dH/dt = F * exp(-t/tau) "Your version":- dH/dt = F(t)−λΔT "My version":- dH/dt = -f(t) "Your version" makes the same assumptions that are built into the Schwartz model, and then one additional assumption, which is the curtailment of higher order temperature terms in the exponential expansion (or in the derivative form of S-B apploied to Ts - it comes to the the same thing). I wasn't suggesting that this is "wrong":- each of these versions is founded on its own assumptions. I am claiming that my solution is more general than Schwartz, and hence more general than "your version". Where are we going with this? With real-world data (CO2), over a wide variety of assumptions imposed on "my version", the rise in OLR cannot be reconciled with the IPCC assertions and assumptions. This whole conversation started because, as I stated, the "hand-waving" arguments about CO2 being the primary driver, BUT thermal emissions overwhelm the OLR response don't hold water for the long equilibration times asserted by the IPCC .
  4. PaulK, Schwartz is the same as mine, you wrote it wrong. But anyways, if we agree that H indicates a variation and that your f(t) (apart from the sign convention) includes the forcing and the radiative thermal emission, we have an agreed starting point. You may not want to linearize the radiative thermal emission and write it as εσ(T^4-Te^4) throughout, but in this way you make the solution considerably harder to find. Afterall next term in the expansion is of the order of 10^-4, i'd say it's negligible.
  5. Nice charts and graphs; but, explain why you think only some minute specific band of IR could explain the energy levels required to accomplish the feats you describe. Fact is, that if you go even one iota outside the range you are trying to limit this to O2 becomes a much larger factor than CO2; and O2 has gone down by the same amount CO2 has gone up, not that it's very much. Another fact, 400ppm of CO2 can't contain anything close to even 1/1000th of a degree, let 7/10ths of a degree. Increases in input energies explain whats been going on far better; and, that includes both the rises and the falls. The primary source of input energy is the Sun. In addition to the direct IR and the visible light we see, it also emits UV and a variety of other forms of energy as well as impacting how we are effected by more distant energy sources. Attempting to claim that it must be CO2 because there aren't enough increases in IR to account for it is more childish than just about any argument out there. If you want to make the lack of input sources argument, then do your homework. Get the UV, CME, Gamma, x-ray, visible spectrum, and other readings, convert them using known atmospheric norms, and add them to the broad spectrum IR increases; then, try the argument again - except, you won't be able to; as, they do explain the temperature variations quite well when taken as a whole. Final thought - Hanson had nice charts and graphs; and, if they had been accurate Manhattan would have sunk by now.
  6. Mr_Obvious at 16:30 PM on 17 July, 2010 "they do explain the temperature variations quite well when taken as a whole" This is news. Please supply some evidence or references to support this.
  7. Ian, from the Oregon Petition stream Also please explain to us how convection and evaporation add energy to the atmosphere. They only move the energy around Convection and Evaporation do not add any energy to any system, they only move energy from areas of high energy pressure (temp) to areas of lower energy pressures with an increasing rate with increasing differences in pressure. This is precisely the core of my argument. the enhanced green house effect is what adds the additional energy. This goes against my understanding. Once you add energy pressure to a system, it is finite and does not add more energy to the system even if a portion is trapped, the 1st and 2nd Laws apply here. Trapped radiation will only serve to slow the radiative cooling exhibited by black body radiation. My understanding is that should a method of energy transfer experience resistance, then another medium available, in this case convection combines with evaporation, will make up for this resistance in the most efficient manner possible. Preserving the 2nd Law.
  8. Ian, 107 continued . . . A true Greenhouse comes to mind. If it has a constant source of energy and it is perfectly sealed, then it will increase in pressure/temp until it is radiating at the same rate it is absorbing, keeping the emissivity and conductivity of the container in mind. Place the greenhouse into an atmosphere, and now you include conduction to the atmosphere at the surface. With a uniform surface and in zero gravity convection is not in play so what you'll end up with is a steady stratification of the atmosphere's temperature moving from higher temps to lower temps the farther you get from the surface. The energy in the system has not increased, we have only added an additional component with the atmosphere. The energy pressure in the greenhouse has decreased by the amount of energy transfered to the atmosphere. By adding gravity to the equation, we introduce convection. Gravity on Earth is 99% the strength at the top of the troposphere as it is at the surface so gravity will pull more strongly on cooler denser air than it will hotter cooler air that is created at and near the surface. The higher the surface temp the quicker the air near the surface heats of and the quicker it becomes less dense than the volume above it. Gravity drags cooler air to the surface which creates a larger difference in temp between it and the warmer it it replaced creating a faster rate of cooling.
  9. Ian, 107, 108 continued . . . A uniform surface creates equal pressure going up and will have equal resistance going down. Varying the surface in both energy pressure and specific heat allows for currents to form which allows for an increased convection rate. Land Sea breezes come to mind. While the surface and atmospheric energy pressures have adjusted, no energy has been created or lost. Now let's place a resistance on the Radiation rate. If 2 Watts is correct for CO2, that is a .005% restriction at 390 Watts. Convection rates within the Earth's atmosphere are highly variable, from dead calm to reaching dozens of meters per second. Thermal currents and hang-gliding come to mind. If a radiative resistance is encountered, and if my understanding of the 2nd Law is accurate, then any energy trapped by radiative resistance will simply be picked up convection given that it very rarely reaches it's physical limits. No energy has been created or lost, the system has only lost it's ability to cool an additional 2 Watts at the speed of light and instead has to cool at the speed available to convection. One more cont . . .
  10. Ian, 107, 108, and 109 cont . . . From 109 No energy has been created or lost, the system has only lost it's ability to cool an additional 2 Watts at the speed of light and instead has to cool at the speed available to convection. Now this begs the question, is the difference in the cooling rate enough to create a back pressure that holds the energy at or near the surface increasing that region's pressure/temp? My understanding is that the elasticity of the atmosphere combined with the readily available room for an increased convection rate will offset that kind of back pressure. No energy was created or lost from my understanding, I believe I have met all Thermodynamic Laws. Will save that in the event I am understanding this incorrect.
  11. theendisfar re 107, 108, 109 and 110. This is just gibberish. I cannot follow what you are trying to say. I doubt that even you know what you are saying. AGW is real and is caused by an increase in CO2 and other green house gases. No other science can dispute that statement.
  12. theendisfar - I am sorry but would appear to have an extremely flawed understanding of physics which is guiding you to some very wrong conclusions. I have no idea where you got this from but short of starting again from a text book, how about you look at science of doom. Its impossible to have a discussion with any meaning in face of this kind of misconception.
  13. theendisfar - Nope. Your posts here, here, in fact all of your posts on this thread - nope. At the top of the atmosphere the only energy pathway is radiation. If you look at Figure 2 at the top of this page, the emissivity of the Earth has decreased due to greenhouse gasses - so that since the 1970's, at the 1970 temperature of the Earth, it wasn't radiating as much as it received. Energy accumulated, the temperature of the Earth/atmosphere increased, and and the energy radiated to space increased as well. It got warmer. In the simplest view possible: The equilibrium temperature of IR cooling bodies (energy output) with equal energy inputs and different emissive spectra depend on the integrated energy of their emission spectra - objects (or planets) with lower integrated spectra will come to equilibrium at a higher temperature than objects closer to a black body spectra. Note that greenhouse gasses tend not to affect the visible wavelength energy coming from the Sun - I think less than 1% (numbers, anyone?) of the solar spectra is in the affected IR bands. So the input energy does not change. CO2 directly reduces the emissive IR spectra of the Earth - and temps go up as a direct response, as the Earth system moves back to equilibrium energy exchange at the top of the atmosphere.
  14. Ian, This is just gibberish. I cannot follow what you are trying to say. I doubt that even you know what you are saying. Sorry, Temperature is a measure of kinetic energy within a given volume. Kinetic energy, when viewed as Force and when divided by Volume, can be represented as Pressure. The higher the density per unit volume, the higher the temperature or 'pressure'. You can simply replace 'energy pressure' with 'Temp...', and that should clear things up. Oh, and to be clear, any references in those posts to 'Law', all apply to the 3 Laws of Thermodynamics. 4 if you count the Zeroth Law, which seems obvious, but I imagine it has some use. You are right to doubt me. AGW is real and is caused by an increase in CO2 and other green house gases. No other science can dispute that statement. ? That is not gibberish, but understand that I am not so experienced as you, so my ability to read between the lines is not as refined. For give me but I only got, basic English mind you; "AGW is real and is caused by an increase in CO2 and other greenhouse gases."
  15. theendisfar, I agree with the other respondents. Your explanations are gibberish. By that I mean "nonsensical." I can't even argue against it, because it is so confused. You really need to set aside everything that you think you know, and read some very basic explanations.
  16. If I may be so bold as to interject and declare that I'm more with Theendisfar.Surface heat loss must be by conduction,convection and radiation.99% is lost by cond/convect. Radiation fom the surface is trifling and as for "back radiation"...no go,completely contrary to 2nd Law of thermodynamics.There is no greenhouse(ie limiting of heat loss by convection)in operation whatsoever.Arrhenius was wrong.The greenhouse effect is a complete misnomer.The model of the atmosphere that is conveyed to the general public is simply wrong.Gravity and surface pressure accounts for the "warm" atmosphere....and not CO2 or any other gas. To mentally image the effect of CO2 on the atmosphere that some claim, then each molecule of CO2 would have to be a 2 bar electric fire with a parabolic mirror directed earthwards.
    Response: If you really want to argue about the 2nd law of thermodynamics, please start by reading the post "2nd law of thermodynamics contradicts greenhouse theory." If you still aren't convinced, read the more detailed material that you get to by clicking the links in the "Further Reading" green box below the post. Still want to argue? Then please read the comments on that post first. To save everybody the time of repeating all that. Again.
  17. KR, Can you please explain what is wrong in 107? Note; Should be "Convection combine with evaporation" and I failed to state the assumption 'where evaporation is available". And Trapped radiation implies an atmosphere where convection is available given Ian's "greenhouse effect". Other than that, I can see no other areas that could be in question. Are you saying that restricting a method of energy transfer where another is available will not increase the rate of the other? Thanks
  18. :) Dang! 107 should say COMBINED with a D, with regards to evaporation. HTML tags are causing trouble :) I can see how COMBINES with an S could cause confusion
  19. #113 KR at 06:39 AM on 22 August, 2010 If you look at Figure 2 at the top of this page, the emissivity of the Earth has decreased due to greenhouse gasses You know that's not true. Figure 2 only shows it has decreased in certain narrow frequency bands relative to the rest. The offset in the figure is arbitrary, it is not measured with acceptable accuracy at all. Also, it would be very interesting to have a look at the curve below wavenumber 710 cm-1. Arctic window, between 400 and 600 cm-1 plays a crucial role under certain circumstances.

    Figure 2

    About that and nooks and crannies of arctic clouds read Delamere & al. 2000. As usual, it is not about actual measurements, but modeling. However, it presents the basic concepts quite clearly.
  20. theendisfar, the problem is not your typo of "combined" instead of "combines." I'm going to make a pretty wild guess at some of your major misconceptions, but again, what you wrote is so bizarre that I might guess wrong. The system at issue is not just the Earth's solid and liquid surface. Nor is it that plus the atmosphere. The "system" is the entire Earth, from the outermost wisps of the atmosphere down to the Earth's core, as seen from outer space. Convection, conduction, and evaporation happen only within that system, and do not--can not--exchange energy with anything that lies outside that system. So convection, conduction, and evaporation cannot cool that system. But radiation from that system can escape that system, thereby carrying energy out of that system, into outer space. Such radiation is emitted toward outer space by everything in that system that has a temperature above absolute zero--which is everything. All those everythings also emit radiation in every other direction at the same time. Radiation that does not head toward outer space eventually gets absorbed by more things in that Earth system. On average, half of the emitted radiation heads toward outer space at least initially. But even radiation headed toward outer space faces obstacles--objects that absorb radiation. (But radiation does not get absorbed by other radiation it encounters. Only objects can absorb.) Some of those obstacles are greenhouse gases. When they absorb radiation, they attain higher energy states. Some of that energy they give to other objects by banging into them (that's "conduction"). But some of that energy they radiate. Just like all the previous radiating, the radiation goes in all directions, but half of it heads toward outer space. But now some of that radiation hits more obstacles in the form of greenhouse gases, and so on. This process causes a smaller proportion of outgoing radiation to make it all the way out of the system (to outer space) than would make it if there were no greenhouse gases in the way.
  21. Tom et al, To those that think 107 - 110 are gibberish, can at least one of you provide an example? Pick the first obvious one, the understanding is progressive so an early error will likely falsify the rest of it. Does Temperature as 'Energy Pressure' not make sense? Makes sense if you take into account that a joule = a newton with regards to Energy. They are equivalent. Energy/Volume = Pressure. Anyone?
  22. There's a phrase by Wolfgang Pauli that I find appropriate, "That's not right - that's not even wrong". What you've posted is nonsense. In particular, "Trapped radiation will only serve to slow the radiative cooling exhibited by black body radiation" is completely insensible - random words that don't mean anything to me. How could energy accumulation slow radiative cooling? That completely reverses cause and effect - slowing of radiative cooling traps energy, not the other way around. You're just not making sense. When you do, great. Until then, well, babble just isn't worth my time. In the meantime, I would suggest reading up on the Greenhouse Effect, and how it actually works.
  23. Berényi - do you think that in other wavelengths the top of atmosphere (TOA) emissivity has increased? Note that the measured TOA emissivity has decreased at all observed wavelengths in that graph. Can you produce evidence to that effect? If not, I can't take your comment seriously.
  24. Berényi - Delamere et al 2000 is an interesting paper. But they fail to show actual data for TOA emissions.
  25. Look, folks the precedent on this site is that assuming you can wrestle him/her to the ground w/regard to basic physics, theendisfar will then apply this dog-eared joker from the skeptic deck: "Without going into motivation, Yes, I think many climate scientists are purposely misrepresenting conjecture as empirical and repeatable evidence quite frequently using subjective terms to provide wiggle room and plugging conjecture into GCM's, passing the predictions off as reliable." Once you actually get back to empirical evidence as opposed to heading back into the 19th century to rehash classic thermodynamics and the like, whatever data you may end up discussing will be dismissed as the result of a vast conspiracy or whatever it would take to support theendisfar's hypothesis of rampant corruption in the scientific community. So I really wouldn't bother w/rejoinders, attempts at correction or the like. I'll hazard a guess that theendisfar is here for kicks, nothing more.

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