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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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How do we know more CO2 is causing warming?

What the science says...

Select a level... Basic Intermediate Advanced

An enhanced greenhouse effect from CO2 has been confirmed by multiple lines of empirical evidence.

Climate Myth...

Increasing CO2 has little to no effect

"While major green house gas H2O substantially warms the Earth, minor green house gases such as CO2 have little effect.... The 6-fold increase in hydrocarbon use since 1940 has had no noticeable effect on atmospheric temperature ... " (Environmental Effects of Increased Atmospheric Carbon Dioxide)

Predicting the Future

Good scientific theories are said to have ‘predictive power’. In other words, armed only with a theory, we should be able to make predictions about a subject. If the theory’s any good, the predictions will come true.

Here’s an example: when the Table of Elements was proposed, many elements were yet to be discovered. Using the theory behind the Periodic Table, the Russian chemist Dmitri Mendeleev was able to predict the properties of germanium, gallium and scandium, despite the fact they hadn’t been discovered.

The effect of adding man-made CO2 is predicted in the theory of greenhouse gases. This theory was first proposed by Swedish scientist Svante Arrhenius in 1896, based on earlier work by Fourier and Tyndall. Many scientist have refined the theory in the last century. Nearly all have reached the same conclusion: if we increase the amount of greenhouse gases in the atmosphere, the Earth will warm up.

What they don’t agree on is by how much. This issue is called ‘climate sensitivity’, the amount the temperatures will increase if CO2 is doubled from pre-industrial levels. Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).

What Goes Down…

The greenhouse effect works like this: Energy arrives from the sun in the form of visible light and ultraviolet radiation.  The Earth then emits some of this energy as infrared radiation. Greenhouse gases in the atmosphere 'capture' some of this heat, then re-emit it in all directions - including back to the Earth's surface.

Through this process, CO2 and other greenhouse gases keep the Earth’s surface 33°Celsius (59.4°F) warmer than it would be without them. We have added 42% more CO2, and temperatures have gone up. There should be some evidence that links CO2 to the temperature rise.

So far, the average global temperature has gone up by about 0.8 degrees C (1.4°F):

"According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS)…the average global temperature on Earth has increased by about 0.8°Celsius (1.4°Fahrenheit) since 1880. Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20°C per decade."

The temperatures are going up, just like the theory predicted. But where’s the connection with CO2, or other greenhouse gases like methane, ozone or nitrous oxide?

The connection can be found in the spectrum of greenhouse radiation. Using high-resolution FTIR spectroscopy, we can measure the exact wavelengths of long-wave (infrared) radiation reaching the ground.

 

Figure 1: Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapour is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

Sure enough, we can see that CO2 is adding considerable warming, along with ozone (O3) and methane (CH4). This is called surface radiative forcing, and the measurements are part of the empirical evidence that CO2 is causing the warming.

...Must Go Up

How long has CO2 been contributing to increased warming? According to NASA, “Two-thirds of the warming has occurred since 1975”. Is there a reliable way to identify CO2’s influence on temperatures over that period?

There is: we can measure the wavelengths of long-wave radiation leaving the Earth (upward radiation). Satellites have recorded the Earth's outbound radiation. We can examine the spectrum of upward long-wave radiation in 1970 and 1997 to see if there are changes.

 

Figure 2: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature (Harries 2001).

This time, we see that during the period when temperatures increased the most, emissions of upward radiation have decreased through radiative trapping at exactly the same wavenumbers as they increased for downward radiation. The same greenhouse gases are identified: CO2, methane, ozone etc.

The Empirical Evidence

As temperatures started to rise, scientists became more and more interested in the cause. Many theories were proposed. All save one have fallen by the wayside, discarded for lack of evidence. One theory alone has stood the test of time, strengthened by experiments. 

We know CO2 absorbs and re-emits longwave radiation (Tyndall). The theory of greenhouse gases predicts that if we increase the proportion of greenhouse gases, more warming will occur (Arrhenius).

Scientists have measured the influence of CO2 on both incoming solar energy and outgoing long-wave radiation. Less longwave radiation is escaping to space at the specific wavelengths of greenhouse gases. Increased longwave radiation is measured at the surface of the Earth at the same wavelengths.

These data provide empirical evidence for the predicted effect of CO2.

Basic rebuttal written by GPWayne


Update July 2015:

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

 

Last updated on 1 August 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.

Further reading

A good summation of the physics of radiative forcing can be found in V. Ramanathan's Trace-Gas Greenhouse Effect and Global Warming.

Comments

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Comments 76 to 100 out of 436:

  1. Phan-tum. Why do you believe we need 1000 times more points?
  2. PHAN-TUM @75, Both of these topics are covered on this site, along with a legion of others. There is a search function, and a helpful list link "Arguments" on the top left of the page. 1) Explaining how the water vapor greenhouse effect works 2) Could cosmic rays be causing global warming?
  3. PHAN TUM: "we need temperature date from 1000 times more points to make a valid conclusion toward surface warming." I agree with the first part of the statement. Will you support this endeavor with your tax dollars? You should really move the modeling comments and questions to the appropriate threads, though. Water vapor, by the way, is not ignored in the models. It has a short atmospheric residence time, though, and the quick cycling stabilizes it as an element of the long-range atmospheric dynamic (climate). Take a good read here and here, and reply on those threads.
  4. PHAN-TUM "this cannot be ignored in a "model" indeed it's not. As for the cosmic rays, take a look and comment here.
  5. phantum#75: "what about the interaction of the Earth's and Sun's magnetic fields (solar wind)? this effects the temperature as well" If you have data to substantiate this claim, please provide links to it on the cosmic rays thread. I've looked and haven't found much of any quantitative information. "maybe we should be more worried about the things we cannot change than the ones we can." That is an interesting suggestion. After we've worried about the things we cannot change for a while, what do you propose we do about them?
  6. CO2 is a heavy gas that tends to stay close to the ground. (otherwise plants couldn't consume it) Heat always rises, it does not fall, even when radiating in all directions, the hotter the air the faster it rises. So explain to me how CO2 is keeping any heat from simply slipping past it and rising into space? Additionally, on a cloudy day, temperatures go down, not up. Heat rising with water vapor in it tends to create more clouds and thunderstorms, which cool things even more. I can see how (at night) cloud cover would keep some heat from escaping the atmosphere, but once you're more than a few feet off the ground (even in the heat of summer) the air temperature drops dramatically. I have personally observed this as a glider pilot. Even 3000' off the ground you can see ice crystals form on your canopy with the sun shining and ground temp at 90 degrees. So someone explain it to me (taking real world observations into account).
    Response: CO2 is well mixed in the atmosphere, as has been repeatedly measured with ground stations at various altitudes, aircraft, and spacecraft. Not an assumption, but a repeated observation. That's different than what happens in a small enclosed system such as a jar. Clouds keep the surface warm at night because they prevent some infrared radiation from escaping to space. During the day they do the same thing but also prevent some of the Sun's radiation from reaching the surface, and the energy prevented from coming in can exceed the energy prevented from getting out to space. Heat does not rise. Hot air does. It cools as it rises because its pressure drops because its surroundings are at lower pressure.
  7. Still, compared to water vapor, CO2 is such a miniscule percentage of the atmosphere. Water vapor and the sun itself seem to play a much greater role in the natural cycle of heating and cooling the planet. Take the sun out of the equation and no amount of man made warming would keep the planet from turning into a frozen world. You can't regulate or outlaw water vapor any more than you can CO2, they are the essential building blocks of life. Has anyone considered that the sheer numbers of humans exhaling has increased several fold over the past 100 years? Where are we now 6 billion? It was barely 2 billion when I was a kid. Could that account for increased CO2?
  8. Alleycat, you would do well to look at (and comment further, if you wish) the following threads : CO2 is a trace gas Water vapour It's the Sun Human exhalation of CO2
  9. Alleycat... You getting a few things wrong here. First, water vapor responds to heat. More heat, more water vapor. This is very basic. The only way you get more water vapor is to turn up the heat somehow. Second, the sun varies only a very tiny amount. Of the ~1370Watts/m2 it only fluctuates about 0.5W/m2 over the course of an 11 year cycle. The sun is very stable, it just doesn't change much. We know with a high degree of accuracy how much the added CO2 has changed over the past century and we know the radiative forcing change for it (and the other man-made greenhouse gases) is about 2.4W/m2. That is a big change that turns up the thermostat and causes more water vapor to be held in the atmosphere. Human's exhaling doesn't actually add to the carbon in the atmosphere because, well, where did that carbon come from? It was already part of the natural carbon cycle. The problem comes from burning ancient carbons in the form of fossil fuels.
  10. #81, 82: Take yourself to the Most Used Climate Myths, you'll see such gems as Water vapour is the most powerful greenhouse gas, and Does breathing add CO2 to the atmosphere. You can search for more as well. I'd also recommend Richard Alley's AGU talk about why CO2 is the biggest control knob on climate despite the small overall percentage in the air. One of the best bits of science communication you'll see, and a better use of an hour of your time than browsing on most climate blogs!
  11. Alleycat, I think you'll dig this Alley cat.
  12. I watched Dr. Alley's speech and it seems to suggest that in the past the climate changed without any human intervention whatsoever. Very interesting stuff but it doesn't do anything to point a finger at man as the cause. I really hate the term "climate denier", it says to me that someone has made up their mind and their is no room for debate. In fact none of this is carved in stone and we're still figuring it out. I have to quote the article "So skeptics are right in saying that water vapor is the dominant greenhouse gas. What they don't mention is that the water vapor feedback loop actually makes temperature changes caused by CO2 even bigger." The fact is that water vapor IS the dominant greenhouse gas, and if all this happened many times before man entered the picture, that says to me that we are not the problem and that this is a natural process. Just because it might not end well for man doesn't mean it's our fault.
  13. Sorry guys, but I remain skeptical. Not denying that the climate is changing, but I am skeptical that it is 100% man made.
  14. Alleycat @88, You really do not seem to understand the science behind climate change and the theory of AGW, and that is confusing you. Also, no reputable climate scientist are claiming that ALL the observed warming is due to increased GHGs from burning fossil fuels and land use change. Nor do the IPCC assessment reports make that claim. As for fingerprints indicating that the warming is because of an enhanced greenhouse effect, well they are everywhere (follow the link below for more information). [Source]
  15. Alleycat#87: "in the past the climate changed without any human intervention whatsoever." No one disputes that. However, this time around we have intervened and the climate is changing, in large part because of that sudden and unprecedented intervention. Using the 'its changed before' argument begs two questions: Do we know what caused climate change in the past? Answer: mostly yes. Do we know if those same mechanisms are making climate change now? Answer: We know very well they are not. Let's use an analogy: The last time your house burned down, it was due to a lightning strike, an entirely natural cause. Does that mean that an arsonist can't burn it down the next time? Does that mean you shouldn't be concerned when you see smoke coming out of the attic?
  16. Albatross, NO ONE seems to understand the science behind climate change, not even the scientists who espouse it. Remember when you were all calling it 'Global Warming'? Now it's 'climate change' because you're all not sure if we'll end up with an ice age instead. A science that relies mostly on computer models as opposed to observation is completely flawed. If this was all so obvious, then the majority of scientists would concur, but they don't. muocounter, one volcano can throw out more CO2 and ash into the upper atmosphere than all the coal power plants on the planet combined. What are your plans to stop that? Outlaw volcanoes and undersea gas jets? The planet is a dynamic system that tends to correct itself. WE ARE A PART OF THAT SYSTEM, not some separate alien invader corrupting it. So even if it is 'man made' it's still 100% natural. The earth created us and it can destroy us just as easily. I'm fine with that, but you all seem to think you're some splendid shepherds of the planet, when in fact we're just another life form on it. Name calling and demonizing someone because they don't agree with your 'deeply held beliefs' is the first sign that you've lost the argument. [snip]
    Response: [Dikran Marsupial] Inflamatory comment deleted, please read the comments policy and make sure your posts conform to it more closely in future. We are very happy here to discuss the science with those who disagree with the mainstream poistion. However, it would be helpful if you were to first find out what the mainstream position is on the science, for instance volcanos do not produce more CO2 than anthropogenic emissions. Many of these facts can be easily checked by looking at the list of most used climate myths (on the left of the page), and then if in doubt, following the links given to the peer reviewed science. There is a possibility that you have been mislead, and some self-skepticism is a valuable quality in anyone wishing to discuss the science.

    By the way, understanding is not binary, there are issues in climatology that are very well understood, others where there is considerable uncertainty. This is much the same as in any other science. The use of computer models is not an indication that science is flawed. For example predicting planetary orbits into the distant future (i.e. astronomical timescales) is a special case of the n-body problem, which is analytically intractable, and as a result you need a computer to do the calculations. Does this mean that the science of planetary motion is flawed? Of course not. (note that the observations we have of planetary motion are the merest blink of an eye on astronomical timescales, so the science of planetary motion is no different from climatology from the point of use of observations).
  17. Alleycat, Your understanding of climate science is deeply flawed. The shift to the term "climate change" because the effect of global warming goes far beyond a simple increase in temperature. It affects rainfall pattern, ocean acidity etc. Global warming is the cause, climate change are the symptoms. No working climatologist believes that we are heading into an ice age. Global warming theory came long before the first computer arrive, and the theory have been verified through multiple observations and experiments. None of the 10 fingerprints albatross cited requires the use of a computer model. Computer models themselves are tested continuously against observations to ensure that the physics are captured correctly. You should also check out How reliable are climate models? Your belief that Volcano emits more CO2 is flat out wrong, as Dikran pointed out.
  18. Alleycat @91:
    "Name calling and demonizing someone because they don't agree with your 'deeply held beliefs' is the first sign that you've lost the argument. "
    Actually, claiming somebody has lost the argument based on some trivia entirely unrelated to the science is a fairly sure sign that you have no substantive argument to offer. As an aside, the whole "global warming"/"climate change" issue is a complete non-issue that just makes you look silly (watch the video - you'll know what I mean).
  19. alleycat#91: "So even if it is 'man made' it's still 100% natural. " What a useless tautology. Cars are man-made and are therefore 100% natural (by your 'logic'). Yet there are laws to control the use of cars. Should nature just self-correct wayward drivers? Volcanoes? Biggest in recent memory was Pinatubo and it did have an effect on climate - for about 2 years. So? You can learn something about the science here. Thus far your claims (such as 'volcanoes emit more CO2') have all been factually incorrect and everyone commenting has politely told you so -- with references. No one is name-calling and demonizing; the first to accuse another of doing that is usually the one who has lost the argument. If you find it necessary to go that path, you'll find the moderators will quickly show you the door.
  20. Alleycat, "The planet is a dynamic system that tends to correct itself. WE ARE A PART OF THAT SYSTEM, not some separate alien invader corrupting it." Alleycat, The idea that the planet tends to correct itself has a definite problem: what is "correct"? What is the ideal state? You don't know, because if you did, and you had evidence, you'd either be considered a major prophet or everyone would already also know. Humans have enjoyed a relatively stable climate for roughly 10,000 years. It's understandable that climate instability is not an element of any original narrative--except perhaps as a distant echo in certain practices and pieces of wisdom. It's also understandable, then, why climate stability might be represented with a Gaia-type character: Earth-and-biosphere-as-being, and as a being in some sort of balance that has recently been upset. No. The earth is what its historical and material conditions (including us) make it. It has been wildly variable in the distant past, and for long periods (very inhospitably so). There is no base climate that the Earth returns to after excursions. There is no base climate that the Earth is heading for (well, there is a set of unpleasant end scenarios involving the "death" of the sun and the "heat death" of the universe). To think that we can't alter climate is to set us outside of nature. We can alter nature, and we make choices every day that increase or decrease the long-range chances for our own happiness, health, security, and freedom (and the same for other parts of the biosphere). I assume from your use of "outlaw" that you are concerned with individual freedom and government regulation. The more that AGW is allowed to become a problem, the more likely a government solution will be necessary--perhaps even a global government solution. At some point, you're going to have to set aside the universals and start up from basic physics. Do you accept that CO2 absorbs and emits (in all directions) at a specific set of pressure-broadened frequencies within the range at which the Earth emits (having been initially warmed by the sun)? If no, then read the advanced version of this page (again) and provide evidence for your position here. If yes, then do you accept that humans are the primary reason why atmospheric carbon has been rapidly increasing over the last 150 years? If no, then provide evidence for your position here and/or here. If yes, then you are pretty much forced to believe that humans are responsible for the activation conditions of a warming mechanism. The question then becomes one of energy balance. Is there enough warming (through positive forcings and feedbacks) to overcome the effects of negative forcings and feedbacks? Answer that question here after reading the article(s).
  21. Hello Alleycat, The climate system is complex, no doubt, but scientists have increased their understanding on it immensely over the last 150 years or so. Not knowing everything, does not equate to knowing nothing. We do not know everything about how the human heart operates, but surgeons operate and save lives each and every day. We have known for some time (and past climate change has aided scientists in understanding this) now that doubling or trebling or even quadrupling CO2 levels in a very short time is going to have a dramatic impact on the climate system and biosphere. Indeed, those changes are already evident in data collected/observed across many scientific disciplines. No models required. You say "The planet is a dynamic system that tends to correct itself." You seem to be trying to apply Le Chatelier's principle, but in the wrong context. I encourage you to read this great article by Dr. Bill Chameides. "If this was all so obvious, then the majority of scientists would concur, but they don't." It has been quite obvious for quite some time now actually (e.g., seminal research by Tyndall,Arrhenius, Callendar etc.). Not that science is done by consensus, but the vast majority of scientists who are working in climate related fields do agree on the theory (not hypothesis) of anthropogenic global warming or anthropogenic climate change. That there is allegedly "no consensus" is currently ranked as Skeptic Myth #4. Regardless, the agreement goes beyond "consensus", it is in fact consilience. Hope that this helps.
  22. Let it be noted that the posters here have been patient and polite to AlleyCat. Yet s/he keeps insisting that people are making comments about his/her persona, when all people are trying to do are point out the problems with his/her logic and understanding of the science. And rather ironic too, given that he/she has been the one making inflammatory comments about people here. But all that aside, even if people here were being rude or whatever, that would not be an excuse to dismiss the physics, the data and the observations.
  23. Alleycat is correct that the planet will most likely "self correct" whatever we do to it. The challenge is how to survive the correction. Better yet, how do we avoid pushing the planet to a point where a self correction is inevitable?
  24. Rob, I would agree that the planet will respond to any human-based forcings. However, if the forcing is rapid and severe enough, the current path of the biosphere will be permanently altered, and the biosphere has a great deal to say about the global carbon balance and the composition of the atmosphere. That idea of "self-correction" disturbs me, because it implies that a "correct" state is definable in the interests of something non-human. Roughly 2.4 billion years ago, cyanobacteria altered the Earth (both the biosphere and indirectly the shape of the surface) permanently. There was no "correction" to this event. There was simply a permanent change, and life had to adapt or disappear.
  25. DSL... I think we're saying the same thing. The point I'm trying to make to Alleycat is to be careful what you ask for. The "correction" may take more the form of a "hard reset" than a nice soft bounce back. I think skeptics sometimes find comfort in the idea that the planet will pull all that carbon back out of the atmosphere naturally. And that's realistic... over the next 10+ million years. I, for one, don't care much about what the planet will be like in that amount of time. What I care about is that we at least try not to create undue (and avoidable) hardships on near term future generations. As it is already, with another, what, 0.6C of warming baked into the system and not a lot of progress towards significantly changing our ways, the latter half of the 21st century is going to be pretty hellish for billions of less well off people. It's awful but I don't think that is avoidable. What I hope is still avoidable is that hard reset.

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