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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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Climate Hustle

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

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.


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Comments 351 to 358 out of 358:

  1. jessicars - when you do the experiment outside and you want to calculate the effect of your plastic bag of CO2, then you need to consider that column of gas under consideration is 55km tall and you have changed the CO2 content of the bottom few centimeters. Yes, that CO2 will trap IR radiation and re-radiate to the surface. Because that surface is receiving more radiation, it will heat up and the surface will heat the bag.  You note that it is surface under that bag that is heated by back radiation, not the gas in bag. (hence thermometer under bag). The question is how much, and in your setup, yes, it is insignificant. To replicate the atmospheric GHE, you need  a column of gas 55km or so high.

    The experiments we pointed you to isolate other effects and applify the CO2 effect to make it measurable with thermometer. Radiation is radiation whether it is coming from heat lamp or a warm surface.

    If you are looking for empirical evidence of GHE, then that paper on direct measurement is one, but see also our reference especially the CO2 traps heat section with papers from Harries, Evans, Griggs , Philpona, Chen who all compared the calculated radiation at either top of atmosphere or surface of earth and compared it to direct measurements.

  2. Hi Eclectic,

    > (A) Firstly, there is the empirical evidence from experimentation during the past 150+ years, showing CO2 absorption of (some) Infra-Red radiation. (B) During the past century there is the empirical evidence of the CO2-related global Green House Effect [GHE] : evidence provided by both expensive and (relatively) cheap experimentations & observations. (Admittedly, "greenhouse" is a poorly-named term — but historically we are now stuck with it, and it is now a widely-understood useful label.)

    Your experiment is inappropriate because of its lack of sensitivity and specificity (too many confounding variables in your experimental set-up).

    Not only do you need to address the question of IR absorption by CO2 gas, but secondly you need to address the mechanism of the planetary GHE (a mechanism which is completely unconducive to backyard experimentation, I think).

    What variables? You can't simply make a statement then not substantiate it.

    I am aware of "evidence" of the planet warming or ocean levels rising or ice caps melting, but the climate has always changed naturally. Why assume it is being changed now by human-caused CO2? If you don't know the cause, measuring the effect doesn't prove anything. You really can't realy on any empirical measurements to prove AGW, you are left with the theory alone i.e. what happens when there is more CO2 in the atmosphere?

    I have done the experiment several times. With different levels of CO2. The most I saw last night, at about 20,000 ppm, was an increase of 0.5 degrees. This is a CO2 doubling of over five times, so if this were in the atmosphere, it would be expected to increase by 5 degrees.

    Why is it not possible to do an experiment in a plastic bag to measure a response?

    I have been linked to similar experiments done to prove CO2 where they put CO2 in a fishtank, then shine a heat lamp. I pointed out that in nature, there is no heat lamp or artifiical source of radiation, so that experiment is not indicative of what will happen in reality. Why is it acceptable to do a fishtank + CO2 experiment to try to prove AGW, but not acceptable to do a plastic bag + CO2 experiment to try to disprove it? I'm happy to accept an explanation, but I don't see why, if you understand the reason, you can't simply explain it to me instead of send me to read hundreds of papers (unless you yourself do not understand it).

    Can you also give an explanation - if the correlation shown between CO2 and temperature on the Vostok Ice Core samples is assumed to be causation, with CO2 causing temperature rise - why CO2 would have a linear effect on temperature (~1 degree per 10 ppm), but now we expect a logarithmic effect (i.e. 1 degree per doubling).

    Can you explain what is going on with the molecular activity of the gas that would allow for such an unintuitive behaviour? Do all gases act this way when absorbing IR? Why? At what point do they go from linear to logarithmic and why? Has this been studied? It seems like it is fundamental to knowing whether climate science is accurate or not. How else can you come up with the prediction of 1 degree per doubling?

    My understanding is that they act logarithmically, which would mean that the Vostok Ice Core samples can not be assumed to be that CO2 causes temperature. The 800 year lag, where temperature changes first also would support that. You know what does have a linear relationship though, and could explain the Vostok Ice Core samples? The CO2 solubility of water (oceans) between 0 to ~23 degrees.


    [DB] Please confine comments to the focus of the thread on which you place them.  Thousands of discussion threads exist at Skeptical Science; using the Search function in the Upper Left of every page can help you find them.  Alternatively, you can click on the Arguments Tab to find different listings of topics (like a Taxonomic Listing or by Popularity).  Area experts and knowledgeable individuals will respond to help you increase your understandings, but you must first be on-topic.  Thanks!

  3. jesscars - I explained why you cant do it with a plastic bag - it is not 55km high. You cant complain about science being wrong when the problem is with your understanding of it. You do not appear to have looked at resources posters have offered you.

    You have now raised a whole bunch of long-debunked talking points which are offtopic here. Please use the search button on top left or the "arguments" to find the appropriate myth and comment there, not here.

    eg "Climate has changed before", "Co2 lags temperature". It would seem that with a very large no. of misunderstandings about climate, that a read through the appropriate section the IPCC WG1 to get a grip on the basics. I would also reiterate the Science of Doom. Just for starters, the ice-age cycle is driven by orbital variation which primarily affect earths albedo. Once temperatures change, CO2 changes also from interactions with ocean and eurasian wetlands amplifying the effect. The detail here is huge - if you want to question the science, then please become familiar with the science first.

  4. Also, just to look at what you expect from your experiment, executed perfectly. Assuming you are around mid-latitude and in summer, that you use a jar of CO2 0.5m high, then changing concentration of CO2 from 100ppm to 20,000ppm should change flux from 414.8W/m2 to 415.4 if I have done calcs correctly. Not enough to change temperature by even 0.1C.

  5. Hi MA,

    > jesscars @343,

    The relative strength of CO2 as a GHG is dependent on the logarithmic nature of its forcing. The first doubling will, molecule for molecule, be twice as 'forceful' as the second doubling and a thousand times more 'forceful' than the tenth doubling. So the 'forcefulness' you measure in the High-CO2 bag will be mainly a thousand-times weaker than the CO2 'forcefulness' involved in AGW. And while the ten doublings of CO2 together will provide a very 'forceful' GHG effect at 15 microns, (By-the-way, I note my 12 microns @340 is wrong - it is 15 microns.) this is achieved by stripping all GHG from everywhere else. This one-step-warmer-one-step-cooler effect for the bag world could well explain the non-result although there could be many other contributing reasons.

    I agree with this. I repeated the experiment last night with about 2% CO2 or 20,000 or 5-6 doublings. (I.e. enough to get an effect from CO2 without diluting the effect of H2O.) The bag with CO2 was 0.5 degrees warmer during the night, outside. The difference disappeared in the morning.

    > jesscars @347,

    Your comparison of the 1ºC of warming for double CO2 (without feedbacks) with the Vostok Ice Core temperature/CO2 graph doesn't properly hold. Firstly, the Vostok temperatures will be subject to polar amplification and Ice Ages result from other non-CO2 'forcings' (CH4, ice albedo) and their feedbacks. The direct CO2 contribution (without feedbacks) to the Ice Age cycles (which are globally some 5ºC) is probably something like 0.5ºC, which fits in with the logarithmic relationship. With feedbacks, the CO2 'forcing' is responsible for about a third of the Ice Age wobbles.

    OK, so you are saying that the effect of CO2 on the temperature is only minor. If so, then what explains the correlation? Why would the other factors that contribute to temperature change move/fall at the same time as CO2? This is obviously not chance, so whatever affects the CO2 must also affect those other factors in order to get that correlation. Has this been proven by empirical research?

    (Also, what is the cause of historic atmospheric CO2 change? I've heard several contradictory answers I.e. Milankovitch Effect or volcanoes. Why does CO2 change over time i.e. where does it come from, where does it go?)


    [DB] Please confine comments to the focus of the thread on which you place them.  Thousands of discussion threads exist at Skeptical Science; using the Search function in the Upper Left of every page can help you find them.  Alternatively, you can click on the Arguments Tab to find different listings of topics (like a Taxonomic Listing or by Popularity).  Area experts and knowledgeable individuals will respond to help you increase your understandings, but you must first be on-topic.  Thanks!

  6. Jesscars, I rather suspect you have been pulling my leg with your story of your plastic bag experiments.   Perhaps you just used that as an entry point for your argumentative disagreement with mainstream science.

    As Scaddenp has said, you really need to educate yourself on basic science before you can seriously start to question things.   Otherwise . . . you embarrass yourself with a Dunning-Kruger performance.

    But fear not — there is time to redeem yourself.   Why not try the intellectually-stimulating exercise of learning some genuine climate science from the articles, videos, etcetera found here at SkepticalScience & similar reputable sites.   Or if you find your dour & angry mood persisting, then try the very entertaining Potholer54 video series I mentioned — all are amusing as well as informative about real science.

    The more you learn, the more you understand reality.   It is not a coincidence that all real scientists are in consensus agreement about AGW !!

  7. jesscars - I have responded on a more appropriate thread here. Any discussion of past climate should go there.

  8. jesscars @355,

    The Ice Age cycles are strange beasts. They are triggered by changes in the solar warming of the Northern Hemisphere (the Milankovitch Cycles) but the climate has to be primed and in an a-stable state for the trigger to work.

    And while the trigger is quite a gentle shove to climate, the triggered 'impacts' are big enough to raise global temperatures some 5ºC. The 'impacts' are technically feedbacks forced by the Milankovitch Cycles but it is these feedbacks that do all the work.

    Simplistically, the main instability is the polar Ice Age ice caps that begin to melt out, this raising temperatures and destabalising further ice caps. And on the back of this warming, the level of atmospheric CO2 and CH4 will rise.

    In the case of CO2, the carbon cycle requires oceans and biosphere to be in equilibrium with the atmosphere. With warming ocean waters, less CO2 can be carried by the warmer water, raising atmospheric CO2 levels which in-turn adds to the warming process. And the frozen biosphere also releases captured CO2 as it melts. (These processes will be at work today under AGW but with only 1ºC of warming in less than a century, the impact of the warming CO2 feedbacks is much less than the 100ppm CO2 Ice Age effect that resulted from much more warming over 8,000 years.) In very simple terms, that answers "Why does CO2 change over time i.e. where does it come from, where does it go?" Volcanoes do emit CO2 but it is only very exceptionally (within the billion year planet's history) that volcanism has elevated atmospheric CO2 by anything of significance.

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