The temperatures and carbon dioxide concentrations have been correlated - see e.g. Petit et al., Nature 1999 - but we know for sure that the temperature was the cause and the concentration was its consequence, not the other way around. This fact has also been explained in The Great Global Warming Swindle. It follows that the C0₂ greenhouse effect has not been important in the history and we shouldn't expect that it will become important in the future.
Special comment for Australian readers on Sep 28, 2007: just yesterday, there was a new paper in Science - Lowell et al., Science 2007 - that showed that CO₂ lagged by about 1,000 years when the last ice age started to end 18,000 years agoThe direction of the causal relationship can be shown in many ways: for example, it is not just CO₂ but other gases such as methane that follow temperature. The hypothesis of CO₂ as the primary reason wouldn't explain why these other gases are correlated, too. Also, we understand how oceans react to temperature changes by releasing gases. Finally, the gas concentrations lag behind the temperature by 800 years, see e.g. this 2003 paper in Science by Caillon et al.
- See also: climate sensitivity & nonlinear relationship between CO₂ and temperature
- MS Word introduction to the climate debate
Main textThe movie of the former future U.S. president - "An Inconvenient Truth" - has impressed many viewers: it is an optimized promotion of the alarmist understanding of the global climate. Moreover, it shows a more attractive Al Gore than the old Al Gore whom we know from the 2000 campaign.
A few years ago, Gore visited Harvard and with Jochen Brocks, my fellow Fellow, we went to see him. Jochen is a leftist, of course, but he claimed that Gore looked repulsive, unhuman, and evil. I am a rightist but paradoxically, I never had terribly serious complaints about Gore's looks.
Don't get me wrong: I certainly think that George Bush is more human and looks like a more trustworthy and more human being than Al Gore, and I wish him the best on his 60th birthday! Nevertheless, their design is not the primary thing that determines my political and scientific opinions.
That's why I am going to discuss more important issues, namely the scientific ones. The most powerful argument in Al Gore's movie were the graphs showing the correlation between the carbon dioxide concentrations and the temperature extracted from ice data in the last 650,000 years.
Figure 1: Correlations between the temperature and the concentrations extracted from the ice cores (click to zoom in). Combined graph by Thomas Stocker of University of Bern, Switzerland
(Incidentally, if you care, the concentration of CO2 and CH4 is determined by a direct chemical analysis of the bubbles. The temperature is reconstructed from the concentration of frozen heavy water - water with the one normal hydrogen atom H replaced by the heavy hydrogen D, known as the deuterium - in the same bubbles. Why? Because the warmer weather there was, the easier it was for heavy water vapor molecules to get to a sufficient altitude - think about the Maxwell-Boltzmann distribution - and to join clouds whose precipitation was adding ice to the ice sheet during the same year.)
No doubt, the correlation between the temperature and CO₂ is nearly perfect. No doubt, the climate on the Earth in the 650,000 years before the industrial revolution can be described very accurately by a single function of time. But if two things, A and B, are correlated, does it imply a particular causal relationship?
In classical physics, the answer is essentially Yes. The perfect correlation must either mean that A is caused by B, or B is caused by A, or both A and B are caused by something else, namely D. It is completely clear what is Al Gore's answer: the temperature was determined by the concentrations of carbon dioxide. That's why all of us are going to die in a hell by The Independence Day 2016 unless all of us accept Al Gore as the ultimate savior, neglecting that he is not a Christ but rather an anti-Christ as Rae Ann has noticed. ;-)
Figure 2: Climate scientists extract the ice cores.
According to Gore, the concentration of carbon dioxide from the ice core records (see the picture above) was evolving according to its free will and does not require any explanation. The concentration could have been caused by oil companies owned by various mammoths. At any rate, Al Gore does not have to answer why the carbon dioxide concentration was changing in the first place. He does not have to answer because he is the savior.
Now imagine that you have the freedom to think about these things rationally, as opposed to metareligious quasithinking under the influence of crazy brainwashing. First, let us try with the following exercise.
What is the cause and what is its effect
Imagine that you find out that whenever you smell methane in the living room, you can also find a certain person in the same room. The correlation is nearly perfect. What is the conclusion? Someone could propose that the methane in the room is the cause whose presence creates the person. I would propose an "alternative" explanation: it is the person who creates the methane whenever he is in the room. Choose any explanation you want.
I picked methane because it will play a role in the main example, too.
You should notice that the graph above shows a perfect correlation not only between the temperature (A) and the carbon dioxide concentration (B), but also between the temperature (A) and the methane concentration (C). What is the cause and what is the consequence if three quantities are correlated so nicely?
Note that the answer can't be unique a priori. At most one of the three quantities - A,B,C - can be the primary cause. Which one? Clearly, if you choose one of the gases, your explanation will be asymmetric and it won't explain all the correlations in a satisfactory way. If you say that the carbon dioxide concentration determines the temperature, you must still explain why the methane concentration (and other concentrations such as N2O, for that matter) follows the same time dependence. You will clearly need a different explanation. If the CO₂ greenhouse effect is primary, you can't explain why the concentrations of CO₂ and CH4 coincide. Unless you find another inevitable explanation of this subtlety, your theory will be very weak.
Actually, we have more than logical arguments of this kind. We know very well why the causal relation is the opposite one. Imagine that you have a small bottle with 385 milliliters of Coke. It originally contained 4 volumes of carbon dioxide: if you extract carbon dioxide from one bottle of Coke to empty bottles at normal conditions, you will fill four bottles. I had to learn these things when we discussed various thermodynamical issues with Brian Greene when he was writing his second excellent book. Now, imagine that the CO₂ has leaked a bit and there is only 1 volume of CO₂ left in the bottle.
Take this bottle to your car whose internal volume is 1 cube meter i.e. 1 million milliliters. The carbon dioxide from the Coke makes 385 ppm (parts per million) of the volume of your car - just like the ratio in the atmosphere.
Suddenly, you notice a strange correlation between the concentration and character of the bubbles in the bottle on one side, and the temperature in your car on the other side. You will have two possible interpretations. Either the leaking CO₂ in the Coke determines the temperature in your car because the Coke with more CO₂ is a bit darker for the Sun that is shining to your windows (or for the infrared rays reflected from the chairs), or the temperature in your car determines how the bubbles behave in the bottle. Which explanation do you choose? ;-)
I think that any sane person obviously chooses the temperature as the cause and the concentrations as a consequence. Everyone who has ever tried to open a bottle of lemonade during a hot day must know why. Hot liquids are not able to absorb gases so well. Warmer oceans are not able to absorb atmospheric gases either. Clearly, if the temperature goes up, less carbon dioxide and methane can be bound to the ocean waters, which is why their concentration in the atmosphere goes up: this process is known as outgassing.
There are many other mechanisms that contribute to the correlation between the temperature and the concentrations. For example, the frequency of fires may increase when temperatures are warmer, and fires create more CO₂. Also, the growth of plants and animals (consumers and producers of CO₂) depends on temperatures - but the most important contributions to the correlation work in such a way that the temperature is primary and the concentrations are secondary. If you think for a while, you will realize that the example with the car is actually pretty much realistic and the ability of water to bind gases is much stronger an effect than the greenhouse effect.
Even if you did not believe that conclusion and preferred the Al Gore's explanation that methane and CO₂ create the person or the warming, you will have problems to predict the future. While the correlation between A,B,C was nearly perfect in the past, we have violated this perfect harmony because we produce CO₂ and CH4 at different rates. We can deliberately do so. You won't get any natural prediction for the temperature because the correlation data itself can't tell you how much the two gases contribute.
Figure 3: A map of the Vostok lake. The deeper you go, the further you get to the history because the ice was being added at the top. Different years look like different layers of ice, in analogy with tree rings.
You should better look at physics, and physics tells you quite clearly that the ability of water to bind gases is more important an effect for the correlation than the greenhouse effect, and this fact will influence the measurements from The Subglacial Lake Vostok System, a Russian center in Antarctica (see drawing above). The temperature is the primary cause of secondary quantities such as various concentrations - and I would expect advocates of a "global warming" theory to agree with me that the temperature should be the fundamental quantity. This description explains all the correlations and not just some of them.
Much like all other potential explanations, it still says nothing about the origin of the "primary" quantity, in this case temperature. If temperature is indeed the primary and fundamental quantity, why was it changing the way it did?
There are many contributions to the temperature variations we partially know - such as various periodic astronomical cycles or solar variation - and there are many others that we don't know well or we don't know at all - such as nonlinear chaotic effects in the formation of different kinds of clouds. But I think that even though we don't know some things for sure and in their entirety, we can still be pretty much sure that certain hypotheses are almost certainly incorrect. The hypothesis that the CO₂ concentration was primary and it determined the CH4 concentrations and the temperature is one of such extremely unlikely hypotheses.
And that's the memo. But let us add a cute and important point.
What does the 800-year lag mean
There exists a simpler way to show that the temperature was the cause and the carbon dioxide concentration was a consequence, not the other way around. If you look carefully at the graphs, you will see that the carbon dioxide concentrations lag behind the temperature by 800 years. There have been many papers that found and reported the lag. One of the newest and most accurate ones is this 2003 paper in Science by Caillon et al. (full text, click).
On the graph above (click to zoom in), the past is on the right side, time goes to the left. You can see that the Antarctic temperature starts to change first, and CO₂ responds with a 800-year lag. Methane is still correlated with both. The graph is not new. Today, we have many more accurate graphs of this kind, many of which are from more distant past. We also have a more detailed analysis by Stott et al. (Science 2007) of the end of the last ice age 19,000 years ago where CO₂ lagged by about 1,000 years, too.
The explanation is obvious: oceans are large and it simply takes centuries for them to warm up or cool down before they release or absorb gases.
The work proving the lag was recently explained in Scientific American as well as RealClimate where they also essentially claim that you can easily produce a time machine as long as you want to travel only 800 years - or anything less than 5,000 years - to your past. ;-)
See also: CO₂ lag and how alarmists thinkI leave it up to you whether you learn just the hard data or also their bizarre interpretation, and whether you will think that the RealClimate people are sane according to this interpretation. I personally don't think so. They would be right if they said that 90% of the time, the temperature and gas concentrations move together, and if you could hide the remaining 10% of the data, you couldn't learn the direction of the causal relationship.
But scientists who don't want to close their eyes can look at these critical 10% of the data, too. The result of such an analysis is that the impact of temperatures on gas concentrations is much stronger than the opposite influences, including the greenhouse effect. This fact can be extracted from the time periods where the trend is changing but because the physical laws themselves don't change, it is very clear that in the remaining periods, it is still true that the influence of temperature on the gases is stronger than the opposite influence. The only way to hide this conclusion is censorship, witch hunts, and burning of heretics at stake. There is no scientific way to deny this clear conclusion from the data.
The comments in some of these articles that the greenhouse effect could still be important is just fog that the authors included in order for their "politically incorrect" scientific conclusions to get published. This fog was probably incorporated into these papers by reviewers-alarmists, but this fog makes no sense whatsoever.
It follows from an analysis of the data that the greenhouse effect couldn't have been too important at the multi-millenium timescale.
Appendix: Gore's lift
If you have seen Al Gore's movie, you may also remember the lift. He argued that because there has been a correlation between CO2 and the temperature during the glaciation cycles, the significant recent growth of CO2 may be directly translated to a huge warming.
But we already know that this prediction is falsified, either by understanding the opposite direction of the causal relationship, as explained above, or simply by looking at the basic numbers:
During the ice ages, the concentration was 180 ppm (parts per million) and it grew to 280 ppm or so during the (warm) interglacials. This increase by 100 ppm of CO2 was accompanied by 8 °C of warming or so. But the same increase of CO2 from 280 ppm in 1800 to 380 ppm in 2005 was only accompanied by the measured 0.7 °C warming or so (even if we assume that all of the observed warming is man-made), more than one order of magnitude smaller than 8 °C. We simply know that the warming caused by CO2 is at least 10 times smaller than Al Gore tries to suggest with his exercise.
Incidentally, if you care, after many centuries or a few millenia, the correlation between CO2 and the temperature will get restored again. But the details how it will happen are inconvenient, too: in a few centuries after we stop adding CO2 to the atmosphere, the oceans will absorb or "suck" most of the "excessive" CO2. Therefore, they will also undo the small warming by 1 °C or so that the excessive CO2 has caused. The oceans have a huge capacity.
Other likable climate articles on The Reference Frame
- January 2008 was the coldest month since 1994 (HadCRUT3)
- Christopher Monckton & warmers
- Temperature changed before CO₂ concentrations, not the other way around
- Intelligence Squared US debate: deniers beat alarmists
- Temperature CO_2 sensitivity is sublinear
- Correlation of sunspots and cosmic rays vs temperature
- 2006: a painful year for chicken little's
- 2006: colder than 2002 - 2005