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Why solar energy trumps coal power: Exclusive new Caldeira analysis explains “the burning of organic carbon warms the Earth about 100,000 times more from climate effects than it does through the release of chemical energy in combustion.”

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"Why solar energy trumps coal power: Exclusive new Caldeira analysis explains “the burning of organic carbon warms the Earth about 100,000 times more from climate effects than it does through the release of chemical energy in combustion.”"

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The color of solar cells — and their short energy payback — are trivial factors when considering the huge climate benefit they provide in avoiding the release of CO2 from the combustion of fossil fuels.

That was a central point in my first post debunking the error-riddled book Superfreakonomics.  By failing to retract the many glaring errors I pointed out in my original post weeks ago — and instead blowing an aerosol smokescreen with false claims that Caldeira did not say the book misrepresented his views (see here) — Levitt brought upon himself the detailed and devastating takedown by Geophysicist Raymond T. Pierrehumbert, which focused on the same exact paragraph in the book that I debunked:

“A lot of the things that people say would be good things probably aren’t,” Myrhvold says.  As an example he points to solar power.  “The problem with solar cells is that they’re black, because they are designed to absorb light from the sun. But only about 12% gets turned into electricity, and the rest is reradiated as heat “” which contributed to global warming.”

In my post, I noted that there were three and a half major howlers in this one tiny paragraph and that California Energy Commissioner Art Rosenfeld called this “patent nonsense” when I read it to him.  Within minutes of my posting, a former lead engineer at Princeton Plasma Physics Lab “emailed me to be sure I don’t miss the forest for the trees here in debunking this,” as I wrote at the time.  He pointed out that climatologist Ken Caldeira, of all people, had an analysis showing it was trivial:

As Ken Caldeira so grippingly points out (and I tried to make graphically clear in my Stanford talk last year), each molecule of CO2 released thermal energy when it was formed “” that’s why we formed it.  In the case of electricity generation, about 1/3 of its thermal energy went out a wire as electric power, the rest was released promptly as waste heat.  But each molecule of CO2, during its subsequent lifetime in the atmosphere, traps 100,000 times more heat than was released during its formation.

A hundred thousand is a big number.  It means that running a handheld electric hairdryer on US grid electricity delivers a planet-warming punch comparable to [the heat given off by] two Boeing 747s operating at full takeoff power for the same time period.  The warming is delivered over time, not promptly, but that don’t matter; the planetary heating is accrued, the accountants would say, the moment you hit the switch.

And so I immediately added that in the original debunking (see here), which Levitt and Dubner obviously read and chose to ignore.

The graphic above is a PowerPoint from the engineer meant to illustrate the factor of 100,000.

Several people asked me for the analysis that derived the factor of 100,000.  Climatologist Ken Caldeira was kind enough to share it with me and give me authority to post it.  It is a previously-unpublished joint analysis by Caldeira and NYU’s Martin Hoffert titled, “Warming from fossil fuels,” which is now posted here.  The abstract reads:

Caldeira abstract

Put another way, as Caldeira and Hoffert write in their final paragraph:

In other words, when we burn carbon and release CO2 to the atmosphere, only 0.001% of the total warming comes directly from the release of chemical energy during burning. The remaining 99.999% of the warming is associated with the trapping of outgoing longwave radiation by that CO2 in the atmosphere.

Thus the color of the solar cells or the heat they reradiate is utterly trivial.  What matters is that they replace the burning of fossil fuels and prevent the fossil carbon from ever being released.  As an aside, Pierrehumbert notes that coal plants also give off massive amounts of waste heat because they are so inefficient, so “That makes the waste heat of solar cells vs. coal basically a wash,” even ignoring this factor of 100,000.

Levitt and Dubner need to retract that entire paragraph, much as they have agreed to correct their claim that Caldeira believes “carbon dioxide is not the right villain” in future editions.

Because they failed to quickly own up to the egregious mistakes in that one paragraph, they left themselves open to Pierrehumbert writing his “An open letter to Steve Levitt,” accusing Levitt of “academic malpractice”:

So, the bottom line here is that the heat-trapping effect of CO2 is the 800-pound gorilla in climate change. In comparison, waste heat is a trivial contribution to global warming whether the waste heat comes from solar cells or from fossil fuels. Moreover, the incremental waste heat from switching from coal to solar is an even more trivial number, even if you allow for some improvement in the efficiency of coal-fired power plants and ignore any possible improvements in the efficiency of solar cells. So: trivial,trivial trivial. Simple, isn’t it?

…  A more substantive (though in the end almost equally trivial) issue is the carbon emitted in the course of manufacturing solar cells, but that is not the matter at hand here. The point here is that really simple arithmetic, which you could not be bothered to do, would have been enough to tell you that the claim that the blackness of solar cells makes solar energy pointless is complete and utter nonsense. I don’t think you would have accepted such laziness and sloppiness in a term paper from one of your students, so why do you accept it from yourself? What does the failure to do such basic thinking with numbers say about the extent to which anything you write can be trusted? How do you think it reflects on the profession of economics when a member of that profession “” somebody who that profession seems to esteem highly “” publicly and noisily shows that he cannot be bothered to do simple arithmetic and elementary background reading? Not even for a subject of such paramount importance as global warming.

And it’s not as if the “black solar cell” gaffe was the only bit of academic malpractice in your book….

Levitt first tried to respond to my debunking of that paragraph by letting Myhrvold reply.  But that backfired when Myhrvold repudiated the core argument of the chapter!  Myhrvold’s “defense” was so lame that Berkeley economist Brad Delong posted on his blog an extensive debunking of it, written by Nicholas Weaver, which ends with perhaps the best one-sentence judgment on the book and its key source that I’ve seen so far:

… what is happening is I have to conclude that anything Myhrvold says has to be assumed to be false until proven otherwise, and by unquestioningly accepting his assumptions, anything Drubner and Levitt say may need to be taken the same way.

On October 30, Levitt replied directly to Pierrehumbert on RealClimate with another attempted aerosol smokescreen:

Raymond,

I enjoyed your intentional misreading of my chapter on global warming! I think it has really contributed to moving towards a solution to these important problems.

Myrhvold’s *main* point was about the energy required to produce the solar cells, not the radiated heat.  He has expanded on it here:

http://freakonomics.blogs.nytimes.com/2009/10/20/are-solar-panels-really-black-and-what-does-that-have-to-do-with-the-climate-debate/

His view is simply that solar panels are not a *short-run* solution to cooling the planet. I doubt you could disagree with that, given the arguments you make in your own blog post.

So he, and we, thought it made sense to explore some solutions that DO cool the earth in the short-run.

That doesn’t mean you don’t work on long run solutions as well.

I’m not sure why that is blasphemy.

Steve Levitt

As anyone can see, it is not an “intentional misreading.”  Quite the reverse.  Just go to the now-searchable Superfreakonomics on Amazon and put “reradiated” into the search engine.  Pierrehumbert replied directly:

Steve, glad to see you’re reading this.

Something I have found rather bizarre about your responses to the criticisms of your climate chapter is the way you continually try to change history about what you actually wrote, which is plainly there for anybody to see. I found it so unbelievable that you included the “black solar cell” meme when I first heard it that I actually went over to Borders and stood there and intentionally read (not misread) the chapter to see if it was true. Anybody reading what you wrote would never, ever guess that the waste heat effect was so trivial unless they already knew the subject from some other source. And as for the “short term vs. long term” issue, here’s something to chew on: if you instantaneously built a solar array big enough to meet the entire world electricity demand, you would only have to wait something under a year before the avoided CO2 radiative forcing paid back the waste heat effect.

The payback time for recouping the carbon cost of manufacturing solar cells is somewhat longer, but still substantially less than the lifetime of the solar cells — and coming down as technology improves. So, there is really no sensible construction I can put on your statement.

Yes, lots of people couldn’t believe the book was as bad as I had asserted — especially since the publisher made me take down PDF of the chapter I had posted and also asked Amazon to end the searchability, so no one could see the contents until the book was actually out.  But now everyone can see it was as error-riddled as I said, and that every single statement I made in the original post was accurate.

I’ll address the energy payback argument further in a later post.

NOTE:  I have updated this post slightly for absolute clarity since some people might not read the first debunking post that I linked to above (click here), which lays out the timeline of how I came to include this factor of 100,000.

That makes the waste heat of solar cells vs. coal basically a wash,

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27 Responses to Why solar energy trumps coal power: Exclusive new Caldeira analysis explains “the burning of organic carbon warms the Earth about 100,000 times more from climate effects than it does through the release of chemical energy in combustion.”

  1. Tim says:

    Joe – Dang, your take-downs of Levitt almost make me feel sorry for the guy . . . NOT! Not half as sorry as I feel for those who read & swallow that lying s.o.s’ b.s. Keep up the good fight!

  2. Mike says:

    I think part of what makes Levitt infuriating is how he so cheerfully refuses to engage in debating his critics on their actual points:

    “I enjoyed your intentional misreading of my chapter on global warming!… Myrhvold’s *main* point was about the energy required to produce the solar cells, not the radiated heat”

    He then feigns sincere interest in trying to solve the problem of global warming and seems baffled we don’t realize he’s really on our side:

    “That doesn’t mean you don’t work on long run solutions as well. I’m not sure why that is blasphemy.”

    In some ways it’s easier to deal with flat out deniers like Inhofe and Kloor.

  3. Jay Turner says:

    This also underscores the importance of careful conservation efforts. The energy we waste means extra CO2 emissions that keep on adding to global warming for decades. So, flipping off that light switch really does matter. All of our energy saving efforts pay big dividends over time.

  4. George B says:

    Levitt has destroyed his credibility by his intellectual dishonesty. He has made himself into butt of jokes.

    But he’s famous with a big platform now so he’s a star.

    Levitt has as much credibility on climate science as George Will.

    None.

  5. Leif says:

    This is a bit off topic but needs better coverage. Tonight on the John Stewart TV show, he reported that the “Fair and Balanced Fox News” covered a recent “Tin Hat” rally in Washington, DC and spliced footage from a summer rally that had better attendance. You got to love it. We should be singing that one from the roof tops. Right along with the forged letters to congress. Any one see a pattern here???

  6. B. Arthur says:

    The factor of 100,000 works both ways. When you say CO2 warming is 100,000 time combustion heat, that’s one perspective. Seems big. On the other hand, when you begin by saying that there is a certain amount of CO2 warming (much of which we want, to establish climate – to avoid freezing) and that combustion heat is 100,000 times smaller, that’s a second perspective. This second perspective is not a surprise to any of us who have built campfires in the depth of winter. You can make a very big fire, and nature is not impressed by the combustion. So the factor of 100,000 from this perspective, that fires are weak relative to the prevailing climate, is not news.

  7. Leif says:

    B. Arthur. The fires that you burn at you camp are, for the most part, from the wood that will decompose to CO2 @ at any rate and be reabsorbed by future growth. It is a closed system. The fires that we burn with fossil energy, coal, oil, etc. is carbon that has been sequestered for 10s and 100s of millions of years and injected into an otherwise stable ecosystem. Notice CO2 concentrations changing from ~260 ppm to ~ 390 ppm. By the by, the last time the earth saw 390 ppm was about 20 million years ago and at that time the earth was a different critter, without people I might add. All of this is in the record for verification if you care to look.
    So as they say at boy scout camp, put that in your pipe and smoke it.

  8. B. Arthur says:

    Leif,

    Why NOT take the time to READ what I SAID – not what you suppose I was saying. I think it is clear enough, if you take about a minute or two. My discussion happened to be about a wood fire because that’s usually what we use for campfires, but it could just as well have been coal or oil or natural gas. You missed that point. Then you should see that I was making a point about the perspective from which humans view things, not really about science, CO2 concentrations, or anything like that which you bring up. I think you really might well agree with what I said, and then, only if you wish, you can apologize.

  9. Mike Roddy says:

    The disturbing thing about the whole episode is that Levitt will continue to appear on television and rake in millions of dollars from his quite damaging book. We live in a world where facts do not govern, in spite of people’s ability to discover them through the internet.

    Speaking of solar, I look forward to another major post on it to add to your previous excellent summaries of the technology. There is a lot going on, but inside information is required to determind the likely winners. I would also like to see an updated busbar cost analysis, which would include recent advances in thin film and storage.

    I know people who are anxious to fund a major solar thermal plant, as you know. I can be reached at greenframe@aol.com.

  10. Hello Joe,

    Your blog post is full of facts and authority,
    you really put a lot of work into your blogging,

    I am new to your blog, but will return, I learn so much
    from your articles( blog posts )
    thank you.

  11. Leland Palmer says:

    Hi Joe-

    Actually, the solar energy that shines on solar arrays does contain a lot of heat, and this heat is added every day. So dark surfaces might actually be a problem, although any realistic calculation would have to include the emissivity of the solar arrays, and how much heat the arrays re-radiate at night to the upper atmosphere.

    Of course, this albedo change problem is easily fixed, by just painting the rooftops that the solar arrays sit on white, or scattering some white gravel around to balance the albedo of the solar arrays.

    But, assuming that solar arrays absorb 10 percent more heat from sunlight than would otherwise be absorbed, and assuming no re-radiation of infrared to the upper atmosphere at night, it looks to me like a one meter solar array could add roughly 0.5 x 10^8 joules of heat to the environment per year. So assuming a one hundred year lifetime for the solar arrays as a dark surface, this one meter of solar array could add something like 0.5 x 10^10 joules of heat to the environment.

    Of course, it is easy to fix this, just by scattering some white gravel around. Electricity produced by solar arrays will also be transformed ultimately into heat at the destination, so it might be a good idea to paint the roofs of the cities using the solar generated electricity white, to balance this by increased reflection of solar heat back out into space. Of course, we ought to do this anyway, and electricity generated by solar is certainly no more guilty of creating urban heating than electricity created by combustion of fossil fuels.

    What is different about solar generated electricity, of course, compared to fossil fuels, is that it is possible to run our society in a heat neutral way, by paying attention to the potential albedo problems, and scattering a little white gravel around, if necessary.

    When solar energy gets big enough for this to be a concern, maybe we ought to pay some attention to the potential albedo problems, which are so easily fixed.

  12. Leif says:

    B. Arthur: Unfortunately,it is science, or rather the lack of understanding there of, that is the problem here. If you notice I said “for the most part” assuming the times when you might find a chunk of coal, etc. to burn at which point you would be adding to the total. The biosphere has dealt with these anomalies quite nicely for the last 20 million years, even adapting to the odd volcano or lighting started coal seam from time to time. As it is attempting to do as we speak, absorbing most of the CO2 into the oceans. Check the ph levels over recent years. While you are at it check the affects of ocean acidification. The science, not that you perceive the water to be clean so what could be wrong with that.
    With regards to an apology, I will say that it is out of character to be quite as “in your face” as I was but it pales in comparison to what we endure daily from FOX News and the like. So, I stand firm on my explanation.

  13. DM says:

    Wasn’t one of the core points being made my the Freakonomics guys that we’re already hosed? That the amount carbon emitted to date (or what will be emitted to date plus the next 25 years) is enough to cause the predicted catastrophic impacts? Hence, we don’t have the time to bring on an entirely need energy creation and distribution systems since we’d be further compounding the issue creating it. By the time it’s ready, we’re cooked.

    That’s one of the key points I got out of the chapter.

    [JR: But that point is erroneous, as this post and RealClimate make clear.]

  14. Mike says:

    Leif: I think you still misunderstand the point B. Arthur was trying to make. To me it sounds like he was just saying that we shouldn’t be surprised that directly heating the atmosphere in any way (the campfire was just one example) has such little impact on the climate compared to the effects of releasing greenhouse gasses. I’m sure he would agree with all the science you bring up. Am I hearing you correctly, Arthur?

    Leland: It’s unclear to me why painting roofs under solar panels white or covering them in white gravel would decrease Albedo. Isn’t the sunlight getting absorbed by the panels before it hits the roof? It seems like working to decrease the Albedo from solar panels still might be a worthwhile pursuit, but I think one of the points of Joe’s post was that the heat reradiated by solar panels (which is roughly equal to the waste heat of the coal plants they are replacing) is trivial compared to the heating produced by greenhouse gas emissions from coal plants.

  15. Mike says:

    DM: I think most climate scientists believe that if we take aggressive action to cut emissions starting now, we can still avoid 2 deg C of warming. It would be one thing if Levitt and Dubner disagreed with this and advocated some geoengineering in the short term to buy us time to stop emitting GHGs. However, they seem to be suggesting geoengineering is an alternative to emission cuts altogether, which would be disastrous 50-100 years from now. Their analysis of the payback time for building renewable energy is also deeply flawed (see Joe’s previous posts). We’re already going to have to build a new energy creation and distribution system to replace the aging system already in place in the developed world and build a new system from the ground up in the developing world. Our choice is what kind of system we want to create.

  16. Leif says:

    Mike: B. Arthur clearly states in his rebuttal, ” My discussion happened to be about a wood fire because that’s usually what we use for campfires, but it could just as well have been coal or oil or natural gas.” My counter is that it can not “just as well have been about coal, oil or natural gas. By burning those fossil fuels you are increasing the CO2 above the present static level, however slightly. Yes the direct BTU output does not amount to a fart in a gale of wind, with that I agree, however my impression remains that Mr Arthur was taking a different track. It was late and to the degree that I may of misinterpreted his intentions and to keep it civil on Joe’s site, I here by apologize, you do not “have to put that in your pipe and smoke it.” On the other hand I hope that Mr Arthur does attempt to understand the difference between perception of a problem and an actual problem which, to me, from his statements is still not all together clear that he does.
    .

  17. DM says:

    Mike, you write:

    “It would be one thing if Levitt and Dubner disagreed with this and advocated some geoengineering in the short term to buy us time to stop emitting GHGs.”

    Interesting. I read this as being part of their message (I don’t like how they wrote the chapter in general, so I can see how this gets lost).

    JR: excuse my ignorance, I can see how this post indicates the cost-benefit of solar energy is better than what D&L assumes, what I don’t see is where this post addressed the idea that “we’re already hoses” (in laymen’s terms).

    [JR: It points out that the energy payback argument that make is also trivially wrong. We can build renewable plants and slash emissions rapidly. I'll do a longer post on this.]

  18. RunawayRose says:

    Kind of a side tangent, but I think I first saw that point mentioned in “The Long Thaw” by David Archer, that we will get much more warming from that molecule of CO2 heating up the atmosphere than the energy we got when we first burned C to CO2. So when I saw this post, I started nodding my head right away. We need to phase out burning carbon fast as we can.

  19. B. Arthur says:

    Mike said:
    November 12, 2009 at 12:16 pm

    Leif: I think you still misunderstand the point B. Arthur was trying to make. To me it sounds like he was just saying that we shouldn’t be surprised that directly heating the atmosphere in any way (the campfire was just one example) has such little impact on the climate compared to the effects of releasing greenhouse gasses. I’m sure he would agree with all the science you bring up. Am I hearing you correctly, Arthur?

    Thanks Mike. You got it EXACTLY right. I hope Leif will go back and read it one more time. ——B. Arthur

  20. DM says:

    JR: Thanks for the response. I look forward to the longer post. I only came about this blog via the Freakonomics controversy, so I’m interested to read further.

  21. Thanks for this knowledgeable post and sharing it with us..I think their are lot of concern on environment I think these kind of innovations are really helpful in solving the problem of global warming and energy consumption as solar power is renewable so I think this is the best way to utilize it.

  22. Edward says:

    It can’t be repeated too many times: Greenhouse gasses, mostly CO2, CONTROL the climate. Energy used and wasted by humans is dissipated into deep space easily because most of the Universe has a temperature near absolute zero. We are fortunate to have the sun to keep us from freezing. We can make our climate whatever we want by controlling greenhouse gasses. If we need more energy, we can compensate by lowering the concentration of greenhouse gasses by a very small percentage. But WE HAD BETTER WANT A CLIMATE IN WHICH WE CAN SURVIVE!!!!! We can’t, for example, get rid of snow everywhere and still expect to eat.
    The easy way to lower the percentage of greenhouse gasses is to not make them in the first place.

  23. C. Hammar says:

    Gentlemen, i refer to Leif and B. Arthur, i claim to know no more about any of this than either one of you, in fact i know that i know less. I know that the issue at hand is about another matter but i had a question for you, i am not sure you know a lot about this in particular but it would seem that you do from your posts. To my question then; I hear a lot about carbon and through my research ( which is solely based off of other scientists research, none of which i performed or had anything to do with) that carbon as far as greenhouse gases are concerned, in a sense, comes in third to the other two leading greenhouse gases Water Vapor and Methane. I simply ask that if you have a way to direct me to science, and scientific proof that this is either true or false it would be greatly appreciated. As a side note, if you could direct me to scientific evidence on both sides of this matter that would be greatly preferred.
    Again Gentlemen, Thank You
    Sincerely
    C. Hammar

  24. Leland Palmer says:

    Hi Mike-

    Leland: It’s unclear to me why painting roofs under solar panels white or covering them in white gravel would decrease Albedo. Isn’t the sunlight getting absorbed by the panels before it hits the roof? It seems like working to decrease the Albedo from solar panels still might be a worthwhile pursuit, but I think one of the points of Joe’s post was that the heat reradiated by solar panels (which is roughly equal to the waste heat of the coal plants they are replacing) is trivial compared to the heating produced by greenhouse gas emissions from coal plants.

    Actually, what the calculation I did shows is that the heat caused by exposing the dark surface of the solar cells to sunlight might not be trivial, if added up over a long enough period of time. Inputs of solar energy to the system are huge, and in general man made dark surfaces appear to me to be bad.

    Manmade dark surfaces, though, do not affect the heat radiating properties of the earth, and so may have a trivial effect on the equilibrium temperature, I’m not sure.

    The idea of covering the ground or the roof close to the solar array with something white is to balance out the dark solar panel with something white, and bring the overall solar energy absorption back to whatever it was originally, in a cheap and durable way. White gravel, or substances with a high visible reflectivity and high infrared emissivity would seem to fix the problem.

    My point is that the Superfreakonomics guys are wrong even if they are right about increased heat production by solar arrays, because the problem is easily fixed.

  25. Leif says:

    B. Arthur, #6 OK it is two against one. I apologize for misinterpreting you statement and being so short yet another time. Now can we move on in our humble attempts to save the world?

  26. Per a discussion elsewhere, the sentence “That was a central point in my first post debunking the error-riddled book Superfreakonomics,” might be better as “That was a central point in my first post, as updated with analysis from Donovan, debunking the error-riddled book Superfreakonomics.”

    I think the difference is relatively minor, though.

    [JR: Thanks. My first post was not entirely clear. I hope this clears it up.]

  27. Donald B says:

    @C. Hammar:

    The most potent Greenhouse Gas in the atmosphere is water vapor, but it is in a constant cycle of evaporation (mainly from the ocean surface) to the tune of about 1 meter of depth each year (approximately 1980 statistics — see “Is the Temperature Rising” by S. George Philander), diffusion through the atmosphere and condensation into clouds and then rain, which averages 1 meter (39.4 inches) a year over the surface of the earth, but with a high variability by location. Hawaii’s Mount Waiʻaleʻale averages 426 inches of rain annually (since 1912) and the Atacama desert gets one fiftieth of the rain of California’s Death Valley exemplify the extremes of this variation. If the temperature of the earth did not change, the evaporation rate would not change (minus effects like changes in cloudiness which can affect mechanisms like sublimation) so it would maintain the equilibrium. As increasing concentrations of other gases heat the earth, the amount of water vapor will increase, raising the temperature equilibrium point (both evaporation and condensation will increase, but evaporation more because the amount of water vapor that the atmosphere can hold will increase (see the term relative humidity) with temperature.

    Thus consider this crude analogy: consider the amplification provided by a transistor. A transistor has three regions: the different types of transistors use different terms, but functionally they can be referred to as a current source, a gate and a current collector. The gate area separates the source and the collector areas and when no current is applied to it, current between the source and collector is cut off. As a small current is applied to the gate it makes it easier for a much bigger (proportionate) current to flow from the source to the collector, thus providing an AMPLIFICATION effect. By effectively controlling the equilibrium point, CO2 and other low concentration Greenhouse Gases are the major forcing functions of the earth temperature. They can be self reinforcing when rising temperature causes more of them to be created. This is particularly true of CO2 and methane which are stored in (frozen) decaying vegetation which are released as the polar regions warm. This does not mean they cannot be driven by other forces, such as a change in the orbit or tilt of the earth to the sun, which can change the amount of the sun’s radiation that strikes the earth. This has been the cause and decline of previous ice ages, and CO2 concentration rises and falls after temperature changes in these cases. Currently the CO2 increase is human caused so it is preceding the temperature increase.

    Methane has twenty-five times the absorbency of CO2 but its concentration is much less and it decomposes into CO2 and other gases relatively quickly.

    Wikipedia ranks the contributions to warming of the greenhouse gases:
    water vapor, which contributes 36–72%
    carbon dioxide, which contributes 9–26%
    methane, which contributes 4–9%
    ozone, which contributes 3–7%

    The ranges are due to the differences in the absorbency frequencies of the different gases. See the article for the source references and a deeper discussion.