Part 5 of 5:  The Only Explanation Left

This is the final installment of a five-part series by Bill Chameides on How We Know Humans Cause Global Warming.

1. A 175-year-old Puzzle
2. What Chemistry Tells Us
3. Causes of Past Climate Change
4. The Medieval Warming Period
5. The Only Explanation Left

Compared to 100 years ago, the temperature of the atmosphere is warmer. No one disputes that. And no one disputes that an extra source of heat must be causing it – that’s a basic law of physics. But how do we know that the source of the heat is increasing levels of greenhouse gases from burning fossil fuels and deforestation?

So far in this series I’ve described how the relationship between atmospheric carbon dioxide (CO2) and global warming was discovered, how carbon isotopes prove that rising CO2 concentrations are from the burning of fossil fuels, and how the orbital shifts that cause ice ages cannot explain our recent warming.

That certainly suggests that global warming is caused by increased CO2 from burning fossil fuels, but how can we be sure?

Besides the greenhouse effect, there are three other heat sources that conceivably could explain global warming:

  1. Increased output from the sun.
  2. Increased absorption of heat from the sun due to a change in the Earth’s reflectivity or "albedo".
  3. An internal variation in the climate system that transfers heat to the atmosphere from another part of the Earth.

We know that the increasing greenhouse gases are warming the atmosphere. What about these other possibilities?

It’s not the sun. Scientists realized early on that increases in solar output could be the culprit, so we launched satellites to monitor the sun. But the data has shown no appreciable change in solar output for the past three decades:

Source: Foukal et al. 2006. Variations in solar luminosity and their effect on the Earth’s climate. Nature 443: 161-166.

It’s not Earth’s reflectivity. You know how you feel cooler in summer when you wear light-colored shirts? That’s because they reflect light rather than absorb heat. The same principle applies to an entire planet. A change in the planet’s "color" or brightness can make it hotter or colder by altering its reflectivity. In fact, this may be occurring on Mars, where dust storms appear to blowing light-colored dust from the surface and darkening the planet.

But it’s not happening on Earth. Satellite data reveal that the Earth’s reflectivity increased (causing cooling instead of warming) from the 1960s to the early 1980s, and has decreased modestly since. The reasons for this are discussed in my earlier post on global dimming. The recent decrease in reflectivity is much too small to account for the current warming.

It’s not internal variations. The ocean is the only source of heat capable of warming the atmosphere on decadal time scales. If this were happening, the ocean would be cooling as the heat from the ocean was transferred to the atmosphere. But in fact the ocean has been warming rather than cooling over the past few decades. Whatever is warming the atmosphere is also warming the ocean.

Source: Warming of the world ocean, 1955-2003 [PDF]

The Greenhouse Effect is the only explanation left. As I said at the beginning of this post, it’s a basic law of physics that if the globe is warming, the additional heat has to be coming from somewhere. So to end this detective story I’ll quote from Sir Arthur Conan Doyle, creator of Sherlock Holmes: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."

The only possible explanation for the global warming we’re currently experiencing is the Greenhouse Effect, from burning fossil fuels and deforestation.

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  1. Kevin Meaney
    Posted July 13, 2007 at 3:14 am | Permalink

    Thanks for this series.

    It is nicely readable. The disproving of other theories does not prove that another theory is valid. What I think would round off this series nicely would be the basic science of how the greenhouse effect works, or at least a link at the end of this article to the same. The physics of absorbtion of infra-red radiation by gases is not difficult and why this results in warming is not difficult.

    Something on the basic physics of energy balance would also be good.


  2. Posted July 13, 2007 at 11:18 am | Permalink

    A post on the science behind the Greenhouse Effect is a great idea. We’ll do that.

    Thanks for the feedback.

  3. Brian
    Posted July 13, 2007 at 2:21 pm | Permalink

    It’s not necessarily as readable as Dr. Chameides’ posts, but this is a decent place to start:

  4. K.C. Weber
    Posted July 13, 2007 at 11:51 pm | Permalink

    Dear Dr. Chameides,

    No, this is not the only explanation left, and no there is not a concensus among scientists as to what is causing global warming. For instance, I believe that the sun is a major contributor right now to global warming (not the only one, but a major one.) You give one site supporting your view. I will give you five supporting my view.

    Could I be wrong? Possibly. But, possibly not.

    Could you be right? I will admit you could, but it is possible you aren’t. And what if your viewpoint IS wrong and something else is causing global warming that we should be doing something about?

    One thing I have found in doing my own research: There are probably at least an equal number of opposing viewpoints made by a great many other scientists. And they have stong facts and equations to back up their viewpoints as well.

    I want to say that I respect your viewpoint. I just disagree with it.

    K.C. Weber

  5. Richard
    Posted July 18, 2007 at 2:16 am | Permalink

    I agree with the earlier comments. I do have a good backgound in heat transfer, thermodynamics and fluids (M.E), so I would like to see something more than the cartoon illustrations I have seen on the web regarding the CO2 driven greenhouse effect. I would like to see a summary of the modeling/process techniques and maybe a discription of the finite element analysis being done. I can not find even cursory info on the web – which makes me a little concerned. One could argue that on a quantum level conduction is radiation – so even a altitude/temperture energy storage/tansfer energy diagram woulld be helpful – with the I/O to any dead states/sinks or out world drivers would be nice.

  6. Posted July 18, 2007 at 10:18 am | Permalink

    Hi Richard,

    I encourage you to read “the motherlode” of all climate science information, the IPCC report. You’ll probably be particularly interested in Chapters 2 (radiative forcing) and 8 (climate models). It is a rich summary of the literature which goes into far more detail than we have room for here. It also gives references to the primary literature.

  7. ChipMcC1
    Posted July 24, 2007 at 8:38 pm | Permalink

    The other contributors to the ‘greenhouse effect’ are volcanos and hydro methane gas. We can’t control these high impact emissions.

    I feel the global warming discussion is dubious and misleading. The focus always seems to circle around if it is true or not and who is more correct. It IS a forgone conclusion as the Earth has been through the cycle many times and will again and again regardless of humans. It IS misleading to intimate a remedy.

    It’s a shame the issue has been given to politics for fund raising with all the hype. Learning is sure to slow to a crawl and the name calling accelerate. I’d rather see funds spent in other areas to improve the environment.

  8. Dr. Kevin Ankoviak
    Posted July 27, 2007 at 12:48 am | Permalink

    Two comments I would like to add. First, why is the emphasis placed on CO2 concentration in the Earth’s atmosphere?? The Earth has an average temperature of say 15 degrees Celsius or 285 degrees Kelvin. This means the Earth will radiate in a band around 10 micrometers. Mostly from 9 micrometers to 12 micrometers. CO2 absorbs radiation at 3 very narrow bands around 2.7, 4.3 and 15 micrometers. Absorption of radiation is governed by quantum mechanical processes…not classical ones!! The primary gas in the Earth’s atmosphere which does have significant absorption bands near 10 micrometers is water vapor. I would assume water vapor concentrations in the atmosphere would be a more obvious candidate. Secondly, you are being disingenuous Dr. Chameides. While overall solar output hasn’t changed much in 100 years, solar particle radiation and sunspot counts are up. Significantly. This increased blast of charge particles would effect cloud formation in the atmosphere. The charge particles ionize the water vapor and dust in the atmosphere allowing them to attract electrically and form clouds. I know cloud cover effects the thermodynamic equations in several ways, but I can tell you, from experience, that the solar astronomy crowd is convinced solar radiation activity plays a significant role in global warming. Just as geologists believe that increasing atmospheric dust levels from volcanic eruptions are a significant cause. In conclusion, I am NOT saying that humans shouldn’t reduce their CO2 emissions as insurance against the possibility that we are the primary cause of global warming. I am worried that with so many possible influences, if it turns out humans are NOT the primary cause of global warming, the non-scientific community will refuse to listen the next time some scientist says “Hey, you are causing a problem.”

  9. Posted July 27, 2007 at 1:07 pm | Permalink

    1. This H2O thing that is constantly brought up really amazes me. Do the people who bring it up think that they have thought of something that scientists, who have been working on this since Fourier’s work in 1827, did not?

    Sure H2O is the primary greenhouse gas. It is so effective that very little terrestrial radiation below ~8 microns escapes to space. CO2, which absorbs at 15 microns, blocks a significant amount of radiation that would otherwise escape, and is therefore an important greenhouse gas. We can directly observe the very significant effect of the CO2 15 micron absorption band on the flux of terrestrial radiation from space. Not a theory, observational fact.

    For more on why CO2 is the important greenhouse gas and not H2O, see my previous post on the water vapor fallacy.

    2. Solar output and sunspots have not increased (see our post debunking this misconception). Moreover, if increased sunspots were the cause of the warming by changing cloud cover, we would have seen a long-term trend in the earth’s albedo (or reflectivity). We have not.

  10. Dr. Kevin Ankoviak
    Posted July 30, 2007 at 6:42 pm | Permalink

    Alright. If you are going to treat that Dr. in front of my name as though it came from a Cracker Jack box, I can play along. Let’s play a game of 20 questions shall we? Everyone listening? Dr. Chameides, in your article about global warming, you say that if the Earth had no atmosphere, it’s surface would be too cold to support life. Could you please tell the class what the surface temperature of the Earth would be if it did indeed not have an atmosphere?? 12 different papers have been published on this very subject, so I have a number in mind.

  11. Dave
    Posted October 6, 2007 at 6:46 pm | Permalink

    To echo one aspect of the comments above: The strongest science relentlessly pursues the falsification of the *best* (or favored) hypothesis at all times. I wish that I could find more articles that explore the specific predictions made by the human-induced global warming theory and show how each has fit the observations or experiments made to test it. The more detailed and specific the predictions, the greater will be our confidence that the theory is correct. But most of what I read about global warming falls into two categories: (1) evidence that warming is occurring (the case for this seems strong), (2) evidence falsifying other explanations of warming. Maybe the problem is that not enough of the stronger science is making it into the more popular literature…

  12. Posted October 10, 2007 at 3:59 pm | Permalink

    Take a look at the IPCC’s Fourth Assessment Report. That gives a very detailed summary of research from around the world, including models with predictions.

  13. rkcannon
    Posted December 17, 2007 at 11:52 pm | Permalink

    Dr. Chameides- Anxiously awaiting your response to Dr. Ankoviak, who I happen to agree with. I have a MS in Engineering btw. Also your explanation of why the warming of the earth cannot be the cause of global warming does not make sense to me. You said the earth temperature would stop increasing if it was warming the atmosphere. But that would be true only if the fire went out so to speak. If the earth temperature is increasing internally for some reason, a nuclear reaction possibly, then it will continue to heat the atmosphere. Or maybe the heat is escaping faster in the northern hemisphere somehow, or is being conducted via the ocean currents, transferring heat from the south.

    Anyway your explanations seem simplistic and missing key details. The science is by no means settled or you would not have comments by the likes of Dr. Ankoviak. There are many mroe like him and he is by far more believable.

    This site is very good.

  14. rkcannon
    Posted December 17, 2007 at 11:55 pm | Permalink

    Note the IPCC Assessment carries no weight with me whatsoever. I think it is a total fraud and those behind it ought to have charges brought against them, similar to war crimes. It is just a bad to try to deceive unsuspecting people for financial gain and political control using scare tactics.

  15. quentinp
    Posted January 29, 2008 at 12:43 pm | Permalink

    “Note the IPCC Assessment carries no weight with me whatsoever. I think it is a total fraud and those behind it ought to have charges brought against them, similar to war crimes. It is just a bad to try to deceive unsuspecting people for financial gain and political control using scare tactics.”

    It’s interesting that you think the IPCC has so much power. Give the general populace more credit – they have scientific education too, and the “unsuspecting” but in fact highly educated people that have picked over the IPCC output find it all stacks up. As it would, because their entire process is open to a fault – so much so that internal discussions and disagreements can be inflated into controversies to fuel the idea of conspiracy / lack of consensus.