How we know recent global warming is not natural

This is a Skeptical Science repost on one of the latest denier talking points.

Dr. Roy Spencer, like Dr. Richard Lindzen (the subject of a few recent articles), is one of very few climate scientists who remain unconvinced that most of the the recent global warming has been caused by humans (anthropogenic).  Dr. Spencer has grown frustrated with the fact that most of his climate scientist colleagues conduct research under the premise that the recent warming is anthropogenic, and in an article on his blog, has thrown down the gauntlet:

“Show me one peer-reviewed paper that has ruled out natural, internal climate cycles as the cause of most of the recent warming in the thermometer record.”

This challenge is problematic for a few reasons.

Firstly, the fact that research has not ruled out a hypothesis does not mean the hypothesis necessarily has any validity.  For example, there have been no peer-reviewed papers ruling out leprechauns as the cause of most of the recent global warming, either.  But perhaps more importantly, our understanding that humans are causing global warming is not based on just one scientific study, but rather a very wide range of scientific evidence.

For example, scientists have measured the amount of heat being re-directed back towards the Earth’s surface due to the increased greenhouse effect.  Quantifying the amount of global warming that this will cause simply involves multiplying the increased downward energy by the climate sensitivity.  As the name suggests, climate sensitivity is a measure of how sensitive the climate is to this build-up in heat – how much the planet will warm in response to an increase in the amount of greenhouse gases in the atmosphere.

Therefore, in order to argue that humans are not the driving force behind the current global warming, skeptics like Spencer and Lindzen require that the climate sensitivity to increasing greenhouse gases is low.  The problem with this position is that there are many lines of evidence that the planet will warm between 2 and 4.5 degrees Celsius (°C) if the amount of carbon dioxide (CO2) in the atmosphere doubles.

For example, some scientists have studied the climate response to recent large volcanic eruptions, which can have a measurable impact on global temperatures.  Other studies have examined how the global temperature has changed in response to changes in solar activity.  Some other research has compared CO2 and global temperature changes over the past thousand years, and tens of thousands of years, and hundreds of thousands of years, and even millions of years ago.  We can even compare how the temperature has changed over the past century to human-caused atmospheric CO2 changes.  In every case we arrive at this same climate sensitivity range of 2 to 4.5°C, and the most likely value is 3°C for a doubling of atmospheric CO2.

If we take the lower end of this range, even a 2°C climate sensitivity would mean that humans have been responsible for more than half of the global warming over the past century.  So in order for Spencer and Lindzen to be right, all of these different lines of evidence which are in agreement with the likely range of climate sensitivity would all have to be somehow wrong, and all biased high.  Not an impossibility, but certainly not a likely scenario, either.

There are also many “fingerprints” of human-caused global warming.  For example, as the Earth’s surface and lower atmosphere have been warming, the upper atmosphere has been cooling.  There are not many mechanisms which can explain these observations, but they are precisely what we would expect to see from human-caused global warming.  As the concentration of greenhouse gases in the lower atmosphere increases, they effectively trap more and more heat in this lower layer, causing it to warm and causing the layers above to cool.  Another human “fingerprint” is the higher rate of warming at night than during the day. This is because at night, when the surface is cooler and not being bombarded by solar energy, the increased amount of greenhouse gases are able to make more of a difference in the surface temperature.

Dr. Spencer has proposed an alternative to the anthropogenic global warming theory.  He suggests that some unknown mechanism has caused global cloud cover to decrease over the past century.  Low-level clouds cause a cooling effect by reflecting sunlight, so if these types of clouds become less prevalent, it can cause the surface to warm.  However, this hypothesis cannot explain the “fingerprints” describe above.  A decrease in cloud cover would not cause the upper atmosphere to cool.  Nor would it cause nights to warm faster than days – quite the opposite.  Cloud reflectivity only plays a significant role during the day when being bombarded by sunlight.

Dr. Spencer also suggested in his blog post that the “null hypothesis” should be that global warming is caused by natural factors.  A null hypothesis is basically the default assumption which a scientific study sets out to disprove.  It’s true that until recently, global warming (and cooling) has been caused by natural factors.  However, even natural climate changes must have a physical mechanism causing them.  Scientists have investigated these natural mechanisms (the Sun, volcanoes, the Earth’s orbital cycles, etc.), and they simply cannot explain the global warming over the past century.  Spencer’s new hypothesis – that some unknown mechanism is causing cloud cover to change, which in turn is driving global temperatures – is a new idea with very little supporting evidence.  Conversely, our understanding that human greenhouse gas emissions are driving global temperatures has a proverbial mountain of supporting evidence.

Skeptics like Spencer and Lindzen believe that the default assumption should be one which requires that a very large body of scientific evidence is wrong.  The only alternative hypothesis they have put forth cannot explain the many empirically-observed “fingerprints” which are consistent with human-caused global warming.  Although Spencer’s unspecified “natural internal cycle” hypothesis has not been explicitly disproved, there is a very low likelihood that it is correct.  For this reason, we should operate under the assumption that humans are causing dangerous global warming – an assumption which is supported by a very large body of evidence – until the skeptics can provide solid reason to believe that this scientific theory is wrong.

Reposted from Skeptical Science.

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42 Responses to How we know recent global warming is not natural

  1. Phil says:

    Could it be phlogiston?

  2. Mark says:

    There is a decrease in low level clouds, and low-level cloud occurrence is an important feedback mechanism that cannot be directly simulated in GCM’s, but must be parameterized. The question becomes what is the forcing mechanism that is causing a change in low-level cloud cover? It could be an unknown physical phenomenon, or it could be the known and quantifiable effects of GHG’s on temperature. Spencer has selected what he probably knows is an important feedback that has wide variability in GCM’s. However, invoking unknown forces reminds me of the hidden variables explanation for quantum weirdness. The article below describes an attempt to quantify the low cloud feedback outside of standard GCM’s. Their conclusion is that low cloud cover is decreasing as a result of warming, adding a positive feedback mechanism and increasing the estimate of climate sensitivity.

  3. Esop says:

    Didn’t the skeptics for a long time claim that we were cooling?
    Was that something they said just for fun, or did they actually do a thorough analysis and concluded that the temperature drivers were causing cooling.
    Their desperation is getting amusing. Spencer is really starting to sound like a creationists.

  4. BBHY says:

    Less cloud cover also would not explain why it is that satellites in space measure a decrease in infrared radiation from the Earth, at exactly the wavelengths that are absorbed by CO2.
    So, for the deniers to be correct, then something must be preventing the CO2 from doing what it has always done; absorb infrared radiation, while also something must be absorbing infrared radiation exactly the way CO2 does. And it has to be something that is not produced by humans. If anyone can find proof of this mysterious something then they deserve not just a Nobel prize, but all the Nobel prizes from now on.

  5. Adam R. says:

    Spencer does not specify which “natural, internal climate cycles” he means, thereby leaving the identity of the possible forcing agent open to an infinity of remotely possible but unknowable influences. Thus his goalpost is infinitely movable; the paper that will satisfy him can never appear.

    This is nothing but the old “We don’t know everything so we know nothing” denier trope in a new suit. The fact that it comes from Spencer reminds us that it is a favorite of creationists, as well.

  6. Mike Roddy says:

    Roy Spencer believes that the earth is somewhere around 8,000 years old. Richard Lindzen tries to make the case that smoking does not cause cancer. These two men clearly suffer from cognitive deficits that disqualify them from making statements about serious scientific matters. Oil and coal companies have determined that Spencer and Lindzen can help them perpetuate their agenda to become wealthier while they are destroying life on earth as we know it.

    All of this is rather obvious to those who have studied these issues in depth. John Cook’s rational dismemberment of their thoughts is important. So is the need to discredit and humiliate two people whose actions are essentially psychopathic, in result if not intention.

  7. 350 Now says:

    Flash Phone Mob for 2.22.11 repost

    I’ve been “on hold” awhile so hopefully many are calling.

    I lost track of this thread and dug it out. Others may wish to act upon it today as well:

    Roger says:
    February 19, 2011 at 10:25 pm
    paulm (#22) mentions (paraphrasing) that we will only be able to take effective action if our leaders can alert the public to this emergency.
    But none seem to have the guts (end of paraphrase).

    To repeat part of my longer comment yesterday (now lost in the ‘memory hole’-of-sorts), no truer words were ever stated. And the person to do this public alerting would, of course, logically be none other than our duly elected chief executive, our US Commander-In-Chief Obama, the one person, more than any other, designated to protect us from harm. Right?

    Think about it: One well-written 40-minute “State of the Climate” speech from the White House Oval Office, with paid repetitions on all TV shows, could change it all, overnight! Climate Hawks could relax.

    So, what’s Obama waiting for? Apparently he needs more political cover from climate activists, in the FDR-like “Now go out and make me do it” sense. So, let’s all give him the support he’s looking for.

    This is an ORDER: Tuesday morning (following the Monday Presidents’ Day holiday), without fail, every caring reader of CP WILL CALL President Obama at his office to ask him to please educate and lead misinformed Americans by giving them a clear, concise, informational call to action in a prime-time, nationally-televised “State of the Climate” address.

    Here’s the number to call: 202-456-1111. Volunteer operators will be standing by for your call between the hours of 9AM and 5PM. If you can’t immediately get through, keep trying. A livable climate is at stake for you, your loved ones, and billions of other. THANK YOU!!

    Warm regards,

    P.S. If you read this and CAN’T muster whatever it takes to make this one simple phone call to express your opinion to the man who could make such a huge difference, then IMHO, you don’t have any right to complain here, nor when the hell and high water hit you and those you care about.

  8. Lewis C says:

    Spencer and Lindzen may or may not be fossil shills, but they certainly are not scientists for they utterly disdain scientific method in applying rational analysis.

    The gist of Spencer’s claim is that an unknown natural mechanism is causing changes in cloud cover that are causing the long-term observed warming – which is mere hypothesis-mongering and thus irrelevant given that hypothes are an infinite resource.

    The absence of any attempt to identify what prevents GHGs from causing the warming while it or some other mechanism also achieves all of their precursor conditions for warming is the nail in these mens’ claim to be scientists. Were they impartial in their researches, they would not publish without providing one or more plausible culprits for this dual effect.



  9. Roy says:

    Spencer is really starting to sound like a creationists.

    Spencer IS a creationist.

  10. I’m of the feeling that global warming is/has been natural until recently. The earth is still bouncing back from the last ice age, Scotland for example is still rising back out of the sea due to bouyancy, having been pushed down by the ice that was on it. Hanging beaches are seen up to 40 feet out of the water due to this effect. Why could the atmosphere and climate not get increasingly warmer and at a faster rate, as “the ball gets rolling” ?

  11. FS says:

    One problem is that such challenges lead to more research to prove something that has been evident for a long time – that anthropogenic climate change is happening. At this point it would be so much more important to focus on the consequences of climate change – on how to mitigate and how to adapt.

  12. Rick Covert says:


    Is there any data collected to demonstrate a decrease in temperature in the upper stratosphere over the same period of time the lower troposphere has demonstrated a warming trend? That would seem to me at least to be the smoking gun that lays this natural warming cycle claim in the hypothetical trash bin.

  13. Jim Eager says:

    Pulse asked: “Why could the atmosphere and climate not get increasingly warmer and at a faster rate, as “the ball gets rolling” ?”

    Because Earth experienced maximum post-glacial warming forcing 6000-8000 years ago. It’s called the Holocene Climate Optimum. Ever since then northern hemisphere high latitude summer solar insolation has been *declining*, as has Earth’s long term average surface temperature trend.

    Until now, that is.

    Why trust a naive “feeling” when there is science?

  14. Anne van der Bom says:

    “Some unknown force is causing climate change. You can not prove that this unknown force doesn’t exist, therefore I am right”

    “God created this planet. You can not prove that God doesn’t exist, therefore I am right”

    You silly CP’ers. Don’t you see? Dr. Spencer is right! And no one of you is smart enough to prove him wrong.

  15. question says:

    @350 Now

    I just left my comment at the White House (202-456-1111). Had to wait a while, but it was very easy. I didn’t even have to listen to horrible “muzak” (there is no hold music)!

    I agree that we have to get more active. I try to make a comment on an article online, or write my representatives (local, state or federal), or otherwise make my opinion heard everyday. Comments on Climate Progress, while fun don’t count! That’s preaching to the choir!

  16. Joan Savage says:

    I’m yearning for some weather forecasting that incorporates the “human fingerprint” features.

    If not forecasting, then at least some near-future testable hypotheses about changes in the PDO and other phenomena that tie “climate” to “weather.”

  17. Tim says:

    It is pretty amusing that Roy Spencer promotes the ‘null hypothesis’ in response to evidence that the world is warming, but promotes the ‘God hypothesis’ in the face of evidence that evolution proceeds via random mutations and natural selection of biological variants with enhanced survivability. And this guy is one of the two leading “Skeptics”? Pretty pathetic.

  18. Rob Honeycutt says:

    I don’t want to spend too much time defending the likes of Spencer, but I’m not sure he’s “Young Earth” creationist. It’s important to be careful how one paints others in broad strokes lest the same be applied to you.

    That said, I think his belief in the dubious Intelligent Design idea mirrors his dubious positions on climate.

  19. Zetetic says:

    Actually all Dr. Spencer has to do is point to one credible scientific study showing a correlation of global warming with a natural cause, too bad his side hasn’t been able to do so for decades now.

    I wonder why the lack of supporting evidence for a natural cause for warming?

    No doubt Dr. Spencer’s position as a cdesign proponentsists came about because there is…. “no one peer-reviewed paper that has ruled out intelligent design, as the cause of all of the changes in the fossil record.” Or something very much like that.

    How very ironic that when there is plenty of evidence for a natural cause for the diversity of life, he ignores it in favor of a belief in an “intelligent designer” (*cough* god *cough*) that has no supporting evidence. Yet but when there is plenty of evidence of “intelligent” life (the intelligence being somewhat debatable) causing warming, he ignores it in favor of an evidence free belief that the warming is natural.

    How odd for a scientist to keep siding with the position that is not supported by evidence, but rather dogma.

  20. Zetetic says:

    @ Rob Honeycutt:

    I agree, just because he has come out as supporting I.D. doesn’t make him a Young Earth Creationist, there are Old Earth Creationists too.

  21. PB says:

    There is considerable supporting evidence for the hypothesis that the warming is anthropogenic, and none for his cloud hypothesis; in fact he has to invoke an “unknown mechanism” to explain things. Until he produces evidence for a mechanism, his hypothesis is not the most parsimonious explanation and should be rejected.

  22. David B. Benson says:

    This has been well understood since before 1979.

    Charney et al. 1979 NRC/NAS report:

  23. Anne van der Bom says:

    “there are Old Earth Creationists too”

    I didn’t know these were to be taken seriously. ;)

  24. Raul M. says:

    One woman went to the natural world
    to become worldly.
    Another woman went to the nicely
    built-up world to become worldly.
    That the two women emerged with
    markedly different views of the world
    should be notable. Then they switched
    paths to see as the other had seen.
    Again the difference each experienced
    Should be notable.

  25. Chris Winter says:

    Pulse oximeters wrote: “I’m of the feeling that global warming is/has been natural until recently.”

    I believe the assessment of the Milankovitch cycles is that they would lead to a gradual cooling without human intercession. Recently retired scientist William Ruddiman hypothesizes that human intercession in climate started some 8,000 years ago, and prevented the growth of ice sheets in Canada. (The book is Plows, Plagues, and Petroleum. See my review here: )

    And, in Storms of My Grandchildren, James Hansen declares that there will be no more ice ages as long as human civilization persists. The reason is twofold: humans will always produce trace levels of greenhouse gases, and the Sun will continue to brighten.

    As for the rest of your idea, natural forces could well add to the warming. The Sun is expected to do just that, once it comes out of its present quiet phase.

  26. Zetetic says:

    PB said:

    Until he produces evidence for a mechanism, his hypothesis is not the most parsimonious explanation and should be rejected.

    I agree completely, but it’s not surprising since so-called “Intelligent Design” has yet to even accept that it needs to define the term “information”, or come up with a mechanism for it’s “designer”. So I’m not surprised that Dr. Spencer makes such an error. It’s just another example of “crank magnetism”.


    Anne van der Bom said:

    I didn’t know these were to be taken seriously.

    LOL! I certainly don’t! ;)
    At least they don’t deny as much of the science as the Young Earth types do. I never could understand understand the insistence that the planet is about half as old as the invention of beer.

  27. knoxpk says:

    In this case the denier is arguing that spewing 700 billion tons of carbon into the atmosphere annually (plus massive amounts of other GHG’s) is neutral — has no effect on climate. If this were an infinite sphere we were perched on, it would make perfect sense. Since it is not seems to me his argument is absurd on its’ face. The natural carbon sinks that exist are saturated i.e. the oceans and hence the increasing acidity of the oceans — also deforestation allows for less carbon to be absorbed. That’s an awful lot of crap being spewed into the atmosphere by mankind and to argue it is having no effect defies credulity and some pretty basic scientific laws it seems to me.

  28. Adam Gallon says:

    Lubos Motl drives a coach and four through the arguments presented by Dana.

    And if our activities prevent another ice age, then we should be eternally thankful. Cold kills.

  29. Rob Honeycutt says:

    Adam Gallon… Wow. It’s amazing how many things Lubos can get wrong in one post.

    “Cold kills.” Really? Funny how species seem to have survived numerous glaciations but extinction events tend to center around hot events. See: Siberian traps.

  30. Dappledwater says:

    Lubos Motl…………let me guess, leprechauns & rainbows?

  31. Marc A says:

    Without saying what he means by “recent”, Spencer is playing to the anti-science choir and deliberately trying to confuse the public. Over any long-term, warming by natural variability is impossible and he knows it. Over an undefined short term, well, sure, a good El Nino has more influence on temperature than the underlying warming trend.

    Be that as it may, it’s the incremental warming that will toast us, not the natural swings.

    There’s no reason to listen to anything he says.

  32. Mickey says:

    I generally concur with much of what is said. The climate sensitivity of around 3C for doubling CO2 sounds about right as the CO2 levels have risen from 280ppm to 380-390ppm today and the temperature during that period has risen by around 0.7C. I do however believe that until recently, the warming was at least partially natually, however if everything was natural, we should be cooling now, not warming. As for certainty, nothing is every certain and if a credible scientific study is able to debunk AGW I would support it, but none really has yet. There may be some scientist that dispute man made global warming but many look at a natural factor and match it with temperatures changes and if shows some correlation they assume it is the reason. The problem with this is nothing will have a perfect correlation with temperature change and likewise there are other things such as ocean oscillations, sunspots, orbital forcing, and volcanos which also have a correlation. The point is CO2 rises do have a strong correlation especially over the last 20 years, while less so before that. I should also note that I tend to think warming will be towards the lower ends of the projection, not because I think Co2 has less impact than it does, but rather I think it is unlikley CO2 will reach 750ppm. I think it will likely be between 450-500 ppm as people are more environmentally conscious and also the developed countries who produce the most CO2 have birth rates well below replacement level. In fact in a 100 years, I think Europe’s population will likely be less than half of what it is today and for those who mention immigration making up the gap, most European countries are bringing in very restrictive immigration policies and I only expect them to get stricter as time passes. Likewise in developing countries, I expect birth rates to fall as they grow economically since economic growth means higher costs of living and the more expensive it is to live the harder it is to live on one income and thus people will have fewer children. The one thing though that could cause warming above the mid point of the IPCC projections is if the permafrost melts and this releases the methane trapped inside which would cause runaway warming as methane is 57X as potent as CO2. And because there is a chance of this it is probably best to take action to ensure it doesn’t happen.

  33. David B. Benson says:

    Chris Winter @25 — Its more subtle than is generally realized. The current minimum in insolation @ 65N is the least minimum in over 5 million years. It might not have been enough, even without human caused concentration changes in global warming gases, to have triggered a glacial; opinions vary.

  34. Mulga Mumblebrain says:

    Spencer and Lindzen are, in my opinion, thoroughly discredited cranks. Their motivations must, I believe, be base. Their utility is in giving a pseudo-scientific gloss to denialist dross so that the denialist Dunning-Kruger rabble can say ‘Look these ‘scientists’ say this, so I’m really clever because I believe it too, so you’re wrong and a Communist watermelon and Al Gore is still fat’. For pity’s sake, it must be long past time to cease mentioning Spencer and Lindzen’s existence. It’s a waste of time addressing their inanities and it just feeds their egos. Ignore them and let them sink into well merited obscurity.

  35. Dana says:

    Cool, thanks for the repost Joe. And thanks to Motl for directing his 7 or so readers to my post, too!

    Spencer’s argument that we should assume warming is natural despite the large anthropogenic radiative forcing is simply unscientific. He’s becoming ever more nonsensical. Probably attending too many “skeptic” conferences.

  36. Andy Bauer says:

    Maybe S and L are channeling an old skit from SNL:

    “I must have stupidly glossed over my Uvula!”

  37. riverat says:

    Rick Covert, #12

    I believe they get stratospheric temperatures from the same satellites they get tropospheric temperatures from.

  38. Zetetic says:

    @ Rick #12:
    Sorry I missed your question earlier.

    On Climate Progress there a chart at little bit down the page in this article about Joe Bastardi.
    How many major scientific misstatements does Joe Bastardi have to make before In-Accuweather fires him as their chief long-range forecaster?

    But there are also links to papers in this article by Skeptical Science.
    The human fingerprint in global warming
    Be sure to check out the “Advanced” tab in the article for more detailed info.

    I hope that helps!

  39. Dana says:

    Rick #12 – as Zetetic noted in #38, I’ve written about the cooling upper atmosphere as well:

  40. Rob Honeycutt says:

    Everyone needs to take note of this one. Dana got onto Lubos Motl’s site and posted several polite responses to where he misunderstands the tropospheric hot spot.

    No more than three posts later Motl bans him for “not being worth him time.”

    Motl folded like a cheap suit at even the lightest of challenges.

    Kudos to Dana!

  41. Zetetic says:

    @ Rob Honeycutt:

    Of course Motl banned Dana.

    Dana cheated by playing both the “Facts” and the “Logic” cards, instead of the “I’m going to rationalize this to fit the Dogma” card that the denialists insist everyone always should use. ;)

  42. Rob Honeycutt says:

    Today Dana has a response to Motl on Skeptical Science.

    It’s appropriately titled Motl-ey Cruel.