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What's causing global warming? Look for the fingerprints

Posted on 17 September 2013 by John Abraham

Scientists are a skeptical bunch. We never accept claims without evidence and we spend large parts of our careers trying to show that other scientist's claims are wrong. This self scrutiny is one of our best traits, and it is a major reason why science advances over time.

With this said, it often surprises people that scientists are in such strong agreement about human impacts on the Earth's climate. Many studies, including research by Doran and Zimmerman, Anderegg and colleagues, and more recently by my colleague's team, Cook et al., have shown conclusively that the world's climate scientists agree, to about 97 percent, that humans are significantly impacting the climate. But many people ask, how can they be so sure?

There are a number of reasons why we know humans are causing many of the changes we are seeing today. Among them, is the use of attribution studies, often called "fingerprinting". Scientists look at the patterns of climate change and ask, do they have the fingerprint of natural variation, or humans?

One of the most well-known climate change attribution scientists is Dr. Benjamin Santer. He and his team have developed tools to separate natural climate variations from human-induced changes by using a number of different tools. Their latest work was just published in the Proceedings of the National Academy of Sciences and is titled "Human and Natural Influences on the Changing Thermal Structure of the Atmosphere".

The method is somewhat complex; it involves the comparison of climate observations with the output of climate models. Specifically, they compared satellite observations from two different groups, with output from 20 climate models that participated in the most recent Coupled Model Intercomparison Project (CMIP-5). In the models, they calculated what the Earth would be like without us. The "world without us" scenarios have natural changes to the environment caused by volcanoes, the Sun, and internal climate variability (phenomena like El Niños and La Niñas). They wanted to know whether the "world without us" could have displayed the types of changes to the climate that we are seeing today.

Next, the scientists calculated what the Earth would be like if human emissions had occurred, but natural variations in volcanoes and the Sun had not. These "human only" simulations tell us what we expect the impact to be from greenhouse emissions alone; they give us an estimate of the human "fingerprint".

Finally, the models were used to estimate the amount of internal variability in the climate, without human impacts or forced changes from volcanoes and the sun. This third step quantifies the impact of things like El Niños, La Niñas, and other natural variations.

With these three calculations complete, the scientists then went to the observational record, extracting data from satellite measurements of the Earth's climate. They searched the measurements for the "human only effect" by comparing the measurements to the three sets of simulations. In particular, they looked at the signal-to-noise ratio, which helps tell them which of the three solutions ("world without us", "human only" or "natural variability") fit the observations best.

What did they find? Certain patterns emerge that are consistent with the "human only" scenario. For instance, the heating of the lower atmosphere and cooling of the upper atmosphere, which satellites clearly see, could only happen if human emissions were the culprit. But the study went further; they actually stacked the deck of cards in favor of nature. They used solar and volcanic variations much larger than those that actually occurred since 1979. The strategy was to see if even a worst case "world without us" could be made to look like the current measurements. But, even that didn't work. The human influence still stood out.

Perhaps the best summary is in the abstract of the paper.

"We show that a human-caused latitude/altitude pattern of atmospheric temperature change can be identified with high statistical confidence in satellite data. Results are robust to current uncertainties in models and observations … Our results provide clear evidence for a discernible human influence on the thermal structure of the atmosphere".

Click here to read the rest

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Comments 1 to 16:

  1. Arguing about climate change is sort of a hobby of mine, and the SkS "fingerprints" list has been a primary weapon in my arsenal since I discovered it. I've done a lot of such arguments, and to this day I've yet to see a "skeptic" who can respond to it.

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  2. Just a heads-up for all SkS readers. The Skeptical Science 'escalator' gets a mention on this Aussi blog. Well worth a look for a few laughs;

    Click the graph.

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  3. hank_ - Indeed, that's hilarious. They posted something attempting to diss the SkS Escalator emphasizing year-to-year variations (strawman, nobody seriously discusses trends from yearly varations) and portray a 12 year time-span for their "rational observers" (which is a prime example of the 'too short for significance' timespans shown in the SkS Escalator, uncertainties of about ±0.168 °C/decade 2σ).

    Quite frankly an "own goal" for ACM, it makes them look quite the fools. 

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  4. KR, I agree... except that their 'fans' are clearly too delusional to understand how stupid that ACM graph is.

    Heck, the graph even winds up showing a small warming trend. Thus, the only 'knock' against SkS in the entire piece is in denouncing our "OMG we're all going to DIE" alarmism... and that's just a rather pathetic lie given the lack of any page like that on this site.

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  5. Not wishing to get into a debate of the merits of climate change as a whole, but i do see a flaw in the modelling used. Firstly, to do a model without human intervention requires a set of assumptions to be made that may or may not be accurate, secondly the Human only model requires a lot of assumptions to be made that may or may be accurate, and then they use the output to produce a model into which they add natural variations..

    That is flawed science in my humble opinion, regardless of the truth about human impacts on the climate. People can pat themselves on the back all they like, but I am skeptical about the accuracy of such models.

    We can argue about the impact of human activity on warming the climate all we like, but it is a proven fact that some human activity actually causes a cooling of the climate, was this taken into account in the models in the SKS study? I'll take a bet that it may have been overlooked or simply ignored as an "inconvenient truth" and the problem with that is that all your models are then inaccurate.

    Lastly, has anyone completed a study to see how the chemistry of humanity and the chemistry of nature interact and impact the cliamte, the assumptions made about natural variations may be wrong because the chemistry if the atmosphere may be vastly different (good or bad) without humans dumping millions of tonnes of gases into the atmosphere each year.

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    Moderator Response:

    [JH] Let's move this discussion to a more appropriate thread, i.e., How reliable are climate models?

  6. LandyJim, can you point to a model of real-world phenomena in any area of science that is not subject to your criticisms.  All models of real-world phenomena fail at accuracy.  Some models are useful; some are not.  You can toss all the climate models out the window and still have excellent confidence that the climate system will store more energy with an increase in atmospheric greenhouse gas concentration.  In fact, you can have high confidence that a doubling of atmospheric CO2 will result in about +1C equilibrium increase at the surface, without adding in any feedbacks.  No GCMs necessary.  

    I don't know what you mean by "human intervention."  Do you mean the different likely emissions scenarios?  Models definitely incorporate those.

    What SkS study?

    What do you mean by the "chemistry of humanity" and the "chemistry of nature"?  What do you think of when you think of each of those?

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    Moderator Response:

    [JH] Let's move this discussion to a more appropriate thread, i.e., How reliable are climate models? Thank you. 

  7. LandyJim - I would suggest reading through the Models are unreliable thread, followed by How do Climate Models work, and if you wish to commenting there. 

    Long story short - climate models are based on physics, fed forcings, and progress from there. Neither global nor regional observations can be generated from those physics unless anthropogenic forcings are part of the input.

    I'll note that claiming that major factors '...may have been overlooked or simply ignored as an "inconvenient truth"', without evidence, is rather contrary to the"No accusations of deception" portion of the site Comments Policy

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    Moderator Response:

    [JH] Let's move this discussion to a more appropriate thread, i.e., How reliable are climate models? Thank you. 

  8. Noting above the commenters who want to talk about the reliability of models, I think the article should mention that model-based fingerprinting is complementary to basic physical fingerprinting like carbon isotopes. Of course one has to accept that CO2 is a GHG to begin with for any of it to make sense. :/
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  9. GFW @8, we should distinguish between fingerprinting the rise in CO2 levels as anthropogenic (which can be done by carbon isotopes), and fingerprinting the rise in temperatures as being caused by increased greenhouse warming which cannot).  I will grant you that SkS discussions of the topic often fail to distinguish this point, but never-the-less we shoud do so.

    Further, the fingerprinting above is not "model based" in any except the most trivial sense.  Any scientific theory is a model.  It specifies certain empirical relationships between some measurable quantities and some other measurable quantities.  In some theories, the models are very simple, and can be solved algebraicly with linear equations.  Others are more complex.  As it happens, the subject matter of climate science is so complex that detailed predictions can only be made by large, complex computer programs.  Use of such programs to determine what the theory predicts, however, is no different in principle than using Newton's second law of motion to determing the force applied to an accelerating body.

    The denier objection to computer models is, fundamentally, an insistence that the predictions of climate science be kept to the level of handwaving, ie, the only level at which their "predictions" can appear to survive empirical tests.

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  10. This is my recent comment at in agreement with Dr. James Stalker. Here I want to report ...

    Simple example to explain James Stalker’s thought of how erroneously climate models are built.

    Lets assume a forest is analogous to a climate model. Like forest has many living things from insects to wild animals, climate models have many physical processes having local scale to global scale, and time scales of fraction of seconds to centuries.

    Some insects have foot-steps of a fraction of micro-meter and other animals have of few meters. Moreover, few animals are brisk and other are slow. That means, different animals have different temporal and spatial steps. For example, if we are to model a forest with all kinds of living things in the forest, we need to take all these steps (temporal and spatial) of all living things into account. For example, first lets assume we have chosen spatial resolution of 10 meters and time step of 2 sec. Lets imagine how reliable these resolutions for all living things in a forest. It may be right for deer but, can an ant jump 10 m in 2 sec? In the same way, each climate process analogous to each living thing in a forest, has different temporal and spatial time step. Hence, if we are to model climate, we need to take all these steps ( temporal and spatial) of all physical processes of climate into account.

    If we don’t take these suitable resolutions(temporal and spatial) for various variables, we are going to ask an ant to jump few meters in second or two, and a deer to rolling on the floor and many more clearly visible blunders with animals. At the present situation, climate community is asking the models to do mistake as this forest model example, and artistic science is saying 97 % community is in agreement with anthropogenic global warming !!!

    I am a novice in climate research. So, lease let me know if any climate model takes care of such things in building model so that we can rely on them. But, from my perspective, the utility of such model are only to understand better about climate system intrecacies with a lot of uncertainty ...

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    Moderator Response:

    [JH] Let's move this discussion to a more appropriate thread, i.e., How reliable are climate models? Thank you. 

  11. Kishoreragi,

    Since you are a novice why do you suppose professoinal scientists who dedicate their lives to making climate models would overlook such basic steps as your example?  The models are adjusted so that different processes are modeled appropriately.  Your analogy to a forrest is simply a way to avoid having to point to a specific problem with the climate models.  Please provide an example of a problem with resolution in a climate model.   If an example cannot be provided, with all the "auditing" that skeptics do, that suggests that your objection is simply an argument from ignorance.  Just because you do not know how to model climate does not mean no-one knows how to make suitable models.  Read some of the modeling threads here or at Real Climate and you will be more informed.

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    Moderator Response:

    [JH] Let's move this discussion to a more appropriate thread, i.e., How reliable are climate models? Thank you. 

  12. Kishoreragi, to add to Michael's comment: that is the way general circulation modeling is done.  The Earth itself is one big experiment, and it has been well-demonstrated that the minor variables vary within certain ranges and rarely, if ever, end up leading global or even regional climate by the nose long enough to significantly alter major elements of general circulation.  There is a strong tendency to regress to the mean, and the mean is driven by the major elements of the climate: continental drift, orbital variation, vulcanism, solar output, collision with significant extra-terrestrial objects, and, now, artificial enhancement of the greenhouse effect.  Everything else is a feedback: something that reacts but is unlikely to change on its own -- biosphere (with rare exceptions), snow/ice albedo, ocean carbon cycle, clouds, water vapor, natural greenhouse effect, etc.  Feedbacks vary and are integrated in different ways, but they regress to the mean of net forcing, with minor temporal variations driven primarily by ocean thermal capacity.

    Thus it's a little inaccurate to describe the climate system as comprised of components that vary but are all of equal or near-equal power in shaping the future of the system. And, in addition to the major forcings, modeling does take into account many of the major and minor feedbacks.  Of course, in the short-term (months to years), the interplay of major and minor feedbacks can produce significant but temporary anomalies, but the resonance of those anomalies across the long-term trend is ultimately insignificant. Arctic sea ice (ASI) is a great example.  ASI is never, with all forcings stabilized, going to vary strongly and consistently to the extent that a glacial cycle is initiated.  Only orbital or solar variation (or a one-timer) can do that (partially through the mechanism of snow/ice albedo feedback).  

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  13. Ok, JH - copying the comment over now.  

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    Moderator Response:

    [JH] Muchas gracias.

  14. Hi all.
    I am a Molecular Biologist, and hence has a reasonable understanding of various sciences.

    I am also passionate about wildlife, both observing for pleasure on visits and also the crucial need to conserve dwindling habitats around the earth.

    The topic of this thread, which I stumbled across in reading the Guardian last week, is just what I have been looking for to pose some questions to see if they can be answered to help me with my utter confusion over the massive consensus with regard to the cause of global warming. Given my background, I hope you see that I have a decent level of knowledge of the science, as well as no agenda.

    1. Firstly, the earth is heating up right now, so no argument there. The earth has always heated and cooled, the distinction is the focus of most arguments these days is that "is this current heating man-made? And if so, is CO2 the main driver?"

    2. The earth has always experienced changing levels of atmospheric gasses. Indeed, when life started about 3bn years ago, almost no oxygen existed as gas, rather a massive amount of CO2 existed along with nitrogen. Billions of years (2.5bn) had to elapse of just pure simple, single-celled organisms changing the atmosphere spectacularly – they generated all the Oxygen we see today in the atmosphere (~21%) and indeed they removed all the CO2 to levels we see today = almost nothing / 0.03% = 390/400ppm by concentration is it right now?

    Indeed it is fairly miraculous that all the plants/photosynthetic organisms on earth – land and sea based - actually manage to get enough CO2 to fix in to sugars and other Carbon-based molecules. It is a miracle of efficiency that they do.

    We should note that many farmers increase CO2 concentrations many times atmospheric concentrations in enclosed environments in which plants thrive. They take up CO2 more quickly and efficiently, event at concentrations 3x, 4x, 5x atmospheric levels, which is still well under 1% of air.

    3. During the 2.5 billions of years when life was in simple form and extracted CO2 from the atmosphere, and indeed accelerated for much for the 500 million years since complex life appeared, this was the period during which all carbon-based ‘organic’ layers formed under seas and oceans. Important to stress that the primary “lock-up” of this carbon during this period is Limestone’s and similar rocks formed by millions of years of sediment layering of organic matter, mainly from foraminifera carbon fixated via CaCO3 and locked away. All the Alps, Himalaya, Andes etc contain massive amount of this rock that was once gaseous CO2 millions and billions of years ago during period when life thrived on earth. A far smaller proportion of later organic matter (500myo-1myo) had the right conditions to enable formation of oils, gasses and coals i.e. fossil fuels.

    4. The earth has always heated and cooled. Is it a problem that ice caps melt and low lying land disappears? In geological terms, “no” is the answer. In human times, “yes” is the answer for those effected, but the old biblical story has to apply here about the wise man not building his house close to the cliffs. The earth changes, life evolves, it always has and always will. Better to plan for and embrace change surely.

    5. CO2 as a concentration of 0.03% of the atmosphere is accused of being the primary driver of man-made global warming. I have to ask – has the climate change movement not perhaps made a huge mistake in blaming CO2 for warming?

    It is colourless and only reflects a very small fraction of heat-wavelengths. On top of this, its tiny concentration in the atmosphere, to any scientist this has to raise serious questions as to it being a key driver? Even if you add in all the carbon-based gasses, they are combined still under 1% of air!

    6. There has been some objection to model accuracy here and indeed there is another thread covering this exact topic. Indeed, I build models. I know their pros and I know their weaknesses. I have never seen a model that is accurate. It is impossible for a model to be accurate. The more complex the subject, the less accurate the model is.
    I see people arguing over super complicated formula and how powerful cray supercomputers are and even saying “look, I back-modelled history and it correlates”. None of this is accurate. Simple reason being is that you will AWLAYS miss a variable input. The best example of this is the recent global financial crash. Thousands of the brightest physicists, mathematicians and generally “clever” people in the world chased money, became bankers and build models to say that asset-based securities were almost risk-free, or AAA-rated. Trillions of dollars piled in to them. EVERYONE agreed that they were AAA risk-free as the super bright people said so as their models said so, everyone followed, no one disputed. Then, one variable the models did not include since there was historically a 0.01% chance of it happening, happened. The simple fact it CAN happen, means one day it will. This is just one variable. There are thousands. This proves models have to be treated with extreme caution. They can NEVER be relied upon fully.

    Indeed, one of the papers I saw on this site that provides the computer model “proof” that a doubling of CO2 from 0.03% to 0.06% would leads to a 2 degree rise in temperature stated an incredible thing. Namely that the results do not include the effect of water in the atmosphere!?! When it is pretty much agreed that water is a FAR greater contributor to the green-house effect than any carbon-gas, it to me is astonishing that everyone seems to ignore this monumental fact.

    Could water not be the cause of global warming? We have chopped down trees over the past 5,000 years on a huge scale. This might have changed water cycle and increased cloud cover and hence global warming perhaps? I don’t know, but am just saying this strikes me as being far more powerful and feasible than a colourless gas that has concentration of 0.03% right now and been historically many, many 1,000+ times higher concentrations on earth and life and earth did just fine during those times!?

    One further point I’d like clarity over, is that CO2 concentrations have risen in the past when temperatures has risen. Yes. But is CO2 a cause, or an EFFECT of warming? We all know what happens to water bodies when heated….they release gas. The delay in it being a cause driver or an effect of heating is impossible to prove by samples, rather people have relied upon the fact CO2 reflects a tiny fraction of heat wave length and they have then used computer models to say it is a cause-driver. Are you sure this is not wrong? Would you bet your house on it like bankers did with their models?

    I’m not being argumentative. I just would appreciate some constructive straight, common-sense answers against any of the above to help me be ‘converted’ as 97% of scientists seem to be. Right now, I’m in the 3% who don’t currently think CO2 is responsible in the slightest.


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    Moderator Response:

    [DB] As noted by KR and DSL below, threads at Skeptical Science have a narrow focus by design, as long comments spanning a multitude of subjects (such as yours) do not contribute to a healthy dilogue, but often serve to impede understanding

    Before you follow up on each salient thread indicated to you by KR, please familiarize yourself with this site's Comments Policy

    Any needed reponses and clarifying questions should be done on those more appropriate threads, and not here.

  15. Undecided Molecular Biologist - That's quite a collection of arguments/questions, most of which show up in the SkS list of 'skeptic' myths. 

    Regarding your points and the associated myths:

    1. Climate has changed before, also  It's not us.
    2. CO2 was higher before.
    3. No idea what you're trying to say here other than "rocks".
    4. It's not bad, Plants and animals can adapt.
    5. CO2 is a trace gas.
    6. Models are unreliable, also It's not us and CO2 lags temperature.

    I believe the moderators would prefer each point be addressed in the appropriate thread(s). Far more importantly in regards to the structure of your comment, you should read about Gish Gallops, as your comment is really just a series of climate myths with out reference to evidence. 

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  16. Yah, what KR says, Molecular Biologist, and I'd add that you'll probably want to engage the point that seems most fundamental to your understanding at this point.  You'll get a response wherever you post, and so if you post in ten places, you're going to be juggling quite a few threads, and those threads will have a good deal of overlap.

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