The Nature of Scientific Consensus

Cross-posted from NextGen Journal

It is common for one to fail to grasp the difference between “consensus” and “unanimity”.

A consensus does not require agreement from absolutely every member involved. Rather, it is a more general measure of extremely high agreement, high enough to accept and base decisions on. It’s stronger than a majority-rules style of democracy, but does not necessarily equal unanimity. In fact, in the area of science, where the concept of consensus is particularly important, unanimity is nearly impossible.

With the exception of pure mathematics, scientific theories cannot be proven beyond a doubt. Every physical process that researchers study has some amount of irreducible uncertainty – because there is always, no matter how small, a chance that our understanding could be completely wrong. Additionally, science is never “settled”, because there is always more to learn, whatever the field. Even a law as basic as gravity is still being studied by physicists, and it turns out that it gets more complicated the more you look at it.

Despite this inherent uncertainty, scientists have developed consensuses around all sorts of topics. The Earth is approximately oblate-spherical in shape. Smoking cigarettes increases one’s risk of lung cancer. HIV causes AIDS. There’s a tiny chance that these statements are incorrect, but researchers can still have confidence in their accuracy. Incomplete knowledge is not the same as no knowledge.

However, when there is room for doubt, there will usually be doubters. Physicist Richard Lindzen continues to dispute the health risks of smoking (a conversation is recounted in a recent book by James Hansen). Peter Duesberg, an active molecular and cell biologist, prominently opposes the link between HIV and AIDS. Believe it or not, the Flat Earth Society was alive and well until the death of its leader in 2001 – and signs of the society’s renewal are emerging.

As these examples suggest, for a layperson to wait for scientific unanimity before accepting a topic would be absurd. When consensus reaches a certain point, the null hypothesis shifts: the burden of proof is on the contrarians, rather than the theory’s advocates.

Another case study that may seem surprising to many is that of anthropogenic global warming. A strong scientific consensus exists that human activity, mainly the burning of fossil fuels, is exerting a warming influence on the planet’s temperature, which is already beginning to show up in the instrumental record. This phenomenon is contested by less than 3% of publishing climatologists, a negligible amount of peer-reviewed scientific studies (so few that not one showed up in a 2004 survey’s random sample of almost one thousand papers), and no major scientific societies internationally. Additionally, scientists who dispute the existence or causes of climate change tend to have lower academic credibility than those who do not. It becomes apparent that this scientific question warrants “consensus” standing: never quite settled, never quite unanimous, but certainly good enough to go by. The mainstream media does not always reflect this consensus accurately, but it nonetheless exists.

As world leaders meet in Cancun this week to discuss a global policy to prevent or limit future climate change – a prospect that looks less likely by the day – science can only offer so much advice. Climatologists can approximate what levels of emissions cuts are required to prevent unacceptable consequences, but only when the governments of the world decide which consequences they are willing to accept. Can we deal with worldwide food shortages? Rising sea levels? What about a mass extinction? Even after we define “dangerous consequences”, scientists are unsure of exactly how much temperature change will trigger these consequences, as well as how much greenhouse gas emissions will need to be cut, and how quickly, to prevent the temperature change. All they can offer is a range of probabilities and most likely scenarios.

But remember, incomplete and uncertain knowledge is not the same as no knowledge. Of one thing climate scientists are sure: the more greenhouse gas emissions we emit, the more the world will warm, and the harder it will be to deal with the consequences. There’s no reason for you and I to doubt that simple correlation any longer.


24 thoughts on “The Nature of Scientific Consensus

  1. For what little impact even a unanimous agreement on climate change will have on world leaders, scientists might just as well be measuring the speed of gravity.

    I assume you mean acceleration. I had to measure the acceleration of gravity in my first physics lab…not so much fun. -Kate

    • I had to measure the acceleration of gravity in my first physics lab…not so much fun.

      Mechanical engineers love to measure the acceleration of gravity, but no one has measured the speed of gravity! Scientists have determined that the speed of electromagnetic radiation and light waves or photons through space is about 300,000,000 meters/second.

      Gravity is a different beast altogether. The theory is that each and every particle of mass in the universe, no matter how small, is attracted to each and every other particle of mass in space by the flow of some undetectable gravitational fluff. What is it, really?

      When I sneeze, my movement should cause a slight gravitational perturbation of a particle a billion light-years away. When will that particle feel the gravitational effect of my sneeze? Instantaneously? A billion light-years later? No one knows, but scientists believe the answer lies somewhere between the two. If the correct answer is instantaneously, then the speed of gravity must be infinite! How else would the message get propagated across such a great distance? If the answer is the speed of light in a vacuum, then, according to many scientists, the universe must go unstable!

      Is it possible that gravitational attraction can travel faster than the speed of light? According to Einstein, the answer should be no. But according to the stability of the universe, the answer should be yes. Now if one believes that the universe is uniformly expanding, then mass at the observable edge of the universe is traveling relative to us at the speed of light, and beyond the detectable edge of the universe, mass should be traveling relative to us faster than the speed of light! If that is impossible, then somebody’s theory must be wrong. Is the universe uniformly expanding or not? Can my sneeze have an effect on mass that’s beyond the edge of the observable universe?

      I guess we’ll have to wait for the scientific consensus on these questions.

      I’m still trying to figure out which is more difficult, climate science or the speed of gravity? I guess the answer will come if we ever see a consensus on one or the other.

      Cool! I had heard of gravity waves, but had no idea that gravity had a speed…makes sense, though. Thanks. -Kate

      • “Information” cannot travel at a speed greater than the speed of light. That includes gravity, under current theories.

        There are experiments in progress to detect and measure gratity waves. They have been unsuccessful so far, but a gravity wave is very weak indeed.

      • I studied gravitational waves and quantum gravity for the first half of my Ph.D., before switching fields.

        In all modern theories of gravity, gravitational changes propagate at the speed of light.

        Although gravitational waves have not yet been detected directly (the Advanced LIGO experiment should be able to do this), their speed has been measured indirectly. The 1993 Nobel Prize in physics was awarded for this work. The measurement was of the orbital decay of a binary star system: the rate of orbital decay is consistent with gravitational energy being carried away from the system at the speed of light, to within the experiment error.

        Gravity propagating at the speed of light does not imply anything about the stability of the universe. (I’m not even sure what that means.) Faster-than-light propagation would be inconsistent with relativity and its experimental tests.

        In a curved spacetime, it’s kind of hard to define “the velocity of a distant object”, or of the cosmological horizon of the observable universe. However, relativity does not impose any lightspeed restriction on how fast a “horizon” can move. It only imposes restrictions on how fast matter or energy can move with respect to other matter of energy at the same location. In particular, it is possible for the universe itself to expand “faster than light”, since there is no restriction on how fast spacetime can expand. See this FAQ.

      • Some Food for Thought

        In the Large Hadron Collider, Once or twice a day, as the protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting dipole magnets will be increased from 0.54 to 8.3 teslas (T). The protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV. At this energy the protons have a Lorentz factor of about 7,500 and move at about 0.999999991 c, or about 3 metres per second slower than the speed of light (c). It will take less than 90 microseconds (μs) for a proton to travel once around the main ring – a speed of about 11,000 revolutions per second.

        There will be two streams of protons traveling in opposite directions, each at 3 metres per second less than the speed of light. The opposing streams of protons will be deflected laterally to collide with each other at a relative speed, I’m guessing, of just 6 metres per second less than twice the speed of light. Or have I been hanging around Newton too long? If the relative speed between these particles must be less than the speed of light, then what has changed? Mass or time? And what is the true relative speed at impact? Could the resulting particle accelerations associated with the impact emit a pulse of gravitational radiation? If the magnitude of gravitational radiation were somehow proportional to mass and acceleration, might the colliding protons emit a measurable gravitational impulse? What if a swarm of neutrinos happened to be passing by at the instant of collision? Might some of these neutrinos be deflected ever so slightly by this gravitational impulse?

        Does Gravity Travel at the Speed of Light?

        Gravitational Radiation

        Yes, what they said… ☺

        A final note

        If Newton was proven to be not always correct, is it not possible that Einstein will someday be proven to be not always correct? Faster-than-light Might it be possible that Super String Theory will worm its way through 10-dimensional space and obsolete Einstein’s whole thought process? Heaven forbid.

      • I don’t know if I should pull the comments further off-topic of the article, but I suppose they loosely have to do with scientific consensus (on the speed of gravity). :)

        Two particle beams, each with a speed of 0.999999991 c (in some frame), have a relative speed of about 0.99999999999999996 c. Their relative speed never exceeds c. Relativistic velocities don’t add the way Newtonian velocities do (see this FAQ). This is an effect of time dilation and length contraction.

        An accelerating particle emits gravitational radiation, but the amount of radiation is indetectably small. We could never detect gravitational radiation in a collider in that manner. (If there are “large extra dimensions”, there are quantum gravitational effects which may be detectable, but I don’t think that’s what you’re referring to.)

        It’s possible that Einsteinian physics may be proven incorrect; in fact, general relativity is almost certainly “incorrect” in that it can’t account for quantum phenomena. But that doesn’t imply that the speed of light restriction is going to be disproven and we’ll return to Newtonian physics. That’s an exceedingly implausible way for relativity to be wrong. The speed of light is deeply tied into the geometry of spacetime and causality itself, and the Lorentz symmetry of relativity is integral to all phenomena in high-energy physics. We’re not in a Newtonian world.

        In particular, string theory (like quantum field theory) is explicitly constructed to obey local Lorentz spacetime symmetry, and so is an inherently relativistic theory by definition.

      • @Roger:

        You’ve been hanging around Newton too long. :)

        We’re dealing with relativistic velocities here. You can’t just add two velocities together: that’s an approximation that only works when the Lorentz factor is damn close to 1. (in lay English, the closer the speeds in question come to lightspeed, the less they behave like you’d expect with Newton.) Wiki actually has a pretty decent article on this (I was surprised) at Velocity Addition Formula; it also explains what’s going on (in a cursory way) and shows the difference in the two perspectives. (Note that the relativistic one reduces to the Galilean one at speeds much lower than lightspeed.)

        Blogs are a horrible format to explain this… I’d suggest reading the Feynman Lectures if you can find the time. It’s been said that they’re almost all any student would need for an undergraduate education in physics, and Feynman was an incredible teacher. (Also a personal hero of mine, but I’m a touch biased on that.)

        But yeah, in a nutshell, current theories show gravity propagating at the speed of light. Nathan’s comment is spot-on. And Einstein wasn’t right, to some extent – relativity doesn’t explain action at quantum scales. (Similarly, quantum mechanics breaks down on larger scales.) However, just as Einstein reduced to (pretty close to) Newton given Newton’s assumptions, I suspect that if anything royally disproves relativity, it’ll be reducable to relativity.

        Oh, and as for string theory… get back to me when it makes a testable prediction. ;)

        I feel very proud of myself because I actually understood that. Off to go study some more for my physics final. -Kate

      • I believe there is a great lesson to be learned from these discussions. Clearly I should not be dabbling in relativistic theory. ☺ And likewise, most politicians and news reporters should not be dabbling in climate science.

  2. People, not just politicians, will be driven to understand something when it affects them in a negative way.
    It took centuries but eventually, the human race did figure out the connection between disease and clean water. You might say, we have never looked back. Would anyone today deny this scientific theory and deregulate public health statutes to allow defecation into drinking water?
    When the connection was figured out in the 19th century, enormous sums were spent to ensure clean water supply and safe removal of sewage in all the world’s major cities.
    It will be the same with fossil fuels, except the vested interests in fossil fuels are stonger. But when the public demand clean energy, they will get it eventually. It is only a matter of time – the sad thing it that it may be too late for many on the planet, and there may be a price in one or more major disasters.

    Well said. -Kate

    • Been there, dude.

      While secrets from WikiLeaks were splashed all over the American newspapers, I couldn’t help but wonder: What if China had a WikiLeaker and we could see what its embassy in Washington was reporting about America? I suspect the cable would read like this: […]

      Along the way the [American] ambassador used his cellphone to call his embassy office, and in one hour he experienced 12 dropped calls — again, we are not making this up. We have a joke in the embassy: “When someone calls you from China today it sounds like they are next door. And when someone calls you from next door in America, it sounds like they are calling from China!” Those of us who worked in China’s embassy in Zambia often note that Africa’s cellphone service was better than America’s.

      But the Americans are oblivious. They travel abroad so rarely that they don’t see how far they are falling behind. Which is why we at the embassy find it funny that Americans are now fighting over how “exceptional” they are. Once again, we are not making this up.

      • Americans have ruined their hearing with loud music; how would they know? ☺ ☺

  3. Consensus: when the losers do not call for a count because they know they have lost by a huge margin.

    Nicely nuanced Kate, pity nuance is somewhat absent in today’s politics.

  4. But exactly how many cigarettes can I smoke before I get lung cancer? If you can’t tell me, then obviously you don’t really know that smoking causes lung cancer and I can keep smoking and your supposed scientific “consensus” is just a money-making scam.

    Or so say the inactivists.

    PS Tony: that’s a great definition.

  5. You got the point, Toby! It is true that you don’t mind things around unless you are personally involved in them. What everything has to be proved and justified so we start to think in a different way? Every reasonable person can reckon that the major impact on Climate change have the human’s activity. Why the others don’t understand this?

  6. While so many are beating up on the so called skeptics, may I remind you of the following.
    Consensus once held that the earth was flat.
    Consensus once held that the universe revolved around the earth.
    Consensus once held that life just appeared where there was no life before
    Consensus says that some will jump all over me for these statements.
    The facts are that continued discussion and experimentation showed that proven science and science by consensus can and has been wrong. Only these discussions and continued experimentation, across scentific lines moves the science along and eventually proves or disproves theories and consensuses. So lets be civil and not strident and let us be supportive of those who “cannot see” because the truth will out. All science is good science in the long run.

    • winnipegman, you keep trying to sound reasoned and open-minded, but in fact you’re repeatedly portraying yourself up as an impartial arbiter of everyone else’s behaviour and science. Given that you don’t actually have any science to offer to this discussion, I see your behaviour as the pinnacle of arrogance.

      (And need I remind you that you’re on record trying to downplay the cyber-attack against the CRU and the ensuing death threats against CRU scientists?)


    • Winnepegman,

      Required reading for you is “The Structure of Scientific Revolutions” by Thomas Kuhn. Science statys within the confines of a particular paradigm (Kuhn invented the word) until pushed out of by the weight of contrary evidence, or by a synthesis of the old and the new theories. Kuhn admits that is not a “scientific” process, you could say it is in the “Context of Discovery” (which can be irrational but innovative) rather than the “Context of Justification” (which must be rigorously scientific – logical and robust).

      But at the moment the global warming paradigm holds the field. It may be refuted or modified, but it explains the empirical data right now. It is surely irrational to reject it on the grounds that another theory MAY come around some day. At the moment, physicists and cosmologists are struggling to explain dark matter, but they have not suspended the Standard Model of the Universe in the interim.

  7. And interestingly, Kuhn got some things wrong too (i.e. in his view, a revised theory must completely replace an older one, instead of incorporating revisions into the old theory, and in his view there was no mechanism for evaluation of theories for replacement apart from accumulation of anomalies, meaning any replacement theory was as good as any other), but that doesn’t invalidate Kuhn so much as revise it. ;)

    Other philosophers that followed Kuh, particularly Laudan, may be more accurate, but Kuhn really is the seminal work and I heartily endorse reading him.

    Another tidbit: Kuhn has also been vastly misread by scientific illiterates to support their Galileo Gambits. Directly to the point, Philip Kitcher (another philosopher of science) has said:

    Thomas Kuhn’s book The Structure of Scientific Revolutions has probably been more widely read—and more widely misinterpreted—than any other book in the recent philosophy of science. The broad circulation of his views has generated a popular caricature of Kuhn’s position. According to this popular caricature, scientists working in a field belong to a club. All club members are required to agree on main points of doctrine. Indeed, the price of admission is several years of graduate education, during which the chief dogmas are inculcated. The views of outsiders are ignored. Now I want to emphasize that this is a hopeless caricature, both of the practice of scientists and of Kuhn’s analysis of the practice. Nevertheless, the caricature has become commonly accepted as a faithful representation, thereby lending support to the Creationists’ claims that their views are arrogantly disregarded.

    Note that this applies as well to any from of group of scientific illiterates who claim conspiracy from The Man to silence their dissent (i.e. climate denialists), especially if they think that Kuhn supports them.

    In order to wear the mantle of Galileo, it is not enough to be persecuted by an unkind establishment. You must also be right. And so far, the denialists aren’t actually making any steps to show that they’re right, and yet they expect the paradigm to shift anyway.

  8. [citations needed – deniers have received death threats too]

    Toby and Brian, thank you. The philosophies of Kuhn and Laudan seem, from what you have written to buttress my statements that we have to be careful in how we present our theories. Science is never settled, and things change as new and more accurate information comes to light. In other words, never hitch your wagon to one star.

    I admit to being a big believer in hard physical evidence, and the trends it may be showing. I do have troubles with theories that have more than one “IF”. Too many if’s spoil the broth. Controlling all of those variables with out injecting some of our own prejudices into the situation would be very difficult. And, after all, scientists are human too.

    • W.,

      I recall an anecdote from a video I saw of the great physicist Richard Feynmann giving a talk about science to a group of laypeople. One gentleman (clearly a bit bored by the factual exposition) challenged Feynmann:

      “Why can’t you invent an anti-gravity machine?”

      Feynmann paused, and replied:

      “That is not the game I’m playing. The science game is to get a far as we can with what we already know. Besides, the chair you are sitting on is a perfectly good anti-gravity device because it stops you falling on the ground.”

      “Get as far as we can with what we already know” – a fine summary of the Kuhnian thesis. Applied to global warming, we must accept the IPCC consensus of unequivocal evidence and act accordingly.

      You seem to have a hankering for anti-gravity machines.

  9. The question of whether science is settled is a scientific issue. In science, nothing is ever certain (see the Problem of Induction), and therefore no science can truly be settled.

    However, science is more than academic – it can identify problems. The science that demonstrated the link between smoking and lung cancer wasn’t “certain” or “settled” in the scientific sense, but there was literally no way of explaining the data using what we knew that didn’t jive with the basic understanding of that science (that is, the only way that understanding could be wrong and the data could be explained were if leprechauns were involved). From the SCIENTIFIC perspective, no, it wasn’t settled. From the POLICY or ACTION perspectives, it was settled enough to warrant action.

    The tobacco companies at the time obscured the distinction between scientific “settled” and policy “settled” to delay action on regulating their product.

    You see exactly the same thing with creationists, obscuring the distinction between a scientific “theory” (a reasonably complete explanatory framework) and the public’s use of “theory” (a guess) to peddle their garbage to credulous creationists. Evolution is “only a theory”, they would say, “not a fact”.

    And now, you are doing the same thing. Yes, climate science is uncertain. However, it’s certain enough to warrant action. You don’t need to have the seventh decimal place filled in to see that the number isn’t looking good.

    This applies to uncertainty exactly as much as it applies to wrongness:

    When people thought the earth was flat, they were wrong. When people thought the earth was spherical, they were wrong. But if you think that thinking the earth is spherical is just as wrong as thinking the earth is flat, then your view is wronger than both of them put together.

    –Isaac Asimov

  10. From Anna’s quote list, uncited but originally from Herman Daly:

    To make the point more simply, if you jump out of an airplane you need a crude parachute more than an accurate altimeter.

    Winnipegman would insist we need to know, precisely, the rate of our descent, the exact distance (to at least one more decimal point) to the ground, and even the likelihood we’ll land in a random haystack and walk away unscathed (straight out of bad movie tropes) before considering even donning a parachute, let alone pulling the cord. For *that* he’d need to see proof that a parachute does in fact slow you down during a fall.

    Meanwhile, we keep falling, and he doesn’t deny that. He’d rather gamble on the haystack landing. Why? Apparently because some parachutes aren’t perfect, a TV weatherman says some scientists have questioned their usefulness, and they’re a product of the old paradigm anyway.

    This is on top of raising basic science questions over and over, which he has a habit of not replying to once answered (essentially a smokescreening trick), while insisting he agrees with “the science” (what science, exactly, do you believe in, Winnipegman? That’d be a huge help to this discussion.). There’s a few examples in that swarm of links above; I’d be happy to provide more.

    All in all, it doesn’t seem very intellectually honest from someone who prides himself in being skeptical or of being educated. Nor does it seem particularly conservative (although I have to admit I am inferring your political stance from a combination of your particular pattern of “skepticism” and a passing derogatory reference to [Canadian Liberal party leader] Stephan Dion; I don’t recall you explicitly pointing out any particular alignment. Note the small-c.).

    But then again, since when has intellectual consistency been a hallmark of climate change inactivism? For instance, I recall one debate (Jan 29, 2010) in which Monckton and Ian Plimer collectively argued Not The IPCC using their usual arguments and agreeing with each other. The problem is that Monckton’s usual argument is that we’ve overestimated climate sensitivity (that is, climate sensitivity is too low to cause significant changes), while Plimer’s argument is that the climate’s changed more dramatically before (that is, climate sensitivity is so high that the current changes aren’t distinguishable from natural causes). The two theses are mutually exclusive, and yet neither they nor their “skeptical” audience appeared to notice this. All that mattered was Not The IPCC.

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