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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Is the science settled?

What the science says...

Select a level... Basic Intermediate

That human CO2 is causing global warming is known with high certainty & confirmed by observations.

Climate Myth...

The science isn't settled

"Many people think the science of climate change is settled. It isn't. And the issue is not whether there has been an overall warming during the past century. There has, although it was not uniform and none was observed during the past decade. The geologic record provides us with abundant evidence for such perpetual natural climate variability, from icecaps reaching almost to the equator to none at all, even at the poles.

The climate debate is, in reality, about a 1.6 watts per square metre or 0.5 per cent discrepancy in the poorly known planetary energy balance." (Jan Veizer)

At a glance

Science, in all of its aspects, is an ongoing matter. It is based on making progress. For a familiar example, everyone knows that the dinosaurs died out suddenly, 65 million years ago. They vanished from the fossil record. The science is settled on that. But how and why that happened is still a really interesting research area. We know a monster asteroid smacked into the planet at roughly the same time. But we cannot yet conclude with 100% certainty that the asteroid bore sole responsibility for everything that followed.

With regard to climate science, the basis of the greenhouse effect was demonstrated in the 19th Century. The effect on global temperature through doubling the concentration of atmospheric CO2 had been calculated before 1900 and was not far off modern estimates. Raising global temperature causes Earth's climatic belts to shift polewards. Higher temperatures reduce the amount of land-ice on the planet. That in turn causes sea levels to rise. These are such simple basic physical principles that we can confidently state the science is settled on all of them.

Where the science is less settled is in the fine detail. For example, if you live in a coastal town at a low elevation, you would obviously like to know when it is likely to be affected by rising seas. But that's difficult.

Difficult because changes in sea levels, variations in the sizes of tides and weather patterns are all factors that operate independently of each other and on different time-scales. We may well know that a big storm-surge hitting the coast at high water on a spring tide is the worst-case scenario, but we don't know exactly when that might happen in the decades ahead. Too many variables.

Such minute but important details are where the science isn't settled. Yes we know that if we carry on spewing out tens of billions of tonnes of CO2 every year, things will get really bad. Where and when is the tricky bit. But if climate change was a deadly pathogen, for which there was a vaccine, most of us would get that jab.

In passing, the myth in the box above illustrates a key tactic of misinformation-practitioners, to mix up a whole bunch of talking-points into a rhetorical torrent. The classic example of the practice is the 'Gish-gallop'.

The term Gish-gallop was coined in reference to a leading American member of the creationist movement, Duane Gish (1921-2013). Gish was well-known for relishing fiery public debates with evolutionists. He perfected the method of presenting multiple arguments in a rapid-fire but scattergun manner so that they are impossible to answer in a structured form. It's the opposite of scientific discussion. The Gish-galloper appears to the viewers or listeners to be winning the debate. 'Appears' is the keyword here, though. If you can recognise a Gish-gallop developing, you can make your own mind up quickly.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

Deniers often claim that the science of anthropogenic global warming (AGW) is not 'settled'. But think about this for a moment. No science is ever completely settled. Science deals in probabilities, not certainties. When the probability of something being correct approaches 100%, though, scientists agree that's the most likely answer. Consensus is achieved.

Thus we agree that certain pathogens can make us extremely unwell. We agree that a big asteroid hitting the planet would be nothing short of catastrophic. We agree that if we live in a district prone to tornadoes, it makes sense to have a good storm-shelter in your home. There are countless other examples, all of which can be filed under the same term, 'obvious'. That's even if we don't know exactly when the next pandemic, impact or tornado outbreak will occur.

Climate science deniers, on the other hand, insist that results must be double-checked, triple-checked and uncertainties must be narrowed before any action is taken. This is basically stalling for time, since the basic principles behind AGW have been staring us in the face for decades. It's also very misleading because by the time scientific results are offered up to policymakers, they already have been quintuple-checked.

Scientists have been predicting AGW with increasing confidence since the 1950s. Indeed, the hypothesis, backed up by detailed calculations, was first proposed in 1896. As science learned more and more about the climate system, a consensus gradually emerged. Many different lines of inquiry all converged into the IPCC’s 2007 conclusion that it is more than 90% certain that anthropogenic greenhouse gases are causing most of the observed global warming.

Some aspects of the science of AGW are known with near 100% certainty. The greenhouse effect itself is as established a phenomenon as any. There is no reasonable doubt that the global climate is warming (fig. 1). And there is also a clear trail of evidence leading to the conclusion that it’s caused by our greenhouse gas emissions. Some aspects are less certain; for example, the net effect of aerosol pollution is known to be negative, but the exact value needs to be better constrained. We're working on it. But it changes nothing regarding the basic principles.

Latest temperature anomalies from four leading datasets.

Fig. 1: the latest temperature anomalies from four leading datasets, relative to a 1951-1980 baseline. The trend continues upwards and upwards. Graphic: Realclimate.

What about those remaining uncertainties? Should we wait for 100% certainty before taking action? No. Outside of logic and mathematics, we do not live in a world of absolute certainties. Science comes to its conclusions based on the balance of evidence. The more independent lines of evidence are found to support a scientific hypothesis, the closer it is likely to be to the truth. Hypotheses are tested to death before they are able to graduate into a theory. If someone tells you something is 'only a theory', they do not know what they are talking about. Theories are extremely robust explanations.

Just because some details about AGW are still not well understood, that should not cast into doubt our understanding of the big picture: humans are causing global warming. It is specifically down to our perturbation of Earth's carbon cycle by chucking some 44 billion tonnes of CO2 into the atmosphere every year. That's around a hundred times more than annual volcanic emissions. It's such a huge amount it's almost incomprehensible.

In most aspects of our lives, we think it rational to make decisions based on incomplete information. We all take out insurance when there is even a slight probability that we will need it. We don't know when that tornado might pay a visit, but we want to be covered for the possibility that the house might get flattened, because we all know tornadoes can flatten houses.

Likewise, we don't know the exact details in terms of when or how disasters may strike due to global warming. Nevertheless, we know it will make more intense rainfall events more likely. We know it will cause more land-ice to melt, further raising sea levels. We know it will make fire-weather more common and intense. We know it will cause agriculture to be compromised, to the point of non-feasibility in some places. We know it will displace human populations. These are all very basic principles based on elementary physics. In other words, they are obvious. Why, then, do we ignore such settled things?

Last updated on 7 April 2024 by John Mason. View Archives

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Comments 1 to 25 out of 107:

  1. This 90% certainty that the IPCC claim is hugely simplified, for the non-techincal masses. I would say it's near 100% for some effect, but as to how much warming corresponds to how much CO2, that 90% figure for most of it is just a shot in the dark by the IPCC. It assumes our understanding of climate-forcing factors is nearly complete. I think there is still quite a lot we are missing.
  2. mistermack, that "90% certainty" is for a very particular claim. It's not just for the non-technical masses. Other claims have less or more certainty. I suggest you read a January 2010 article by Trenberth, "More Knowledge, Less Certainty", about the lower certainties that will come along with the IPCC's next report's projections.
  3. If there was a 90% certainty, based on scientific analysis, that a given model of plane would fall out of the sky, would you board that plane? Or would you dismiss those claims because there is still quite a lot we are missing in the study of aerodynamics?
  4. mistermack, there is a condensed list of claims with their certainties on this EPA page "State of Knowledge". If you are unhappy with the lack of detail there, then look in the IPCC Working Group I Summary for Policymakers. If you are unhappy the lack of detail there, then dig into the body of that IPCC report. You might also want to ask for a copy of the content of a poster presented at the AGU conference in 2009, by Martin Vezer, titled A Defence of the AR4's Bayesian Approach to Quantifying Uncertainty.
  5. Bibliovermis, I'm not mocking, but you made me chuckle there. I would't board the plane if it was the other way around, and there was a 10% chance of coming to grief. Or even 1%. I'm not sure how that affects AGW though. The truth is that the 90/10 is just someone's guess. The reality is that CO2 has some affect, but we don't know if it can push temperatures significantly higher than todays. The 90/10 is not based on experience of previous events, we haven't had this level of co2 during an ice-age.
  6. @mistermack: I would't board the plane if it was the other way around, and there was a 10% chance of coming to grief." So, in essence you're arguing that, even if there's a 10% chance current AGW theory is real, it would still be too much of a risk? :-) "The truth is that the 90/10 is just someone's guess." You need to learn about statistical trends. "The reality is that CO2 has some affect, but we don't know if it can push temperatures significantly higher than todays." Actually, we have no reason of believing it can't - and yet you seem ready to gamble on the fact it might not. If you're going to err, isn't it better to err on the side of caution? "The 90/10 is not based on experience of previous events, we haven't had this level of co2 during an ice-age." Exactly, so why would you expect the result to be the same if the parameters are so different now? You're not making logical sense.
  7. The plane example is interesting. I wouldn't get on a new kind of aircraft, that had never flown before, but a panel of experts said that it was 90% sure to fly.
  8. @mistermack: exactly. Even a 10% threat seems too high for you, so a 90% threat should be even worse - and yet you seem fine taking a gamble on that 10% when it comes to the climate...
  9. Archie, the difference is, a plane crash is almost 100% bad. A bit of warming isn't. (Certainly not for Britain). So it's not black and white like a plane crash. And considering that without MMGW, if you go by the previous cycles, we are due to drift into a full ice-age, a moderate amount of warming might be a good thing for the rest of the planet too.
    Response: Further comments on warming's benefits versus costs must be made on It's Not Bad. Further comments on the need to prevent the next ice age must be on We’re Heading Into an Ice Age.
  10. @mistermack: please read the "It's not bad" article. At least now you're acknowledging that AGW is real. It took some time to get you to admit it, but your last sentence leaves no room for interpretation.
  11. Mistermack, Stating "A bit of warming isn't [bad]", is called argument by assertion. You aren't going to get very far by engaging the commenters on this site with logical fallacies.
  12. This comment of mine is a response to comments by Norman on the thread Real Experts Don't Know Everything. Norman, you are incorrect that there is a sharp distinction between "empirical" science that reveals "facts" versus "fuzzy" science that yields only probabilistic statements. All sciences yield probabilistic statements. "Facts" simply have very high probabilities of being true--so high that it is downright silly to constantly refer to them as "maybes." That is why consensus among scientists is important. I suggest you watch Naomi Oreskes's "Consensus in Science: How Do We Know We're Not Wrong?" I suggest you also read my comments 195 and 197 on the consensus thread. If you want to learn more, check out this paper by Duffy Hutcheon just as a start.
  13. Norman, the fact that all of scientific judgment is probabilistic is easy to discover. You don't have to take my word for it. Here is just one of many places you might start learning about that.
  14. #12 Tom Dayton at 15:20 PM on 12 November, 2010 All sciences yield probabilistic statements. "Facts" simply have very high probabilities of being true--so high that it is downright silly to constantly refer to them as "maybes." That's not so. Some scientific statements are simply true, with no reference to probability whatsoever. For example in 1610 Galileo has discovered (using his improved telescope) that the surface of the Moon was not smooth, but had mountains and valleys on it. This statement can not be translated as "the surface of Moon being smooth has zero (or extremely low) probability", because there is no reasonable definition of a sample space with a probability measure on it that would fit the situation. The "translation" simply does not make sense. Galileo's proposition is either true or false. It turned out to be true, verified by the observed behavior of shadows on the lunar surface (which also made possible to measure the height of those mountains).
  15. BP, of course Galileo's conclusion of mountains on the moon could be expressed probabilistically. That particular case would be one in which most people would not bother using that terminology, because the probability was so high that such terms would be "downright silly," as I wrote. Contrast with early scientists' conclusions of the existence of canals on Mars, based on their telescopic observations.
  16. #15 Tom Dayton at 01:02 AM on 13 November, 2010 of course Galileo's conclusion of mountains on the moon could be expressed probabilistically OK, give it a try. I'm listening.
  17. There is a mildly interesting philosophical argument here. By observational evidence and using some basic laws of physics, there are mountains on the moon. That fact cannot be expressed as a probability. Our observations may be wrong along with some basic physics, but that possibility cannot be expressed as a probability. By laws of physics, CO2 causes warming, but it is not "near 100% certain" in any scientific sense but only as a figure of speech. It is an established fact that CO2 and increases in CO2 cause warming unless a lot of physics is wrong. The correctness of the physics cannot be expressed as a probability. There is not a "90% certainty" that manmade GHG is causing "most" of the observed warming. That number is a meaningless invention. There about as much support for the statement that there is a 60% probability that this post will be deleted because it is purely philosophical and philosophy is dangerously close to politics. It is far better to drop the fake probabilities and make statements about theories and supporting lines of evidence. The best evidence is empirical, e.g. Various of these observations and measurements may arguably have alternative explanations. But none of those measurements or counter-arguments have any kind of probability associated with them.
  18. BP, it's easy: "Galileo was 100% certain that there are mountains on the moon." Or "Galileo was 99.9% certain that there are mountains on the moon." Or "I am 99.9999... to so many decimal places of 9s certain that there are mountains on the moon, that for practical purposes I am 100% certain."
  19. Eric (skeptic), what do you think is impossible about expressions such as "I am 100% certain that there are mountains on the moon?" There is nothing "fake" about subjective probabilities. All humans operate on the basis of their subjective probabilities. Click the links inside my comment #12 above. Then read the short essay Probability and Induction: The Very Foundations of Science. For an overview of subjective probability see the New School page on The Concept of Subjective Probability. If you want more detail, here is an article I ran across after a quick internet search: Updating Subjective Probability. It is easy to find a great deal more free material on subjective probability, subjective utility, decision making, and their roles in science.
  20. Martin Vezer's poster from the American Geophysical Union 2009 conference now is available: A Philosophical Defense of the IPCC's AR4 Bayesian Methodology.
  21. #18 Tom Dayton at 14:38 PM on 13 November, 2010 "Galileo was 100% certain that there are mountains on the moon." I am sure he was. He actually says it in his booklet Sidereus Nuncius: "I have been led to that opinion which I have expressed, namely, that I feel sure that the surface of the Moon is not perfectly smooth, free from inequalities and exactly spherical, as a large school of philosophers considers with regard to the Moon and the other heavenly bodies, but that, on the contrary, it is full of inequalities, uneven, full of hollows and protuberances, just like the surface of the Earth itself, which is varied everywhere by lofty mountains and deep valleys." However, you miss the point. Galileo's state of mind may be interesting from a historic point of view, but it is absolutely irrelevant to science. What matters is the evidence he gives in subsequent sections starting with "The appearances from which we may gather these conclusions are of the following nature: [etc. etc.]" (and also the detailed description of the instrument used for observation, given in previous sections). That's what makes his observations repeatable and his conclusions verifiable. This is what constitutes the scientific method and makes him a scientist. Without it he would be just another Moongazer disseminating vague Witchy Wisdom of Gaia’s Sacred Circle.
  22. Tom, thanks for the links. The proper role of probability and statistics in science is the evaluation of multiple samples of imprecise data (e.g. regression analysis of imprecise measurements, multiple model runs, sensitivity analysis, etc). That is basically what you would probably call "objective probability". See an explanation of that here: The "defense of IPCC's Bayesian methodology" claims that "Climatological analysis of the AR4 requires subjective assessment." I disagree. There is no requirement for subjective assessment in science. Science builds from a conceptual framework of theories based on observations. If the theories fit observations and each other, then they are certain. If the theories don't fit the observations or the theories conflict, then new theories are required. Those are the two states of science, there are no states in between.
  23. No, Eric, you are very wrong that theories have only the two states of certain and uncertain. If somehow you did not get that point out of the links I provided earlier, try this page for an overview, and if you object to any of its claims, please do read the sources cited there for those claims: The Nature and Philosophy of Science. There is nothing special about that particular web page; you can find the same information easily in textbooks and in multiple places on the internet.
  24. Tom, theories have one state, certain, otherwise they are not theories but notions.
  25. Tom, most of your links suggest that science has been separated from, or progressed beyond what they call "Baconian inductivism". This appears to be a modern consequence of the need for theories of phenomena that are not directly observable. Your latest link gives examples of "atomic theory and the theory of gravity". The observations are thought to be "theory-laden" therefore unsuitable for inductive reasoning. The paper proposes using the principles of parsimony and "how well a theory ties in with other theories". But those are simply principles of concept formation. There is no difference between concept formation in all nonscientific realms and theory formation in science. The attempt to posit a difference leads to absurdities like the example in your paper: rejecting the theory that the moon is made of green cheese because of the "law" (no longer just a principle) of parsimony. In fact the moon is not made of green cheese because of a large number of theories and observations that conflict with that theory. No (falsely elevated) "law" of parsimony is necessary to reach that conclusion. Ultimately the real reason for such acceptance of subjectivism in science is revealed in your link: "Scientists (and regular human beings) are also affected by cultural, social, and personal beliefs.... Rather than the traditional view that science is to be protected from biases and other imperfections of people, it turns out that science is inescapably infected with humanness." That notion might be a good way to study past errors in science or science history, but it has not scientific purpose, is not required and must be rejected. This paper has a concise explanation of induction as used in modern science.

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