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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

Science is never 100% settled - science is about narrowing uncertainty. Different areas of science are understood with varying degrees of certainty. For example, we have a lower understanding of the effect of aerosols while we have a high understanding of the warming effect of carbon dioxide. Poorly understood aspects of climate change do not change the fact that a great deal of climate science is well understood.

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)

A common skeptic refrain is that "the science isn't settled", meaning there are still uncertainties in climate science and therefore action to cut CO2 emissions is premature. This line of argument betrays a fundamental misunderstanding of the nature of science. Firstly, it presumes science exists in a binary state - that science isn't settled until it crosses some imaginary line after which it's finally settled. On the contrary, science by its very nature is never 100% settled. Secondly, it presumes that poor understanding in one area invalidates good understanding in other areas. This is not the case. To properly answer the question, "is the science settled?", an understanding of how science works is first required.

Science is not about absolute proofs. It never reaches 100% certainty. This is the domain of mathematics and logic. Science is about improving our understanding by narrowing uncertainty. Different areas of science are understood with varying degrees of confidence. For example, while some areas of climate science are understood with high confidence, there are some areas understood with lower confidence, such as the effect on climate from atmospheric aerosols (liquid or solid particles suspended in the air). Aerosols cool climate by blocking sunlight. But they also serve as nuclei for condensation which leads to cloud formation. The question of the net effect of aerosols is one of the greater sources of uncertainty in climate science.

What do we know with high confidence? We have a high degree of confidence that humans are raising carbon dioxide levels in the atmosphere. The amount of CO2 emissions can be accurately calculated using international energy statistics (CDIAC). This is double checked using measurements of carbon isotopes in the atmosphere (Ghosh & Brand 2003). We can also triple check these results using observations of falling oxygen levels due to the burning of fossil fuels (Manning & Keeling 2006). Multiple lines of empirical evidence increase our confidence that humans are responsible for rising CO2 levels.

We also have a high degree of confidence in the amount of heat trapped by increased carbon dioxide and other greenhouse gases. This is otherwise known as radiative forcing, a disturbance in the planet's energy balance. We can calculate with relatively high accuracy how much heat is trapped by greenhouse gases using line-by-line models which determine infrared radiation absorption at each wavelength of the infrared spectrum. The model results can then be directly compared to satellite observations which measure the change in infrared radiation escaping to space. What we find in Figure 1 is the observed increased greenhouse effect (black line) is consistent with theoretical expectations (red line) (Chen et al. 2007). These results can also be double checked by surface measurements that observe more infrared radiation returning to Earth at greenhouse gas wavelengths (Evans & Puckrin 2006). Again, independent observations raise our confidence in the increased greenhouse effect.

Increased greenhouse effect - models vs observations
Figure 1: Increased greenhouse effect from 1970 to 2006. Black line is satellite observations. Red line is modelled results (Chen et al. 2007).

So we have a lower understanding of aerosol forcing and a higher understanding of greenhouse gas forcing. This contrast is reflected in Figure 2 which displays the probability of the radiative forcing from greenhouse gases (dashed red line) and aerosol forcing (dashed blue line). Greenhouse gas forcing has a much higher probability constrained to a narrow uncertainty range. Conversely, the aerosol forcing has a lower probability and is spread over a broader uncertainty range.


Figure 2: Probability distribution functions (PDFs) from man-made forcings. Greenhouse gases are the dashed red curve. Aerosol forcings (direct and indirect cloud albedo) are the blue dashed curve. The total man-made forcing is the solid red curve (IPCC AR4 Figure 2.20b)

The important point to make here is that a lower understanding of aerosols doesn't invalidate our higher understanding of the warming effect of increased greenhouse gases. Poorly understood aspects of climate change do not change the fact that a great deal of climate science is well understood. To argue that the 5% that is poorly understood disproves the 95% that is well understood betrays an incorrect understanding of the nature of science.

Last updated on 9 July 2010 by John Cook. View Archives

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Comments 1 to 25 out of 81:

  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. http://www.skepticalscience.com/Empirically-observed-fingerprints-of-anthropogenic-global-warming.html 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: http://plato.stanford.edu/entries/science-theory-observation/ 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 http://www.johnmccaskey.com/Induction%20and%20Concepts%20in%20Bacon%20and%20Whewell.pdf has a concise explanation of induction as used in modern science.

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