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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 101 to 107 out of 107:

  1. TWFA @ 100:

    I see where your problem is. You have no idea what you are talking about.

    Determining trends is "processing". The data that goes into that trend determination will affect the trends that are calculated. Noise does matter, and uncertainty in trend calculation takes that into account.

    Of course, you are probably determining "trend" using the eyecrometer, so it is easy for you to just see what  you want to see, and filter out anything you don't like.

    ...and this may be news to you, but "global sea level" is not something that is measured using a single data value. It requires a bunch of values at different locations, and  you need to combine those data values together properly to get an estimate of a global value. You know: that "processing" step?

    ...and because sea level measurements are not evenly distributed, you can't just average the individual locations with equal weighting. The weighting method (more "processing") does matter, and that is what Jevrejeva does poorly.

    As for your focus on the 1700s - there are many factors that affect sea level, both locally and globally. If one factor affects sea level in 1700, that does not mean that other factors can't affect sea level at another time. I think you need to read the "Climate's change before" rebuttal. There is a reason it is #1 in the "Most Used Climate Myths" list.

  2. So the answer is, "What was going on prior to 1850 is not important", or all that data is suspect, or has not been presented properly, or some such thing?

    I don't understand how what is now deemed to be abnormal can be so determined if prior normal cannot be. Again, the simple question is, what was the temperature doing in 1600, 1650, 1700, 1750 and 1800, as best can be estimated? It's not an unreasonable or silly thing to ask.

  3. TWFA @102 :

    Best if you look at the bigger picture ~ not just the past 500 years.

    As you know, back before about 150-200 years, the various temperatures must be assessed by means of proxies.  And the more proxies over a wider area, then the smaller the amount of uncertainty.

    The proxy evidence points to the past temperature normals [note: plural] following a cycle [Milankovitch] during the past million years.  For instance, during the Holocene period, world temperatures were at a high plateau for roughly 5,000 years . . . followed by a slow decline of [all rough figures of course] about 0.7 degreesC for the past 4.000 years.    ~All this, due to natural factors.

    During that slow decline, there were small deviations from the line of decline [owing to certain transient natural causes].   E.g. the Medieval Warm Period of several centuries showed a world warming of about 0.3 degreesC ;  and the Little Ice Age (including the 1600-1800 period you express interest in) showed a downward "blip" of about 0.3 C  . . . and thus was about 0.6 C below the peak of the MWP.

    The LIA finished roughly 1850 ~ by which time world temperatures had reached the "normal"  i.e. were near the average line of decline from the Holocene peak.  And by 1850, thermometer readings were starting to be used (outside the Central England Temperature region).   And so we know, quite accurately, that the world temperatures have then "shot up like a rocket" ~ way above the (Milankovitch-related) line of natural decline . . . and world temperatures today are roughly 0.5 C above the peak of the Holocene.

    # A very impressive sudden warming change (and with more to come) ~ and all this is clearly "not natural" in its causation.

    TWFA ~ so where is your "not unreasonable" objection?

  4. OK, so in 1850 whatever forcing nature was doing to bring temperatures up stopped and we took over, the sun had nothing to do with anything, right? 

  5. TWFA @104 :

    (Thanks ~ good timing ~ I was about to leave the house.)

    Your question would be better expressed, not as "nature bringing temperature up stopped [in 1850]" . . . but rather as : nature reducing the greater downward pressure (by about 1850).  Of course, from a Milankovitch-cycle aspect, we would expect the slow gradual line of temperature decline . . . to continue for about 15,000 years, until "the ice really hits the fan" . . . ;-) . . . and the world plunges deep into the next Glacial Age (a genuine Ice Age).

    [ So there was no rush for humans to burn all their coal to keep the next glaciation at bay. ]

    TWFA, the forcing from the sun ~ is only one factor in the big picture.  And as best I currently understand it, the Little Ice Age was caused by two roughly equal factors.  Those factors being (A) the Grand Solar Minima [Spoerer, Maunder, etc] . . . and [B] a period of greater frequency of major volcanic eruptions [stratospheric particulates causing cooling ].   A Grand Solar Minimum, by itself, is rather weak in its cooling effect.

    The major factors causing climate change are : Albedo, Sun, Particulates, and CO2  (currently!)

    Yeah, it's complicated.  But the scientists have been doing good work in getting an understanding of it.

    Fair to say : the science is settled enough for our current practical purposes.   It is the politics of how to tackle our self-made problem . . . which is the difficult part to carry out efficiently.

  6. TWFA @104,

    I assume your presentation of somebody's helpful re-drawing of Delague & Bard (2010) fig 5 (below) is your true message and your comment is being ironic.Delaygue&Bard(2010)FIG5

    There are some seriously dodgy proxy TSI reconstructions that survive zombie-like through the decades. 

    Bard et al (2000) has been criticised for finding larger wobbles in TSI than other studies using the same proxy data (eg Vieira et al (2011) [fig5 below] or Wu et al 2018) but I don't see that criticism is greatly extending to the Delaygue & Bard (2010) graph you post.

    Given there is no dispute with the measured TSI record over the last half-century (so after your graphic runs out) during which we have also seen rising global temperatures, would an increase in +1.5Wm*-2 in TSI 1650-1950, that equal to a spherical forcing of +0.375Wm^-2 so less than a decade's-worth of the GHG forcings seen of late; would such a TSI forcing applied over half a millenia be particularly significant?Vieira et al 2011 fig5

  7. MA Rodger @106 :  Thanks for that.

    TWFA hasn't yet said where he gets his background info from ~ but it seems he had been lurking in some mighty crappy corners of the internet.   Corners where the order of the day is to supply doctored graphs and other deceptive half-truths to the Man-in-the-Street.   And in corners where the products marketed are heavily slanted toward conspiracy, outrage, and FUD.

    Still ~ as Pollyanna would say ~ he may be on the path to thinking things through, and picking up better-quality information.

  8. I agree with Eclectic that TWFA seems to be getting some rather bad information from dubious sources. Given that TWFA often seems to just jump to a different "talking point" when challenged on his interpretation or argument, it seems that he lacks understanding of exactly what point his snippets of information are supposed to represent.

    As an example, after arguing about the features of the Jevrejeva sea level reconstruction, in comment 99 I pointed to a RealClimate post that shows the Jevrajeva methodology is suspect. In comment 100, TWFA did not make any attempt to justify the use of Jevrajeva - instead, he made a bogus general argument about trends and processing, and did a "Look! Squirrel!" about comparing 1600 with 1750. After I commented in #101, he continued with more Just Asking Questions.

    I will attempt to respond to TWFA's comment 102 in two ways. First, to address his general question about past climates, what we know, and what does it tell us.

    • The information we have about past climates is limited, and often requires use of proxies (geological records, tree rings, ice cores, etc.) That does not mean we "know nothing". though. In essence, the proxies are the result of past climates, rather than direct measurements of the temperature, precipitation, etc.
    • By understanding the physics of climate (including physics of solar output, etc.), we can use the evidence we do have about past climates to determine what factors were playing a role at that time. And we can compare that to what we can directly measure about those factors now.
    • ...and we see that the best explanation for current trends must include greenhouse gas changes (mostly CO2 from fossil fuel use) to get things anywhere close to right. Other factors were active in the past to a sufficient degree to cause changes we see in the past - but they are not sufficient now to cause the changes we are seeing now.
    • To directly respond to TWFA's "I don't understand how what is now deemed to be abnormal can be so determined if prior normal cannot be",
      • We can determine what "prior normal" was - at least to some limited extent. But that limited extent contains a range of uncertainty due to our limited information. (Even today, we have limits on what is measured.)
      • When we interpret our evidence of the past, we have to include that uncertainty range. Hence Eclectic's question in comment 98: the broad mauve band versus the smooth calculated curve in the graphs that were being discussed.

    The second approach I'll take is by analogy. A thought experiment.

    • Let's assume I am on trial for stealing money from TWFA's bank account.
    • The prosecution has shown evidence of an electronic transfer of $10k from his account to mine on a particular date last month, and evidence that this transfer was initiated for a login from my IP address. At the time, TWFA was on vacation in central Africa, with no internet access.
    • I have presented evidence that TWFA's bank account balance in the past has gone up and down by thousands of dollars from month to month. I do not have information about individual transfers in the past, but I do have evidence of TWFA's approximate income and typical monthly expenses.
      • I argue that this past range of bank balances raises doubt that I stole the money. How can we be sure that some expense that existed in the past did not cause the removal of $10k?
      • On cross, the prosecution presents detailed records that show each transaction for the past year (when detailed records are available). None of the historical  expenses that cause $10k changes in the older historical bank balances were happening during the period I am accused of stealing money. They again point out that the current detailed records include a transfer to my account.
    • The judge ends up saying "it's settled - guilty as charged".

    Climate scientists have spent a lot of time looking at past climates, using the available (albeit limited) evidence. We've spent time to understand the physics, analyze the data, and determine the range of effects that have caused past climate changes. And now we've looked in detail at the role of CO2, and we are observing the effects of increased CO2 that are in broad agreement with theory.

    There are things we still want to learn (always), but the fact that CO2 is a greenhouse gas, has caused most (if not all) the warming in the recent past, and will continue to cause warming in the future is settled science.

  9. To follow up on MA Rodger's comment (#106) on TWFA's comment (#104) that presents data from Delague and Bard (2010).

    • MAR has provided a link to a free copy of the paper.
    • The journal page is here,
    • That journal page includes a link to Supplementary data, which is a CSV file that includes their TSI reconstruction (discussed, but not graphed or presented in the paper).

    With respect to Delague and Bard's TSI reconstruction, it is worth noting:

    • It provides values on a roughly 10-year interval.
    • The first value is for the year 695.
    • The last value is for 1982.
    • The graph presented by TWFA says "5-per running mean", so it is a smoothed graph where each point represents roughly 50 years.
    • The difference between the maximum and minimum in Delague and Bard's TSI data is 1.2 W/m2. You need to divide by 4 to compare it to the CO2 forcing, to get 0.3 W/m2. You need to then adjust for the earth's albedo, since 30% of TSI is reflected, further reducing the absorbed radiation to 0.21 W/m2.

    We can graph the original data (no smoothing) for the period 1900 to present. It looks like this:

    Delague and Bard TSI


    Question for TWFA:

    How much of the warming observed since 1900 do you think is accounted for by the changes in TSI, as indicated by your source (Delague and Bard)?

  10. Bob Loblaw @108 , 109 : Thanks.

    Bob, you and MAR have a great deal of technical knowledge ~ certainly well above my pay-grade.

    As you both are doubtless suspecting, poster TWFA probably does not have that level of knowledge.  Even so, he wishes to challenge the mainstream science of climate, for his own personal reasons.  (Yet possibly, part of him does wish to learn more.)

    But TWFA has boldly jumped into a gunfight, while he's only carrying a knife.

    ~Or to re-frame the old cliche more humorously :-  TWFA has gone into a gunfight while carrying a rolled-up Forbes magazine.  

    [Future readers can be forgiven for missing my reference to a current May 2024 American politico-legal item of notoriety & humor].

    Still, I welcome TWFA's presence in this thread !

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