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I just read the IPCC's Fourth Assessment Report and noticed that this is the first (or first very significant) report to state that climate change exists and can no longer be scientifically disproved. However, humankind has known since 1824, since the discovery of Jean-Baptiste Joseph Fourier, that greenhouse gases are changing the atmosphere. So I ask myself, why did it take so long to gain knowledge and what were the major theories that speak against human-made climate change?

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    $\begingroup$ I believe the difference is that since 1824 anthropogenic global warming had been predicted, but in IPCC AR4 it had for the first time been measured beyond doubt. $\endgroup$
    – gerrit
    Jul 20, 2022 at 9:04
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    $\begingroup$ 'Why has climate change still not been proven beyond doubt?' - 'this is the first very significant report to state' ... that's kinda rude to everyone else's work for the last 200y. - "Why did it take so long to gain knowledge and what were the major theories that speak against man-made climate change?" Money, Money, and Money $\endgroup$
    – Mazura
    Jul 20, 2022 at 21:50
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    $\begingroup$ The discovery of greenhouse effect as such has nothing to do with the theory of climate change. The latter concerns the dynamics of it, due to the change of the atmosphere composition. The effect itself is not dangerous per se, in fact, it provides hospitable conditions for us on Earth. $\endgroup$
    – Zeus
    Jul 21, 2022 at 1:43
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    $\begingroup$ This: people just cried wolf without proving there is a wolf, for too long. So over time, they lost credit and other people stopped believing them. That's how I see it, at least. $\endgroup$ Jul 21, 2022 at 6:45
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    $\begingroup$ @Valorum That's a widespread myth based on media distortions that did not reflect the scientific consensus at the time. Climatologists in the 1970s were interested in the long-term potential for ice ages, but even then the imminent threat of anthropogenic global warming was considered much more important: journals.ametsoc.org/view/journals/bams/89/9/2008bams2370_1.xml $\endgroup$
    – David
    Jul 22, 2022 at 15:26

5 Answers 5

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"Why has climate change not been proven beyond doubt for so long?" to a good degree has the same answer as "why has a connection between smoking and lung cancer not been proven beyond doubt for so long?".

Because there is a lot of money in return for prolonging the doubt. It takes considerable effort establishing the structures cementing global enterprises, and change disrupts many people's livelihoods.

So while there is some money to be made by paying scientists to advance the knowledge about some global phenomenon, sometimes paying scientists to corrupt knowledge has larger payoffs, with the actually incurred costs getting borne by society in general.

And for politicians it tends to be a favorable deal to trail established scientists considerably with regard to recognizing the undeniable. It's popular with both voters and lobbyists.

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    $\begingroup$ There is a reason why the law has the standard "proof beyond reasonable doubt", not "proof beyond doubt". $\endgroup$ Jul 20, 2022 at 20:36
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    $\begingroup$ I remember reading that the PR company that spent decades trying to sow doubt regarding the effect fossil fuels had on climate change is the very same PR company that spent decades trying to sow doubt regaridng the healthy risks of tobacco. $\endgroup$
    – VGR
    Jul 21, 2022 at 19:39
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    $\begingroup$ as @SimonCrase said, the evidence has been beyond reasonable doubt for many decades. Because we aren't gods, we will never know anything beyond all doubt; I can claim gravity might stop working tomorrow and noone can fully prove me wrong. . . but the pattern of gravity's persistence up to this point makes the weight of evidence say we'll still be on the ground. When the evidence keeps piling up in one direction, and the risk is genuine apocalypse --- the reasonable answer is clear. -------- and I see there's already an answer below saying the same :-) Read that ~ $\endgroup$
    – Mike M
    Jul 22, 2022 at 10:28
  • $\begingroup$ Why doesn't every smoker get lung cancer is the million dollar question $\endgroup$
    – Valorum
    Jul 23, 2022 at 9:28
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    $\begingroup$ @Valorum because not all mutations cause cancer, tobacco contains carcinogens and mutagens but lots of mutations does not automatically cancer make, it just makes it vastly more likely. $\endgroup$
    – John
    Jul 23, 2022 at 12:54
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It's the first very significant report to make that claim because the claim is unscientific in nature.

As Albert Einstein once said:

The scientific theorist is not to be envied. For Nature, or more precisely experiment, is an inexorable and not very friendly judge of his work. It never says "Yes" to a theory. In the most favorable cases it says "Maybe," and in the great majority of cases simply "No." If an experiment agrees with a theory it means for the latter "Maybe," and if it does not agree it means "No." Probably every theory will someday experience its "No"—most theories, soon after conception.

Or, more recently, as Liz Gerjbine writes in Scientific American "[S]cience cannot provide certainties."

This isn't an attempt to cast doubt on climate change, in fact I think a better comparison would be saying we're about as certain as we are about gravity.

So they're the first to publish that statement because a core tenet of science, if not the core tenet, is doubt.

I'm actually quite shocked any serious scientist or group would make the statement "can no longer be scientifically disproved" about anything. Seems more like political grandstanding than genuine science.

The other answers so far do a fine job explaining the slow dissemination and acceptance of scientific findings.

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    $\begingroup$ Gravity is a good comparison really. We know it exists but science still gets fuzzy on the precise details, and the modeling of it gets shaky fast (see: three body problem). Same with climate change. It's one thing to observe that it exists. The quibbling is mostly over the nature of it, the origin, the effects, the timetable, and the fact that the modeling is always inaccurate to a variety of degrees. (We have had global warming but nothing like the models from 20-30 years ago said we were going to have in 20-30 years.) $\endgroup$
    – JamieB
    Jul 21, 2022 at 13:51
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    $\begingroup$ @JamieB I had a long parenthetical disclaimer after originally writing the theory of gravity, then decided on this word choice instead. Couldn't agree more with your analysis. $\endgroup$
    – TCooper
    Jul 21, 2022 at 16:49
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    $\begingroup$ Scientists tend to be bad at public speaking. When asked something, they say "probably", meaning +90%, yet the "average Joe" will think that's more like 50-60%. "Statistically insignificant" goes from < 0.001% to < 20% in the public eye. "Almost certainly" becomes "this is a lie". Bill Nye, Neil deGrasse Tyson, and others have furthered scientific speaking enormously with their smooth and natural speaking styles, immense intelligences, specific use of terms, and defining the terms they use. They don't just assume "John Doe" understands the scientific terms the same way they do. $\endgroup$ Jul 21, 2022 at 20:28
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    $\begingroup$ One of the things that bothered me for decades on this issue has been the failure to make testable predictions with a reasonable defined null hypothesis. The climate modelers predicted that CO2 forcing would cause temperatures to keep rising, but then when temperatures fell (contrary to predictions) the climate modelers said that too was a result of CO2 forcing. Given the their initial long run of inaccurate predictions, scientific rigor would suggest that climate modelers should need to produce a good run of accurate predictions before they should be taken seriously. $\endgroup$
    – supercat
    Jul 22, 2022 at 17:13
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    $\begingroup$ "Seems more like political grandstanding than genuine science." I cannot blame the IPCC. They wrote 5 reports over 30 years, explaining us very clearly "There's a large concrete wall just ahead of us. We might want to not crash into it". And all they got as an answer was "Braking is bad for the economy", "There's no wall", "The wall has always been here", "How do you know crashing into a wall is bad for us?", "Is the wall 2m or 3m thick?", "How will my car look like, exactly, after it hits the wall?". No wonder the IPCC now writes "CRASH IS IMMINENT. EMERGENCY BRAKE, NOW!!!!" $\endgroup$ Jul 23, 2022 at 9:19
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Chapter 9 of the Fourth Assessment Report gives a comprehensive overview of the difficulties involved in 'detection and attribution'. It is worth reading in detail - it gives a much better idea of the complexity and residual uncertainties of the conclusions than the newspaper headlines do.

There are two basic problems: first, the natural background level of variation of the climate on longer timescales (decades and centuries) is not precisely known, since we don't have accurate global measurements for very long, so it is very hard to tell whether any given observed change in the weather is natural or artificial. There is geological evidence of large and sudden changes of climate in the past, long before humans industrialised, and we don't know how they all work or how big they can get.

The second problem is that while the opacity of CO2 to infrared has been known about for a long time, this is only part of the story. There are a large number of feedback mechanisms that can magnify or reduce the effect of CO2, and these are often poorly understood. For example, warming due to CO2 is thought to cause more water vapour to evaporate, and water is also a powerful greenhouse gas - a positive feedback. But water vapour also reduces the lapse rate in the atmosphere (the rate at which temperature drops with height), which is a negative feedback. The biggest uncertainty is the effect on clouds. Current climate models have the effect of multiplying the effect of CO2 alone by about 3; around two thirds of the projected effect are thus due to other causes. But the error bars on that estimate are huge, and there is an ongoing scientific debate about whether the empirical measurements agree.

From p688 of the linked report:

Both detection and attribution require knowledge of the internal climate variability on the time scales considered, usually decades or longer. The residual variability that remains in instrumental observations after the estimated effects of external forcing have been removed is sometimes used to estimate internal variability. However, these estimates are uncertain because the instrumental record is too short to give a well-constrained estimate of internal variability, and because of uncertainties in the forcings and the estimated responses.

From p669:

The approaches used in detection and attribution research described above cannot fully account for all uncertainties, and thus ultimately expert judgement is required to give a calibrated assessment of whether a specific cause is responsible for a given climate change. The assessment approach used in this chapter is to consider results from multiple studies using a variety of observational data sets, models, forcings and analysis techniques. The assessment based on these results typically takes into account the number of studies, the extent to which there is consensus among studies on the significance of detection results, the extent to which there is consensus on the consistency between the observed change and the change expected from forcing, the degree of consistency with other types of evidence, the extent to which known uncertainties are accounted for in and between studies, and whether there might be other physically plausible explanations for the given climate change.

As an aside, while Fourier did publish some predictions on the effect of CO2 on temperature, the mechanism he described does not apply in a convective atmosphere. The correct mechanism was actually identified during the study of the atmosphere of stars in the 1940s, and was applied to planetary atmospheres by Manabe and Strickler in the early 1960s.

When gases are compressed, they warm up, and when gases are allowed to expand, they cool. So as the atmosphere circulates convectively, rising air cools and descending air warms. This sets a particular temperature gradient in the atmosphere, called the adiabatic lapse rate. Temperature gradients much larger than this cannot normally be maintained, as convection starts up and transports heat vertically to cancel out the excess. Moist air has a lower adiabatic lapse rate than dry air, about 6.5 C/km on average.

To a very close approximation, all the energy from the sun that arrives on Earth is subsequently radiated as infrared to outer space - the black body temperature needed to radiate this much energy is about -20 C. However, when the atmosphere is opaque to infrared, it radiates not from the ground, but from the first 10 km of the atmosphere. The average altitude of emission to space is about 5 km. So it is the air at this altitude settles at -20 C. And because of the adiabatic lapse rate, the surface is roughly 6.5*5 = 32.5 C warmer (i.e. about +13 C.) This is the greenhouse effect in a convective atmosphere. (In an atmosphere without convection, using Fourier's mechanism, the surface temperture would be about 60 C.) More CO2 and water vapour raises the average altitude of infrared emission to space, thus raising the surface temperature. But more water vapour reduces the vertical temperature gradient, reducing the surface temperature rise and at the same time increasing the temperature of the upper troposphere 10 km up. If you look at figure 9.1 panels (c) and (f) on p675 of the IPCC Report linked, which show where the models say the warming will be seen, you will see this as a big red blob in the middle of the picture. This is the 'fingerprint' of water vapour feedback.

Please note - everything I say here comes from the AR4 IPCC Report or the mainstream climate literature.

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    $\begingroup$ This answer should be the accepted answer $\endgroup$
    – Peter A
    Jul 23, 2022 at 9:54
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    $\begingroup$ @PeterA I think this is a fantastic answer providing great insights and I upvoted myself. Although I think at a certain point it becomes more an explanation of the current state of climate science than directly answering the question. I think it more so says "yeah... they shouldn't be claiming anything with certainty because x,y,z" more than "Here's why this report makes a claim of certainty". Either way, has my +1 and I'm glad I got to read it in response to the question. $\endgroup$
    – TCooper
    Jul 24, 2022 at 14:30
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Just because a "small number" of scientists knew about something in the mid 1820s doesn't mean it was initially accepted by other scientists. You also have to consider the means of disseminating information and knowledge then and since. You also have to consider what other attitudes prevailed.

Charles Darwin's Theory of Evolution was vigorously opposed by many eminent people because of religious beliefs. In some places it still is.

Personally, I first became aware of global warming via the mass media many years after I completed my university studies. I was taught other scientific things - there's a lot to learn - and I still think I had a good education.

I became aware of Darwin's Theory of Evolution in high school, but no-one mentioned global warming or the heat retaining potential of carbon dioxide or methane, or whatever else. It wasn't part of the science curriculum, other things were.

People may have been warning about global warming before that, but from what I'm aware of they were few in number, they didn't captivate the attention of the mass media and they were essentially voices in the wilderness. There's a saying "the squeaky wheel gets the oil", they weren't squeaky enough.

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    $\begingroup$ "but no-one mentioned global warming or the heat retaining potential of carbon dioxide or methane" - greenhouse gasses were part of my high school curriculum in the 70s - water vapor, then methane, then CO2. $\endgroup$ Jul 20, 2022 at 21:10
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    $\begingroup$ Planet Earth, aired on PBS in 1984. I've watched it at least once a year for the last ~30y. It's so jammed packed with information I'm still learning things from it. Via that series, I knew all these things before HS, but it's still an ongoing process to understand all their affects on our world. $\endgroup$
    – Mazura
    Jul 20, 2022 at 22:09
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    $\begingroup$ Titles of some of the episodes: "The Living Machine" (the Biosphere), "The Climate Puzzle" (the effect of on biosphere), "Gifts from the Earth" (our effects), "The Fate of the Earth" (and the end.) ... Then there's the original Cosmos, but that's hellbent on worrying about nuclear war. $\endgroup$
    – Mazura
    Jul 20, 2022 at 22:09
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    $\begingroup$ When wheels are squeaky enough, they tend to get arrested, as we are seeing now with groups like Letzte Generation. $\endgroup$
    – user253751
    Jul 21, 2022 at 9:10
  • $\begingroup$ @Mazura Oh yeah, Sagan was very vocal about it, and there was his famous and memorable address to the US congress in 1985. $\endgroup$
    – Luaan
    Jul 21, 2022 at 17:30
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Svante Arhennius' crude calculations published in 1896 of the likely global temperature changes from halving and doubling CO2 (based on prior work by Tyndall, Foote and Fourier) were considered speculative - amongst other speculations about climate - and were not taken seriously at the time or for a long time after. In part because the prevailing view was nothing humans could do could change the climate but also, in science terms, through incorrect understandings around "saturation" of Infrared absorption and ocean uptake (drawdown?) of CO2.

The re-emission of IR from greenhouse gas molecules within the atmosphere was not fully accounted for, leading to the incorrect conclusion that with enough CO2 to catch 100% of IR on the way to space adding more would make no difference.

With respect to ocean uptake of CO2, the Revelle Factor - resistance of the ocean surface layer to taking in CO2 - was not known until 1957; it was assumed rapid ocean uptake would prevent CO2 concentrations rising significantly in the atmosphere.

By the early 1970's most factors that are now deemed capable of causing "inadvertent climate modification" were known - including that aerosols from fossil fuel burning had a reflective effect that reduced solar intensity, ie a cooling influence - but not it's relative strength over time compared to enhanced greenhouse effect. The relative strengths of all the known factors and the impacts of interactions and feedbacks within the global climate system were not known with sufficient precision for it to be possible to be sure, even into the 1970's.

From "Understanding Climatic Change: a Program for Action" - US National Academy of Sciences, 1975 -

"Unfortunately, we do not have a good quantitative understanding of our climate machine and what determines its course. Without this fundamental understanding, it does not seem possible to predict climate — neither in its short-term variations nor in its larger long-term changes."

The "Program for Action" was intended to achieve that better "quantitative understanding" and by the 1980's had done so well enough for then still current "little ice age" fears to be put to rest. Unfortunately the reason why - warming was expected to dominate - was not so reassuring as was hoped.

A lot of debate and dispute has followed but every high level science based report and study from the world's leading science and climate science institutions since then (about 4 decades so far) has consistently concluded that we have global warming and can expect more so long as we keep burning fossil fuels and adding CO2 (and methane) to the atmosphere.

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