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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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What does past climate change tell us about global warming?

What the science says...

Select a level... Basic Intermediate

Greenhouse gasses, principally CO2, have controlled most ancient climate changes. This time around humans are the cause, mainly by our CO2 emissions.

Climate Myth...

Climate's changed before

Climate is always changing. We have had ice ages and warmer periods when alligators were found in Spitzbergen. Ice ages have occurred in a hundred thousand year cycle for the last 700 thousand years, and there have been previous periods that appear to have been warmer than the present despite CO2 levels being lower than they are now. More recently, we have had the medieval warm period and the little ice age. (Richard Lindzen)

At a glance

Just imagine for a moment. You fancy having a picnic tomorrow, or you're a farmer needing a dry day to harvest a ripe crop. So naturally, you tune in for a weather-forecast. But what you get is:

“Here is the weather forecast. There will be weather today and tomorrow. Good morning.”

That's a fat lot of use, isn't it? The same applies to, “the climate's changed before”. It's a useless statement. Why? Because it omits details. It doesn't tell you what happened.

Climate has indeed changed in the past with various impacts depending on the speed and type of that change. Such results have included everything from slow changes to ecosystems over millions of years - through to sudden mass-extinctions. Rapid climate change, of the type we're causing through our enormous carbon dioxide emissions, falls into the very dangerous camp. That's because the faster the change, the harder it is for nature to cope. We are part of nature so if it goes down, it takes us with it.

So anyone who dismissively tells you, “the climate has always changed”, either does not know what they are talking about or they are deliberately trying to mislead you.

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

Past changes in climate, for which hard evidence is preserved throughout the geological record, have had a number of drivers usually acting in combination. Plate tectonics and volcanism, perturbations in Earth's slow carbon cycle and cyclic changes in Earth's orbit have all played their part. The orbital changes, described by the Milankovitch Cycles, are sufficient to initiate the flips from glacials (when ice-sheets spread over much of Northern Europe and the North American continent) to interglacials (conditions like the past few thousand years) and back  – but only with assistance from other climate feedbacks.

The key driver that forces the climate from Hothouse to Icehouse and back is instead the slow carbon cycle. The slow carbon cycle can be regarded as Earth's thermostat. It involves the movement of carbon between vast geological reservoirs and Earth's atmosphere. Reservoirs include the fossil fuels (coal/oil/gas) and limestone (made up of calcium carbonate). They can store the carbon safely over tens of millions of years or more. But such storage systems can be disturbed.

Carbon can be released from such geological reservoirs by a variety of processes. If rocks are uplifted to form mountain ranges, erosion occurs and the rocks are broken down. Metamorphism – changes inflicted on rocks due to high temperatures and pressures – causes some minerals to chemically break down. New minerals are formed but the carbon may be released. Plate tectonic movements are also associated with volcanism that releases carbon from deep inside Earth's mantle. Today it is estimated by the U.S. Geological Survey that the world's volcanoes release between 180 and 440 million tonnes of carbon dioxide per year - as opposed to the ~35 billion tonnes we release.

Epic carbon releases in the geological past

An extreme carbon-releasing mechanism can occur when magma invades a sedimentary basin containing extensive deposits of fossil fuels. Fortunately, this is an infrequent phenomenon. But it has nevertheless happened at times, including an episode 250 million years ago at the end of the Permian Period. In what is now known as Siberia, a vast volcanic plumbing-system became established, within a large sedimentary basin. Strata spanning hundreds of millions of years filled that basin, including many large coal, oil, gas and salt deposits. The copious rising magma encountered these deposits and quite literally cooked them (fig. 1).

Fig. 1: schematic cross section though just a part of the Siberian Traps Large Igneous Province, showing what science has determined was going on back then, at the end of the Permian Period.

Now laden with a heavy payload of gases, boiled out of the fossil fuel deposits, some of the magma carried on up to the surface to be erupted on a massive scale. The eruptions – volcanism on a scale Mankind has never witnessed - produced lavas that cover an area hundreds of kilometres across. Known as the Siberian Traps, because of the distinctive stepped landforms produced by the multiple flows, it has been calculated that the eruptions produced at least three million cubic kilometres of volcanic products. Just for a moment think of Mount St Helens and its cataclysmic May 1980 eruption, captured on film. How many cubic kilometres with that one? Less than ten.

Recently, geologists working in this part of Siberia have found and documented numerous masses of part-combusted coal entrapped in the lavas (Elkins-Tanton et al. 2020; fig. 2). In the same district are abundant mineral deposits formed in large pipes of shattered rock as the boiling waters and gases were driven upwards by the heat from the magma.

Fig. 2: an end-Permian smoking gun? One of countless masses of part-combusted coal enclosed by basalt of the Siberian Traps. Photo: Scott Simper, courtesy of Lindy Elkins-Tanton.

It has been calculated that as a consequence of the Siberian Traps eruptions, between ten trillion and one hundred trillion tons of carbon dioxide were released to the atmosphere over just a few tens of thousands of years. The estimated CO2 emission-rate ranges between 500 and 5000 billion tonnes per century. Pollution from the Siberian Traps eruptions caused rapid global warming and the greatest mass-extinction in the fossil record (Burgess et al, 2017). There are multiple lines of hard geological evidence to support that statement.

We simply break into those ancient carbon reservoirs via opencast or underground mines and oil/gas wells. Through such infrastructure, the ancient carbon is extracted and burned. At what rate? Our current carbon dioxide emissions are not dissimilar to the estimated range for the Siberian Traps eruptions, at more than 3,000 billion tons per century. The warning could not be more clear. Those telling you the climate's changed before are omitting the critical bit – the details. And when you look at the details, it's not always a pretty sight.

Last updated on 14 February 2023 by John Mason. View Archives

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Further reading

RealClimate article published by Prof. Stefan Rahmstorf on July 20, 2017:

The climate has always changed. What do you conclude?


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Comments 276 to 300 out of 420:

  1. 275, lancelot,
    Just another question though, how can you be certain of "no geological signs" in the ocean floor from 55ma ago? Lot of sediment since then, and it's a big ocean.
    Research ocean cores. See how they are used to establish various aspects of climate, life, etc. See how they are dated (by looking at the different, distinct layers of sediment). For example, consider this excerpt about how cores can be taken:
    The Chikyu (Japanese for planet "Earth") seen above, cost $540 million and is a colossal 57,500-ton, 210-meter-long white ship developed to drill deep below the ocean floor. Besides being the most sophisticated laboratory on the seas, the science vessel boasts the tallest drilling derrick at 112 meters above the waterline and a drill pipe that is 9.5 kilometers long--22 times the height of the Empire State Building. The borer drills through 7,000 metres of crust while floating in seas up to 2,500 meters deep. Its drilling system uses a 380-ton protective casing over the wellhead that is about the size of a six-story office building. It shields the vessel against eruptions of methane gas and pressurized fluids and allows for the secure retrieval of nine-meter-long core samples.
  2. 275, lancelot, Oh, and... a lot of real deniers come here trying to "trip up" the science by offering half-baked ideas that they assume scientists haven't considered. A lot of your questions come across in that vein, because of the way they are phrased. You'll probably raise less hackles if you take a little more care in how you phrase your questions, by making it clear that it's an honest question as opposed to a rhetorical question intended to convey doubt to lurkers (i.e. aimed at other readers rather than the people of whom you are asking the question). For example, your "how can you be certain..." question might be better phrased as "how have scientists ascertained that..." I know it may seem annoying, but it's important to come across as truly interested in the answers, and giving scientists their due respect, rather than making it appear that you have, in your spare time, out-thought the professionals and come up with something they can't possibly have considered.
  3. lancelot - Forminifera species ratios, sedimentation types/rates, isotopic balances, other signs. Forminifera species ratios are quite sensitive to environmental conditions, for example, and have been used to construct multiple paleotemperature records, including sea rise on the order of millimeters/year. And if the ocean floor heated up non-uniformly, the signs of such heating would be even more clear where it did happen. But seriously - there is no evidence to suggest a 5 °C heating of the ocean floor lasting 200 ka. That's the evidentiary equivalent of fairies, of claiming climate change is due to wars among the Mole Men, of magic wands. Actually reading Katz, Dickens, et al 1999, they state: "Long-term global warming during the late Paleocene pushed the ocean atmosphere system past a critical threshold, causing warm surface waters to sink and intermediate to deep ocean temperatures to rise by 4 to 8C. This warming propagated into the sediments, converting once solid CH4 hydrates into free gas bubbles. This dissociation resulted in an increase in pore pressure at depth, leading to sediment failure and the release of massive quantities of CH4 into the ocean." (emphasis added) From this they feel a circulation change pushed surface water quite quickly into the depths, warming the benthic waters and triggering the clathrate release. . "Methane release would have occurred on continental slopes ... adding carbon to all reservoirs of the global exogenic carbon cycle and substantially shoaling the depth of carbonate dissolution in the ocean. ... Over several hundred thousand years, global carbon and oxygen cycles gradually retumed to equilibrium conditions after the LPTM, although marine and terrestrial ecosystems were forever changed. " Again, note the carbon cycle disturbance. There are also in this paper descriptions of ocean cores that show sedimentation supporting this scenario. I would strongly suggest, lancelot, that you read the papers you have pointed to - it looks more like you're grabbing the first interesting quote without looking at the context, and hence presenting a misinterpretation of the work. There's a big difference between asking "Hey, somebody said XXX, I don't know if it's right or wrong?", and presenting skeptic arguments and (in this case) misinterpretations of papers as your opinion.
  4. Lancelot - have a look at this article here for more on PETM. The evidence for CO2 being culprit is in the ocean acidification which also constrains the size of change. I would also suggest looking at "Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming" Richard E. Zeebe, James C. Zachos and Gerald R. Dickens 2009 for an idea of the constraints (though I disagree with some of their conclusions).
  5. Sphaeriica, if I make a statement, and someone says "how can you be certain?", I dont take offence. I explain why I feel confident in making the statement. So I don't quite see your point. KR please note the preface in my post 269. I clearly stated my ignorance at the outset, I quoted in full the basis for my question. Anyway, question raised, answer given, thanks for the scientific evidence which you have given to negate ocean floor heat release as a possible partial contribution to the PETM warming. Have you considered that every time some non expert says , maybe it isn't CO2, and you give a reasoned answer to explain why it is, it actually strengthens the public perception of the Co2 theories? Scaddenp will take a look, thanks,
  6. 276 , by the way, amazing engineering, really fascinating to hear what goes on in deep ocean exploration. Was honestly interested in how one would could possibly know what happened 55mya and 7 miles down under the ocean. Impressive.
  7. 280, lancelot, This isn't a normal venue. In a classroom, or a party, or at lunch, when you do as you describe you are right. But in climate change there are a large number of very, very arrogant, angry and ignorant people. They waltz in here with personal theories based on a gross misunderstanding of the science, stating everything from proof that the globe hasn't warmed to the idea that GHG theory violates the laws of thermodynamics and more. One (-self-snip-) even claims that he's proven that current warming is coming from the interior of the earth. Most people who comment here but do not already understand the science are not here to learn, or asking reasonable questions and looking for answers (as I presume you are). Most people who comment here but do not already understand the science instead have an axe to grind, and believe that they have all of the real answers, and that they'll show those dang, silly scientists a thing or two about logix and clear-headed thanking. Hence... a perfectly ordinary sounding question is likely to touch some nerves with people who have already been rubbed raw with what could, at best, be called a parade utter and complete nonsense. Hence, a sensitivity to such a response is advised. You might not want to go to the trouble to carefully word your questions, but you might give someone like KR a break when they presume that you are possibly asking these questions with a hidden agenda, hoping to trip people up or to sow doubt. At the same time... people here at SkS probably should back off and give newcomers a little more slack before flipping out on them.

    [DB] "At the same time... people here at SkS probably should back off and give newcomers a little more slack before flipping out on them."

    Agreed.  Give the benefit of the doubt, with the action mentioned after the word "before" in your back pocket.

  8. Sphaericq, thanks for the gentle advice. I note that KR told me I was raising a Skeptical Argument , my caps. I suspect that is the touchy point. I have read nothing up on the PETM except Brian Lovells book. My question honestly came out of my own head, as an immediate reaction to the events as described, not off some website. But if the question, as I now suspect from KR's phrase, is a regularly used SA, I can see how KR would react to someone apparently churning out an old mantra. I do appreciate being given the benefit of the doubt. My purpose in using this forum (for which many thanks for allowing me) is to clarify my personal understanding. I entered here rating significant AGW as around 50% certain, now I would rate it as 80 to 95% certain. Lastly, I have seen criticism of some of my posts on the lines of , why dont you read up in depth on the subject before coming here with questions. Simple answer, I would love to, but life isn't long enough. That is why one asks the experts.
  9. lancelot - If I've misinterpreted your posts, overreacted, my apologies. What I was reacting to is in large part the series of different skeptic arguments you have presented, pointing every which way. That can be very hard to distinguish from the behavior of a Concern Troll. I will note, however, that the vast well of knowledge does reward those who put in some effort. In my last post I spent ~20 seconds on Google Scholar using the search term "Dickens G R 1999, Nature" from your post (clicking on the first PDF), roughly two minutes looking through the abstract, initial sections, and conclusions - and found clear information completely dismissing a 'heat from the crust' scenario. That's two and a half minutes. And this is not my professional field. Alternatively, using the "Search" box here on SkS, with the term "crust", links to some relevant threads discussing particular climate issues, such as the recent Heat from the Earth’s interior does not control climate. I'm glad you are actually looking for information on climate change. Far too many take a personally/ideologically attractive position, stick their fingers in their ears and sing 'lalala", as I'm sure you've seen. I would encourage both your curiosity - and perhaps a bit more use of search capabilities.
  10. KR I will use google scholar in future, it looks good. iI have in the past tried to download papers such as Dickens from the AAAS site and for some reason the site repeatedly fails to register me. So only the abstracts are viewable, for me. When I have seen full articles, often as a non expert the language and terminology can get quite dense and very hard to follow. I simply don't have the hours in the day to learn the language. At the same time, i need to make some decisions which are to a large extent dependent on theIPCC advice for policy makers. For that reason I have taken time over the last 6 months to fully understand the subject in as much depth as possible, in order to feel confident about applying the advice. Learning mainly on the web, that inevitably exposes one to a lot of arguments and apparently valid questions, from all sides, some of them very plausible. Muller, Svensmark, Kirkby.. Appinsys... I felt it was quite important to be clear in my mind and to subject the questions in my mind (some of them independently arrived at by the way) to expert grilling. I wish of course I had the time to do a climate science degree and even to understand a tenth of the content of some of the papers! But this forum has been very helpful, and patient. I have actually gone through my finite list of 5 query subjects now so I doubt that I will need to ask any more, unless some surprises come up. Of course as we know, life is full of surprises!
  11. 285, lancelot, Three things. First, while it seems daunting at first, you will find that as you look at more and more actual papers (just briefly!) it will get easier and easier. Admittedly, KR's ability to do what he did in 2 1/2 minutes did come from a lot of practice. But it does get easier. Second, on finding papers... I've found that while most papers are behind paywalls, you can almost always find a copy for free download from somewhere. Usually including the term "+pdf" in the Google search terms will find you an option or two... but not always. Third, on learning on the Internet... that can be very, very dangerous, especially with the vast number of denial sites out there who want to spin the science from their own perturbed (and wrong) point of view, but do so with the confident tone of someone who is teaching you fact rather than their interpretation, misinterpretation or misrepresentation of the facts. You need to be very leery of what you read and what you trust.
  12. 285, lancelot, For learning properly I might recommend Raypierre Humbert's book Principles of Planetary Climate. I have not read it myself yet. I simply have a whole lot of respect for Raypierre. Amazon lets you look at a fair number of pages inside the book to give you a feel for it. If you do pick it up, let me know how it reads. I won't have time to do so myself for quite a while (looking at my schedule, I'm planning on getting to it by my fourth reincarnation from now).
  13. lancelot - One thing I've found helpful on Google Scholar is the links to "All N version". Often this points to alternative sources, pre-prints, author's copies, etc., when just an abstract is available on a journal page. Full text isn't always available, but it's helpful. If nothing else works, I follow the "Cited by N" link, and try to get a feel for that particular topic from those either using that paper as background, expanding upon or refuting the results.
  14. lancelot : "Muller, Svensmark, Kirkby.. Appinsys..." Seeing that you go to Appinsys, I can now see how you could have been confused or uncertain previously ! A website whose 'Global Warming' page contains 28 instances of 'alarm' and 3 uses of the word 'lie', should give pause to anyone thinking of using that site. Not only that, the About page ends up by calling Obama a "liar" ! How could you (or anyone) consider that site a useful and unbiased one ?
  15. Lancelot: Rather than attempting to access and read a slew of peer-reviewed papers, I recommend that you first read and digest books and reports that essentially synthesize the science of climate change. You really need to understand the forst before you start analyzing each and every tree in it. In terms of textbooks, I highly recommend the Fourth Edition of Global Warming: The Complete Briefing by John Hougton, Cambridge University Press, 2009. It is written in plain English so to speak. Another excellent resource is the recent set of reports prepared by the National Academies of Science under their America's Climate Choices initiative. To access these reports, click here.
  16. lancelot - I would also recommend The Discovery of Global Warming for an overview of the topic, not requiring a technical background. And in looking at the 'series of tubes', the interweb, I would suggest discounting any sites with obvious political or ad hominem ranting. Such as, for example, "Appinsys". The ability for anyone to publish to the web means that quality checking is ever more important.
  17. 292, lancelot, Didn't notice Appinsys in your list. The site is a travesty. You will get nothing but confusion and misinformation there.
  18. Thank you all for the wealth of suggestions for further reading. To be honest, From now on I will tend to rely on IPCC advice and reports, in which I now have much more confidence.
  19. Out of interest, and for future reference in any discussions with colleagues, what would be specific examples of wrong information (as distinct from polemic) in the appinsys site? It was the most convincing skeptic site i came across, the arguments being supported by a lot of impressive graphics. Sorry if off topic here, but I am never quite sure which thread to move to for digressions.
  20. lancelot#254: Replying here, a thread that deals in facts; a strong counterpoint to appinsys.
  21. 294, lancelot, I dont' want to waste too much time dissecting their nonsense, because just looking at it makes my skin crawl, but you should note that they are very, very good at taking actual scientific data that is accurate, but presenting only that data that fits the conclusions they wish to establish. I'll just do a couple. Their page trying to claim that warming is not global. This page goes to great lengths cherry picking plots to try to show that warming is merely a northern hemisphere problem. Their conclusion:
    Conclusion: The empirical data show that warming in recent decades is a northern hemisphere phenomenon – in particular an Arctic phenomenon –with no significant warming in the tropics or southern hemisphere.   It is not a global phenomenon.
    The fact is that because of the unequal distribution of land masses the warming in the two hemispheres is always dramatically different. This does not mean that the southern hemisphere is immune or not involved, merely that it responds differently in the short term. The conclusion that warming will be greater in the Arctic is, in fact, a tenet of the current science -- research "polar amplification." At the same time, much of their evidence relates to the differences between the Arctic and Antarctic ice and temperatures, but these are apples and oranges. One is a large sea virtually hemmed in by land, while the other is a large, mountainous area of land surrounded on all sides by ocean. The southern hemisphere is dominated by ocean, while the northern hemisphere is dominated by land. Of course they are responding differently!
  22. 294, lancelot, appinsys MWP This section goes to great length to try to claim that the MWP is global and had temperatures greater than we are seeing currently, when this is patently false. Part of their argument:
    This IPCC statement is at odds with the findings of other scientists. For example, research at the Harvard-Smithsonian Center for Astrophysics report on a recent paper using proxies, which verifies the occurrence of the MWP: []
    The linked paper by Willie Soon (an astrophysicist, not a climate scientist) is notorious for it's failings, and the resulting resignation of half of the publishing journal's editorial board in the ensuing controversy. They go on to say:
    Many studies can be found exhibiting the MWP. One example is shown in the following figure.
    This is a common skeptic trick, and a great example of presenting individual proxies that for that one location seemingly show similar warming, yet their graphs stop before more recent temperatures. Another common problem is that the MWP is very loosely defined as occurring from 950 to 1250 AD. The warming in any particular location might be a 50 year span peaking at 1000, 1100, or 1200 AD. They're all treated as contemporaneous. Can you imagine if current temperatures were computed by taking the highest temperature in a 300 year span from each separate location on the globe? In their example shown here the peak is just prior to 1000 AD. They also term this a "Northern Hemisphere" reconstruction even though 9 of the 14 proxy sites used lie above the 60˚ north, and 13 of the 14 above 45˚ north. It's more appropriately a sub-arctic reconstruction. And yet even their graph hows that current global temperatures exceed those around 1000 AD at that latitude. If you were to instead plot temperatures in the same band, you'd see this: [Source: GISTEMP... click on the image to view] Imagine the 1 to 3 degree increase added to the Cook graph, which are representative of the same sites used in his study and on his graph, instead of the temps appinsys added. The Cook paper is available here.
  23. 294, lancelot, I don't have any more time for appinsys... have to chauffeur the teen daughter around town. But if you have any particular section of appinsys you'd like me to rip up, just let me know. It can be a fun diversion.
  24. Sphaerica, Thanks , I guess that writing that response was the equivalent of root canal work for you. I get the point!
  25. Lance @ 293 Over the past couple months I have been reading both here and at JoNova's blog trying to acquaint myself with the state of the debate. Even if I were absolutely scientifically, numerically and logically illiterate, one thing would stand out. One side is full of desperation, anger, ideology, rhetoric and ignores questions. The other side is a little testy at times, a little left-biased at times but generally patient, responsive and comprehensive. SkS makes it really easy to pick who to trust.

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