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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 101 to 125 out of 136:

  1. Actually Roger you could learn a lot by just shoving the lawyers' arguments through this site. How about something from a scientist that understand atmospheric physics instead?
  2. Roger, Ok how about this: you are correct, technically speaking, no proof of AGW exists. However, this is true for any empirical scientific knowledge, including the theory of gravity or Newton's laws. So saying that AGW is not "proven", is somewhat of a meaningless statement. Here and here are a couple links that go into more detail about what "proof" means to science. Hopefully they will clarify why your usage of the word is somewhat off target.
  3. yocta, Its not supposed to be a scientific paper, its looking at the arguments and evidence to see if they stack up from a legal proof point of view. Like any lawyer or judge, such a cross examiner does not have to understand the technical side of things, but takes an unbiased view of the evidence and what is presented. Cheers Roger
  4. Roger, >Like any lawyer or judge, such a cross examiner ... takes an unbiased view of the evidence and what is presented. Cross examinations are biased by definition. They are an examination of your opponents witness, hence the "cross" part. That paper is just a summary of AGW skeptic talking points. If you are going to claim an unbiased view of the evidence, at least listen to what the other side has to say in rebuttal. In any case, science isn't determined by lawyers, it's determined by scientists publishing peer-reviewed research. Take a look around this site and the others linked for you to get a feel for what the science actually has to say.
  5. Roger, if a lawyer or judge does not understand what he's reading or hearing he can be lied to without being aware of it. Inability to discriminate truth from fiction implies that useful judgment is suspended. The conclusions of an ignorant jurist are unreliable even as they are necessarily unbiased.
  6. The legal system is not really a good model for the scientific method. Thus, it's perhaps not surprising that I'm very unimpressed by the link Roger provides to a document written by a law professor at the University of Pennsylvania. I picked one area with which I am somewhat familiar (sea level rise, discussed on pp. 65-68) and examined it. Unfortunately, it turns out to be exactly what one would expect from someone trained in the adversarial legal system rather than in the scientific method -- he is making a case rather than skeptically evaluating the evidence. (Roger, do you understand the actual meaning of the word "skepticism"?) It also seems to have been heavily influenced by a recent error-riddled paper on sea level rise by Madhav Khandekar in Energy & Environment, which might explain some of Johnston's problems. Bottom line, I would file Johnston's paper in the recycling bin. If there are any valid points in there, they're obscured by a lot of obvious garbage. There are much better resources out there for understanding climate change.
  7. Ned wrote : Thus, it's perhaps not surprising that I'm very unimpressed by the link Roger provides to a document written by a law professor at the University of Pennsylvania. You're not the only one. Not only does the author of that piece start off by thanking McKitrick, Lindzen and Pielke Jnr for their assistance, but the use of terms like "group of activist scientists" and "faith in the climate establishment" give a good flavour of the author's pre-conceived views. Also, the first foot-noted link goes to the American Enterprise Institute for Public Policy Research, which has an article using academic papers which have been countered (e.g. Lindzen & Choi, 2009; Mclean, de Freitas & Carter, 2009); attempts to discredit the peer-review process; accepts the views of the Wegman Report with regard to 'tribalism' in climate science, and uses examples from the leaked CRU emails. Only the committed so-called skeptic would advance that paper as 'evidence' for anything other than his/her personal preference.
  8. Upon further investigation, the "recycling" here is kind of fascinating. Johnston's document relies heavily on a 2007 paper by Lindzen in E&E (yes, another E&E paper ... Johnston is looking worse and worse). That E&E paper was recycled by Lindzen in a 2009 blog post. That blog post by Lindzen, in turn ... is the very same "skeptic argument" quoted by John Cook and then debunked at the top of this thread! In other words, we've come full circle ...
  9. It never seems to occur to the people who make the 'climate has changed before over the last 700,000 years ' argument that almost no-one was around at those times to suffer the consequences. The few that were around ( about 5 million, ten thousand years ago according to NOAA) ) could move to follow the climate that suited them anyway. Before a few 100,000 years ago Homo Sapiens didn't even exist in any case.
  10. Firstly I would like to thank you and your website for generating valuable discussion on AGW. Secondly, I would like to say that I don't think it is impossible for humans to change the climate, but that the change we bring about through AGW will be less significant than claimed by your article. In other words, my research has shown that our climate isn't as sensitive to CO2 as most AGW proponents claim. In your article, you claim that "we" can calculate that a doubling of CO2 will result in approximately 3 degrees centigrade. I hope you can enlighten me on the methodology of this calculation. The empirical evidence does not support your conclusions. You can take any era from history, but I will take the Jurassic and Triassic to make my point. The raw data is taken from encyclopedia Britannica 11'th edition. During the Jurassic Period CO2 concentration was 1950ppm (7 times greater than preindustrial levels of 250 ppm). You claim that for every doubling of CO2 temp increases by 3 degrees centigrade. With that logic, the Jurassic period should have been warmer by 9 degrees. It was only warmer by 3 degrees. Triassic period had CO2 concentrations of 1750ppm and was also only hotter by 3 degrees. I understand that other variables were different during that period as well. But, if you look anywhere on the geological timescale you will have a hard time proving that a doubling of CO2 directly results in a 3 degrees increase in temp. Thank you for the article!
    Response: Thanks for the kind comments. Climate sensitivity is essentially the change in temperature in response to a change in the planet's energy balance. So to calculate climate sensitivity, you need to work out how much global temperature has changed in the past and the changes in the planet's energy balance at the time. So if we can obtain records that give temperature (eg - from ice cores) and couple that with records that give changes in solar activity, atmospheric composition and volcanic activity, it's possible to calculate climate sensitivity empirically.

    Climate sensitivity is around 3 degrees warming for a doubling of CO2. A more technically correct definition is 3 degrees warming for a radiative forcing of 2.7 watts per square metre (which is the radiative forcing from a doubling of CO2). So if you want to look at the planet's energy imbalance over past periods, you need to include other variables which affect the planet's energy balance. Specifically, we find that as you go further in the past, the sun is less bright. So you need to consider the combined effect of a dimmer sun with higher CO2 in past periods. When we do that, we find a close correlation between the net radiative forcing and climate.
  11. Svettypoo - I'm afraid your analysis is rather simplistic, for instance you fail to account for reduced solar luminosity further back in time, and also new analysis is casting some doubt on the extremely high CO2 levels once thought to have existed. Atmospheric CO2 concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100 Furthermore, how exactly do you think the estimates for climate sensitivity came about, if not from the study of Earth's previous climates?
  12. OK, I'll make this short and quick. CO2 levels are about 380 per million here. They are about 965,000 per million in the Venetian atmosphere. Ya think that about .04% of the same concentration is going to throw us way out of whack? The big dog, which no one wants to talk about when it comes to greenhouse gases is water vapor. It goes from about nil when we are in the depths of an Ice age to a worldwide average of 2% during recovery and the following times. THAT is our basic greenhouse gas. CO2 is a bit player. You drop CO2 levels to under 1% on Venus and that place would turn into an iceball without water vapor. Yes climate just keeps on changing. We are in a period where the temperature has fluctuated, sometimes rather quickly, within a 4C range over the last 10,000 years. We are near the middle of that range now. Sun activity is relatively high, higher than it has been for the last 2,000 years. The long range trend has been up since about 1500. We have seen temperatures rise about 0.5C since 1900. Yawn, I hate to say it but a rise of about 3C in about 100-200 years was seen about 8,000 years ago. That's a quick warm-up. BTW, it has been warmer than this about half the time since we snapped out of our last ice age. At least 4 times it was 1C warmer than now and once it was 2C warmer. Bottom line. Climate changes and I am not alarmed.
  13. cruzn246 wrote : "Bottom line. Climate changes and I am not alarmed. Bottom line is assertions and beliefs are worth nothing. If you have any linkable evidence for even half of the figures you posted concerning temperatures, please let us see them.
  14. Look up the stuff for Venus and the earth yourself. It's easy to find. I derived my temp and sun activity from easy to access sites. Here you go. The temp data was derived from ice cores. I took the temp graph and blew it up to capture the Holocene maximum and other features. Here is the page for the solar activities
  15. The basic stuff about changing Venus' atmosphere is just common sense stuff I derived from taking meteorology. Yes, I took a class. The 2% worldwide average is from the same place. I know in any spot on earth it ranges from near zero to 4%. I just averaged it. It could be 0.5 either way, but it would still be at least 37 times more by mass than CO2(0.04%) at lowest average values (1.5%).
  16. Well Christ man it is a continuation of a discussion. Where do I have to go to discuss electromagnetic absorption? I guess you just want to split up everything so there is no real big linking pool of info.
    Response: The big list of Arguments is the linking pool of info. There is also a "Newcomers Start Here" page linked in the left side of this page.
  17. cruzn246 wrote : "Here is the page for the solar activities" As well as looking at the picture, how about reading the relevant WIKIPEDIA page for context : The scientific consensus is that solar variations do not play a major role in determining present-day observed climate change. Also, the increase of temperature is a little more than you suggest : The updated 100-year linear trend (1906 to 2005) of 0.74°C [0.56°C to 0.92°C] is therefore larger than the corresponding trend for 1901 to 2000 given in the TAR of 0.6°C [0.4°C to 0.8°C]. The linear warming trend over the last 50 years (0.13°C [0.10°C to 0.16°C] per decade) is nearly twice that for the last 100 years. The total temperature increase from 1850–1899 to 2001–2005 is 0.76°C [0.57°C to 0.95°C]. Summary for Policymakers Finally, this picture (also from WIKIPEDIA) seems to disagree with your assertion about temperatures since we "snapped out of our last ice age" : Holocene Temperature Variations
  18. I commented a little more than a week ago and I received some kind responses. Sorry I haven't answered yet, I've been camping in beautiful British Columbia with the family. The OP was nice enough to leave me a comment. I have to first respond that there are no records of solar activity that go back more than 1000 years (and even those are unreliable) There aren't even available proxies we can use to estimate solar activity. You must be refering to the young dim sun hypothesis... As explained earlier, this would make it impossible to find 3 degrees of warming through a doubling of CO2 "empirically." Proving something empirically means that you would have to prove it through records or experimentation. There is no empirical way to show any trends in the sun's history. Perhaps the dim sun hypothesis is correct and the sun has been warming (I believe this is probable although uncertain) even then CO2 doesn't tell doesn't even come close to telling the rest of the story. The solar irridiance has to make some pretty funky and unscientific loops to make CO2 be strongly correlated with temperature in the long term. Perhaps in the short term there is a strong correlation between CO2 and temp but I believe the 800 year lag weakens a CO2 - temp causation. I would like to know where you find estimates of detailed solar irridiance in the past. Furthermore I am even more curious of the methodology of how you combine these "guesstimates" with CO2 proxies to find a close correlation with temp. Lastly, I couldn't access your first link. I'm not sure if its broken or I am trying to open it wrongly. I am new to this site. Thank you
  19. To Dappledwater: Sadly your link was broken and I couldn't open it. To quote you, "Furthermore, how exactly do you think the estimates for climate sensitivity came about, if not from the study of Earth's previous climates?" You try and mock me with this statement... but I couldn't agree with you more. I only wish estimates for climate sensitivity came from the Earth's previous climate. This however, isn't the case. Today's climate sensitivity and models (at least issued by the IPCC) place an unprecedented emphasis on CO2, which simply isn't supported by the empirical record.
  20. Beautiful blazing BC, burning bark and beetles. Today's climate sensitivity and models (at least issued by the IPCC) place an unprecedented emphasis on CO2, which simply isn't supported by the empirical record. Levels of C02 unprecedented since ~15 million years ago when global temperatures were several degrees higher, is that the missing empirical evidence you're talking about, svettypoo? I suppose we can dodge forming a conclusion by sticking with an appropriate definition of "empirical evidence."
  21. doug_bostrom, I hope you are not trying to argue paleoclimatic analysis with forest fire and beetle phenomena of the last several decades. I don't mean to be rude but please stay on topic. Secondly, I don't think you completed your thought in the third paragraph. You wrote "Levels of C02 unprecedented since ~15 million years ago when global temperatures were several degrees higher, is that the missing empirical evidence you're talking about, svettypoo?" I am going to assume you mean current levels of CO2 are unprecedented since ~15 mil years ago when global temperatures were several degrees higher. The logical question is if the CO2 is the major climate driver and ppm was matched 15 mil years ago and the average temp was several degrees warmer, why isn't it several degrees warmer now? I have not confirmed your statements anywhere but if they are true, they would only strengthen a AGW sceptic argument... I will give you a chance to explain what you meant with no judgement... I did form a conclusion. Did you not read my previous post? If I wasn't clear enough i will state it now. I concluded that CO2 has never been empirically shown with statistical significance to be the driver of climate and yet the IPCC models place it in that very position. I would also like to address my emphasis on empirical evidence. One can not claim to have a record of solar activity for the past 500 million years when such a record doesn't exist. If you would have kept on reading my post you would have read that even if we were to use the dim young sun hypothesized irridiation and combine it with the CO2 proxies as the major climate drivers we would not be able to explain prove causation of CO2 driven temperature. Lastly, I have tried to be as clear as possible in describing my argument and conclusion. Perhaps you could share some of your conclusions? Thanks.
  22. Just so I'm clear on your position, svettypoo, what would you consider evidence of CO2 being the primary driver of the currently-occurring climate change? What would that evidence be? What would make you say, "Aha! Ok, I see it now"? Or do you think it's fundamentally impossible to provide, no matter what the conditions?
  23. svettypoo - CO2 has, in the paleo record, been a component of climate change. In warming, as CO2 is forced out of warmer waters, it's been a positive feedback. When CO2 weathers into newly exposed rocks, dropping temperatures, it's been a forcing. Currently we're changing CO2 levels at 100x any paleo recorded rate. That makes CO2 the first to change, and thus it's the forcing on the climate. Doubling CO2 causes ~1°C, and with the various water vapor and other feedbacks, an estimated 3°C warming. As to solar activity - we have quite literally millions (billions?) of data points as to the evolution of stars of our sun's mass - via astronomy. Not to mention the various paleo proxies. We know that stars of our sun's type evolve warming over time. Unless you have solid evidence indicating basic astronomy and stellar lifetimes are wrong? As to your graph - You have quite clearly not incorporated the 'warming sun' into it. The evidence others have presented indicate that CO2 levels and solar forcing together match quite well to temperature.
  24. Svettypoo I'm guessing you're pretty new to this topic, that or you're practicing some form of impressionism by ignoring important features of this topic. Notice that we've been adding C02 to the atmosphere in significant quantities only for the past 150 years, reaching close to 400ppm only in the past couple of decades. This means atmospheric temperature has not come close to equilibrating to the temperature it can be expected to reach at the present concentration, let alone where it'll eventually settle assuming we end up in the 800-1000ppm level where my intuition (cynicism?) suggests we'll sit given our indolence, complacency and proclivity to believe in comforting fairy tales. As to your loner perspective on solar evolution, yes I did read to the end of your version of "I doubt it" and found that part unpersuasive. Start here w/Spencer Weart's informative book on the topic of global warming to get caught up, if indeed you're fresh.
  25. I do not have a loner perspective on solar evolution doug_bostrom and KR, just like you I think that the young dim sun hypothesis is likely true. To quote myself "Perhaps the dim sun hypothesis is correct and the sun has been warming,I believe this is probable" Please don't commit a straw man. I stated it wasn't empirical because there is no record of it, which is also true. Which one of my points do you disagree with? The only reason why I mentioned it was not empirically proven was because the OP claimed that CO2 effect on the climate can be proven empirically. KR, there is a statistical reason why i wanted to leave out the warming sun. Its the whole point of the argument. Here is the statistical explanation. Radiative forcing hypothesis claims CO2 combined with solar irridiance to provide the closest correlation with temperature proxies. statistically speaking, this would mean that both CO2 and solar irridiance should be positively correlated with temp proxies when calculated alone. I however agree 100% that solar irridiance is heavily correlated with temperature proxies. I wanted to show the lack of correlation that CO2 had with temperature proxies in paleoclimate. This makes perfect sense statistically and in the context of the argument. Please try and understand the argument and the post before making a rebuttal. KR, Sadly, those graphs that you posted do not show temperature on any axis. So I don't know how you can claim that "the evidence others have presented indicate that CO2 levels and solar forcing together match quite well to temperature." I am beginning to think that you post first and think later. Actually, simply by looking at the graph you can see the holes in the solar forcing combined with CO2 argument. The Ordovician, Devonian, early Carboniferous, and Permian ice ages are not correlated with troughs in the graph.(the Permian ice age happens on a mini crest, look closely) Thats 4 out of six ice ages... This blows a pretty gaping hole in the argument... Also, the interglacial between the 4th and 5th ice ages makes absolutely no sense at the level of radiative forcing at that moment.

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