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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)

Greenhouse gasses – mainly CO2, but also methane – were involved in most of the climate changes in Earth’s past. When they were reduced, the global climate became colder. When they were increased, the global climate became warmer. When CO2 levels jumped rapidly, the global warming that resulted was highly disruptive and sometimes caused mass extinctions. Humans today are emitting prodigious quantities of CO2, at a rate faster than even the most destructive climate changes in earth's past.

Abrupt vs slow change.

Life flourished in the Eocene, the Cretaceous and other times of high COin the atmosphere because the greenhouse gasses were in balance with the carbon in the oceans and the weathering of rocks. Life, ocean chemistry, and atmospheric gasses had millions of years to adjust to those levels.

Lush Eocene Arctic 50 million years ago

Lush life in the Arctic during the Eocene, 50 million years ago (original art - Stephen C. Quinn, The American Museum of Natural History, N.Y.C)

But there have been several times in Earth’s past when Earth's temperature jumped abruptly, in much the same way as they are doing today. Those times were caused by large and rapid greenhouse gas emissions, just like humans are causing today.

Those abrupt global warming events were almost always highly destructive for life, causing mass extinctions such as at the end of the PermianTriassic, or even mid-Cambrian periods. The symptoms from those events (a big, rapid jump in global temperatures, rising sea levels, and ocean acidification) are all happening today with human-caused climate change.

So yes, the climate has changed before humans, and in most cases scientists know why. In all cases we see the same association between CO2 levels and global temperatures. And past examples of rapid carbon emissions (just like today) were generally highly destructive to life on Earth.

Basic rebuttal written by howardlee

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 6 August 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Myth Deconstruction

Related resource: Myth Deconstruction as animated GIF

MD Past

Please check the related blog post for background information about this graphics resource.


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Comments 351 to 375 out of 883:

  1. I think one of the "strengths" of this argument (from the Denialist point of view) is that there are actually two possible implied arguments.  One is (as others have pointed out above) is

    Climate has changed before [naturally and so the current instance of climate change must be natural too]

    The second is this

    Climate has changed before [and "the planet" survived that and so will survive the current climate change, irrespective of how its caused]

    I've put the incorrect implication in []. The first case is a failure of logic, the second case mis-identifies both the risk of climate change, and what "taking action" on GHG emissions seeks to accomplish.

    So when responding to the simple statement "Climate's changed before",  you can't be sure which one of these two arguments is being implied, if you rebute one the other can still be thought to be valid, if you try and rebute both it can get very involved.

    Perhaps this helps explains why, as an argument, it is so popular with contrarians.

    A final thought: It seems to me that as soon as the implied arguments are stated, the flaws in them are very obvious, and this, together with the fact that the prima-face statement is true and uncontestible, explains why the implied arguments are always just that - implied - which is why SKS haven't found a quote to adorn this acticle that exposes the real myth(s)

  2. Phil:

    Yet another "advantage" of not stating the implied argument is that it leaves the imagination of the listener free to come up with any number of arguments. I was going to phrase that as "possible arguments", but then I realized that they don't even need to be possible - at least in a real sense. As long as the listener thinks they are "possible", then the speaker has sown the seeds of doubt.

    Twenty listeners can come up with 20 mutually-exclusive implied arguments, and as long as nobody goes into detail everyone is happy (except for the scientists and policy people that actually want to examine the logic, validity, and strength of the argument).

    As you say, getting the person to explicitly state the argument, rather than leaving it implied, allows you to examine it properly. Unfortunately, you often end up playing whack-a-mole as the fake skeptic refuses to provide a proper argument, and keeps saying "that's not what I meant" as you cycle through the possibilities and explain why they are wrong.

  3. In conversation I actually enjoy this kind of misleading statement;

    Denier: 'You know, climate has changed before.......'

    Me: 'Good point! The link between past CO2 changes and past climate changes gives us a clear idea of what to expect with our current CO2 emissions.'

  4. Is climate sensitivity a constant or dependant on other variables? Meaning, as the earth gets warmer (or cooler) does the climate sensitivity stay the same or increase/decrease (or maybe it's dependant on some other variables as well)?

  5. Kristjan, I think it is expected to be more or less constant within a particular range of temperature/states. Ie once all the ice is melted, then the albedo feedback becomes a lot more subtle. Ditto, on an iceball earth, you would expect sensitivity to be high at point where ice melts at tropic.

  6. "The combined evidence indicates that the net feedback to radiative forcing is significantly positive."

    I would say that the fact that the planet survived several cycles of glaciation over the last half a million years, and that the planet's sensitivity can be computed, is an indication that the net feedback is negative.  Positive feedback would cause a runaway temperature, even after the forcing function is removed (See amplifier squeaks.)

    However I would like to verify the list of the various feedback mechanisms.

    I read a lot about poitive feedback processes.  Water vapor, atmospheric CO2, polar ice reflection, and maybe more.  As for the negative feedback I saw only brief mention of clouds and black body radiation.

    Are there any other negative feedbac mechanisms?  

    What is the dominant negatve feedback mechanism?

    Is there evidence of its occurence at the peak of the glacial warming cycles?


    (Rob P) Please see this SkS post by Neal King:Why positive feedback doesn't necessarily lead to runaway warming. Further comments on positive feedback should be directed there - where it is considered to be on-topic.  

  7. I stumbled on this webpage and appreciate the huge amount of work, data and analysis that goes into it. I have a degree in Geology and understand the arguments yet I'm not convinced and I'll site two examples. 1. The text posted alongside a graph showing sea level rise cautions the reader not to focus on a small set of points (with a ruler) as any trend line can be visible in a small data set. We are cautioned further to look at the entirety of the data. The first thing I noticed is that the data is only available to the mid-1800s. This ~180 year span is a tiny sliver of geologic time- even since the last ice age. We don’t seem to have a data set covering a span of time long enough to be meaningful. Why draw a conclusion from incomplete data? 2. Ice cores are presented as providing data on volcanic dust, atmospheric gases and other forces that worked to cause previous climate change. I have not see any proof that we understand whether the delta in the ‘forces’ caused the climate change or themselves were caused by the climate change (or were coincidental to it). The inference I took is that we fully understand the mechanics of the earth’s climate and can account for the causes of past climate change. Given our meager ability currently to forecast the weather and predict storms this does not seem to be true. I’m not a ‘denier’ nor is my head in the sand. I’m offended by these terms and refuse to hold a civil discussion with anyone resorting to name calling. My position, to paraphrase, is that incredible claims require incredible evidence. I don’t deny anything- but I don’t see adequate evidence showing mans’ actions as the cause. To the supporters of man-mad climate change- I hope for all our sake you are wrong. P.S. I don't seem to have mastered the formatting capabilities of the software- please forgive the lack of structure in my writing.

  8. Skeptical Still... 

    First I have to ask, do you believe that the researchers who have spent decades studying this subject understand their field of science?  Those researcher are extremely clear about how serious this issue is.  What I get from your comments is that, not only are you "not yet convinced," you seem to have already made up your mind before fully understanding the issue.

    This is definitely a field of science where there are considerable uncertainties in many areas.  Thus, it requires getting to know many more of the elements of the research in order to grasp how all the pieces fit together.

    This is the point I usually direct people to Richard Alley's AGU lecture.

    "I don’t deny anything- but I don’t see adequate evidence showing mans’ actions as the cause."

    And this is where I usually point out the basic radiative physics of CO2.

    "To the supporters of man-mad climate change- I hope for all our sake you are wrong."

    And this is where I agree wholeheartedly.  But the concern is that the evidence is overwhelming at this point.  It's very unlikely to be wrong.  The remaining question is merely, is it going to be bad, or is it going to be really bad?  Or, hope beyond hope, can we stomp on the brakes and get our trajectory under control fast?

  9. Skeptical,

    If you have a degree in Geology you will be aware that changing sea level leaves geological records.  This link has data you might appreciate covering the last 40 million years.  Is that enough for you?   The Romans, Greeks and Egyptians all left records of sea level.  These records extend the well documented record back 4,000 years. You cannot expect to have world wide tide gauge records longer than 150 years, scientific measurements have only been done for that long.

    You are confusing climate and weather.  Climate is much easier to forecast than weather.  I do not know if there will be a storm in two weeks.  It will certaily be colder in January than it is now in Florida.  Read the background information before you challenge people who are much more informed than you are.   If you do not understand the basics you cannot hope to master more complex subjects.  The ice core data is compared to model projections to determine how the various forcings interact.  This is one of the ways the models are validated.  If you do not understand how the data is examined, how can you expect to understand "incredible" results?

    It is not the responsibility of this web site to spoon feed you all climate science.  You must read the background information so that you can ask reasonable questions.  I suggest you go to the "start here" button at the top of the page.  Ask a few questions about items you do not understand.  You will sound like you want to learn more with a better attitude.

    If you do not look at the data you will never see "incredible" evidence.  That does not mean that the data does not exist, just that you have not looked for it yet.  Arhennius predicted in 1896 that the increase of CO2 would increase temperatures more in winter than summer, more at night than in the day, more over land than sea, more in the arctic than the tropics and more in the Northern Hemisphere than the Southern Hemisphere.  All these predictions have been measured in the last several decades.  That sounds pretty "incredible" to me.  What is your alternate explaination for the collapse of Arctic sea ice (keep in mind that the "skeptics" still deny the ice is melting and predict a recovery every year)?

    We all wish that AGW theory was wrong.  Unfortunately, the data indicate that is not the case.



  10. Skeptical Still.

    Okay, how about looking at a longer (2000 years say). The important thing is actually to take a long enough record to be able to distinguish the signal from noise. This depends on S/N ratio.

    "I have not see any proof that we understand whether the delta in the ‘forces’ caused the climate change or themselves were caused by the climate change (or were coincidental to it)." I find this strange coming from a fellow geologist. Firstly its not normal to talk of proof in science, only maths gets that. Second, there is a vast geological literature on the paleoclimate and forces. For starters you should look at IPCC WG1 Chp 6 for the basic background but follow cites from key papers and you will see new research. Hansen and Sato 2012 is discussed here. Note Fig 3  especially.

    "Cant predict weather so cant predict climate" is a very old myth. See here for pointers to the science. I would strongly suggest you work your way through the "arguments" button on the top left of this page so see what science has to say about these myths.

  11. Yah, I second what Rob points out. 

    Skeptical Still: "I don’t see adequate evidence showing mans’ actions as the cause."

    Start with the greenhouse effect.  If it's highly probable (it's been measured in lab for over a century, inferred from satellite for decades, directly measured from the surface for a couple of decades (e.g. Puckrin et al. 2004), and successfully accounted for in products such as air-to-air missiles), then we have a basic energy imbalance situation.  Add GHGs, and the climate system stores more energy.  No matter what any other forcing is doing, and no matter what alternative is offered, the enhanced greenhouse effect must be accounted for.  If you don't believe that humans are responsible for the rapid rise in CO2, there are threads for that.  If you don't think that CO2 is a powerful greenhouse gas, Lacis et al. 2010 is a fairly readable explanation why it is.

    The paleo data simply helps constrain the net feedback both in terms of strength and timing.  It does not provide evidence for the basic theory.  That's just physics.    

  12. Thanks to those that responded to my post- I will look through the references and continue to visit this site; it’s the best representation I’ve found for the pro-manmade argument. I am open to the idea and willing to review more data but I remain unconvinced. Thanks again.

  13. Sorry if it's been replied to on other pages, but where is the link to the peer-reviewed research? And where is the skeptics research on the same subject, so I can pick whichever suits my beliefs better? Isn't what science is about?

    Thank you.

  14. justmoi - try clicking on the tab to see the intermediate version of the article and/or look at the notes at the bottom of the page, there are plenty of references to the peer-reviewed research.

  15. @ justmoi: 

    In response to your question, Nope, that is not what science is all about.

    The vast majority of "skeptic research" is pseudo-science poppycock.

  16. Oops, sorry for the duplicate (F5 shouldn't do this, computer science is really one!). John, what is science about if not making really sure that the "other side" isn't right when assuming theories about the future? Tagging their arguments as "myths" to begin with is not what I'd call a scientist approach. And I'm glad you admit that not all (let's say 95% ;) of "their" research is rubbish, dare to enlighten me on the remaining 5%? Thanks!


    [JH] Your duplciate comment has been deleted.

  17. @justmoi #366:

    In response to your question, I decline to engage you in a game of "Gotcha."

    BTW, I just realized that your comments are "off topic" for this thread. Please post your comments on the thread to an article which directly relates to your concerns. 

  18. I have read that CO2 ppm have been much higher in the distant past,as much as 1000ppm. Over what period of time did CO2 reach that level? Is the current rate of increase faster,or can we tell?

  19. John Wise


    Yes CO2 has been higher in the distant past, even much higher than 1000 ppm. Over the very longest timescales (100's of millions to billions of years) CO2 levels have been trending downwards. 400-500 million years ago CO2 levels were perhaps 4000-8000 ppm. But what we need to bear in mind is that this is actually compensating for the fact that the Sun was cooler in the past. The Sun's heat output has grown by around 40% over its 4.5 billion year history.


    As a rough calculation, Solar intensity today is around 341 Watts/M2 at the earth. Allow for albedo reflecting around 30% of that and the Earth absorbs around 238 Watts/M2 . If we go back 500 million years that would 230 Watts/m2. A doubling of CO2 wil produce around 3.7 Watts/M2 of warming so around 2.5 doubling of CO2 would be needed back then just to compensate for a cooler Sun. In fact the level was more like 3.5-4 doublings of current levels. And temperatures back then were warmer than now - perhaps 5-8 DegC warmer. Suggesting some of the past higher CO2 levels were compensating for a cooler Sun and some were actually producing a warmer climate.

    As for the rate of increase, yes it is currently perhaps the fatsest in Earth's history. Some numbers.

    • Current CO2 concentrations are rising at a bit over 2 ppm/yr
    • CO2 concentration over a Glacial cycle changes by around 100 ppm over perhaps 10-20,000 years. So 0.005 to 0.01 ppm/yr
    • A period around 55 million years ago called the Paleocene/Eocene Thermal Maximum (PETM) is regarded as the point in the past most analgous to today. A large, geologically rapid release of CO2 & Methane occurred and temps climbed 5-8 Deg C. Recent research by Lee Kump and colleagues has obtained a good estimate for the release of CO2 back then - it occurred over around 20,000 years. We are raising CO2 levels today 10 times faster than that. See this report
    • Much of the Coal we burn was laid down during the late Devonian and Carboniferous periods. Perhaps 60 million years. And we are burning this cola up in the space of centuries. Coal that took centuries to be laid down is burnt by us in 1 day. So yes, we are likely releasing CO2 at unprecedented rates.
  20. Glen Tamblyn

    Thanks for taking the time to provide such interesting and useful information.

  21. Ok, I asssume this forum will be eager to slap this down but I am honestly trying to do the math based on geological evidence. First, there is little doubt that the recent Co2 increases can be attributed to anything else but human impact. What i don't get is that the math doesn't seem to add up to any serious problem unless measured in Ma.  I will use an average increase of 10% per year based on the numbers in the original header. I note that the Dacaan traps are briefy cited in one of the posts. Dewey M Mclean in his 1985 paper, " Deccan Traps Mantle Degassinging the Terminal Cretaceous Marine Extinctions," has established the mechanism of volcanic outgassing being the proximate cuase of several mass extinctions(besides the one cited in this paper). This theory has now largely surplanted the impact theory and should rightly be taken as a blue print of how excess Co2 could cuase mass extinction events. The problem I am having in looking at the numbers is trying to understand why the current increase is seen with such alarm and portrayed as immently catstrophic in affects. If you read this paper, you will see that he asserts that this geological event increased the rate of annual Co2 outgassing over baseline by 10-25% . This increase is comparable if you take his lower number and far greater if you take the higher numbers. Becuase of the intermittent periods of eruptive events of the traps, the anual numbers probably ran up and down this scale of averages. Still the mean increase would be substantialy higher than the modern rate of increase. The problem I have with these numbers comes down to this: this paper asserts that these increase took place in a .53-1.32 Ma time frame. For the sake of simplicity, lets use 1,000,000 years as a round number.  I just can't imagine what is so different now about the earth that it could handle 1,000,000 years of continual co2 increase before pitching a fit, but now is only seen as capable of going only a few hundred years before an extinction event is seen looming. Help me out here.

  22. pheidius @371, the background rate of natural emissions of CO2 over the last several million years has been 0.09 Gt C per annum.  That compares with a current rate of industrial emissions of 8.2 Gt C per annum.  That is, rather than a 10-25% increase of emissions, there has been a 90 fold and counting increase.  If you feed that into David Archer's GeoCarb model by setting the transition CO2 spike to 0, and the simulation CO2 degassing rate to 682.5 x 10^12 mol/yr, then look at the result after a million years, you will see that the CO2 concentration is still increasing linearly after a million years.  Effectively, the current rates of emissions of CO2 are so high that they cannot be stabilized by geophysical processes.  Only be radically reducing emissions can we stabilize.

    In contrast to the current rate of anthropogenic emissions, a 25% increase in the base background rate of emissions (simulated by setting the degassing rate to 9.4) results in an atmospheric increase to only 510 ppmv after a million years.  Importantly, after 500 years there has only been an increase in CO2 concentration from 273 to 276 ppmv, and a corresponding temperature increase of 0.1 C.  In contrast, in half that time, from 1850-2100, anthropogenic emissions will raise CO2 levels and temperatures by approximately the equivalent of a million years of Deccan traps outgassing.

    What makes this fact worse is that species adjust to changes in temperatures either by migration or adaption.  For most species the potential for migration is low, and the speeds of migration are slow.  For some species, at high altitudes or at the poles, there is no potential for adjusting by migration at all.  Species in those regions face a future of extinction as better migration by species better adapted to the new, warmer conditions results in their being out competed in the areas they formerly dominated.

    For those species that must rely on adaption, we face the conumdrum that evolution is slow relative to human scales, though rapid in geological terms.  An indication of how slow evolution is is the fact that many humans are still ill adapted to diets high in milk (lactose intolerance) or grains (glutten intolerance) 12 thousand years after the invention of agriculture.  As it happens, most species are currently adapted to conditions colder than those that prevailed pre-industrialization.  That follows from the slow pace of adaption and the fact that just 10 thousand years ago the world was much colder for 100 thousand years.  So, for adaption, species are already behind the eightball.  And now they are faced with the prospect of adapting to a million years of warming in just 250 years.

    Given these facts, it is probable that the current anthropogenic warming will result in extinctions far greater in number than those caused by the Deccan traps.

  23. I admit to some difficulty with Mclean's math as he was using moles as a measurement while current measurements use ppm(petagrams). I could not find any online calculator as moles and metric measurements are apples and oranges. He cites the figure of 5 x 10 to the 17th moles of co2 as the total Deccan release. I made that out to be 500,000,000,000,000,000(500 quadrillion moles). He gave a current mean figure of 4.1 x 10 to the 12th moles as the annual realease from all sources. (410,000,000,000 410 billion moles) At first, I thought I would try to convert from moles to the modern atomic unit by dividing by 12 and then trying to convert to ppm but then got muddled into thinking I could just multiply the moles by 1,000,000 to get PPM. Then I got tired and decided to post in the forum before going further. So how does Maclean's  1985 math fare against more recent calculations?

  24. As a footnote to the above, I am reading another paper, "An Atmospheric pco2 Reconstruction across the Cretaceous_Tertiary Boundary from Leaf Megafosils."  This paper compromises the usual bolide vrs. outgassing argument by accepting the linear increase from the Daccan traps but postulates a bolide colision as well that threw 4,600 GT c into the air rasing the ppm from 500 PPM to 2,300 ppm in only 10,000 additional years. I think were the two authors to duke it out in person, Mclean would argue that 500 PPM was the tipping point where the carbon sinks failed causing the rapid rise and consequent extinction while the author(s) of this other paper argue that another cause was neccssary for such a quick and dramatic increase.


  25. pheidius, 1 mole of CO2 contains 12 grammes of carbon.  Thus, 5 x 10^17 moles of CO2 equals 60 x 10^17 grammes, or 6,000 petagrammes of Carbon.  A petagramme is also a Gigatonne, so 6,000 petagrammes of Carbon is 6,000 Gigatonnes of Carbon.  4.1 x 10^12 moles equals 49.2 x 10^12 grammes, or 0.0492 Gigatonnes Carbon.  In contrast, the IPCC cites a value of 9.5 Petagrammes of Carbon (or Gigatonnes of Carbon) in 2011.  Given the large discrepancy, it is likely the figure you cite from McLean is an estimate of natural emissions only.

    The unit, ppm does not stand for a given mass of CO2, but for part per million, ie, a concentration of the gas within the atmosphere.  Strictly what is called ppm in climate science is actually ppmv, ie, parts per million by volume - ie, the ratio of numbers of molecules in the atmosphere rather than the ratio of the total mass of each component of the atmosphere.   Given the mass of the atmosphere, 2.13 Gigatonnes of Carbon = 1 ppmv of CO2.  However, about half of all emissions are taken up by the ocean or biosphere rather than staying in the atmosphere.

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