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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)

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

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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.

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 426 to 450 out of 611:

  1. @423, The data points in this graph seem to be quite equally spaced and the trend of climate instability increase is still quite obvious I think


    from wikipedia



    [RH] Shortened link

  2. skeptic123 @425:

    1)  The long term global temperature trend over the last 8000 years has been flat, or slightly downward:

    From this we would expect the CO2 concentration to have declined by about 10 ppmv from the early holocene levels of 260 ppmv of CO2.  Instead, it rose to 280pmmv - most probably due to preindustrial activities of humans:

    Arguably, even that rise was sufficient to prevent the Earth declining into a new glacial. Ergo your hypothesis is getting well ahead of the evidence even there.

    2)  More importantly, while restoration of zero net emissions will eventually result in CO2 concentrations declining to pre-industrial levels, it is likely to take much more than 10,000 years to do so:

    Indeed, even if we were to return to zero net emissions prior to 2020, temperatures would still be elevated above preindustrial levels by a degree Centigrade 10,000 years from now, assuming no major alteration in natural forcings.  Consequently concerns about targetting zero emissions are entirely misplaced.  (They may be valid concerns for aggressive CO2 sequestration programs that aim at negative net emissions.)

    3)  The ice core record is very clear that the slide into a glacial occurs, not over decades but over millenium.  Even were CO2 levels low enough for that to be a genuine risk, there would still be ample time to react to such an event by reinitiating the consumption of fossil fuels to raise CO2 levels by the 20-40 ppmv necessary to prevent the onset of the glacial.  So, even if you were not ignoring relevant facts (1 and 2 above), preventing anthropogenic global warming would still remain the priority issue, with concerns about the ideal post mitigation CO2 concentration being a third or fourth order issue that can be left to be dealt with several decades or even centuries from now, when our understanding of the science will also be that much better.

  3. @427
    About 3) "The return to the cold conditions of the Younger Dryas from the incipient inter-glacial warming 13,000 years ago took place within a few decades or less (Alley et al., 1993)."

    "Over the last 400,000 years the Earth's climate has been unstable, with very significant temperature changes, going from a warm climate to an ice age in as rapidly as a few decades."

    While the overall slide into a full-blown ice age indeed could take a millenium, it does not happen smoothly but in a step-wise manner of a few decades with some steps spanning a few degrees of temperature drop. I am also highly suspicious that an increase of 20-40ppm CO2 could prevent the onset of an ice age, your graph from 1) is quite clear that a 20ppm increase had little effect.

    About 2) The estimates in that paper might be a bit too pessimistic, who would have guessed that plants would start eating more CO2 as concentration increased
    Also, past CO2 concentrations seem to show that levels could fall as quickly as in a few centuries

     About 1) The last 8000 years have indeed been quite uneventful, luckily for us, however it depends how much smoothing one applies to the data, for example

    An it should actually be worrying that the overall trend was still downwards despite the 20ppm CO2 increase.

    Finally, I would have been really worried if we were currently at preindustrial levels of CO2, but even at 400ppm we are still below the level when the ice ages started and with the current trend in solar activity and orbital cycles pointing downwards it might take just one major volcanic eruption to trigger the positive feedbacks.


    [RH] The David Lappi graph should be disregarded altogether for the mere fact that he states that GISP2 runs up to the year 2000. This is factually incorrect. The "years before present" represented in the GISP2 data uses 1950 to represent "present." Thus the data only runs up to 1855 (95 years before present).

  4. skeptic123:  Sorry, my mentioning of the spacing of data points obscured rather than clarified my point about temporal resolution of temperature records. The measurement techniques average across years, decades, centuries, millennia,....  For example, sediments and snow/ice compress over time so the years are not as physically separated, so a sample will be an average across a larger span of time, which will smooth across variations in temperature.  That lack of resolution is reflected in the sparcity of data points the farther back in time you look in graphs, because the placement of a datapoint in time is done at the midpoint of the estimated range of times for that temperature.

  5. skeptic123, further to Tom Curtis's points, see the post "Are We Heading Into a New Ice Age," the Intermediate version.  Read the Archer 2005 paper referenced there. 

  6. On the general topic of climate variability, Science of Doom has a series of interesting posts called Ghosts of Climates Past.

  7. @429, Surely there is some averaging in the ice cores resulting from compaction, and I am not an ice core expert, so I don't really know how much it is, but a quick search on the web shows it might not be that much

    This 19 cm long of GISP2 ice core from 1855 m depth shows annual layers in the ice. This section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers. From the US National Oceanic and Atmospheric Administration

    Besides, the fact that the ice ages started only about 3 million years ago still remains.

  8. @430, I am not saying that an ice age is imminent, what I am saying is that with an unstable climate we can not really tell when an ice age would come. Surely ice ages have in general been triggered by drops in solar activity and orbital changes, but there are other events too, e.g. major volcanic eruptions, which are even more unpredictable than solar activity. In the past there have been events when glaciation occured unexpectedly and within decades, not centuries, see my post 428. Also, we do not really know the exact CO2 level vs solar irradiation drop that would prevent an ice age from happening. The author in Archer 2005 himself admits that the models are very sensitive to how the parameters are set and this is to be expected since strong positive feedbacks are involved. Finally, the current slow down (if not reversal) of the warming and the last few winters of heavy snow are not very reassuring.

  9. @431,"The huge changes in past climate demonstrate the sensitivity nature of our climate. Small changes in solar output and minor variations in the distribution of solar energy across seasons (from minor changes in the earth’s orbit) have created climate changes that would be catastrophic today. Climate models can explain these past changes. And if we compare the radiative forcing from anthropogenic CO2 with those minor variations we see what incredible danger we have created for our planet."

    I agree about all of the above except the last sentence. The thing is that in an interglacial period such as today, one of the major warming feed-backs is not present, ice sheets are small enough so that an increase in temperature would not decrease the albedo noticeably. The opposite, however, is much stronger, a relatively small drop in temperature may increase the ice sheets and snow cover sufficiently to trigger run-away albedo increase.

  10. skeptic1223, you are cherry picking quotes to support your intuition, and explicitly rejecting the data-and-model-based conclusions of scientists who have carefully studied glaciations and backed up their conclusions with evidence.  Ari has compiled a bibliography of just some of the many other papers explaining that and why we have delayed the next glaciation for many tens of thousands of years.

  11. @434, You make some intelligent points yet I can't help but feel you conveniently neglect rates of change aka the time factor.

  12. By the way, the link to the Archer 2005 paper (Archer & Ganopolski, 2005, "A Moveable Trigger...") is stale in the "Are We Heading Into a New Ice Age" post.  The full text of the paper is available elsewhere now.

  13. skeptic1223, your worry about climate instability is backward.  The past 10,000 years have been unusually stable compared to at least the previous 40,000 years, allowing the rise of agriculture (see also here and later here) and civilization.  Humans now have violently spun a major control knob of the climate all the way to 11

  14. Ari also has a short but older-than-his-excellent-bibliography article "Rising Carbon Dioxide Concentration Stops the Glacial/Interglacial Cycle."

  15. @435, I am not saying that abrupt glaciation is the norm, what I am saying is that abrupt glaciation is possible. The bibliography you mentioned discusses the normal process and relies heavily on models. Some events such as solar activity or volcanic eruptions can not be modelled reliably. As for the cherry picking, well, I don't have the actual numbers of the temperature measurements, and unfortunately the publically available graphs don't have sufficient time resolution to draw a positive conclusion, so I have to rely on other people who do have the actual numbers, and I thought that such respected institutions as the IPCC and GRIDA would be trusted with what they say.

    @437, yes, I googled it and found it also here


  16. @438, about the control knob, the past violent climate changes are due to positive feed-back effects, forcings only trigger the feed-back process. The major positive feed-back effect is the albedo acting through the area of ice sheets and snow cover. In an interglacial period the albedo is mostly a cooling positive feed-back due to the small starting size of the ice/snow cover, while in a glacial period it is mostly a warming positive feed-back due to the large area of ice/snow cover. So, unless there is some other major warming positive feed-back effect today, the current climate is more prone to a swing to cold than a swing to hot. It is of course entirely possible and even probable to continue gradually warming with the increase of CO2 if there is no cooling event to trigger the albedo feed-back. However, my worry is if there is a cooling event that we can not model reliably and the albedo feed-back gets triggered. So, just to be on the safe side it might be a good idea to increase CO2 by another 1-200 ppm to get to pre-ice ages epoch levels.

  17. skeptic1223, you are correct that at any moment a Vogon constructor fleet might accidentally or haphazardly or maliciously destroy the Sun, thereby inducing a glaciation.  But otherwise, solar changes within the empirically and modelled very well supported range of probabilities will barely and temporarily make a dent in the warming that our increased CO2 level is causing and will continue to cause.

  18. skeptic1223, when you say things like "Finally, the current slow down (if not reversal) of the warming and the last few winters of heavy snow are not very reassuring," strongly suggest a lack of understanding in the basics.

    There has been no slowdown in  the rate of energy accumulating in the climate system.  There has been a slowdown in the rate of surface warming over the last 8-10 years.  If you're having a hard time understanding this, there are plenty of threads to help.

    Why is heavy snow in some parts of the world at certain times of the year an indication that the climate system is cooling?   It is, rather, an indicator of warming, as warming puts more precipitable water vapor (pp. 201) into the atmosphere.  

  19. skeptic1223, amazingly, you wrote regarding ice and snow albedo: "unless there is some other major warming positive feed-back effect today."  Let's see... how about water vapor?!  You really should learn the basics.  I suggest you enroll (sorry, Aussies--"enrol") in the Making Sense of the Climate Science Denial course.

  20. @439, "Rising Carbon Dioxide Concentration Stops the Glacial/Interglacial Cycle." - that's exactly what I mean too, it seems we only disagree in the safe level of CO2. As I said in 441, just to be on the safe side it might be a good idea to increase CO2 by another 1-200 ppm to get to pre-ice ages epoch levels.

  21. skeptic1223, your wild guessing about the possible catastrophic cooling effect of volcanic eruptions is, like your wild guessing about solar-caused cooling, wildly out of scale.  One way to start replacing your uninformed intuition with data is by reading John Mason's series.

  22. @443, I haven't seen any conclusive evidence about the state of the surface temperatures for the last 15 years (that's why I put it only as a feeling and not a statement), if the mechanism is so obvious why it wasn't included in the climate models and they failed to predict it?
    About the precipitable water vapor I could of course counterargue with the drought in California and my child memories of heavily snowy winters during the 1970s dip, but again there isn't conclusive evidence in either direction, so I've put it just as a feeling and not a statement.


    [JH] Your "feelings" carry no weight on this website. Please cease and desist from posting them. 

  23. skeptic1223, just because glacial periods suck bad, that doesn't mean that periods of rapid warming are good.  Keep in mind that atmospheric CO2 hasn't risen this quickly in at least 300 million years, and arguably ever.  That could easily mean that the climate system is now warming at an unprecedented rate.  Remember that life has reached equilibrium with Holocene conditions, and more generally with Pleistocene conditions.  A rapid change to Carboniferous conditions (600+ppm) will put the current biosphere up against the wall.  Because the climate system is comprehensively integrated, it's not as easy as just popping up the carb-o-stat to 600ppm and popping the cap off a beer: "No problem, mate!  No more glacial periods!  Let's kick it!"  The problems that result from extremely rapid warming may make questions of glacial periods irrelevant.

  24. Skeptic1223, click on the links I provided.  Try not to assume that increased global water vapor means more rain for everyone.  I said "precipitation intensity" not "more widespread precipitation."  You might check out the observed and modeled expansion of the Hadley circulation as well.

    Why wasn't the "pause" in surface temp "predicted" by climate models?  Because climate models aren't designed to project sub-decadal trends.  The temporal resolution is getting better, and the "pause" has inspired focused science that's been quite fruitful, but the bald fact of the matter--and something that fake skeptics aren't willing to get--is that climate modeling isn't designed for accuracy over the short-term.  Do you understand why that might be?  

  25. @444, so, did you all have to pass that course first :)
    There is a lot of controversy regarding water vapor, there is the strong green-house effect, but there are also the clouds which reflect sunlight. Also, water vapor is mainly present in the low troposphere where the greenhouse effect is already saturated, above 10km there is virtually no water vapor, that's why there are no clouds above when flying with a passenger jet. And according to skepticalscience "It is the change in what happens at the top of the atmosphere that matters, not what happens down here near the surface."

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