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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

Printable Version  |  Offline PDF Version  |  Link to this page

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Related Arguments

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 201 to 225 out of 611:

  1. What if heat causes CO2, which I gather is an emerging theory?
  2. stickybeak, I think you're probably referring to this, Murry Salby Confused About The Carbon Cycle. Not very promising.
  3. #201 "What if heat causes CO2, which I gather is an emerging theory?" No, it's an old falsified claim that pops up every now and then in different guises by some "skeptics". I assume you mean the claim that the rapid rise in CO2 over the last 150 or so years is mostly due to rising temperature. That higher temperature will over a long time result in outgassing of CO2 from the oceans is not controversial and has no bearing on the recent spike.
  4. CBDunkerson and DB Thank you for your replies and graphs! Very helpful indeed! The "How reliable are CO2 measurements?" article was also interesting, I particularly like the animation at the bottom of the page! :-)

    [DB] If you liked that video then you should love this one, from SkS author Robert Way:

  5. I was wondering what is the consequence of what and if their is a proof for that... I'm not expert and it looks like a big assumption of all(or most?) sutdies is that the increase of Earth superficial temperatura is a consequence of the increase of CO2. I red once that in the past, the CO2 seemed to be a consequence of Earth superficial temperature increase and that CO2 increase generally occurs after temperature increase. That would be great also if some selected bibliography could be suggested at the end of article. I'd like too know about a good book about past climate, glaciation and atmospheric composition... Even, if I a bit skeptical, the precaution principle makes me still keep on using my bicycle as much as possible to commute...

    [DB] If a book is what you ask for a book is what ye will receive:

    Spencer Weart's: The Discovery of Global Warming

    Highly recommended.

  6. " I red once that in the past, the CO2 seemed to be a consequence of Earth superficial temperature increase and that CO2 increase generally occurs after temperature increase." Natural CO2 increase occurs that way, which has the effect of amplifying warming. For more detail see CO2 lags temperature
  7. So we accept that global warming has happened before. We accept that it is happening again now - and that man is contributing to it. In the past, global warming may have been the cause (or a major contributing factor to) mass extinction events. If previous global warming events have shown a positive feedback - more water vapour causing more warming - what eventually turned things around? Is it already too late to take action? I think the focus on "per capita" CO2 production is counter-productive. It encourages countries like Australia to increase their population in order to reduce their per capita pollution. This actually results in more pollution in total. I believe the focus should be on "per area" pollution. This would encourage reductions in population and a reduction in total CO2 production. Of course it helps if we individually reduce our production of CO2 but the big problem is overpopulation. Our planet does not have enough free oxygen (O2 as opposed to CO2) for its current population.

    [DB] "Our planet does not have enough free oxygen (O2 as opposed to CO2) for its current population."

    Umm, nope.  Of all the resources consumed by mankind, O2 is the most ample and in no danger of running out.

    Scarce items:

    • Food
    • Water
    • Housing
    • Open land
    • On-topic comments
  8. And, whatdoctor, whatever the merits or problems with a per capita approach, it cannot encourage population growth. People don't think about having children in that way. I can't provide a citation, but I suspect you can't either, and I've never heard anyone cite lowering a tax as a reason for having children. Now, true, it might encourage some people to have children because they understand mitigation as foundational to a brighter future.
  9. whatdocter @207: 1) Your claim about oxygen is simply absurd. Humans do not begin to suffer from oxygen deprivation due to altitude until 2000 meters, with most unaffected to 2400 meters. That represents a 22% reduction in available oxygen, or about 4.5% of the total atmosphere. Consequently, for the reduction in oxygen due to combustion to effect human health, CO2 levels would have to increase from their current 0.04% to 4.5% of the atmosphere. 2) An international agreement based on restricting per capita CO2 production would need to set limits based on a a benchmark year, with national targets based on the population during that benchmark year. Failure to do so would either penalize countries with a low population growth with respect to those with a large population growth, or result in population growth forcing total emissions above the absolute limit required to mitigate climate change. Such a benchmark approach would encourage limiting population growth.
  10. I've been following global warming for quite a while, with growing interest. I have discovered many informative posts on this site and decided to register. I can grasp the idea that changes in orbit and solar activity can force climate change, and that greenhouse gases amplify the trend. I can also grasp that our massive dump of CO2 into the atmosphere could force a climate change. My question is this, can anybody point out a time in the past when CO2 did the forcing, rather than an orbital cycle or solar activity?
  11. The PETM event (Paleocene-Eocene Thermal Maximum). Skeptical Science: CO2 Currently Rising Faster Than The PETM Extinction Event
  12. stonefly @210, orbital forcings are only as significant as they have been when the Earth is cool enough to have ice caps. Throughout most of Earth's history CO2 has been the major determiner of the surface temperature (thanks to the remarkable stability of our sun). I suggest you watch Richard Alley's 2009 lecture to the AGU for a brief summary of the details.
  13. Bibliovermis at 09:18 AM The reading gets deep. I'm doing a lot of googling for definitions. After I struggle with it for a while, maybe I'll get an understanding. One question I have now, though. They talk about carbon 13. I know that carbon 13 makes up only between 1% and 2% of carbon. I don't understand that. Does carbon 13 separate from carbon 12 and form CO2 that is all carbon 13? I know I'm going off on a little bit of a tangent. Anything you know about it would help, or if you could steer me in the right direction. Thanks.
  14. Tom Curtis at 09:47 AM on 10 September, 2011 stonefly @210, orbital forcings are only as significant as they have been when the Earth is cool enough to have ice caps. Throughout most of Earth's history CO2 has been the major determiner of the surface temperature (thanks to the remarkable stability of our sun). I can tell this is going to take a lot of studying, but I want to understand this. I looked at a chart which showed glaciations approximately every 100,000 years going back to about a half million. Are there any glaciation charts that go back further? **** I suggest you watch Richard Alley's 2009 lecture to the AGU for a brief summary of the details. I did watch that lecture. It was interesting and informative. Only I wish I could have seen the laser on the slides.
  15. Bibliovermis at 09:18 AM Sorry I missed your link. I went right to google. Your link explains the carbon 12 - carbon 13 deal right away. I'll read it now.
  16. stonefly @214, below is a proxy of temperature over the last 5 million years. As you can see, significant low temperature episodes (glaciations) only occur the last 3.5 million years, and the large glaciations which we are familiar with from popular culture are a feature only of the last on million years. Here is a temperature reconstruction of the last 65 million years to put that into perspective. Note that the oxygen isotope "thermometer" is differently calibrated depending on the level of ice, so the first section is not strictly comparable to the last section of the graph. Over the last 600 million years,, every period of glaciation has coincided with CO2 concentrations less than 1000 ppmv. Note that this graph has an effective resolution of 10 million years. The entire period of the recent glaciation (double the length of the first graph) would appear as just one point on that graph. Obviously a lot can happen in 10 million years, and more detailed measurements have indeed shown in the episodes of glaciation during high CO2 levels as shown above, the duration of the glacition was short (< 10 million years) and that during the glaciation, CO2 levels where low (< 1,000 ppmv).
  17. Thanks, Tom, Very informative. It brings up new questions for me, but I'll save 'em while I do more reading.
  18. Mankind's influence aside, should we otherwise be in a cooling phase of a Milankovitch cycle at present, in other words, on a downward slope, chart-wise?

    [DB] In the absence of anthropogenic forcings, yes.  Since the Holocene Climatic Optimum some 6,000+ years ago the net forcings (without man) have been negative and the overall temperature trend downward.

    The long, slow slide back to glacial conditions had already begun.  Now evidence shows we have little to worry about (the next cold phase of the ice age cycle).

  19. Thanks.
  20. If we are on a slide back to a glaciation, perhaps similar the the last one, and we left peak temperature 6000 years ago, then we should not be seeing continuing glacial retreat or the continual loss of Arctic ice, should we? I know there may be inertia, for lack of a better word, which may carry past the optimum temperature, but 6000 years later, to the extent we are seeing? Are there any other likely reasons for the loss of ice we are seeing other than anthropogenic?

    [DB] "Are there any other likely reasons for the loss of ice we are seeing other than anthropogenic?"

    Absent near-mythological musings such as massive volcanic eruptions along the Arctic Ocean seafloor (no evidence), increased submarine patrols under the ice causing turbidity which then causes the ice to break up & melt (huh?) or magical as-yet-undetected cycles...none that I'm aware of.  Please let me know if I missed something.

  21. Another question: When the Earth is reaching the optimum in a Milankovitch cycle, when Co2 is high, the creation of an ice cap through the warmer and therefore wetter winter is enough to offset the greenhouse effect of high Co2 levels, and in spite of those levels nudge a cooling cycle?

    [DB] "when Co2 is high"

    See the label on the graph below:



    The last time CO2 levels were as high as now (the Pliocene) the world was a dramatically different place, with much higher sea levels:


    [Archer 2006]

    I am not aware of any plausible mechanism that would allow formation of an Arctic ice cap during a period of high Milankovich orbital forcings and elevated CO2 levels such as at present.

  22. Stonefly - the change in the solar forcing from 6000 years is small especially compared to the anthropogenic forcing. Even without these forcings another iceage would not have happened for around 50,000 years. Berger & Loutre. The increase is CO2 that goes with the Millankovich forcing is a slow feedback and at any time is close to equilibrium. Once you are at peak, then as the solar forcing wanes, the feedbacks work in reverse, removing CH4 and CO2 and amplifying the cooling. If you are asking why not working at the moment with solar in decline over last 6000 year, then you need to look at the magnitude of the respective forcings. Milankovitch is very slow - at least 10 times slower than present rate of CO2 forcing - and small by comparison to CO2 from FF burning. Note also that Milankovitch cycles still happened in pre-Quaternary times but only have much affect on climate when CO2 is low enough for NH snow pack to form. When the earth was last at 450ppm, we didnt have the glacial cycle. You might also find the article at Are we heading into an ice age helpful.
  23. "Please let me know if I missed something." No, I think I'm beginning to get a better grasp of the climate picture. We'd be headed for another glaciation. Instead, I think we're gonna be headed in the other direction. I'm a truck driver. I drive an 18 wheeler all over the continental USA. I see rush hour in every major city. For a long time I've thought, "This must be gonna have a big effect...on way or another."

    [DB] And this, then, the most telling graphic of all:


  24. scaddenp, Thanks. I can grasp what you're saying. Also, thanks for the links. I'm gradually getting the picture.
  25. Great site! Questions which I swear I have searched for, but this is also a vast site, and I do have a day job: 1 is there any climate model that can simulate in full using natural forcings alone the generally accepted natural temperature variations of the last 1100 years, up to 1850.? (the variations appear to be +/- 0.5 deg c.) 2 if not, what is the magnitude of the gap in degrees c? 3 How much in degrees c is current temperature above the historic mean of the last 2000 years (proxy measurements of course)? Thanks for any answers.

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