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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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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 876 to 883 out of 883:

  1. TVC15.

    Your interlocutor seems to be using a combination of "Climate has changed before" (this thread) and "we're coming out of an ice age", which is an odd twist on "we're going into an ice age". Maybe he's confused with "we're coming out of the Little Ice Age".

    There is a post here at SkS that talks about sea level over the past 150,000 years. It includes the following graph of sea level, covering the last glaclal/interglacial cycle:

    Sea level past 150000 years

     

    I do not see anything on that graph to support his claim that sea level is normally 4 to 14m higher than now during interglacials. The peaks of that graph may be a touch higher, but there sure isn't a steady period of higher sea levels in the last interglacial. I suspect his claim is completely bogus, but you could always try to get him to provide his source of data.

    On very long time periods, ocean basin size/shape changes have dramatic effects on relative sea level. On short time scales (glacial/interglacial cycles), local sea level in areas subject to the weight of ice have serous effects due to isostatic depression and rebound.

    Of course, the answer to "why should this interglacial be different?" is "because we're adding CO2 to higher levels than seen in hundreds of thousands of years, and we're looking at rising temperatures and conditions not seen in past interglacials".

  2. Eclectic @874, 875

    Hi Electric,

    Thanks for the responses!

    I don’t understand why this denier is trying to make it a fact that the Western half of the US always undergoes a drought during Inter-Glacial Periods when I don’t think we really know, or do we?

    They mention the other 11 interglacial periods, during which modern humans did not exist until the Holocene. I’ve also head a new epoch called the Anthropocene, with the start date of this new epoch still in debate. Not sure if this is recognized as the most current epoch.

    Since humans did not occupy most of those interglacial cycles, I don’t understand the deniers argument. There is nothing to connect the dots between past interglacials and our current situation.

    Now instead of saying climate always changes, deniers are claiming that what we are witnessing all over the globe today is typical during an interglacial cycle.

    How can we know the conditions of ancient interglacials? Did they all cause drought in the Western half of the US?

    Thanks!

  3. Bob Loblaw@876

    [Sea levels are normally 4 meters to 14 meters higher than they are now during Inter-Glacial Periods.]

    Thanks so much for responding to this deiners claim and providing that graph!

     

  4. TVC15 , from experience, you know how anything & everything is grist for the deniers' mill, in their attempts to minimize and/or deny the climate science.   It's always ABCD ~ Anything But Carbon Dioxide.

    And their excuses come in cycles of excuse ~ first: It's Not Warming . . . then: It's only warming a bit . . . then: Yes, it's warming a lot, but it's Not caused by humans . . . then: Well yes it's half-caused by humans, but the Warming is really very very good for us and is saving us from disastrous cooling.

    Then it's back to: the Warming has Stopped and it's cooling now (for at least 6 years' cooling, says the deluded Monckton) . . . and a Colossal Grand Solar Minimum will have all of Canada under a mile-thick ice sheet by 2050 or somesuch date.

    For millennia, the US Southwest has been arid - the opposite of the Northeast.  And you can find other regions of the world likewise arid.  All it needs is a slight variation in climate, and you've got a mega-drought (a drought defined as >20 years) or a super-mega-drought (for centuries).   Nothing very new about that  ~ except that now our Anthropogenic global warming is exacerbating the droughty tendency.  (The exacerbation being the point your denier wishes to deny.)

    I haven't studied past Interglacials w.r.t. aridity & droughts.  Presumably similar overall conditions in the past have caused rather similar episodes. But that's all rather irrelevant to the current situation, which must be dealt with on its own terms.  And the current droughty tendency doesn't disprove the climate science, nor does it fail to point to more of the same trouble in future, as AGW worsens.

    Of course, as we look back in time, the proxy evidence (in past Interglacials) gets fuzzier & fuzzier ~ so conclusions of any sort get more difficult to make.

    The "Anthropocene" is a semi-humorous label.  Not official.  But the label does get up the deniers' noses.  Still, it does rhetorically emphasize that the Holocene is transitioning into something significantly different from the "natural".

  5. A follow-up to the "sea level was higher in past interglacials" claim.

    Is this person claiming that sea level is indeed rising to dangerous levels, but it's natural?

    Is this person claiming that sea level has risen naturally in the past, so rising CO2 and rising temperatures cannot change this previous pattern?

    The part TVC15 has quoted is the usualy mishmash of vague, unspecific claims. Good luck trying to pin the denier down to a specific claim or statement that can be examined by evidence - my experience is that such individuals often are incapable of expressing themselves clearly enough.

  6. TVC15 @873,
    Your denialist is actually making four bold statements that are patently nonsense with the rather pathetic request that you "Tell us why this Inter-Glacial Period should be different."

    "Ice sheets and glaciers always melt during Inter-Glacial Periods." The melting actually happens in the run-up to the "Inter-Glacial Periods" which is what makes them "Inter-Glacial Periods" so in one respect this is entirely straw man territory. If the bold assertion is that glaciers and ice sheets shrink as they do today throughout an inter-glacial, that is false as sea levels of past millennia demonstrate.

    "Sea levels are normally 4 meters to 14 meters higher than they are now during Inter-Glacial Periods." This is not supported by the evidence that
    suggests only two or three of the eight had higher sea levels. (The graphic is from here but originates from this web engine.)
    800,000 years sea level

    "Global temperatures in the other 8 previous Inter-Glacial Periods were at least 7°F warmer than present." Again not supported by the evidence. A google search provides many graphical representations of 800,000y temperatures and globally the present interglacial has been warmer than all but three of them (although AGW may be on course to change that ranking).
    800,000 year temperature

    "The West always undergoes a drought during Inter-Glacial Periods." This is a more specialist assertion. That there has been "a drought" in "the West" through the Holocene is potentially correct. It isn't a place with massive rainfall. But more accurately there are periods of drought and periods when the rain is heavier. What we see to make sense of that is a bit of a Hockey Stick situation with drought conditions becoming more wide-spread. The graphic comes from here an account which does address the question "Will anthropogenic climate change cause the West to get drier or wetter?"
    West drought since 800AD

  7. IIRC, the Last Glacial Maximum saw a greater buildup of land-based ice than in previous glacial phases.  By the time maximum warming in the Holocene had been achieved and a natural cooling initiated, a great deal of ice remained in Greenland and Antarctica that had already been lost by that same point in previous interglacials. 

    Given time to reach equilibrium with the modern forcing, global sea levels will eventually surpass any levels achieved in previous interglacials spanning the previous 800,000 years+, and likely far longer than that.

    From Dutton 2015, Figure 5:

    Dutton 2015 - Figure 5

  8. Many thanks to Eclectic, Bob Loblaw, MA Rodger and Daniel Bailey.  

    I gain so much from each of you every time I post denier claims.

    Thank you all for your time and efforts to help educate us! 

  9. How valid is the claim, in refuting man-made climate change, that the Earth's climate has changed before?

    Your honor, I can’t be convicted of murder.
    You see, people have been dying since before I was born.
    So death is a natural thing, not caused by me. And probably nothing to worry about.

    Plus, you can’t prove that I killed that guy.
    Sure, I stabbed him, but all you can prove is that this caused localized cell death at the micro scale, not that it caused him to die.
    People survive stabbings all the time, in fact, stabbing a scalpel in people is often healthy, doctors do it all the time!
    I only stabbed 0.4% of his body, so it can’t be an issue, my stab is barely 3% of his body cavities.

    And the models are so unreliable.
    Doctors can’t even predict with 100% accuracy if someone will survive a surgery or not, how can they claim to suddenly know that this particular stab was bad?
    Science has been wrong before!
    So you see, I can’t be convicted of murder.

    Now stop trying to push this radical anti-stabbing agenda.

    Source

  10. When we say that CO2 and Temperature are correlated, which one comes first in that interaction ? I mean, is it the temperature that influences de CO2 or the CO2 that influences temperature ? Which one is the most significant, which one drives the most between the two ? Also, where can we find a comprehensive study, comparing the natural causes for climate changes over the last few millions years (or more if possible) compared to recent data, which would include the human factor, let's say since 1850 ? 

  11. paulbegin @885,
    Increased concentrations of CO2 in the atmosphere will increase global temperatures directly through the greenhouse effect. An increase in temperature can influence CO2 concentrations but less directly. Thus colder oceans are more able to absorb CO2 so CO2 will be drawn from the atmosphere into the oceans during an ice age, this increasing the ice age cooling. But there are other temperature-CO2 correlations, for instance during the ENSO cycle with temperature and CO2 increasing in the aftermath of an El Niño. Yet in the ENSO cycle the fluctuating CO2 levels are not due to temperature but due to changing patterns of rainfall causing changes in vegitation growth in the Amazon basin.

    So it is CO2 that drives temperture while temperature has a small influence on CO2.

    I'm not emtirely sure what you are asking for with your last question. The various drivers of climate change, be they forcings or the resulting feedbacks, are reasonably well understood for the past few million years but there is more difficulty going back into the more distant past as the drivers and the climate are less well understood.
    The big difference between the last few-million-years and the last century-or-so is that the major climate forcing has resulted from human activity and that said, I'm not sure what you expect from comparing the last century-or-so with the last few-million-years.

  12. I agree with the statement. CO2 and global warming are definitely correlated, the more CO2 we have will indeed warmup the Earth. And your rebuttal to the myth is excellent in my opinion, what happened in the past surely doesn't mean that what's happening now is the same. Us humans are changing the climate. Lastly, I do have a question regarding the CO2 level, until how much would think that the climate can handle?

  13. There were changes in climate that caused species extinctions, but those changes were caused by physical impact in scale and duration far beyond what we can do by adding CO2 to the atmosphere.

  14. Welcome to Skeptical Science, hmmyes and Fya.

    hmmyes: you ask about how much CO2 the climate can handle? The earth-atmosphere system can handle a lot, and climate will always be there - it is simply what is observed, however cold or hot things are. From a human perspective, though, there are an awful lot of climates that earth can tolerate that humans cannot. 18,000 years ago was 4-6 C cooler than now, and cie covered huge areas of current land. Running today's modern civilation in that earth would be very difficult. On the flip side, a 4-6C warming would also be very expensive for humans, if not intolerable. The past changes in climate tell us that 4-6C of change is a big change.

    Fya: mass extinctions from things like meteor impacts are indeed a very abrupt and severe form of change, and CO2 wll not have that rapid effect. But the effects of a 4-6C rise in temperatures (the high end of esitmates for a doubling of CO2 from 300 to 600ppm) will not be gentle.

  15. Global warming has always been one of the most concerning and dangerous things that humans and the climate can experience. Global warming is when the concentration of greenhouse gases like carbon dioxide increases in the atmosphere, making the planet heat up.

    Although global warming is genuinely concerning, a lot of people all over the world still look over it and do not seem to really care. This might be because people think that global warming is impossible to solve since we still need to use the stuff that is causing the problem, like burning fossil fuels. But for me, I do not think that is the case.

    I think that global warming is especially important because it affects us as human beings e.g., extremely hot weather can damage our lungs. Moreover, it affects a variety of biodiversity across the world. An example would be in the polar regions where it starts to get warm which leads to animal deaths. I believe that we can help solve this problem and make the planet a good place to live in again. We just need to put our hands together and fight through it. We can start off by raising awareness and using fewer fossil fuels.

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