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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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Comments 776 to 800 out of 883:

  1. A question from a denier.

     

    The Northern Part of the US was covered in Ice 20,000 years ago and the Great lakes were only formed 10-14,000 years ago.

    Alaskan Glaciers started receding big time around 1750 to 1900.

    What caused all of that and why is now any different ?

    All insights are welcome. :)

  2. TVC15 @776,

    The denialist asks about the features of two different times and asks what is different today.

    20,000 years ago was the maximum glaciation of the last ice age, this the result of reduced solar radiation for the high-latitude Northern summers which 100,000 years ago triggered an increase in the amount of ice-covered land/ocean in high Northern latitudes. The level of ice was amplified by the increased albedo of the ice reflecting away greater levels of solar radiation and by the reduced GHGs (CO2 & methane) caused by the cooling global climate having a net draw-down of such gases. Thus we find the Laurentide & Cordilleran ice sheets covering mainly Canada and beyond, extending to cover the sites of today's Great Lakes. (The map below ignores changing coast lines.) It thus requires the Laurentide Ice Sheet to melt considerably before the Great Lakes can exist, their formation reportedly beginning 14,000 years ago.

    Ice age N America

    We now leap forward to a time when the Cordilleran ice sheet has long gone. Over recent times the dynamics of glaciers is not always determined by local temperature. A sea-terminating glacier will likely spend most of its days slowing advancing and then, becoming unstable, undergo a short period of rapid retreat. And reduced/increased snowfall can cause a glacier to shrink/expand.

    So is there evidence that "Alaskan Glaciers started receding big time around 1750 to 1900"? Solomina et al (2016) who, in an analysis of global glaciers over the last two millenia, examine land-terminated glaciers in Alaska and see no sign of it. Their Fig 2 shows a GEI index indicating a fluctuation in local glacier size which peaked at about 1880AD. This would explain a "receding big time around ... 1900" but not the earlier 1750 date. Other research may give differing timings but it seems unlikely that there is any proper support for a 1750 to 1900 date. Can the denialist provide support for this bold assertion of his?

    And "why is now any different"? The unprecidented global rise in temperatures will impact glaciers globally, including Alaska.

    Response:

    [DB] Fixed image display issue

  3. "Alaskan Glaciers started receding big time around 1750 to 1900"

    As opposed to those promoting misinformation, looking at the full context of the Holocene, Alaska glaciers have only been recently declining, reversing a 8,000 year period of growth and expansion:

    Per McKay et al 2018 - The Onset and Rate of Holocene Neoglacial Cooling in the Arctic

    "Arctic summer temperatures have decreased for the past 8,000 years, before rapidly warming over the past century. As temperatures cooled, glaciers that had melted began to regrow throughout the Arctic, a phenomenon and a time interval known as Neoglaciation.

    This study seeks to understand the nature of this cooling and whether or not this indicates a tipping point in the climate system. Specifically, we use a large database of records from ice cores, lakes, ocean sediment, and more paleoclimate archives to detect patterns of cooling. We investigate these patterns, and climate model simulations, to determine what parts of the Arctic experienced Neoglaciation at the same time, how rapidly it cooled, and what climate models indicate about the causes of cooling.

    We find that the Arctic did not cool simultaneously, but different regions cooled at different times and that the climate models perform well when simulating both the timing and amount of Arctic cooling."

    Full copy available here.

     

    Further, recent climate warming in the central Yukon region has surpassed the warmest temperatures experienced in the previous 13,600 years (and therefore likely the past 100,000 years).

    Porter et al 2019 - Recent summer warming in northwestern Canada exceeds the Holocene thermal maximum

    Yukon temps

    News release here.

  4. "the Great lakes were only formed 10-14,000 years ago"

    In the spirit of transparency and full context, the basins of the Great Lakes are the product of repeated and successive glaciations, and not just the most recent.  From Larson and Schaetzl, 2001:

    "The basins that contain the Great Lakes are the product of repeated scour and erosion of relatively weak bedrock by continental glaciers that advanced into the Great Lakes watershed beginning perhaps as early as 2.4 Ma. Most of the scouring, however, probably occurred after about 0.78 Ma when episodic glaciation of North America was much more extensive, with ice cover sometimes extending as far south as Kentucky."

    A full perusal of this fine document reveals that the southern lobes of the Great Lakes were ice-free at the near peak of the Last Glacial Maximum at 25,000 years BP and that the most southern advance of the ice during the last glacial phase occurred well after the LGM and during the deglaciation phase of the Laurentide Ice Sheet, when cold (dry) ice processes had shifted over to wet processes (promoting ice flow).  From Figure 8:

    Great Lakes Ice Cover

    Full copy available here.

    The point is, it's pretty easy to show the interested onlooker just how uneducated your particular pretend-skeptic actually is by doing some digging.  The plus side to all that digging is in the self-learning you accomplish.

  5. Thank you both so very much!

    I shared the information that you both offered and a snarky denier came back as spouted off this at me.

    Nothing is unprecedented. Try science.The science is settled:

    Palaeo data suggest that Greenland must have been largely ice free during Marine Isotope Stage 11 (MIS-11). The globally averaged MIS-11 sea level is estimated to have reached between 6–13 m above that of today.

    [emphasis mine]

    https://www.nature.com/articles/ncomms16008

    “Even though the warm Eemian period was a period when the oceans were four to eight meters higher than today, the ice sheet in northwest Greenland was only a few hundred meters lower than the current level, which indicates that the contribution from the Greenland ice sheet was less than half the total sea-level rise during that period,” says Dorthe Dahl-Jensen, Professor at the Niels Bohr Institute, University of Copenhagen, and leader of the NEEM-project.

    [emphasis mine]

    https://www.nbi.ku.dk/english/news/n...e-of-the-past/

    The sea levels are going to rise and you can't stop it, so stop pretending you can.

    What the heck can I make of this denier's snarky reponse?

  6. What the heck indeed. That they confuse MIS-11 with the Emenian maybe? That they think causes of sealevel rise and ice melt in past are the same as the forcing at work today?

    Or more likely: their beliefs are not based on any rational analysis and that they are making 2+2=5 with motivated reasoning.

  7. @ 781 scaddenp,

    Yes indeed what the heck! This denier portrays himself as being scintifically literate but he does not have me fooled. He's pompous and always results to insulting anyone's intelligence if they challenge the rubbish he put out.

    That's a great angel you provide in asking does he think the causes of SL rise and ice melt from the past are the result of the forcing's at work today.

  8. I meant angle!  

  9. TVC15 @780,

    Sea level during previous intergalacials has been the subject denialist blather preented on this thread before. This version is a little different from that @715 & @780 which asserted that all eight previous interglacials saw sea level metres above today's level. Of course there are only two previous interglacials that saw such sea levels - the Eemian (MIS-5e) and 400,000y bp the MIS-11.

    The Eemian was significantly warmer than today in northern latitudes. The link @780 states the temperature became +8ºC warmer than today in northern Greenland. So if temperatures do rise like that, we should expect significant SLR. The denialist appears to be saying that such temperatures are going to arise naturally. Of course, all interglacials are different. That is why only two of the last eight had sea level higher than today.

    MIS-11 is interesting because it was of longer duration than other interglacials. This resulted from the milankovitch cycle that triggered the interglacial being followed 20,000 years later by a stronger peak in the cycling extending the interglacial accordingly. A comparison of MIS-11 & the present Holocene is provided by Rohling et al (2010). The milankovitch cycles do not provide that extra boost for the Holocene so again there is no reason to have expected 20,000 more years of interglacial with sea level increasing above today's levels - not without AGW.

  10. MA Rodger @780 

    Thank you so much. I learn so much from you all when I post the denialist blather that deniers challenge me with.

    Thanks for the link to the Rohling paper.

  11. I have to agree with TVC.  The exchanges between the experts and the well informed and the deniers has given me a more detailed look at the different aspects of AGW.  

    I've been here lurkinga couple times a day since the inception of this blog site.  I read most of the articles and love going to the commets where things get fleshed out. 

      

  12. MA Rodger @ 780

    Where can I find easy to understand data that shows that only two of the last 8 eight had sea level rise higher than today?

    Also a denier is trying to say that  Milankovitch Cycles are irrelevant.  Where do they get idea that from I wonder?

  13. TVC15 @787,

    800,000 RSL

    This graphic has already appeared up-thread and is snatched from the web but originates from this webpage. (You need to click the top-left icon to get the 800,000 year version.) The underlying study is Spratt & Lisiecki (2015).

    If your denier who is "trying to say that Milankovitch Cycles are irrelevant" actually manages to achieve such an assertion, perhaps he should be asked what would trigger ice ages if not milankovitch cycles?

  14. MA Rodger @ 788

    Thank you so much!!!!!

  15. Bølling-Allerød seems to be a period when rapid warming far beyound today's was powered by  heat from warm waters originating from the deep North Atlantic Ocean.  It seems to have nothing to do with CO2.  This was pointed out to me by a skeptic after I claimed that present day warming is unprecented.  Reading up I discovered that from what researchers could determine the temperature rose more than a degree in less than a year and 3 to 4 degrees Celcius over a short period of time.  Needless to say I was surprised by this as well as finding nothing in the Skeptical Science search.

    Can you recommend an article on the subject.

  16. Estoma @790 , 

    I am not sure whether you are comparing arctic-localized (e.g. Greenland ice-core temperature proxies) changes with worldwide changes of smaller degree.

    There were also steep rises in Greenhouse gasses (especially methane) and giant meltwater pulses affecting sea-level & the oceanic heat transfer from the tropics.  All producing rapid fluctuation (see the Younger Dryas ).  Still, one needs to ask: what caused the melting of land ice-sheets  ~ if it wasn't caused by cumulative effect of gradually increasing Greenhouse effect?  (That is a question that the "skeptics" don't wish to ask.)

    The present-day rapid worldwide warming is not part of a transient up-and-down fluctuation, but is a strong & steady rise with a clear ongoing causation by CO2.  World temperature has (compared with the gentle & slight variations of the past 10,000 years of the Holocene) now abruptly risen higher than the Holocene Maximum and is still rising steeply.  And has occurred at a time (of roughly 5,000 years' duration) where the natural background tendency is toward ongoing global cooling.

    Altogether, the two cases (of B-A events versus nowadays) are so very different in their long-term importance, that it is perfectly fair to say that current-day gobal warming is unprecedented in its character ( and in its human causation !! )

    I think your "skeptic" is indulging in some Motivated Reasoning, and playing with words, in order to conceal the plain physical truth of what is happening globally.

  17. I figured the genisus was "from the strong & steady rise with a clear ongoing causation by CO2."  What flumoxed me was that the warming had occured at light speed; world wide from what I read.  I didn't realize that had been any  case like this during this inter glacial period. I'll have to add a cavet from now on when I make the unprecedented warming statement.

    It appears that today's temperature is still higher than it was 13,000 years ago but I was unable to find temperature  graph for the world outside of the Arctic that went beyound 2000.

  18. "I was unable to find temperature graph for the world outside of the Arctic that went beyound 2000"

    That can seemingly be an obstacle, but global reconstructions are becoming more common, depending upon the time period you're interested in.  For example, these are useful summaries (sources used listed on each):

    The last 10,000 years

    10,000 years

     

    The last 20,000 years

    20,000 years

     

    The last 800,000 years

    800,000 years

     

    As always, clicking on the images should make them bigger. 

    Images from Bruce Railsback's Fundamentals of Quaternary Science.

  19. Thank you Danial for the graphs.  You come up with lots of good stuff.

  20. Here's some more climate change denier blather I came across today.

    When you are using a complicated explanation to detail why something is happening, or is believed to be happening, you have almost certainly overlooked a much simpler reason for it.

    In the case of CO2, we already know from the fossil record that levels have been much higher in the past and also lower.

    During periods of low atmospheric CO2 levels, surface temperatures have been significantly higher.

    Likewise, during periods of very high CO2, temperatures were cooler than we are experiencing today.

    In fact, the only consistent relationship between CO2 and surface temperature seems to be the slight increase that follows a warming period as the oceans release stored CO2.

    So, when doubling the atmospheric CO2 level results in a claimed surface temperature change that is essentially imperceptible to the senses, the question becomes, how do we know the temperature has actually increased.

    Assuming an increase in surface temperature can be verified (good luck with that), the next question is, is it really caused by elevated CO2 levels and to what degree?

    What other factors are in play and, most importantly, what other factors cannot be measured and their influence accounted for?

    On the issue of cloud cover, NASA essentially punts.

    They admit that past cloud cover was never measured in any meaningful way and that measuring cloud cover, even with the technology available today, is more or less a scientific fool's errant.

    Of course, the problem is that we can't account for the effect of CO2 unless we also account for the effect of cloud cover, then and now.

    I could go into refrigerants in the stratosphere, the albedo effect, rain and humidity and other factors that are clearly ignored by the settled science crowd, but why bother.

    CAGW is a religion dressed up as a pseudo science trying to pass itself off as legitimate science.

    The reason questions that ought to be asked are not asked is because we would have to admit that the data is insufficient, unavailable and probably unknowable.

    There are so much illogical claims here I don't know where to start!

  21. TVC15 @795 ,

    I am assuming that you are dealing with a single denialist who tries to bury you under a blathering gush of Gish Gallop.

    IMO, best to challenge him on just a few points of his gush :-

    (A)  Planetary surface temperatures show a good correlation with the *combined* effects of changes in solar output and variations in atmospheric CO2 level.  Examples: look at the cooler/warmer periods of the Ordovician Age and also the Carboniferous Age, as well as the Cenozoic recently.  Sometimes the CO2 rise was caused by the temperature rise, and sometimes (such as the last 150 years) the CO2 rise preceded & caused the temperature rise.

    [to him] "Question: Why are you apparently unaware of all that paleo evidence?"

     

    (B)  We know the surface temperature is rising, because [i] satellite evidence shows land and sea ice is melting, and [ii] satellite and tide-gauge evidence shows rising sea level, and [iii] thermometers confirm the rising temperature (a rise now of almost 1 degree since reliable general measurements commenced).

    [to him] 'If you cannot understand such basic science, then your "can't feel it" opinion is worth nothing.'

     

    (C)  The scientists have looked into all sorts of factors that might influence climate (ranging from clouds and cosmic rays, to greenhouse gasses and albedo changes).

    [to him] "If you yourself have discovered a vitally important factor they overlooked, then [i] tell us what it is, and [ii] write to the National Academy of Sciences, and explain to them what morons they all have been for missing such a basic factor, and (iii) start planning how to spend all the Nobel Prize money that you will be awarded from the astounded and delighted Nobel Committee !   .....And good luck with that, Mr Unappreciated Genius."

    (Doubtless he will start shifting goalposts, and/or go into strawman arguments, and/or go into Conspiracy Mode.)

    .

  22. TVC15 @795,

    Your denialist runs through quite a complex argument, so probably well rehearsed but it is not very concerned with reality.

    The talk of the decoupling of global temperature & CO2 is probably going back hundreds of millions of years. There is an SkS post on this here (& probably other too) but I would punt the ball back into the denialist's court and ask which era they are talking about. They probably have some ancient denialist graph showing CO2 & temperature less than strongly coupled but addressing specific eras is the way forward, not trying to debunk some bogus lines on a graph scribbled out years ago, (usually with temperature a parody of this Scotese graph).

    The denialist presumably dismisses the tight coupling of CO2 through the last million years of ice ages with their "slight increase that follows a warming period". Then they plunge into the bold assertion that denies temperatures have been increasing over recent decades. If they require it to be "verified" they obviously haven't understood how thermometers work, how calendars work (winters and shorter, summers longer under AGW) and a whole ot more. This is village idiot talk.

    Their final run is in arguing for there being other causes for the temperature increase which is exceptional over thousands of years but that they deny exists. It would probably be better responding that the cause of such an exceptional global temperature change requires a cause, not just a list of mechanisms that impact the climate system. So if they want to invoke rain or humidity or clouds, what has set these off to cause this exceptional warming now?

    One of those they point to as a potential cause is, of course, manmade. CFCs & HCFCs would have been as bad as CO2 at forcing AGW if they hadn't been banned. There is also albedo which could be forced by land-use change as well as cloud (& there are denialists that have argued this as the cause of warming, although measured albedo shows nothing to support such an assertion).

    Their final talk of "questions that ought to be asked" could be another approach to a reply - what questions? Be specific!!

  23. Eclectic @796

    MA Rodger @ 797

    Thank you both so much! I learn so much everytime I come here and post the weird things that denialists state.

    Thank you both so much!

  24. MA Rodger @788

    OK so I shared the link showing that only two of the last 8 eight had sea level rise higher than today and this is what he came back with.

    No, it's every single one of them.

    Here's more science showing sea levels in MIS-7 and MIS 9:

    The presence of a sea-level highstand at E180 ka represents a challenge to the idea that Pleistocene climate is driven by summer insolation at 651N. Sea-level is increasing (and therefore ice is melting) when 651N summer insolation is at one of its lowest points of the last 400 ka.

    [emphasis his the denier]

    Recognition of non-Milankovitch sea-level highstands at 185 and 343 thousand years ago from U–Th dating of Bahamas sediment

    https://www.whoi.edu/cms/files/Hende...2006_21693.pdf

    Here's another on MIS-7...from your own government....omigod...how can you debunk your own government?

    Thermoluminescence (TL) and electron spin resonance (ESR) ages from sediments and fossil shells point to an age of ?220 ka for the end of this marine transgression, thus correlating it to MIS 7 (substage 7e). Altimetric data point to a maximum amplitude of about 10 meters above present-day mean sea-level, but tectonic processes may be involved.

    https://www.ncbi.nlm.nih.gov/pubmed/25590701

    Sucks to be you.....you can't scream, "Oil company funded research!"

    The last interglacial (LIG; also known as the Eemian or MIS5e) lasted from 129 to 116 thousand years before present (ka).

    If we look at EPICA Ice Core Data for CO2 levels in ppm, we find:

    YBP ..... ppm CO2

    116,501.... 262.5
    117,750.... 267.6
    118,649.... 273.7
    119,672.....271.9
    120,382.... 265.2
    121,017.... 277.6
    122,344.... 272.1
    123,070.... 276.4
    124,213.... 268.7
    124,257.....266.6
    124,789.... 266.3
    125,081.... 279.7
    125,262.... 273.0
    125,434.....277.1
    126,347.....273.7
    126,598.....267.1
    126,886.... 262.5
    127,132.....262.6
    127,622.... 275.3
    127,907.... 275.6
    128,344.... 274.0
    128,372.....287.1
    128,609.....286.8
    128,866.....282.6
    129,146.....264.1
    129,340.....263.4
    129,652.... 257.9
    129,736.....259.0

    For a brief period, CO2 levels were in the 280s ppm CO2, but for the most part below that and the average is only 271.7 ppm CO2.

    That, is Science, and Science says sea levels are going to rise and you can't stop it.

    I've never stated that sea levels are not going to rise so him tossing that in at the end is a non sequitur.

    I don't think that either link he posted is refering to sea-levels being higher than the two interglacials you mentioned in 784. (only two previous interglacials that saw such sea levels - the Eemian (MIS-5e) and 400,000y bp the MIS-11).


  25. TVC15 @799,

    It is a rather incoherent rant from your denialist. When you raise the point that only two of the last 8 interglacials were higher than today, he responds "No, it's every single one of them." Yet nothing he presents (his "science") is saying that all past interglacials had sea level above today. Indeed none are saying that global sea level was any different to your assertion (2 of 8 higher).

    He presents two references.
    Henderson et al (2006) is discussing the timing of the interglacial high stand in the Bahamas during MIS7 & MIS9 (note MIS9 is also one of the 2 past interglacials that were higher than today). They were certainly not considering the height of the peak sea level globally. Even if Henderson et al were discussing the height of the highstand, the isostatic movement of land means a local measure of relative sea level is not a reliable marker of global sea level. Nowhere do Henderson et al mention isostatic land movement. The height of the highstand is not their thesis, the timing is. (Note in the following reference Lopes et al, fig 6 shows the MIS1 holocene highstand 4 metres above present sea level. This would be down to local isostatic movement.)

    The denialist's second serving of "science" is Lopes et al (2014) is again primerally concerned with dating and having concluded that it is an MIS7 highstand they have identified, do not proclaim that MIS7 sea level has been underestimated (as the denialist concludes). Instead they look to find reason for their highstand being higher than they would have expected, their suggestion being that plate tectonics may be at work.

    The denialist then sets out non-controversial CO2 levels from the Eemian. Quite how this links to his final grand "science says" assertion can only be speculated. Is he saying that if CO2 was below 300ppm in the Eemian with its higher sea level, then (sarcastically perhaps) with CO2 at 400ppm today sea level must rise?  Strangely, this is the case. The warming resulting from increased CO2 will add perhaps 2.3m/degree C to global sea level, according to IPCC AR5.

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