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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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Comments 801 to 850 out of 872:

  1. TVC15 , if I may add a little to MA Rodger's Post #800 :

    your denialist friend's 2nd scientific paper quoted, is dealing with a small coastal area at the southern tip of Brazil.  The paper specifically says it has not allowed for "tectonic processes" that might modify the results.

    When you add in a quantum of uncertainty of timings locally, and at the Vostok site also . . . then really he is making no useful comparison with changes in insolation at 65N  [was your 651N a typo?] . . . nor any useful comparison with CO2 levels.  And looking at temperature changes over the course of each MIS or interglacial period, you see that each cycle shows its own unique pattern of variations against the grand sweep of glaciation/deglaciation.  From "minor" variations in insolation, ocean currents, vegetation changes, etcetera ~ and variation in sea-levels.

    Your denialist friend has not shown any evidence to disprove the role of CO2 in connection with planetary surface temperature.

    And he seems to have quite misunderstood the papers he quoted : indeed he is wasting the time of the readers.

    And tell him, regarding sea-level highstands . . . he's out of his depth!

  2. Again many thanks to both MA Rodgers and Ecletic!

    I am also out of my depths with respect to sea-level highstands but I knew that this denier was as well and I knew enough to know that neither of his links disproved anything I stated.

    Without this site I would have to return to college and go an additional 2 years with intense focus on the science mythologies of climate change.

    Thank you so much for broadening my scientific knowledge! :)

  3. TVC,

    I'll add that I looked at both links in your comment above and can not understand the allusion to "government." One study from Oxford University was funded by the Gary Comer Abrupt Climate Change Fellowship, the other is from the Annals of the Brazilian Academy of Science. Unless you're from Brazil, it would suggest your interlocutor did not spend much time looking at the studies; it would be a pity because the full text is available for free on both of them.

  4. Its a matter of history that as global tempatures rise evaporation increases and so dose plant matter that ends up traping it in soil or algea or bactria humans are a small pawn in this game. they only real major contributor to co2 is volcano's there is no way humans will ever elvate co2 levels to 4500 ppm

  5. Jkss97 @804 ,

    did you really mean 4500 ppm ?!?!

    I ask, because you made nine other errors/typos in your four lines.

    And that's not counting your major scientific errors.

    Please improve your climate science education !

  6. Eclectic @805,

    I think the reason for the 4,500ppm is probably that such levels of atmospheric CO2 have been found in studies for the early Phanerozoic, indeed perhaps higher. And if all the fossil fuels were burnt [they reckon coal reserves are perhaps 1,000Gt(C) with oil & gas perhaps another 500Gt(C)] you'd 'only' raise CO2 by some 350ppm with perhaps natural feedbacks adding as much again. So mankind would be hard pressed to 'achieve' a 4,500ppm level of CO2 by fossil-fuel-use alone.

  7. Thanks, MA Rodger.  It was mysterious to me, where he ( Jkss97 ) had gotten the 4500ppm figure from.  As you say, a future  4500ppm level would require the decomposition of vast amounts of carbonate rock, or maybe some other almost unimaginable cataclysmic event.

    And yet if he had meant to type 450ppm, then this figure is easily achievable by mankind continuing "business as usual" ~ and probably will be reached in a few decades' time.

    But with so many typo errors (and scientific errors) in his post #804 , there was a great deal of uncertainty about what he was on about.  ( I gave a marking of nine errors there . . . though a stricter assessment might have scored thirteen errors, not counting the 4500 and the scientific errors).

    It is less than 24 hours [now] so it is just possible that #804 might not be a room-temperature-level typical "drive-by" comment: and Jkss97 may return and explain what he means.  

    My other thought was of some translational difficulties perhaps ~ but then again, a non-native English speaker would likely be too careful in his typing to make such a large number of typographical errors . . . and also, the peculiar grocers' plural - volcano's - is an error unlikely to be made by a foreign learner of English.

    We shall see.

  8. Especially the meaning of the chart from the University of Copenhagen remains unclear to me:

    1) CO2 has been measured in the ice of the antarctica. It should therefore reflect the CO2 content of the sea in the past. Warmer oceans can dissolve less CO2. Wouldn't we then expect a CO2 content in the ice indirect proportional to the temperature?

    2) It is a common fallacy in science to assume a causal relationship between two parameters just because they show a similar behaviour over time. If we assume there is a direct causal relationship between the greenhouse gases and temperature, we would expect a much higher temperature in the present in contrast to what we see in the chart. Looking at the graphic from an unbiased perspective I would assume that both greenhous gases and temperature are correlated to other parameters. According to the data CO2 and methane don't seem to have a significant influence on temperature therefore.

    All in all, temperatures in the antarctica seem to be rather unobtrusive with no visible deviations since the industrial age.

  9. The Skeptic:

    1) Your question is hard to read.  Indirectly proportional means as one variable goes up the other goes down.  Directly proportional means as one variable goes up the other goes up.

    We expect that as ocean temperature goes up less CO2 will be dissolved.  The CO2 goes from the ocean into the atmosphere.  We see in the Copenhagen graph that as temperature goes up, CO2 also goes up.  That is exactly what was predicted by scientists decades before the ice core was obtained.

    2) Scientists first predicted that increasing CO2 would result in increasing temperature in around 1855.  The ice core measurements made over 100 years later confirmed this prediction.  A prediction made over 100 years in advance of the data is not the same as "assum[ing] there is a direct causal relationship between the greenhouse gases and temperature".  Validation of predictions is strong confirmation that the theory is correct.

    The ice core data end before the start of the industrial age.  Note that the first line on the timeline is 50,000 years ago.  You need to look at the Hocky Stick to see the changes due to the industrial age.  That data shows a clear link between dramatic increases in temperature and release of CO2.  This temperature increase is known to cause sea level rise, unprecedented wild fires and extreme weather. 

  10. theSkeptik @808,

    You refer to the plot of CO2, Methane & temperature found in the 'intermediate' OP which is derived from Antarctic ice cores.

    (1) The CO2 and Methane data is obtained from the content of air bubbles so is not a measutre of ocean CO2 but of atmospheric CO2.

    (2) You compare the increase in CO2 through recent ice age cycles with the modern anthropogenic increase and, given the larger temperature swings plotted through the ice ages you suggest  "we would expect a much higher temperature in the present in contrast to what we see in the chart." Do note firstly that the climate forcing from GHGs through the ice ages still exceeds the modern anthropogenic forcing which have had little-enough time to act and that GHGs were not the sole forcing agent through the ice ages. Secondly, the temperatures plotted are from the Antarctic which hasn't regionally seen as much warming as there has been globally, and that despite polar amplification which would have 'amplified' the ice age temperature swings in Antarctica.

    You also dismiss CO2 and methane as significant forcers of climate because you "assume that both greenhous gases and temperature are correlated to other parameters." What are these other 'parameters' you talk of and their means of 'correlation' to GHGs and to temperature?

  11. theSkeptik - " I would assume that both greenhous gases and temperature are correlated to other parameters"

    Well, yes, because physics. Long lived non-precipitating greenhouse gases in the atmosphere (CO2, CFCs, Methane) reduce IR emissions to space from the top of the atmosphere, which causes an imbalance of energy in the climate against incoming sunlight, and the climate therefore warms until radiated energy once again equals incoming.

    Along the way there are feedbacks both (mostly) positive and (some) negative which on the whole amplify the temperature response, such as changing CO2 solubility in the oceans, changing Earth albedo by melting snow over darker landscapes, methane releases from warming permafrost, and as a fast response changes in absolute humidity due to warm air holding more water vapor (itself a greenhouse gas, although as a feedback, not a driver).

    Physics comes first - correlation analysis to determine the exact amplitude of the response between drivers and the climate comes later.

  12. What has happened in the past is easily accounted for within our current theory of climate. No unexplained physics though there are unfortunately plenty of underconstrained problems (where more than one possible cause for observations and insufficient measurable to distinquish).

    What we can say with confidence is that increasing GHG will increase the amount of radiation received at the surface because we can unambiguously measure it. Unless you can dream up some other way to account for the observations, then I dont see how you can dispute this.

    Now, if the sun increased its output so that an extra 4W/m2 reached the surface, do you think the planet would warm? (If you dont think so, then I am fascinated to hear your theory of seasons...). If you accept that, then why dont think an extra 4W/m2 from GHG would also warm the surface.

  13. @michal sweet, MA Rodger and KR:

    First of all I very much appreciate your quick comments on my post.

    to 1) yes, I realize now that ice core measurements have been taken in the antarctica (not arctica) which I assume means it's about CO2 from the atmosphere, not the sea water. The shown direct relationship to the temperature is therefore plausible to me.

    to 2) I am discussing solely the meaning of the presented data from the University of Copenhagen. The article claims it supports the assumption that there is a causal relationship between the GHG and global temperature. Don't get me wrong, there may be other evidence for that claim but that's not my point here.

    @michael sweet

    The predicion you mentioned about the global warming 100 years ago is outside the scope of the discussed data. Apart from that, there are only two possible outcomes from such a prediction: a) It can turn out right - temperatures are rising or b) it can turn out wrong - temperatures are falling. So even with an uneducated guess one would have a 50% chance to be right. Finally, the graphic doesn't even show any evidence of global warming, though it does show a very significant raise in methane and CO2 in the last decades.

    @MA Rodger

    I see your first argument is in line with another claim of the article that recent data show a phase shift in GHG and temperature. Since 2012 GHG movement is said to no longer lag behind temperature data. I agree this would be an indication of a significant change. Unfortunately this data is not shown in the article and it can't be seen in the presented graph. Your second argument just seems to support my concern: Global warming can't be seen in the antarctica according to the chart so far. It is possible that it shows up in the future, but the shown data gives no evidence to that assumption.

    Finally I do not make any claims about any relationships between GHG and temperature or other related parameters. I am just looking for unbiased information and constantly come across overinterpreted data and conclusions driven by preoccupation. If one claims a causal relationship between two parameters its up to them to give evidence, not to me to proof otherwise.

    @KR

    As a physicist working for several decades in RnD companies I am not easily convinced of simple models describing the behaviour of complex reality. At least it's not obvious to me that a gas in a concentration of only several hundreds of ppm is likely to have such a significant influence on global temperature. It might not be impossible and I will surely have a closer look at this matter in due course. However, as I mentioned before this is not my current point. I am discussing the presented graphic which seems to support nothing of the claims about global warming apart from unusual high greenhouse gas concentrations.

  14. theSkeptik - Please see How substances in trace amounts can cause large effects regarding increasing the ppm of CO2. Raising the primary greenhouse gas concentration by more than 30% has a very significant effect.

    And those unusually high greenhouse gas concentrations are exactly the issue - based upon simple spectroscopy, supported by direct empirical measurements of surface and orbital temperatures and radiation, and by examining all physically plausible temperature forcings (observed changes in natural forcings would have cooled the climate over the last 50 years, only anthropogenic greenhouse gas increases have warmed it).

    So yes, the change in greenhouse gas concentrations which that graphic displays is hugely significant.

  15. theSkeptic - See also the Argument from Incredulity regarding CO2 concentrations.

  16. TheSkeptic - It is amusing to see a self-professed scientific-minded individual claim that "I will surely have a closer look at this matter in due course" in reference to The Key Apsect of the Issue being discussed.

    How is it you have developed so many thoughts on this matter without first developing a robust understanding of its most fundamental basis?

  17. "The article claims it supports the assumption that there is a causal relationship between the GHG and global temperature"

    It does, but you cannot isolate the diagram from the rest of physics and I do not believe the article purports to do so. The claim is not that "because CO2 levels move lock step with temperature, ergo CO2 is causing temperatures to rise", which would indeed be a misstep. The claim is more that these observation support what we would predict from the underlying physics and chemistry at work. The tricky thing with iceage cycle is that it is strongly correlated with insolation at 65N. While the forcing at that latitude is strong, the forcing globally is weak (can be antiphased in southern hemisphere). However, the feedback loop that change the GHG composition with temperatures at in high northern latitude easily accounts for converting a local change into a global event.

    I would also say, that if you are a physicist, do you go with your gut or do you do the maths?

  18. OnePlanetOF @816  ~ well stated, sir.   I also admire the droll delicacy of your final sentence.

    Like you, I enjoy the obfuscatory sophistry that "TheSkeptic" is employing . . . and reading between the lines of his own comments, it appears that he enjoys constructing these obfuscatory sophistries.

    TheSkeptic  ~ sir, when you have finished with your footwork, would you please make a straightforward presentation of how, where & why you consider the modern mainstream science to be wrong in matters of climate.   ( I would like to think you have something definite in mind . . . and are not just trying to bluff while holding a completely valueless hand of cards.)

    Response:

    [PS] Thanks. Fixed.

  19. @theSkeptic 813

    "At least it's not obvious to me that a gas in a concentration of only several hundreds of ppm is likely to have such a significant influence on global temperature."

    It was not obvious to anyone until the work had been done and the conclusions drawn. A skeptic, of course, asks the question is it possible that a small amount of a substance can have a large effect? And then researches it.

    Looking around we can see hundreds of examples in the real world where small amounts have a large effect. For instance the cascade effect in the body of a small amount of hormone triggers reactions very speedily. I doubt you would eat fuga fish if you felt the chef had done less than a stellar job preparing it.

    You are an experienced R&D scientist. So why are you not more skeptical?

  20. theSkeptik @813,

    Such is the composition of your specific responses (not least to my comment @810) that I feel you should be made aware of how far you are from grasping the reality of the climatology you criticise. This makes addressing the substance of your comment (which actually has some merit) an impossibility.

    Thus (& specific to you reply to my comment @810), what you call my "first argument" is correcting your error @808 by pointing out the well-known situation that the CO2 measured from ice cores is measuring trapped air. You move on from this 'correction' and on to the so-far-unmentioned-by-you problem of the difference between the age of the ice and the age of the air entrapped within the ice which as you correctly say is not addressed in this SkS post. It is addressed on a different SkS post which is linked within the above SkS post. "Unfortunately" you are unable to cope with that situation.

    Similarly, you use part of what I present within what you call my "second argument" to begin anew with a different argument that an absence of Antarctic warming is equivalent to there being no global warming. (Actually if this were the issue, more up-to-date temperature data, so for instance the warming below -70ºS measured by GISTEMP, records a great deal of Antarctic warming over recent years.)

    Finally you are flat wrong to suggest that you "do not make any claims about any relationships between GHG and temperature or other related parameters." Whatever your experience in "just looking for unbiased information," do not deny that you yourself come here with "overinterpreted data and conclusions driven by preoccupation," and I would suggest your two comments @808 & @813 show you are more pre-occupied than those you criticise.

    The SkS post above, addresses the nonsense myth set out by denialist Richard Lindzen that "climate is always changing" and thus "wasting resources on symbolically fighting ever present climate change is no substitute for prudence."  You may be unsatisfied that this SkS post properly addresses Lindzen's denialist argument. And I may agree with you on that specific-but-narrow point. But such a deficiency does not, as you attempt to argue, make the underlying thesis wrong. And you failure to present consistent and trustworthy analysis suggests proper discussion of all this likely a little pointless.

  21. Skeptic @813:

    I am astonished that you will not consider conclusions over 100 years old.  It is no wonder that you conclude that the scientists over generalize their conclusions when you restrict discussion to only the data in their paper.  I note that the paper provides no evidence that the Earth is round, that matter is made of atoms or that the Sun will come up tomorrow.  Normal scientific discussion includes data previously gathered.  Your claims that only the data in the paper can be considered is in contravention to all scientific and normal conversation.

    You make no attempt to provide a scientific justification for your wild claims.  Since many others are commenting to you I will withdraw to prevent dogpiling.

  22. Wow, I am overwhelmed by all this aggression, I never intended to offend anyone and I don't dare to imagine what would have happened if I said that I don't believe in anthropogenic climate change - which I haven't so far.

    Please understand that I am no longer able to respond to every comment in detail. Especially I won't comment any postings from people who are apparently not able to formulate a single argument.

    My second point was that the graphic from the University of Copenhagen does not show any evidence or even indication for a direct temperature GHG relation. I explicitely stated that there may be other evidence for that apart from the graphic. The simple question "what information can be derived from the graphic" obviously can and must be answered only in the context of the graphic. Just assume that I agree with you about the direct causal relationship between GHG and temperature. Would it change the meaning and information of the shown data? Certainly not within the boundaries of science.

    I noticed one single argument related to that question so far. The lack of correlation between temperature and GHG in the presence may be due to a lag of the temperature change since the last couple of years. It therefore doesen't show up in the graphic. Well, fair enough. But that even more so underlines my point: It's something the graphic does not show. So my statement "why presenting that data which does not show what the article intends to" stays unchallanged.

    Response:

    [DB] Inflammatory and baiting rhetoric snipped.

    Please note that posting comments here at SkS is a privilege, not a right.  This privilege can be rescinded if the posting individual treats adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Please take the time to review the policy and ensure future comments are in full compliance with it.  Thanks for your understanding and compliance in this matter.

    Finally, please understand that moderation policies are not open for discussion.  If you find yourself incapable of abiding by these common set of rules that everyone else observes, then a change of venues is probably best.  Electrons are plentiful and abundant.

    NOTE:  T his user has requested to recuse themselves from further participation here.

  23. "Does not show any evidence of direct ...for a direct GHG temperature relationship"

    Physics and chemistry predict that temperature will correlate with GHG for milankovich forcings. The data supports that prediction.

    Here they are from Epica directly overlain.

    And here is a correlation plot, though the GHG forcing instead of CO2 concentration since there is non-linear relationship between CO2 concentration and forcing.

    Source:

    Shows a pretty good relationship if you ask me - or statistically calculate it.  I dont think you are articulating your problem with the graphic well enough.

    "The lack of correlation between temperature and GHG in the presence may be due to a lag of the temperature change since the last couple of years."

    I suspect English is not your first language, but I cannot parse your meaning here. "a couple of years" is not something discernable on UoC graphic. If you mean present time, (since say 1970), then climate (not weather) is strongly correlated with GHG levels. Even more strongly correlated with total radiative forcing (taking into a account all influences on climate). Note of course that decadal-level variations in temperatures are not driven by GHG. Claiming that science predicts constant, temperature/GHG relationship is a straw-man fallacy.

  24. This is may first post and I am not able to repeat the 823 POST chart.  What ir says to me is that changes in temperature clearly ocurr in advance of changing CO2 levels.  Which makes sense since with warming teperatures the ocean will release CO2.  

    Does CO2 in the atmosphere reflect the sunlight?  Similar to clouds.  If so then the greater effect orf increased CO2 is to cool the Earth because there is much more energy reflected toward space than toward Earth.

    This is a simplistic view, but CO2 from the ocean and from decompostion of organic matter are so dominant, does it not make sense?  Also, man has done awful things to the ocean so mightened this contribute to the warming and therefore the CO2 release.  "See this: Is CO2 causing climate change?"

    Response:

    [DB]  Hyperlinked source that was breaking page formatting.

  25. Bruce Monk, the nature of both your questions and remarks clearly show that you are nowhere near the level of knowledge and understanding that you would need in order to formulate any kind of opinion that could be of interest to others. There is plenty to learn on this site and it refers other sources as well; you have lots of reading to do if you want to contribute.

  26. Bruce. Hard to know where to begin.

    CO2 does not reflect sunlight - the gas is transparent to the frequencies of radiation coming from sun. However, the gas absorbs infrared radiation leaving the surface. So GHG lets energy in but slows energy going out.

    A warming ocean will emit CO2 (melting permafrost and temperate wetlands are other sources of GHG as temperature rise), but the oceans will not become net emitters of CO2 for hundreds of years. Currently they are absorbing CO2 (and becoming less alkaline).

    The situation at the end of an ice age is different - the changing distribution of sun energy (milankovitch cycles) result in summer melt in high northern latitudes reducing the albedo (and thus the amount of sunlight reflected directly back to space from ice). The warming releases GHG by the various mechamisms and as a result whole planet warms.

  27. As this is your first post, Skeptical Science respectfully reminds you to please follow our comments policy. Thank You!

  28. Has there been a gradual rise in atmospheric C0s during the last 10 millenia followed by a spike following the industrial revolution?  Does the gradual rise correlate with human agiculture and pastoralism?  Does the spike correlate with conversion of North American grasslands to agriculture and pastoralism with the aid of machinery powered by fossil fuels?  This makes sense to me because grassland soils contain more carbon than soils of forestland or brushland. 

  29. Hal Kantrud

    Atmospheric CO2 levels reached about 265 ppm about 11,000 years ago, near the end of the last glacial phase and the start of the current Holocene interglacial.  From then until just before preindustrial (1850), CO2 levels slowly increased to about 280 ppm, an increase of 15 or so ppm.  

    The last 10,000 years

    (bigger image here)

    What this doesn't take into account is that human activities starting around the development of agriculture until preindustrial times added about 25 ppm to those atmospheric levels.  This implies that, without the human impacts, atmospheric CO2 levels would have naturally dropped by some 10 ppm over the same interval.

    In more depth, human activities have been modifying the climate system for far longer than most people realize. Evidence exists that humans have been doing so since the development of agriculture more than 10,000 years ago, contributing as much as 25 ppm to existing, preindustrial atmospheric CO2 levels. During periods of previous pandemics, reforestation of formerly cultivated lands have drawn down atmospheric carbon dioxide levels enough to measurably lower global temperatures.

    "Scientists understand that the so-called Little Ice Age was caused by several factors - a drop in atmospheric carbon dioxide levels, a series of large volcanic eruptions, changes in land use and a temporary decline in solar activity.

    This new study demonstrates that the drop in CO₂ is itself partly due the settlement of the Americas and resulting collapse of the indigenous population, allowing regrowth of natural vegetation. It demonstrates that human activities affected the climate well before the industrial revolution began."

    Link1
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  30. "n more depth, human activities have been modifying the climate system for far longer than most people realize. Evidence exists that humans have been doing so since the development of agriculture more than 10,000 years ago, contributing as much as 25 ppm to existing, preindustrial atmospheric CO2 levels.'"

    But even from the Antarctic ice data it looks like a gradual rise began about 7000 years ago. This could correlate with the increased use of the high-carbon content grassland soils for cultivation of annual crops such as small grains and also for pasturage.

    "During periods of previous pandemics, reforestation of formerly cultivated lands have drawn down atmospheric carbon dioxide levels enough to measurably lower global temperatures."

    I see no dips in atmospheric CO2 following any of the world's worst pandemics. "Reforestation" probably means abandonment of cropland where forests once stood, where weeds and annual grasses quickly become dominant. So this perhaps accounts for the lack of dips. I doubt if large scale abandonment of cropland occurred in fire-derived ecosystems like grasslands where forests did not originally exist as these areas would be the first to be returned to agiculture or for domestic livestock by the survivors. Unlike grasslands, forests are shallow rooted and store little carbon, other nutrients, or water underground, so I would think long-term effects of reforestation on atmospheric CO2 would be quite low.

    "Scientists understand that the so-called Little Ice Age was caused by several factors - a drop in atmospheric carbon dioxide levels, a series of large volcanic eruptions, changes in land use and a temporary decline in solar activity."

    Thought I read where regular changes in Earth's axis of rotation ("wobble") may also be involved here.

    "This new study demonstrates that the drop in CO₂ is itself partly due the settlement of the Americas and resulting collapse of the indigenous population, allowing regrowth of natural vegetation."

    Whew! Now you are saying that Amerindians had more land under cultivation and overgrazed more acreage than European man?

    "it demonstrates that human activities affected the climate well before the industrial revolution began."

    I agree with that, but believe the recent upward blip was caused as much by convesion of New World grasslands to cropland as it was by the industrial hydrocarbons used to accompish that task.

  31. Hal Kantrud, you wrote that you thinks you read that changes in Earth's axis of rotation may be involved in the Little Ice Age. You might well have read that, because misunderstanding is widespread. But the "Little Ice Age" was not actually an ice age. Indeed, what popularly are called "ice ages" actually are glacial periods nested within actual "ice ages." Within each actual ice age there is a series of glacial periods and "interglacial periods." Currently we are in an interglacial period within an ice age. The Little Ice age in fact was merely a short period of regional cooling within the current interglacial period. See this relevant post--first the Basic, then the Intermediate, and finally the Advanced tabbed pane.

    "Wobbles" in the Earth's axis are so slow that they operate on the time scale of triggering the glacial and interglacial periods.  See this post about Milankovich cycles.

  32. Thanks.  So if the "wobble" that triggered the Pleistocene glaciation, and less extensive glaciations occur during the interglacial, I guess the proper name would be Interglacial Subglaciations.  These must be what misinformed laypersons like myself have termed "Little Ice Ages".  How many have there been during the last 12,000 years and could they have dampened atmospheric CO2 levels? Did the extent of polar ice increase during these lesser glaciations?  

  33. Hal Kantrud: The "Little Ice Age" (LIA) was not a glaciation in any sense. It was a brief period within which some particular regions got colder for a little while before getting warmer again, but not all of them at the same time. From the PAGES 2K study:

    "Our regional temperature reconstructions also show little evidence for globally synchronized multi-decadal shifts that would mark well-defined worldwide MWP and LIA intervals. Instead, the specific timing of peak warm and cold intervals varies regionally, with multi-decadal variability resulting in regionally specific temperature departures from an underlying global cooling trend."

  34. Hal Kantrud, the planetary "wobbles" are much too slow to cause any brief effect such as the Little Ice Age [LIA].

    Have a look at the PAGES 2K study.  

    Many people hear the name "Little Ice Age" ~ and combine it in their mind with old illustrations of Dickensian snow and London "Ice Fairs" on the frozen Thames, and suchlike Christmassy freezes.

    But in reality, the LIA was very minor.  Less than 0.5 degreesC colder than the usual background for the Northern Hemisphere, and more like 0.3 degreesC cooler for the global whole.

    Even the Medieval Warm Period [MWP] was only around 0.3 degreesC warmer than the global historic background.  Despite some of the trumpet-blowing about the MWP and the LIA, they were both pretty minor events overall.  Their names do greatly exaggerate their size.  And they are insignificant compared with the level of warmth of the Holocene Maximum (about 8000 years ago) and the even higher temperature levels of recent decades (which are around 0.5 degreesC hotter than the Holocene Maximum).

  35. Eclectic @834,

    Do be aware that London's frozen River Thames was a very rare event and if anything provides evidence against the Little Ice Age being something exceptional with reported freeze-ups occurring even during the Medieval Warm Period. There were perhaps only a half dozen Frost Fairs listed in the records and they stopped appearing, not because of warmer winters but because the old London Bridge was demolished and the river embanked.

    Given such reasons for the absence of  Frost Fairs since 1813, perhaps a better river to look for evidence of a Little Ice Age (or lack ofevidence) is the Rhine which is recorded freezing 14 times since 1784, the last time in 1963. Of those 14 freezes, most occurred well after any Little Ice Age with seven during the 20th century.

  36. I didn't say the 'wobbles' caused any LIA's. But the evidence seems to show "wobbles" are assoicated with Ice Ages. What threw me off was your statement that LIA's were glacial periods, hence my question about possible evidence of expanding ice caps during these periods.

    It is interesting that the Holocene Macimum occurred about the same time as the dawn of agriculture and pastoralism, concentrated on the carbon- and nutrient-rich grasslands of the Old World.  So perhaps the slow atmospheric CO2 buildup shown by the ice core studies could have been caused by such 'mining' of grassland soils, followed by the spike in CO2 during the Industrial Revolution as the grasslands of the New World suffered the same fate, aided by mechanical power rather than the draft animals of old?

  37. Hal Kantrud, you wrote "What threw me off was your statement that LIA's were glacial periods." Who stated that, where?

  38. "I see no dips in atmospheric CO2 following any of the world's worst pandemics"

    Others disagree. See Ruddman and Carmichael 2006, vanHoof et al 2006.

    Whether such short term effects like pandemics last long enough to affect climate is more debatable (Pongratz et al 2011)

  39. Tom on the 19th. "Within each actual ice age there is a series of glacial periods and interglacial periods"   Guess I took that to mean an ice age extends from one peak glaciation to the next.  Perhaps you were referring to the 'surges' in glaciers during the peaks.

  40. Hal Kantrud, by definition an ice age is any period with continental-scale ice sheets on land (like now).  Within an ice age are warmer periods called interglacials and colder periods, called glacial periods (or glacial phases).  The Little Ice Age nor any cool episode in the past 13,000 years do not rise to the standard of a glacial phase.

    The last 20,000 years

    (bigger image here)

    As can be seen below, glacial and interglacial periods are self-evident:

    The last 800,000 years

    (bigger image here)

    When it comes to the modern warming forcing from human activities, it's already comparable to the warming which lifted the world out of the last glacial maximum 24,000 years ago to the height of the Holocene Climate Optimum 8,000 years ago:

    "About 2.3W/m2 (from CO2), a few tenths more from CH4 and N2O.

    Anthropogenic GHG forcing is ~2 W/m2 (CO2) and ~0.5 W/m2 from CH4+N2O+CFCs.

    So they are comparable - ice sheets were a bigger term in the deglaciation tho."

    (source)

    Humans are inducing a phase transition from an interglacial world to a no-glacial world.  So we are ending the ice age itself.

  41. MA Rodger @835,

    Thanks.  Yes, I had heard that the "frozen Thames" events had occurred even during the Medieval Warm Period (though those are never mentioned by Denialists).

    I was interested in the "meme" of Thames freezings being held up as an example of the world-chilling severity of the Little Ice Age.  And as I was saying to Hal Kantrud (who seems just starting out on learning about climate science) . . . the main point to remember is that the LIA and the MWP were pretty small beer compared with earlier climate changes.

    As you yourself know very well, the LIA is greatly misrepresented by the climate-science Deniers :-

    (a)  Firstly, they exaggerate its severity ;

    (b)  Secondly, they falsely claim that our modern rapid warming is (somehow)  "just a rebound from the LIA" .

    (c)  Thirdly - with amusingly unintended irony - they claim that the huge temperature excursions of MWP & LIA make the modern warming look insignificant . . . and yet at the same time they claim that the planet's Climate Sensitivity is so very low that we need not be concerned about the "slight" warming effect of anthropogenic CO2 emissions.   Superb!

    MA Rodger, you might not have seen it . . . but on one of the Denialist blogs recently, a particular Denier asserted that (by his calculation) Earth's Equilibrium Climate Sensitivity was around 0.4 degreesC.   Improving on that, he then (based on the negligibly-small rise in CO2 which he attributed to humans) calculated that, of the modern warming, only 0.02 degreesC was human-caused.  To repeat: 0.02 degreesC.   Not a misprint.   (Ah, who needs to pursue comedy, when so much is freely available on the Denier blogs! )

  42. Thanks.  So we are in a period of the current Ice Age where continent-size ice sheets cover land at the poles.  While waiting for the next Ice Age, we experience cooling periods, (phases) but these are too small to significantly increase the size of the continental ice sheets.  Does this imply that the warming periods or phases will not significantly reduce the size of the ice sheets?  

    Am I correct in reading the graphs that during the Holocene, Earth reached peak temperatures about 8000 YBP and has since cooled down to temperatures reached about 13,000 YBP, but during the last 200 years has spiked to levels far above those observed during the Holocene peak?

    Can I also say that atmospheric CO2 levels followed an opposite pattern, slowly increasing during the last 8000 years, only to spike upwards during the last 200 years to unprecedented highs? 

    My main question concerns the latter.  What portion of the recent spike in CO2 can be attricuted to the conversion of New World grasslands to agriculture and pasturage and what portion can be attriubted to the increased use of hydrocarbons worldwide?  

  43. Hal Kantrud @842,

    To address your three requested points of clarification/confirmation.

    (1) The only actual continent-sized ice sheet is Antarctica and that remains unaltered in size through an interglacial and through a glacial maimum. The glacial maximum see the growth of ice sheets across the northern half of N America, Greenland and N Europe. The Greenland ice sheet has survived the present interglacial but was melted out in the previous one.

    The impact of small wobbles in global temperature is not significant within this process as the temperature change is small and it doesn't last very long. The ice melt is a slow process. Thus, while global temperatures stopped rising 10,000 years ago, the melt continued strongly for a further 2,500 years and less strongly for another 4,000 years, this shown by the sea level record.

    Sea level holocene

    (2) Your timings are a little off. After the Holocene peak temperture (best considered as a plateau 10,000y to 6,000y bp), global temperature has been dropping but only to the equivalent of 11,000y bp. 13,000y bp would have you back in the Younger Dryas event when it was very cold.

    (3) The CO2 record from ice cores does show previous interglacials with CO2 (& CH4) levels falling quickly from the peak of the interglacial. This is not the case for the present interglacial when CO2 (& CH4) levels are shown to rise not fall. This has led to some interesting work setting out the idea that the activities of mankind are responsible for this early rise, for CO2 perhaps dating back to 8,000y bp (& 5,000y bp for CH4).

    While this work remains speculative, the CO2 (& CH4) levels through this interglacial would act to slow the drop back into a glacial maximum.

    The unprecedented CO2 levels likely now top the CO2 levels seen 3 million years ago (this was back when  N America was joined S America at Panama and initiated the Arctic ice)  and are thus uprecedented in 13 million years, thus back to a time when weathering of the newly-formed Himalayas caused reducing CO2 levels.

    ....

    And addressing your main question which concerns the CO2 levels of the last few centuries rather than those of the late stone age because any increase pre-industrial cannot be the result of fossil fuel use.

    According to the Global Carbon Project, the anthropogenic CO2 emissions since pre-industrial amount to some 650Gt(C) of which 450Gt(C) results from fossil fuel use and 200 Gt(C) due to Land Use Change, but note this is mainly cutting trees down not "the conversion of New World grasslands".

  44. Hal Kantrud , I would like to add a few disparate points which may be of interest to you.  (And you may already have come across some of them.)  As always, I shall be grateful if MA Rodger (who is extremely well-informed on climate matters) sees fit to make any corrective comment!

    1.  The term "BP" / bp  stands for Before Present, but does not mean "up until right now this year of [2020]".   BP is a convention used by the paleo scientists to standardize the reference to past ages - whether centuries, millennia or mega-years [ma].  BP at point zero is taken as year 1950.AD

    Some "contrarians" have not been aware of this convention (for instance the slightly-contrarian scientist Loehle has had to go back and correct some of his work, because he was initially unaware of the paleo convention).

    Hal, this paleo convention is enormously important, since there has been a huge rise in global surface temperature since 1950.   Even today, some Denialist blogsites are publishing graphs which misrepresent reality, and are showing a graph's final temperature as 2000.AD or 2010.AD . . . when the original graph only went up to 1950.AD  . . . and worse, the denialists have sometimes doctored or airbrushed-out the most modern temperatures.  Sometimes this deliberate deception is outright concealed - and sometimes the deception is camouflaged under the term "Adapted from [a certain scientific paper]" .

    Another small point is that some of the ice-core temperatures are recorded up until around 1855.AD , since later/shallower levels of ice are unrepresentative of their ambient conditions.

    [You will have noticed how almost all science-deniers are still falsely (and vehemently) asserting that both the Holocene Maximum and the MWP were hotter than 2000.AD and current years.]

    2.  The Holocene Optimum [sometimes called Holocene Maximum] was roughly 8000 years ago, but as MA Rodger rightly points out, the Maximum was more of a plateau of roughly 5 millennia.   Over the succeeding 4 or 5 thousand years, the temperature has dropped roughly 0.7 degreesC as part of the background cooling which would eventually lead into the next glaciation.  But AGW has intervened - with global temperature rising like a rocket in the past 100-200 years (dare I say like the end of a Hockey Stick?)   Hockey Stick is yet another term which causes Denialists to choke on their cornflakes.

    As a consequence of the natural cooling down from the Holocene Maximum, the global sea level has reduced by about 1 or 2 meters . . . and that fall should have continued onwards as we slide into the next glaciation.  Except for the modern AGW-caused rise in sea level, a rise which is slow but accelerating.

    3.  Each glaciation cycle of the past 800,000 has been subtly different, owing to differences in the variations of the Milankovitch cyclings.  That makes it difficult to predict when the next glaciation would have occurred in the absence of human influence.  One figure I recall seeing, is the next chilly glaciation being due in roughly 16,000 years.  So we humans have plenty of time to fine-tune our climatic effects before any threat of severe glaciation!   (Some denialists maintain that the "New Ice Age" was due in a few centuries from now . . . and our anthropogenic CO2 has fortuitously been raised only in the nick of time... )

    4.  I won't comment on your point of interest about the New World grasslands.  The changes there would be quite minor in the overall picture.

  45. Thanks. To narrow down, the graphs show temperatures dropped during the last 6 millenia, while sea levels rose about 2m. One would think sea levels would rise, so is there a time lag working here? 

    CO2 increased during this period till the recent spike.  There is no data on NH4 and SO2, but I thought this mix tended to prevent reflection of the sun's rays and thus increase temperatures.

    I am not convinced that 'cutting trees' in the New World was as important a source of human CO2 emissions as the switch of grasslands from the production of perennial grasses to annual crop plants and domestic animals.  Forest soils are notoriously poor in carbon and most is sequestered in the trees themselves, whereas perennial grasslands sequester most carbon deep underground while evolving in cycles of frequent fire and intense herbivory. Soils dominated by grasses have always been the first to be heavily exploited for food production ("the land of milk and honey") and were where our staple foods such as wheat, rice, barley, etc., were domesticated.   So to seriously tackle the problem of high CO2 levels, it would be more efficient to seed perennial grasses, whose root systems  remain the only viable net  CO2 sink.  Trouble is those areas are the source of most of our food!

  46. Hal Kantrud @845 , your first paragraph is crossed-up.

    As the planet cools 0.7 degreesC during the past 5-ish millennia, more land ice forms, and so the sea-level falls.  MA Rodger's graph (above) is a broadbrush illustration of sea-level, yet data fuzziness does not illustrate the small fall (roughly in the region of 1 maybe 2 meters in the past few thousand years).

    As far as I have gathered, the broad scientific opinion favors a return to atmospheric CO2 level around 350 ppm.    Incorporating carbon into deeper soil is a worth goal, but probably will be too slow (and limited) to achieve a negation of all the recent & continuing fossil fuel usage.

  47. Eclectic @844,

    I'm not sure where you get the metre drop in late Holocene sea levels. There have been dropping sea levels in some locations through the late Holocene but that is due to isostatic rebound caused by the redistribiution of mass - melted ice sheet flowing into tropical seas. The accepted wisdom as I understand it is still as per IPCC AR5 Ch5 5.6:-

    "Ocean volume between about 7 ka and 3 ka is likely to have increased by an equivalent sea level rise of 2 to 3 m."

    "For the past 5 millennia the most complete sea level record from a single location consists of microatoll evidence from Kiritimati that reveals with medium confidence that amplitudes of any fluctuations in GMSL during this interval did not exceed approximately ±25 cm on time scales of a few hundred years. Proxy data from other localities with quasi-continuous records for parts of this pre-industrial period, likewise, do not identify significant global oscillations on centennial time scales."

  48. Hal Kantrud @845,

    There is certainly a timelag between temperature rise and ice loss and with big ice sheets the lag can also be big (but not necessarily). The current level of AGW is put at 1ºC and the sea level rise so far at 20 or 30 cms. Yet the anticipated sea level rise per 1ºC AGW is put at 230cm over a period of a couple of millenia. But additional to that 230cm/1ºC is Greenland which maintains its ice sheet solely becuse its summit is high up surrounded by cold atmosphere. It is anticipated that somewhere between 1ºC and 2ºC of AGW, the summit of Greenland's ice will drop into an unstoppable melt-out as the summit decends into warmer atmospheres, this adding a further 600cm to sea level over perhaps ten millenia. I say "unstoppable" in that it would require a return to ice age conditions to stop the melt and build the summit back up into colder airs.

    Regarding the chopping down of woodland, this is globally not New World.

  49. MA Rodger @847 ,

    thanks for that information.   Re late Holocene MSL decline, I must confess I was relying on memory of seeing (several years ago) a graph of the Holocene highstand declining by 1-2m during the most recent 4-5000 years, as the global temperature reduced by around 0.7 degreesC.   As you say, I might have been rather faultily recollecting something which lacked land "rebound" compensation.

    On the other hand ~ a quick googling turns up SE Lewis et al. (2008)  showing an eastern Australian fall in MSL of 1-1.5m over the period 7000-2000 BP.   Eastern Australia (excluding Tasmania) had very little burden of ice sheet at the last glacial maximum (to rebound from) . . . and Australia has been tectonically relatively stable, as well ~  so it is a useful basis for Holocene MSL trends.

    I think I may be misunderstanding your IPCC reference where: "Ocean volume between about 7 ka and 3 ka is likely to have increased by an equivalent sea level rise of 2 to 3 m."    If the lagging effect of Holocene warming produced a likely 2-3m MSL rise over the period about 7000-3000 BP . . . is that inconsistent with a 1-1.5m MSL fall in the last 3000 years? [Assuming some fuzziness/uncertainty in the Lewis et al.  dating]

    As a matter of interest, I did a quick back-of-envelope calculation:  indicating that for a 1m fall in MSL, the depth of ice on Greenland/Antarctica would need to increase by about 30m.   This ignores oceanic thermal contraction and glacier expansion in non-polar regions.

  50. Eclectic @849,

    In the past, I do recall SLR free of significant tectonic movement being claimed as a good indicator of global SLR and that it usually concerns Australian data but I'm not sure such claims usually attach to late Holocene SLR. Lewis et al (2008) and indeed Lewis et al (2013) and Lewis et al (2015) are concerned with the late Holocene and more so the Australian record than establishing a global record. And their findings are not so clear cut although a significant drop in sea level post-7,500y bp has been established. The 2013 paper concludes:-

    "A clearer understanding of past sea-level changes and their causes is urgently needed to better inform our ability to forecast future changes. A concerted effort is required ... to address the issues of whether there have been oscillations of the sea surface and if so, of what magnitude. The pattern and rate of fall from the Holocene highstand to modern levels, and of the contributions of the various factors to this change, both global ‘eustatic’ or ‘steric’ components and local geophysical, tectonic and land instability issues also need to be addressed."

    The work to unravel the late Holocene global SLR record is far from complete and I see no evidence of the Australia (Oceania) data providing a short-cut to providing a conclusion. Thus, the conclusion from, for instance,  Khan et al (2015):-

    "Far-field Relative SL records exhibit a mid-Holocene highstand, the timing (between 8 and 4 ka) and magnitude (between <1m and 6 m) of which varies among South America, Africa, Asia, and Oceania regions."

    And on the reasons:-

    "The Relative SL signal of many far-field locations is characterized by a mid-Holocene sea-level maximum, or highstand, at the time meltwater production decreased. The fall in Relative SL to present is due to hydro-isostatic loading (continental levering) and a global fall in the ocean surface due to both hydro- and glacio-isostatic loading of the Earth’s surface (equatorial ocean siphoning). Perturbations to Earth’s rotation driven by mass redistribution also cause Relative SL changes in far-field regions to depart from the eustatic value. These processes occur during the deglacial period but are not manifested in far-field RSL records until the early to mid-Holocene because the eustatic signal is dominant prior to this time. Far-field locations are characterized by present-day rates of Relative SL change that are near constant or show a slight fall (<0.3 mm/a) in [rate of] Relative SL (Fig 1)."

    relative Sea Level map

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