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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)

At a glance

Just imagine for a moment. You fancy having a picnic tomorrow, or you're a farmer needing a dry day to harvest a ripe crop. So naturally, you tune in for a weather-forecast. But what you get is:

“Here is the weather forecast. There will be weather today and tomorrow. Good morning.”

That's a fat lot of use, isn't it? The same applies to, “the climate's changed before”. It's a useless statement. Why? Because it omits details. It doesn't tell you what happened.

Climate has indeed changed in the past with various impacts depending on the speed and type of that change. Such results have included everything from slow changes to ecosystems over millions of years - through to sudden mass-extinctions. Rapid climate change, of the type we're causing through our enormous carbon dioxide emissions, falls into the very dangerous camp. That's because the faster the change, the harder it is for nature to cope. We are part of nature so if it goes down, it takes us with it.

So anyone who dismissively tells you, “the climate has always changed”, either does not know what they are talking about or they are deliberately trying to mislead you.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further Details

Past changes in climate, for which hard evidence is preserved throughout the geological record, have had a number of drivers usually acting in combination. Plate tectonics and volcanism, perturbations in Earth's slow carbon cycle and cyclic changes in Earth's orbit have all played their part. The orbital changes, described by the Milankovitch Cycles, are sufficient to initiate the flips from glacials (when ice-sheets spread over much of Northern Europe and the North American continent) to interglacials (conditions like the past few thousand years) and back  – but only with assistance from other climate feedbacks.

The key driver that forces the climate from Hothouse to Icehouse and back is instead the slow carbon cycle. The slow carbon cycle can be regarded as Earth's thermostat. It involves the movement of carbon between vast geological reservoirs and Earth's atmosphere. Reservoirs include the fossil fuels (coal/oil/gas) and limestone (made up of calcium carbonate). They can store the carbon safely over tens of millions of years or more. But such storage systems can be disturbed.

Carbon can be released from such geological reservoirs by a variety of processes. If rocks are uplifted to form mountain ranges, erosion occurs and the rocks are broken down. Metamorphism – changes inflicted on rocks due to high temperatures and pressures – causes some minerals to chemically break down. New minerals are formed but the carbon may be released. Plate tectonic movements are also associated with volcanism that releases carbon from deep inside Earth's mantle. Today it is estimated by the U.S. Geological Survey that the world's volcanoes release between 180 and 440 million tonnes of carbon dioxide per year - as opposed to the ~35 billion tonnes we release.

Epic carbon releases in the geological past

An extreme carbon-releasing mechanism can occur when magma invades a sedimentary basin containing extensive deposits of fossil fuels. Fortunately, this is an infrequent phenomenon. But it has nevertheless happened at times, including an episode 250 million years ago at the end of the Permian Period. In what is now known as Siberia, a vast volcanic plumbing-system became established, within a large sedimentary basin. Strata spanning hundreds of millions of years filled that basin, including many large coal, oil, gas and salt deposits. The copious rising magma encountered these deposits and quite literally cooked them (fig. 1).

Fig. 1: schematic cross section though just a part of the Siberian Traps Large Igneous Province, showing what science has determined was going on back then, at the end of the Permian Period.

Now laden with a heavy payload of gases, boiled out of the fossil fuel deposits, some of the magma carried on up to the surface to be erupted on a massive scale. The eruptions – volcanism on a scale Mankind has never witnessed - produced lavas that cover an area hundreds of kilometres across. Known as the Siberian Traps, because of the distinctive stepped landforms produced by the multiple flows, it has been calculated that the eruptions produced at least three million cubic kilometres of volcanic products. Just for a moment think of Mount St Helens and its cataclysmic May 1980 eruption, captured on film. How many cubic kilometres with that one? Less than ten.

Recently, geologists working in this part of Siberia have found and documented numerous masses of part-combusted coal entrapped in the lavas (Elkins-Tanton et al. 2020; fig. 2). In the same district are abundant mineral deposits formed in large pipes of shattered rock as the boiling waters and gases were driven upwards by the heat from the magma.

Fig. 2: an end-Permian smoking gun? One of countless masses of part-combusted coal enclosed by basalt of the Siberian Traps. Photo: Scott Simper, courtesy of Lindy Elkins-Tanton.

It has been calculated that as a consequence of the Siberian Traps eruptions, between ten trillion and one hundred trillion tons of carbon dioxide were released to the atmosphere over just a few tens of thousands of years. The estimated CO2 emission-rate ranges between 500 and 5000 billion tonnes per century. Pollution from the Siberian Traps eruptions caused rapid global warming and the greatest mass-extinction in the fossil record (Burgess et al, 2017). There are multiple lines of hard geological evidence to support that statement.

We simply break into those ancient carbon reservoirs via opencast or underground mines and oil/gas wells. Through such infrastructure, the ancient carbon is extracted and burned. At what rate? Our current carbon dioxide emissions are not dissimilar to the estimated range for the Siberian Traps eruptions, at more than 3,000 billion tons per century. The warning could not be more clear. Those telling you the climate's changed before are omitting the critical bit – the details. And when you look at the details, it's not always a pretty sight.

Last updated on 14 February 2023 by John Mason. View Archives

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Further reading

RealClimate article published by Prof. Stefan Rahmstorf on July 20, 2017:

The climate has always changed. What do you conclude?


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Comments 726 to 750 out of 901:

  1. Thanks MA Rodger @ 724

    The climage deneir was the one who made this statment: And as you know, nature's impact on climate can and has been EXTREME prior to man, and man's industrialization. How do you account for that?

    In my orgional post I had it indented so others would know this was not my statment.

  2. Thanks again MA Rodger @ 724

    The deniers worship the US climate denier blog sites such as WUWT. It's ridiculous what that site puts out. The US also has the Heartland Institute whose mission is to instill doubt in American minds about the science of climate change than the Heartland Institute.

    This is a tyical US denier rant:

    "Hot places get hotter in the summer. Weather is not climate, but climate changes naturally due to the Sun, and other factors. If you want to live like a Stone Age Stooge, plus pay exorbitant prices for energy and ALL products and services, be my guest, but don't force others to follow your cult like religion to make yourself feel relevant in the Universe."

  3. Hi again,

    I'm not sure where to post this but I think this might be the correct thread since it deals with past changing climate.

    I'm dealing with deniers who are questioning what caused the end of the last ice age and the melting of the Laurentide ice sheet?

    I came across this 2015 article with respect to the Laurentide ice sheet and wanted to know if there are other studies you can point me to.

    However with respect to the Younger Dryas I came across this: The Younger Dryas.

    For some reason the deniers seem to think these two events somehow "prove" that human caused global warming is a hoax generated to "take our money". 

    However these two events are not relevent to our current climate situation because we are not at the end of a glacial period.  

  4. TVC15 @728,
    This is probably not the answer you were expecting.
    You're asking about three seperate things, two of which are far from straightforward - ☻ The trigger that ended the last ice age, ☻ The melting of the Laurentide ice sheet and ☻ The Younger Dryas episode - and these  linked to some denialist argument which is not so evident.
    Thinking about the linkage to denialist argument:-
    Is it that it shows climate can change rapidly without humans emitting CO2 (but which does require millions of cubic kilometres of strategically-placed ice that would be difficult to miss)? Is it that we do not know exactly what happened 20k & 13k ago so how can we be sure about today (when we do see exactly what is happening today, or exactly-enough)?
    I have to say I cannot see how any denialist argument would begin to stand up.

    So what triggers a de-glaciation. Milancovitch cycles of course. But can you see these triggers. (Image from here.)
    Milankovitch cycle graph
    The triggers aren't so obvious as the ice-age needs to be primed as well as triggered. Ice-ages used to occur every 40ky but over the last 1,000ky they last 100ky. (I think there's been some progress towards answering why - possibly more exposed bedrock.)
    And within ice-ages there are other events that still come with significant questions - Dansgaard–Oeschger events, Heinrich events and the Bølling-Allerød warming that immediately preceeded the Younger Dryas.

    While you asked for some reading, let's start with a 32-second video, the melt-out of the Laurentide ice-sheet.
    Note in the video the big lakes that build in the millennia before 13kybp. The initial take on the Younger Dryas was that it was the sudden draining of these lakes that caused the AMOC to collapse & precipitate the Yonger Dryas. But evidence for such an outflow remains elusive (the Wikithing references to a Mackenzie River outflow are Murton et al 2010 & Keigwin et al 2018), but some research suggests it has to be more complex than that.

    And in all that, there is the wonderous Impact Hypothesis (& apparently others according to Wikithing Younger Dryas page). So it all gets a bit heated at times, but probably not enough to melt out an ice sheet.

  5. Hi MA Rodger @ 729

    I truly appreciate that response! I'm learning so much from your responses!

    However I don't know what you mean by this: 

    (but which does require millions of cubic kilometres of strategically-placed ice that would be difficult to miss)?

    Thank you!

  6. TVC15 @730 , he was referring to the vast amount of ice in the Laurentide ice sheet and the subsequent formation & draining of Lake Agassiz (the outflow of cold water, thought to be the main triggering of the Younger Dryas event ~ i.e. that brief hiccup during the initial warming-up phase of the Holocene).

  7. TVC15 @730,

    I wrote @729 "climate can change rapidly without humans emitting CO2" but added that this "does require millions of cubic kilometres of strategically-placed ice that would be difficult to miss". I was thinking a little more broadly than suggested by Eclectic @731. In very simple terms, rapid bits of climate change results from obvious causes.

    A big volcano (like Mt Toba c73ky bp) or an asteroid strike fall into that category but the climate quickly reverts back afterwards. Big ice sheets can cause rapid change which lasts far longer. I had in mind two different ice-induced phenomenon, a big one and a rapid one although properly I was only thinking of the "rapid" one.

    By the "big" one, I mean the ice-age cycle itself which swings global average temperatures by perhaps 6ºC but takes millennia to achieve this (not very rapid) as it requires the melting of millions of cubic kilometres of ice (43 million in the last deglaciation). The major factor in the swing is the change in albedo due to the growing/shrinking ice sheets. CO2 as a factor is smaller, and the result of what Ganopolski1 & Brovki (2017) [PDF] call a complex "stew" of many mechanisms which don't all work to increase the ice-age effect.
    So for ice-age cycles to happen, we do require tens-of-milions of cu kms of ice to melt/freeze on top of the correct bits of land.

    Significant & "rapid" climate change (at least on a regional scale) can be seen in the Younger Dryas and in Dansgaard–Oeschger events. While there is some remaining cotroversy with the Younger Dryas (so let's not go there), it appears reasonably uncontested that the Dansgaard–Oeschger events result from the AMOC suddenly switching back on having been previously slowly strangled by big unstable ice sheets melting/discharging icebergs. The AMOC-forced-by  the ice melt/discharge switching on & off messes up regional climate and produce the big and rapid changes in regional temperature, Greenland ice cores recording a number of regional swings of +5°C in less than half a century. (Note that wIth polar amplification, you'd expect "humans emitting CO2" under BAU to manage a similar-sized swing.) But when the ice-age melts away & "without humans emitting CO2", there is little ice to mess with the AMOC during the less-dramatic Bond events which have little impact on even local temperature.

  8. it should also be noted that D-O and Bond type events are observed when emerging from an ice-age, not during interglacials.

  9. Much appreciated MA Rodger, Electric and Scaddenp!


  10. Can you guys help me to understand how to respond to these types of claims?


    CO2 levels were 24,000 ppm CO2 for nearly 3 Billion years and there was no runaway greenhouse effect.

    CO2 levels were 8,000 ppm CO2 for several Million years and there was no runaway greenhouse effect.

    CO2 levels were 2,000 ppm CO2 for several Million years and there was no runaway greenhouse effect.

    So, why would 1,200 ppm CO2 over a few centuries cause a runaway greenhouse effect?

    It wouldn't. It's just fear-mongering alarmism by a man who makes his money proffering this nonsense.




    [PS] Just a little bit effort with the search button would find the answers to most of these as would a read of the IPCC WG1.

  11. TVC @735 , there's no scientific study [to my knowledge] supporting "Runaway" greenhouse effect being possible on Earth.   I think those friends of yours are suffering from a fantasy life of runaway strawman arguments.

    Perhap they misunderstood something they heard somewhere. 

  12. TVC15 @735,

    I think I would respond to such a silly comment by asking for the name of this man they are talking about, this because such knowledge may assist in sorting out why they are asking such silly questions.

    But a more direct approach, but more involved could be:-

    There is perhaps a philisophical aspect to the "runaway greenhouse effect." CO2 emissions/CO2 levels that are directly due to man's actions will result in an elevated global temperature and this will result in a further CO2 emissions that are NOT directly due to mankind, this extra CO2 resulting in yet further warming.

    So when would that "further warming" be considered as "runaway"?

    If human emissions totalled 4,000 Gt(C) before we stopped, which is eight-times what we've done so far, that would increase global temperature by perhaps +6ºC which would cause natural emissions of let's say another 4,000Gt(C). These "feedback" natural emissions from a 1,200ppm CO2 world would cause further warming, resulting a total of say +9ºC in a 2,400ppm CO2 world. And then the warming would stop. So is +6ºC with +3ºC of that feedback, is that "runaway"?

    Consider if the physics were such that it didn't stop there, that creating CO2 levels of 1,200ppm would result in say 40,000Gt(C) extra CO2 in the atmosphere - roughly 20,000ppm - which is all the carbon in the oceans & soils (but the rocks would still contain the bulk of the planet's carbon), the temperature would ratchet up to who-knows what temperature and all would see this as runaway warming.

    But at some level it would stop. Any runaway system will eventually stop. Always it will stop somewhere.

    The important thing is whether the runaway effect is so significant that it presents a "wheels-fallen-off" situation. Back in the days of the Hadean or Archean, there may well have been far more than 24,000ppm CO2. Such levels are argued because of the faint young sun paradox. But the Hadean earth was not back-then a "wheels-fallen-off" situation because there was no humans requiring a climate compatable with their needs, while a return to the Hadean climate today would obviously be a "wheels-fallen-off" situation.

    But the physics isn't like that. While a directly-human-caused 1,200ppm CO2 world would result in an additional CO2 boost from warmer oceans & Arctic, any resulting additional temperature rise will be limited so a result like Venus or the Hadean is an impossibility. But that additional CO2 boost will be big enough to make what is an already-very-very-difficult situation for humanity very-very much worse. I would suggest that the increase in suffering from that additional CO2 boost would be enough for some to call it a "runaway" situation.

    But some may disagree. Boosting a warming of +6ºC up to +9ºC perhaps would not constitute "runaway" if human civilisation will have been ajudged to have already suffered that "wheels-fallen-off" situation without the additional natural feedbacks.

    I don't know if that is helpful in the response set out @735.

  13. Agreed with MA Rodger.

    No Venus-syndrome for the Earth:

    "With the more realistic physics in the Russell model the runaway water vapor feedback that exists with idealized concepts does not occur. However, the high climate sensitivity has implications for the habitability of the planet, should all fossil fuels actually be burned.

    Furthermore, we show that the calculated climate sensitivity is consistent with global temperature and CO2 amounts that are estimated to have existed at earlier times in Earth's history when the planet was ice-free.

    One implication is that if we should "succeed" in digging up and burning all fossil fuels, some parts of the planet would become literally uninhabitable, with some time in the year having wet bulb temperature exceeding 35°C.

    At such temperatures, for reasons of physiology and physics, humans cannot survive, because even under ideal conditions of rest and ventilation, it is physically impossible for the environment to carry away the 100 W of metabolic heat that a human body generates when it is at rest. Thus even a person lying quietly naked in hurricane force winds would be unable to survive.

    Temperatures even several degrees below this extreme limit would be sufficient to make a region practically uninhabitable for living and working.

    The picture that emerges for Earth sometime in the distant future, if we should dig up and burn every fossil fuel, is thus consistent with that depicted in "Storms" — an ice-free Antarctica and a desolate planet without human inhabitants"

    So no runaway. But Hansen notes that it won't take a runaway to basically completely eradicate civilization as we know it.  Supported by this:

    "While dominated by anthropogenic forcing in these recent times, solar variability in prior eras caused much larger relative influences.

    The early Sun was approximately 70% as bright as at the present when it joined the main sequence about 4.6 billion years ago with a current rate of increase in luminosity of 0.009% per million year (Hecht 1994). At this rate, it will take 10 million years for the background solar brightness to increase by the 0.1% typical of a solar-cycle variation, and another 3.5 billion years for heating from the Sun to create Earth-surface conditions similar to those of the present-day Venus; although additional effects, such as feedback from enhanced ocean evaporation, may accelerate this warming and make the Earth uninhabitable (at least to present-day complex lifeforms) in about one-billion years."

  14. If CO2 caused the temperature to change, (and is resposible for the climate historically) what caused the CO2 to change? Volcanoes? Is there any geological evidence to support this theory?

    Why do the ice-ages and deglacial period occur cyclically, approximately every 100,000 years? Is there some geological pattern identified on earth that would explain cyclical volcanic activity and CO22 emission?

    N.B. The ice-ages coincide with one of the Milankovitch cycles - the eccentricity of the earth's orbit, which goes through cycles of approx. 100,000 years. If the earth periodically gets further away from or closer to a hot object, this should have some influence on the earth's temperature, correct? 


    [PS] See here for more on milankovich. In the iceage cycle CO2 is a feedback from an initial albedo-driven driver. Water vapour, albedo and the very slow CO2 feedbacks convert a small change at 65N into a global effect.

  15. Thank you to everyone who responed!

    737@ MA Rodger the man they are talking about is Jim Hansen.

  16. TVC15 @740,

    If it is Jim Hansen, we can track down where he stands on this runaway matter. He said in his book 'Storms of My Grandchildren':-

    “After the ice has gone, would the Earth proceed to the Venus syndrome, a runaway greenhouse effect that would destroy all life on the planet, perhaps permanently? While that is difficult to say based on present information, I’ve come to conclude that if we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty”

    This paragraph (I see from Google Books) is a bit of a throw-away at the end of Chapter 10 following a discussion of a boost to AGW from methane hydrates:-

    "If we burn all the fossil fuels, the ice sheets almost surely will melt entirely, ... Methane hydrates are likely to be more extensive and vulnerable now than they were in the early Cenozoic. It is difficult to imagine how the methane hydrates could survive, once the ocean has ahd time to warm. In that event a PTEM-like warming could be added on top of the fossil fuel warming."
    And later, in a Columbia Uni blog page, Hansen explains that he is talking of two things - ☻ The Venus Syndrome runaway (as per the top quote) and ☻ A "mini-runaway" or hyperthermal (as per the lower quote).
    The mini-runaway he decribes as occuring on a millennial scale and which, if we were foolish enough to initiate such an event, could be countered by sequestrating the GHG releases, although there would be a lot to sequester.
    The Venus Syndrome runaway he describes as possibly following on from an un-countered mini-runaway but he ends saying that "Venus-like conditions in the sense of 90 bar surface pressure and surface temperature of several hundred degrees are only plausible on billion-year time scales."

    So this leads to an answer to the question posed by your denialist trolls - Why would 1,200ppm CO2 (Hansen talks of 1,000ppm to 2,000ppm) lead to the Venus Syndrome when in the past it did not?
    Hansen is saying that the 'early Cenozoic' (so before 20 million yr bp) had less methane hydrate which could potentially boost temperatures enough to overwhelm the tropopause and thus allow the leakage of the planet's hydrogen into space, a road that would lead to a Venus-style climate. But importantly, this is on top of AGW. Back in the PETM (44my bp) the fossil fuels were safely tucked away within the geology greatly limiting the potential warming.
    As for previous CO2 levels, there was periods in the past when the fossil fuels of today were still yet to be buried out of harms way & thus CO2 was far higher than 1,200ppm. But that was a long time ago when the sun was a lot fainter and the combined forcing (see this SkS page) would thus be lower than potentially from a mini-runaway today.
  17. jesscars @739,

    The variation of CO2 levels in very ancient times generally resulted from a balance between the amount of carbon being drawn down into the geology by rock-forming and the amount of carbon being ejected by volcanoes. Periods of mountain-building have an impact on that balance, as do periods of extreme volcanism. The results of modelling of very ancient CO2 levels (using GEOCARB III) have been pretty-much supported by the geological evidence.

    The mechanisms that are at work at a more detailed level can be much more complex. Thus the last de-glaciation saw a rise in CO2 levels but that was the product of many different mechanisms, many of which didn't actually work to increase CO2 levels. Thus warming oceans increase CO2 levels but the significant increase in ocean volume as the ice melts into the oceans decreases it. Peat exposed to clmate change releases CO2 while increased bio-activity buries it. (See Ganopolski & Brovkin (2017) for a study of these mechanisms.)

    You also ask specifically about the ice-age cycles. These have been the major feature of global climate for the last 3 million years and until about 1 million years ago they occurred every 40,000 years but now occur every 100,000 years. It is probably best to see ice-ages as being caused by the unstable nature of the full glacial climate. When triggered by the Milankovitch cycles heating the high northern latitudes, an interglacial will result from northern ice sheets melting out. Thus today, if the melt on Greenland were to become enough to drop the summit significantly (a likely event if AGW reached +1.5ºC for a few centuries), the lowered icy-cold top would warm enough to allow more melting and lower it further. Building it back up with new snowfall is a far slower process, so without the return of an ice-age, once ice sheets like the Greenland one begin to go, they go all the way.

    The reason for the change from 40ky to 100ky ice-age timing is not truly understood. The trigger is the Milancovitch cycle, the 40ky from the tilt in axis & the 100ky from a component of the eccentricity variation. One theory is that dust (which increases the sunlight absorbed by ice) was greater in past 40yr ice-ages but now the soils that created that level of dust have been scoured away leaving un-dusty bedrock.

  18. @ 737 MA Rodger

    Dear MA Rodger,

    I deeply appreciate all your knowledgeable responses to my posts with respect to the deniers I deal with! I have learned so much from you!
    With respect to that denier who loves to misrepresent Jim Hansen he also loves to quote mine him and this is exactly what he posts about Jim.  

    I don't understand the mindset of folks who behave in this disingenuous manner.

    James Hansen is the Grand Imperial Kleagle Wizard of Global Warming and he had this to say:

    In his book Storms of my Grandchildren, noted climate scientist James Hansen issued the following warning: "[i]f we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty."

  19. TVC15 @743,

    Hansen's book 'Storms of my Grandchildren' is not a scientific work and I have been critical of it for not being scientifc while making overly-bold statements on Sea Level Rise, statements which others take-&-use as being scientific statements. But with both SLR and this Venus Syndrome issue Hansen has made good by later publishing the science. With respect to the Venus Syndrome issue, he references in the Colombia Uni blog (linked @741) the forth-coming paper Hansen et al (2013) 'Climate sensitivity, sea level and atmospheric carbon dioxide' and in particular Fig 7 (below) which shows that above 16 x 310ppm (5,000ppm) the tropopause disappears (the atmosphere stops getting warmer through the stratosphere) which would see the Earth's water begin to leach out into space.

    Hansen et al 2013 Fig 7

    I wouldn't be sure whether burning all the fossil fuel reserves and then precipitating CO2 emissions from the biosphere etc would manage to achieve 5,000ppm CO2 but it is all rather academic. The damage to humanity, indeed to life on Earth would be unconscionable a long way before 5,000ppm CO2 is reached.

  20. LOL the denier is back with his silly statments.

    The entire global warming hypothesis is nonsense.

    If there was any truth to it, then Inter-Glacial Periods would never end.

    The CO2 that supposedly accumulates and causes temperatures to rise ending a Glacial Period should continue to cause temperatures to rise, except it doesn't.

    No matter how much CO2 accumulates, you always end up right back in a Glacial Period.

    Is there any truth to anything this denier is stating?

  21. Deniers favourite tactics are strawman and cherry pick. In case, a strawman. Just because your denier doesnt understand the science of how glacial and interglacial feedback cycles work, doesnt mean that scientists dont either. Insist that your denier quotes the actual science that he is supposedly refuting. The missing link here is suppression of natural methane and CO2 emissions as land (especially eurasian wetland) becomes frozen; and importantly, the increased solution of CO2 in oceans as they cool. Of course, scientist do the hard yards of measurement, modelling (check numbers work), and cross-checking, whereas denier are only interested in hand-wavy dismissal.

  22. TVC15 - if are you continuing to engage with deniers, then please, please take the time to read the IPCC WG1 report so you have a grounding in what the science says. At very least, read SPM.

  23. Thank you Scaddenp!  I will read it! 

  24. TV- he's using flawed logic from word one... your denier pal is not entertaining complexity and (supposedly, lol) thinks the world works in straight lines. Just a rich idiot who does know better because when it comes to money all of a sudden he understands there are multiple factors at play...

  25. TVC15 @745,

    Your denier is of course spouting nonsense. But perhaps it would be helpful to know how to respond to him other than just asking him to generally explain his nonsense.

    I would ask your denialist troll how long he expects the effects of AGW to last. Does he expect CO2 levels to remain for ever? That would be very wrong. Or perhaps only a few tens-of-thousands of years which would be more correct? That, of course, is the time-scale that ice-ages operate on.
    Ice ages are, of course, mainly driven by changing albedo (due to the changing levels of ice reflecting changing amounts of sunlight back into space). CO2 is not the primary driver.

    For a bit more background, we can look back at those ice-age CO2 levels.

    During the last interglacial (the Eemian) the measured peak-CO2 was 287ppm back 128,400 years before present. (This is from EPIC Dome C ice core data.) From this peak, CO2 dropped to 262ppm in the following 1,240 year, a drop which was the first part of a set of oscillations measured between 280ppm and 260ppm that continued for 15,000 years after the peak. It was only following those oscillations that CO2 began to fall back towards 200ppm, the bulk of this decline (a drop to 230ppm) taking 7,500 years.

    We can compare the drop from that ice-age driven CO2 pertubation with the expected future of our own CO2 anthropogenic pertubation. That ice-age pertubation was (287 - 195 =) +102ppm over 8,000 years while out anthropogeinc pertubation is so-far (410 - 280 =) +130ppm over roughly 100 years.
    The likes of Archer et al (2009) 'Atmospheric Lifetime of Fossil Fuel Carbon Dioxide' suggest that roughly 80% of an instant CO2 pertubation would be absorbed into the oceans in roughly 1,000 years. (Lord et al 2016 Fig 4 suggests it would be a little higher for our present 600Gt(C) level of emissions, perhaps 87%.) About 55% of our anthropogenic pertubation has already been absorbed so if our CO2 emissions stopped we would expect today's CO2 levels to drop roughly 70ppm over 1,000 years or so, being absorbed mainly within the oceans. But the rise of the ice-age pertubation of the Eemian was far slower than our pertubation (8,00y against 100y) so we can simplistically assume that all the +102ppm represents that remainng 20% of the actual ice-aged-forced emissions. (In reality, much of the CO2 in the ice-age pertubation has been driven from the oceans so will not be re-absorbed there over such timescales.)

    That remaining 20% (& bulk of the Eemian +105ppm) is expected would slowly be absorbed over following millennia, but surely not as quickly that 7,500 year Eemian period which saw perhaps a 10% drop (of the assumed total ice-age pertubation). This would concur with the proposed reversal of much of the pre-Eemian ice-age driven CO2 increase as the new ice-age develops, when the oceans begin to re-absorb CO2, along with a whole lot of other mechanisms that operate on CO2 through the ice-age cycles.

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