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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 476 to 500 out of 893:

  1. there are many learned fellows on this site much more educated about this issue than a layman like me.. But there are some basic issues that I have an "issue" with (and let's face it unless laymen like me are convinced either way there won't be any action anywhere. It's great to see all the point scoring and technical data etc.. But (I am in the skeptical camp for now).. There are some irrefutable facts .. Things are heating up.. And.. It's happened many times before.  There are also a lot of emotional stuff being thrown around.  

    Things have heated before.. scary camp say "yes but this time it's much faster" there is absolutely no way to prove this as we are only talking about less than a 200 years.. And many many times we can see shifts from hot to cold millions perhaps billions of years ago.. But no way we can prove the speed of that heating or cooling within only 100 years or so.  Perhaps it is a rapid increase that then wane off .. A natural occurrence .. Can we really do something about it.. Unlikely.. Even if we could .. How? We can't stop the entire world from its day to day use in the next 10 years .. Let alone in the next 100.. So it is what it is.  Between humans, volcanoes.. Farting sheep and cows,  Position to the sun and how many plants grow at any given time on the planet.. It's probably fair to say whatever is about to happen .. Will happen anyway.. 

    even IF.. Humans were totally responsible (which simply .. We aren't) there is no way.. No way at all .. ever.. You will be able to stop 8 billion people doing whatever they are doing.

    so let's agree it's happening.. It's happened before slower or faster will still happen .. Wether due to us or nature and universe or all of the above .. we can't stop it no matter what! Let's just ride it out the best we can.


    [Rob P] - See the No.1 climate myth on Skeptical Science - Climate's changed before.

  2. RoamerNZ, if, as you say, you are a layperson, why would you trust your own interpretation of the facts more than you'd trust the interpretations of those who spend their lives reading and performing climate science?
    Why would you assume that your commentary contains insight that they haven't examined themselves?
    When you say 'there is absolutely no way to prove this', are you informing climate scientists of something? Have you read, and understood, many paleoclimatology papers?
    Why do you think we don't understand what causes the planet to warm and cool? It's not just a mysterious variable that wiggles up and down every so often. Climate has physical causes and physicial constraints. Our expectation that the world will keep warming is not because we look at the wiggle and say 'It's gonna keep wiggling up', it's because, physically speaking, it is actually impossible for the world to not keep warming, given current conditions.

  3. RoamerNZ,

    You have forgotten that as little as 5 years ago the deniers consistently said that it was not warming.  It is only when warming became obvious to everyone that they have  shifted their argument to "it has warmed before".

    In addition to Tristans points, you need to consider that the currrent warming was predicted 120 years ago by Arhennius.  How could this rapid warming be natural when it was predicted decades in advance?  The globe was actually cooling when Arhennius predicted warming and correctly estimated the amount of warming we would get.  Try to inform yourself about the data.  When you do not know what the facts are it is easy to be confused.

  4. Dear Roamernz,

    Have you ever heard of joining issues? It means you are dealing with a complex system you are defeated by !! Luckily there are other people to take care of you!!!

  5. Tristan .. I did not say they were my interpretations.. It is an opinion formed after listening to climatologist. Yes some are of a different persuasion than the one you proclaim., And they are compelling.

    Each of us must form our own opinion there is no "carte Blanche" to be filled with your version.

    i also did did not say anything about you or any one else not knowing why.the heating is taking place.  What I am not buying into is that we are solely responsible especially in the light that this happens over and over on this planet long before the blink of an eyelid that we have been here.

    i would also never presume or assume to give insight to any true scientist.. I was merely expressing an opinion.. Based on what I believe to be plausible and for you this is a problem as I am not convinced otherwise.

    Because of the wiggle.. We are heating up.. But you seem to say because of the current conditions the planet cannot do anything but.. I presume by conditions you mean human activity.

    And you can categorically guarantee that if all human co2 emissions stopped today .. The heating up would stop permanently? I find that impossible to believe knowing what we know of earth and her idiosyncratic movements. It's akin to saying if humans stop moving around ..the magnetic polar shift won't happen again.

    Michael.. Sure, deniers at the time did not see the warming as alarmingly large.. Since they now have irrefutable evidence it is heating.. The question then came.. Why?  And this is where argument is. (Btw the deniers aren't necessarily the same people). And what is wrong with the argument .. It's happened before? .. It has.. Many times!

    Yes predictions for global warming happened 120 years ago.. But for every yay sayer there is an opposite.. In 1837 louis agazzi presented the opinion the planet had cooled and then warmed only to cool again with a new ice age on the horizon.  (In fact earth cooled a whopping 4 degrees from 1450 to 1850 in fact earth has warmed up to 4 degrees 4 times in the last 450 thousand years a fairly regular cycle at that of about 100 thousand years.. We enter the fifth cycle now) .. This concept was met by skepticism only to later be accepted. Many papers have been produced predicting severe cooling 2042 onwards one such scientist is russian astrophysics habibullo abussamatov

    so you see some one is predicting cooling when it is now warming.. Opposite to your point. But as bozza would say.. These are joining issues and the warming is the precursor for cooling even the possibility of a "snowball" earth.. as it once was and may happen again Have you informed yourself about this data?

    the earth is heating and will cool again.. But it has nothing to do with our activities.. It will do it regardless as it has for billions of years. It's all to do with Jupiter apparently

    Im glad you'll be looking after me bozza .. very comforting :)


    [JH] Personal opinion carries very little weight on this site. Please provide appropriate scientific documentation of your assertions in your future posts. 

    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.

  6. Roamernz,

    You seem to be trying to advance the idea that people seeking to determine who to believe regarding the likely impacts of antrhopogenic influences on the atmosphere just cannot figure it out because a) climate changes on various timescales for varioius reasons, and b) one can find experts and supposed experts that disagree on causes and whether warming or cooling is more likely. 

    This information above addresses well a).  On b), simply put, you simply need to have apropriate descretion to weed out quacks.  Wikipedia nots that Abdussamatov, who you apparantly give as an example of why we just can't determine whether we will warm or cool in the future,"holds that Earth's atmosphere does not produce a greenhouse effect." 

  7. by 'nformation above', I meant this Sks page

  8. RoamerNZ, I find your post rather offensive. You confessed earlier having limited knowledge and understanding of the subject at hand, yet you address pretty much all contributors here with such thing as "have you informed yourself about this data?" without specifying what such data is or providing any useable reference. People participating to this forum are usually very well informed about climate science. Many of the contributors/moderators here have published papers in reputable journals. Since you are, by your own admission, the one rather less informed about the whole field, your condescending tone is unwarranted.

    References are needed for many of your assertions, especially the ones that are more specific or mention numbers:

    "But for every yay sayer there is an opposite." The various consensus studies so far indicate that, in fact, the "opposites" are outnumbered by approximately 97 to 3. There are more appropriate threads to discuss this, if you want to provide credible evidence to the contrary.

    This part here: "In fact earth cooled a whopping 4 degrees from 1450 to 1850 in fact earth has warmed up to 4 degrees 4 times in the last 450 thousand years a fairly regular cycle at that of about 100 thousand years.. We enter the fifth cycle now" That requires some serious backing by real scientific work, the kind published in well established science journals. I'm waiting to see it before taking it seriously. If it is an attempt to refer to the alternance of glacial/interglacial periods, the subject is well studied, consistent with the consensus model of Earth climate, and you should go to the appropriate threads.

    "It's all to do with Jupiter apparently." This was one the very first pieces of nonsense that I personally debunked on SkS during the early years of the site. Some cited also Mars, and even Pluto, which took us to the height of stupidity. It is complete nonsense. But by all means, cite scientific evidence on the proper thread if you have any.

    On SkS, you don't get to make up stuff or equate opinion with evidence. The preponderance of scientific evidence is what is considered. It takes some work to determine in which direction that points, but it's worth doing that work, because that is most likely where reality resides. If you're unwilling to do that work, you essentially give up and rationalize your choice of opinion with unfounded ideas of competing theories and competing experts of equal value, which couldn't be farther from reality. Not everything is a matter of opinion, especially in the physical world.

  9. This explains the climate changes in our county, i remember in my childhood weather was very predictable in seasons but nowadays this has changed completely.



  10. Roamernz - Previous causes are not relevant because they are not the cause today.

    Yes it has happened before. When it has happened slowly, life adjusted. When it has happened quickly, it was an extinction event.

    Now it is happening quickly yet there are no natural causes that anyone can find that are emitting CO2 above their historical normal levels. There are no Deccan Traps being created. There are no Siberian Traps to be found.

    Volcanoes continue to emit around 1% of the CO2 that human are currently emitting via the burning of fossil fuels.

    No increase in solar radiation has been detected, the solar cycle is sticking to its regular pattern. Cosmic ray collisions in the upper atmosphere have not increased.

  11. Climate change?In human terms and gong by conditions on this planet , our place is getting warmer and drier .Species are disapperaring by the day and very quickly going extinct as a result.While some humans will make it through desert conditions , the majority will disappear. It may have to take the genius of a genetic engineer to produce our successor . Humans are pretty smart and some will live to tell the tale. Scientifically, we are facing unprecidented and some  man made microcosmic changes in an already hostile  environment .Up to  now it is not obvious how much time we have for life on earth .We need to consult on how much time the energy or plasma  from the sun will last , how many magnetic cycles it will take earth before the atmosphere starts to dissipate like happened on mars, what our chances are for a second life  assuming we can beat cosmic radiation  effects. Anyway we need to deal with many factors just looking at climate change .Details are welcome. 

  12. Andreya @486, with AGW, conditions on planet Earth are on average getting wetter, not drier.  This is complicated by the fact that the Hadley Cells are extending to higher latitudes resulting in the band of dry descending air at the zone between the Hadle and Ferrel Cells to also move to higher latitudes, drying those areas previously under the Ferrel Cell but now under that zone of dry descending air.

    It is further complicated by greater evaporation, which means that dry spells will more rapidly result in drought conditions.

    The net effect of this is that agriculture is likely to be disrupted in many locations, but we will not experience anything like universal desert conditions.  In fact, the political disruption caused by uncertain food supplies is likely to be more dangerous to humans than the uncertain food supplies itself.  And those uncertain food supplies are as likely to be from conditions becoming too wet, or too warm for traditional crops as from their becoming too dry (and will mostly come from year to year variation being less predictable making adaption of agriculture very difficult).  Apart from the top end risks of that political disruption (ie, nuclear war), there is no risk of AGW making humans extinct.

    Finally, there is zero risk from AGW of the Earth's atmosphere becoming as thin as that of Mars, or in fact varying noticably from its current thickness.  Risks on that score are only possible from massive repeated planetary bombardment by infalling meteors (so unlikely as to not be meaningfully expressible as a risk factor); or eventually the gradual warming or possibly the expansion of Sun over time (for which it is not a threat for billions of years).

    AGW is a grave risk for which we should immediately take suitable measures of mitigation.  It is not armageddon.

  13. I find this particular image in AR5 WG1, figure 5.03 about climate over the past 800,000 years, a bit scary:

    Its purpose is to demonstrate the correlation between past climates and insolation changes, which is why it shows precession, obliquity and eccentricity. But these orbital factors only kick off each period of heating, it's clear that CO2 does most of the work in raising temperatures, and it is the correlation between CO2 and temperatue in the diagram which is what interests me.

    CO2 has each time risen from about 200 ppm to about 280 ppm, or thereabouts, which is roughly a 40% rise. What I find worrying is that this 40% CO2 rise seems to correlate consistently with a rise in Antarctic temperature of about 10C. That is a large effect. (And it's just 40%; a doubling of CO2 would be even larger.)

    Anthopogenic CO2 is now 40% above pre-industrial levels (280 ppm up to 400 ppm) so does that mean we already have 10C locked in for the polar regions?

  14. Fig 5.03

    Apologies, here's the image correctly linked


  15. Jutland @488, you are correct that orbital forcings only kick of the variation between glacial and interglacial; and also that CO2 forcing is a major driver of the temperature change.  It is, however, not the major driver.  Rather, albedo effects from changes in sea ice extent, growth of ice sheets and reduction in forest cover.  Hansen quantifies the differences in his well known paper, Target Atmospheric CO2, saying:

    "Climate forcing in the LGM equilibrium state due to the slow-feedback ice age surface properties, i.e., increased ice area, different vegetation distribution, and continental shelf exposure, was -3.5 ± 1 W/m2relative to the Holocene. Additional forcing due to reduced amounts of long-lived GHGs (CO2, CH4, N2O), including the indirect effects of CH4 on tropospheric ozone and stratospheric water vapor (fig. S1) was -3 ± 0.5 W/m2. Global forcing due to slight changes in the Earth’s orbit is a negligible fraction of 1 W/m2(fig. S2). The total 6.5 W/m2forcing and global surface temperature change of 5 ± 1°C relative to the Holocene yield an empirical sensitivity ~¾ ± ¼ °C per W/m2forcing, i.e., a Charney sensitivity of 3 ± 1 °C for the 4 W/m2forcing of doubled CO2. This empirical fast-feedback climate sensitivity allows water vapor, clouds, aerosols, sea ice, and all other fast feedbacks that exist in the real world to respond naturally to global climate change."

    6.5 W/m^2 is the equivalent to the forcing of a 3.4-fold increase in CO2 concentration.  In otherwords, we should expect a 10 C increase in Antarctic temperatures with an increase of CO2 from 280 ppmv to 952 ppmv.  That, of course, is just the Equilibrium Climate Sensitivity (ECS) which does not include the impacts of long term feedbacks such as the melting of ice sheets.  Including those will result in higher temperatures, but only after the course of many centuries.  What is more, if we are sensible and end all CO2 emissions, CO2 concentrations will fall significantly so that we will never experience the Earth System response to the peak CO2 levels. 

  16. Tom @490, ah that's helpful, thank you. I had not realised that albedo changes had such substantial historic effects on past climates. I will read Hansen's paper. 

    It is reassuring to know that we need to reach c 950 ppm before Antarctica warms by 10C above pre-industrial, because that's about 183 years away (assuming a linear 3 ppm annual rise in CO2) and human energy use will presumably have moved off fossil fuels sometime well before then. At least I hope so.

  17. Okay, I am a skeptic, but not foolhardy. There is no need to temp "Mother Nature", as that would be indeed very foolish. I follow some simple rules. First and foremost is to "Live simply so others may simply live". Hope that is not too offensive.

    I was in Newport RI, 36 years ago. I walking south along Ochre Point Ave looking for "The Breakers". To my left was a tall hedge row that was so dense that light could not be seen through it. I saw some people in a group not too far down the road. I suspected that maybe this was where I wanted to be. Sure enough, as I approached, I could tell there was a access point. My first view inside was to the right in a southerly direction and all I saw was vast open grassy field. I was not until I reached a point past the left of the entrance did the building to the left come into view. My first gut reaction was that I became nauseated. To this day, segments on TV, such as "Living Large", disgust me and I do not watch them. There is no reason that this "dwelling" should have existed as a home of a single family. For the same reason, I do not see why any CEO should make 1000 times more than his highest paid employee. Just a few years ago, I recall that a CEO received a total compensation equal to almost 3,700 times that of the companies managers and that year the business not only lost money, it would have gone bankrupt without a bailout. But, I digress.

    I have had hot air solar panels on my home since the time of Carter. I do not own a lawn mower, but when I did, it was a single "man" powered hand reel. I have since a abandoned the idea of have a lawn all together. When I had a lawn, all I can tell you is that it was "green", as I never applied fertilizer or weed killer and God watered it. I walk to Church on Sundays, every Sunday no matter what the weather. I only use the A/C when I have to, usually when we have company. I have replaced almost all of my incandescent light with CFL and I an now in the process of replacing the CFL with LED lights and am now using more than 90% less power than I did with the incandescent bulbs. We never waste food-ever. We walk whenever practical. I gave up golf long ago, because every round of golf requires 1,200 gallons of water. Water is second only to air when it comes to survival. Speaking of air, while college in 1967, I circulated a petition to have smoking banned from the cafeteria area, which if adopted, would have made the building smoke free, as the other area was the student lounge, which was smoke free. (My petition failed even though I had substantial support). That is my story. Think globally and act locally for the good of the planet. It is my guess that Earth will be my only home. Apparently, not everyone thinks the same as I do. If everyone is so sure the current spike in Global temperature is a direct result of human activity and for all intent it is "settled science", then why are we not doing something.

    The US imports or consumes an extra 350 million barrels of oil to produce an extra 6.5 billion gallons of gasoline that when burned produces 61 million metric tons of CO2. And how is this 6.5 billion gallons of fuel consumed here is the great wasteful US of A. IDLING!

    My proposals. #1. Idling will not be tolerated ever. Idling laws will be rigidly enforced and fines for idling will be so draconian that no one would even considering idling their vehicle. 2. Idler lose licenses. Period. 3. Remote starters totally illegal. 4. End of all drive through banking except for those who are disabled. 5. End of all fast food drive and all other drive through services.(accept for the disabled) You want to eat at Taco Bell, Burger King or McDonalds, etc., fine, park your car and walk inside.

    Proposal number two: Eliminate all organic garbage from landfills. Organic garbage in the US produces more than 10 % in terms of CO2 of all global warming gases as methane. Methane is 21 times more potent than CO2 as a greenhouse gas. Add to this the 400 million exta barrels of oil required to produce all the extra fuel to produce the estimated 31 million tons of food that is produced, transported, stored and then just dumped in the garbage. You might not believe it, but many household do not even own containers to store left overs. Everything goes into the garbage.  


    Proposal three: Stop all cutting down of the rain forests. For all other tree farms, for every tree harvested for timber, three trees would have be planted.

    Proposal four: Limit car size and engine size.

    I could go on, but I know no one is really concerned about "global warming" otherwise we would be doing something NOW!  

  18. RayfromNY, your proposals 1 & 4 would have neglible impact on emissions and seem unlikely to happen. We'll switch to electric vehicles long before such 'lifestyle' changes would ever be approved.

    Proposal two doesn't specify where all this organic garbage would go once you banned it from landfills... and would again have virtually no impact on global warming.

    I'm all for some variation of proposal three, though it would only be a stopgap measure.

  19. My observation on warming

    Why to take el Nino effect together with CO2 effect, is there any connection with El Ninios and CO2 levels in the past?

    El Nino and tectonic activity may be linked, effect of el Nino is definitely more influential then CO2 as trend is shorter with more effects.

    How to blame CO2 when there were periods in the past of the planet when it was warmer with lower CO2 level and colder with CO2 higher.
    Another question is: How to get reliable trend out of 200 years of observation, when this planet has trend of warming or cooling which lasts several hundreds thousands years. It may give short trends at the best but hardly indicate global planet trend in long time perspective.

    There are sometimes polar wortex - when good part of continent freezes for several weeks, anyone tried to link to global trends.

  20. NN1953VAN-CA, one thing you need to take into account in your reasoning: increasing CO2 in the atmosphere causes the climate system to warm.  No paleo data is needed to confirm this.  We have direct surface observations of the greenhouse effect (downwelling longwave radiation), and the physical mechanism has been established for well over a century.  This is not about finding something to blame for the warming.  The theory was established first.  The warming was measured decades later.  

  21. DSL,
    If CO2 concentration lags behind temperature raise for  800 ± 200 years than, when temperature starts dropping, CO2 still raises so temperature would not drop ever. Temperature drops eventually, what  in that theory explains how CO2 concentration drops and what makes it drop.

    And on cooling cycle if CO2 lags, how higher CO2 level alows cooling, when it should be warming factor.That makes me think that  CO2 is not as significant as it looks in GHG theory,  there are more factors to take in calculation. Maybe physical mechanism established over a century ago needs some new  aspect to warming. 


    [PS] Please see the myth "positive feedback leads to runaway warming". The basics of feedback apply here and the effective CO2 feedback parameter is way less than 1. Rather than being century old science, ice age cycles are very active research areas. I strongly advice you at least read Chapter 5 of Ar5 to see latest work and understanding before leaping to unwarranted conclusions. The paleoclimate chapter in Ar4 is also a good index to key results and papers. The negative feedbacks associated with milankovich-induced ice sheet growth include: increased albedo, reduced production of wetland methane, and greater absorption of CO2 by cooling ocean.

  22. NN1953VAN-CA, where are you getting the 800 year figure?  I suggest you read this post on the science of the lag.

  23. Real sceptics ask the question, "Are the changes in climate that have occurred since significant human CO2 emission fundamentally different from those that occurred before?". If yes, that would support the CAGW view. If not, CAGW *could* still be true, but the recent climate changes could not be cited as support for it except in a weak, probabalistic way.

    The problem is, for sceptics, that the support from the historical record has been repeatedly miserpresented and even falsified.

    Firstly, the Michael Mann's "hockeystick graph", given prominence in Al Gore's propaganda work, is still cited from tie to time in spite of having been debunked. It shows climate as virtually constant for a millenium before human CO2 and in doing so ignores important changes such as the Medieval Warming Period, the Little Ice age, as well as a noticable warming trend in the hundred years or so before human CO2 emissions.

    Other, more recent graphs are a good deal more honest about such things. However, the "trick" being used now is more subtle. These graphs are, without exception, smoothed to a far greater degree during pre-CO2 periods than the recent period. For example, you will see 100-year smoothing up to 1950, followed by 10-year or even unsmoothed data. It should not surprise that the less-smoothed period is less smooth - and the more bumpt section looks more alarming!

    Graphs going back over many thousands of years are often even more heavily smoothed. I saw one recently that went back 400,000 years, meaning that it gave only 1 or 2 pixels to each 1000-year period, and yet the last 50-years was shown explicitly, with probably only 10-year smoothing. Now, if the underlying data has more resolution, then the correct way to show the data is with a shaded region between min and max values (so the fine variation may be seen), rather than a saeries of averages (which is just another way of hiding detail on shorter timescales).

    To be fair, many studies will be intrinsically unable to uncover that fine detail. But when presenting that data, the fact that past rapid changes will not be present should be honestly pointed out. Not kept quiet. 

    Nearly every graph you now see of past trends is, whether by manipulation or limitations of the underlying study, not showing past rises of the kind seen in the last 50 years but only averages that would hide it. As a result, it would be incorrect to assume from them that such rises have not occurred.

    Suppose for example we see a cold day followed by a hot day. Someone micht claim that is an unprecidented event. Suppose they show you a graph of the last year's temperature, and smooth that graph out by taking weekly averages. Of course you won't see any examples of the event having happened before - the smoothing process rubbed them out!

  24. A.R.S.Says @ #498 :

    Alas, you seem to have tied yourself in a knot. Please go back to the issue, and think about it more carefully.

    This website's educational material is there to educate you ~ but you need to approach it calmly & logically.

    (btw, I must commend your sense of humour in choice of your nom-de-plume ~ the abbreviation is priceless.)


    [JH] Inflamatory & off-topic.

  25. A Real Skeptic would know, having examined the issues, that Mann Bradley and Hughes 1999 (MBH99), aka, the Hockey Stick, has not been debunked despite strenuous efforts by McIntyre and McKittrick.  Rather, McIntyre and McKittrick have used a statistical measure that calls a straight line plus white noise a Hockey Stick to argue that random noise generates Hockey Sticks in an attempt to debunk MBH99.  

    This is not to say MBH99 is without flaws (many first pointed out by Mann himself in later publications).  Rather, it is a reasonably accurate first attempt to generate a multi proxy paleo temperature index with a calculated uncertainty.  Because it was a first attempt, Mann, Bradely and Hughes had things to learn and made some mistakes, but the mistakes do not undermine the fundamental conclusions.  This is seen by comparing MBH98 to later resonstructions that have eliminated some of those mistakes.  Of those, the PAGES consortium reconstruction has used the most data and robust methods, and hence represents the best global reconstruction over recent times:

    Please note the green dots (ie the PAGES reconstruction) all represent 30 year averages, so that the graph is not smoothed at a different rate for the recent period, yet the 20th century still stands out for the rapidity and magnitude of the temperature rise, not mention the reversal of the long term cooling trend and the magnitude of the final value (1971-2000) which exceeds all others.  There is, of course, a qualification on that final result:

    "Many of the proxy records used in the reconstructions were generated some years ago and end before the year 2000. The reconstruction for North America, Asia and South America do not extend to the 21st century. The instrumental record shows that the last several decades have generally been warmer than previous decades. Therefore, the most recent 30-year period in our analysis (1971-2000) underestimates the actual temperature of the last 30 years."

    (From the PAGES FAQ, my emphasis)

    Finally, it is my instinctive reaction to treat "persuasive names" such as "A Real Skeptic Says" as indicating that the person choosing the name doesn't think their real skepcism will come across without their first telling us their presumed status.  That instinctive reaction is rarely wrong.  So rather than flagging the uncommon skepticism of the person involved, it tends to flag the opposite.  Just a word to the wise.

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