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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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What does past climate change tell us about global warming?

What the science says...

Select a level... Basic Intermediate

Greenhouse gasses, principally CO2, have controlled most ancient climate changes. This time around humans are the cause, mainly by our CO2 emissions.

Climate Myth...

Climate's changed before

Climate is always changing. We have had ice ages and warmer periods when alligators were found in Spitzbergen. Ice ages have occurred in a hundred thousand year cycle for the last 700 thousand years, and there have been previous periods that appear to have been warmer than the present despite CO2 levels being lower than they are now. More recently, we have had the medieval warm period and the little ice age. (Richard Lindzen)

Greenhouse gasses – mainly CO2, but also methane – were involved in most of the climate changes in Earth’s past. When they were reduced, the global climate became colder. When they were increased, the global climate became warmer. When CO2 levels jumped rapidly, the global warming that resulted was highly disruptive and sometimes caused mass extinctions. Humans today are emitting prodigious quantities of CO2, at a rate faster than even the most destructive climate changes in earth's past.

Abrupt vs slow change.

Life flourished in the Eocene, the Cretaceous and other times of high COin the atmosphere because the greenhouse gasses were in balance with the carbon in the oceans and the weathering of rocks. Life, ocean chemistry, and atmospheric gasses had millions of years to adjust to those levels.

Lush Eocene Arctic 50 million years ago

Lush life in the Arctic during the Eocene, 50 million years ago (original art - Stephen C. Quinn, The American Museum of Natural History, N.Y.C)


But there have been several times in Earth’s past when Earth's temperature jumped abruptly, in much the same way as they are doing today. Those times were caused by large and rapid greenhouse gas emissions, just like humans are causing today.

Those abrupt global warming events were almost always highly destructive for life, causing mass extinctions such as at the end of the PermianTriassic, or even mid-Cambrian periods. The symptoms from those events (a big, rapid jump in global temperatures, rising sea levels, and ocean acidification) are all happening today with human-caused climate change.

So yes, the climate has changed before humans, and in most cases scientists know why. In all cases we see the same association between CO2 levels and global temperatures. And past examples of rapid carbon emissions (just like today) were generally highly destructive to life on Earth.

Basic rebuttal written by howardlee


Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 6 August 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Myth Deconstruction

Related resource: Myth Deconstruction as animated GIF

MD Past

Please check the related blog post for background information about this graphics resource.

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 526 to 550 out of 883:

  1. Doodad @522, the primary effect of warmer weather on cold blooded creatures is that they become faster in their actions.  There is a limit on this, primarilly based on the fact that they must use different enzymes in different temperature ranges, and above 60 C they do not have appropriate enzymes.  But you are correct, for lizards, and cockroaches, and flies and mosquitoes, and other cold blooded creatures, warm conditions are good conditions.

    Likewise, warm conditions tend to be good conditions for small warm blooded creatures.  This is because they have a higher skin area to volume ratio, allowing more efficient cooling; and because they have higher basal temperatures so that wet bulb temperatures need to be more elevated to cause heat protration and death than they do for large warm blooded creatures (such as humans, and human livestock).  So, within limits, warmer climates are good for rats and other rodents.

    In general, warm weather is good for all of hour pests and parasites.

    I'm not sure why you think that is a good thing.

    In contrast, for humans in the tropics, summer temperatures already reach or exceed the range that allows efficient cooling on a regular basis.  This can be ameliorated by drinking large quantities of water, finding shade, and resting - the combined effect of which is to significantly lower work efficiency.  So, already in the tropics warmer climates will have a negative effect on the economy just from the direct effect of heat on humans.  However, with a sufficient warming, the tropics will regularly reach wet bulb temperatures leading to death of humans and large livestock (anything larger than a chook).  Put simply, most of China, all of India and all of South East Asia, the north part of Australia, most of Africa, the mediterrainian and Middle East, Central America and the north part of South America including essentiall all of Brazil will become seasonally uninhabitable.

    This massive catastrophe is very unlikely if we take serious measures to limit climate change, but laregely unavoidable if we insist on burning our conventional fossil fuel reserves to exhaustion.  Currently we are doing something towards mitigating climate change, but branching out to burn unconventional fossil fuels as well.

  2. Doodad - nice tag by the way :-)

    Is this a drive-by or do you have specific points to make.

    And by the way, current CO2 is more like 400, not 350 ppm.

    Yes there are mechanisms that draw down CO2, just as there are mechanisms that can raise it. The key question is over what timescales. Come back in a million years and our disruption of the carbon cycle will have disappeared from the climate system. However its signature will be visible for 10's to 100's of millions of years in the geology.

    How could an Ice Age have happened when CO2 was 5000 ppm (very approximately)? Perhaps if they dropped!

    Your figures of 4% lower solar and 5000 ppm suggest you are referring to the End Ordovician Ice Age (and Mass Extinction Event). You might help discussion if you clarify that.

    So using the standard definition of the impact of changing CO2 levels - similar impacts for each doubling, 350 ppm (your number) to 5000 ppm, is under 4 doublings - 700, 1400, 2800, 5600. So somewhat under the impact of one doubling. One doubling changes radiative balance by 3.7 w/m^2 so around 14.8 w/m^2 in total. Actually when we look over larger ranges of CO2 concentrations this change is lower, so less than 14.8.

    Then solar was 1310.8 rather than 1365.4 watts per square meter today - your figures. The current estimates are more like 1361, the 1365 figure was likely a small calibration error from an earlier satellite. Small detail.

    So on your figures, a difference of -54.6 w/m^2. But we need to divide by the ratio of the Earths frontal area to its total area, so divide by 4. -13.65 w/m^2.

    Hmmm... Not much difference. So if CO2 dropped back then, an ice age is entirely plausible. A drop from 280 to 180 ppm today is associated with a swing into an ice age. That is the equivalent of a swing from 5000 to 3200 ppm back then. What might trigger that? The evolution of vascular plants drawing down CO2? Major volcanic events sequestering CO2 due to weathering?

    We don't know everything about the past but it sure fits with CO2 being a significant factor.

  3. Glenn Tamblyn @527, here is the relevant paper showing low CO2 levels at the start of the Ordovician glaciation.  It is discussed in passing by Richard Alley in his famous lecture.

  4. Interesting. When you say warmer climates are benificial to cold blooded animals, you only highlight the animals that everyone dislikes(trying to make it sound aweful?). In case you didn't know not all cold blooded animals are lizards, cockroaches, flies and mosquitoes. Similarly, rats and rodents are not the only small mammals either. You could say, the death of pests is a much larger catastrophy than the prospering of pests. No rodents or rats? What are snakes and eagles going to eat? No bugs or pests? A vast majority of animals rely on bugs as their staple diet. So what if the amount of pests increases? Nature has a magical ability called balancing itself. Too much bugs or pests will cause the thriving of birds and other small animals. Or the advancement of pesticides. 

  5. Doodad

    Here are a few animals that definitely aren't cold blooded that might be of passing relevance to human well being. 1.4 billion cattle and domestic buffalo, 1.9 billion sheep and goats, 1 billion pigs. And 7.4 billion homo sapiens.

    Humans and our domestic animals make up around 95% of the mass of larger land animals. Climate change will effect many aspects of the world. But its impact on us is the primary concern!

  6. Doodad @529, the global ecology may well take a hit from global warming, but it is likely to take bigger hits from over fishing, over logging, over population and free trade (with the consequent invading species).  Regardless of all of these, it will survive in one form or another, and likely fully recover within 5 million years (ie, approx 0.1% of the age of the Earth).

    Humans, on the other hands are not so lucky.  We are likely to take major hits from global warming, and the direct impact on warm blooded animals of high body mass will hit us very hard, as will the impacts of disease and on our crops of the flourishing of small creatures and cold blooded creatures.  Warm animals of high body mass make up an unusually large proportion of our non plant food sources (with the exception of fish, nearly all of it), and provide us significant services besides.  Small warm blooded creatures and cold blooded creatures make up an unusually large portion of our disease vectors and predators on our crops.  The distinction is not particularly important ecologically, but for us it is vital.

  7. Doodad,

    Pesticides are invariably a tempory fix that have numerous bad side effect.  I am old enough to remember when doctors gave children penicillin for sore throats and the sore throat went away immediately.  Now penicillin no longer works against most sore throats and stronger drugs are required.  Resistance to Roudup is already (after less than 20 years of widespread use) causing problems in farms using large amounts of herbicides.  There are only a limited amount of pesticides available, after resistance develops to all of them they will no longer work.  After less than 60 years of widespread use resistance to all antibacterials has developed.  In another 60 years these drugs will be even less effective.  Claiming that new pesticides will be developed to solve our AGW caused problems is simply false.

  8. I agree that climate change will be observed whether humans are present on Earth or not. Just as historical records indicate, ice ages, which are significant climate change events, have been documented well before humans existed. The Earth will always undergo natural temperature variations and fluctuations. However, what skeptics fail to recognize is that humans contribute *significantly* to the rate of climate change, increasing these rates to an extent that is far beyond a natural cycle of climate variation. Of course, atmospheric carbon dioxide will always exist and contribute to the GHG Effect, with or without anthropogenic intervention; however, anthropogenic carbon dioxide emissions are sky-rocketing, resulting in the propagation of global warming at concerningly high rates. The following article published in Nature provides excellent evidence that anthropogenic carbon dioxide emissions directly precede maximum peaks in global temperatures.

    Abstract

    The covariation of carbon dioxide (CO2) concentration and temperature in Antarctic ice-core records suggests a close link between CO2 and climate during the Pleistocene ice ages. The role and relative importance of CO2 in producing these climate changes remains unclear, however, in part because the ice-core deuterium record reflects local rather than global temperature. Here we construct a record of global surface temperature from 80 proxy records and show that temperature is correlated with and generally lags CO2 during the last (that is, the most recent) deglaciation. Differences between the respective temperature changes of the Northern Hemisphere and Southern Hemisphere parallel variations in the strength of the Atlantic meridional overturning circulation recorded in marine sediments. These observations, together with transient global climate model simulations, support the conclusion that an antiphased hemispheric temperature response to ocean circulation changes superimposed on globally in-phase warming driven by increasing CO2 concentrations is an explanation for much of the temperature change at the end of the most recent ice age. 

    Source: http://www.nature.com/nature/journal/v484/n7392/full/nature10915.html      

    Response:

    [PS] Fixed link. Please learn how to create links yourself with the link icon in the comments editor.

  9. I really enjoyed that this article had scientific information to back up some of its claims even if some of the links were not the most concrete sources. I agree with the article saying that the climate has changed before; there is too much scientific evidence for anyone to claim that it has not. One example I can think of is ice cores. I believe this article could have gone more in depth when explaining its points. However, I also believe humans have some impact on increasing the rate of climate change. The anthropogenic carbon dioxide emissions are increasing at such a rapid rate, I have no choice but to believe they will influence. I have posted a paper that I think will help sway some people to believe that humans do have an effect on climate change. 

    Zhang XB (2007) "Detection of human influence on twentieth-century precipitation trends" Nature 448, 461-465. 

    Abstract: "Human influence on climate has been detected in surface air temperature(1-5), sea level pressure(6), free atmospheric temperature(7),tropopause height(8) and ocean heat content(9). Human-induced changes have not, however, previously been detected in precipitation at the global scale(10-12), partly because changes in precipitation in different regions cancel each other out and thereby reduce the strength of the global average signal(13-19). Models suggest that anthropogenic forcing should have caused a small increase in global mean precipitation and a latitudinal redistribution of precipitation, increasing precipitation at high latitudes, decreasing precipitation at sub-tropical latitudes(15,18,19), and possibly changing the distribution of precipitation within the tropics by shifting the position of the Intertropical Convergence Zone(20). Here we compare observed changes in land precipitation during the twentieth century averaged over latitudinal bands with changes simulated by fourteen climate models. We show that anthropogenic forcing has had a detectable influence on observed changes in average precipitation within latitudinal bands, and that these changes cannot be explained by internal climate variability or natural forcing. We estimate that anthropogenic forcing contributed significantly to observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics. The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation, such as the Sahel."

  10. Apologies if this is a dumb question, but you say that the cooling of the upper atmosphere is a fingerprint of human-caused global warming. Why?

    Response:

    [PS[ This has nothing to do with past climate. Could follow ups please go to either Fingerprints of AGW or better still Stratospheric cooling. Not a dumb question though  - it is a complicated things to get your head around.

  11. shelleyratcliffe: (1) The increased insulation by greenhouse gasses, of the stratosphere from the energy radiating from the surface. (2) Greenhouse gasses gather energy and radiate energy. In the lower atmosphere, there are more greenhouse gas molecules above those, to absorb those photons being radiated, and most of that absorbed energy is transferred to other molecules (greenhouse and not). But at the top of the atmosphere, there are few molecules to absorb that energy, so that radiated energy vanishes to space.

  12. Has anyone done a study on this possibility? There is a theory that the magnetic field is weakening because earth is in a period of magnetic poles shifting. What is its effect on global warming?

  13. ergodicity - this is a pretty weird idea since past magetic pole reversals arent associated with any climate change. Can you give us a link to where this theory myth is being expounded least the Sks team need to add it to the list of skeptic myths. It is strange how people might expect some unknown mechanism to be causing warming while ignoring that the amout of radiation at the earth surface is measurably increasing, and in the spectrum and intensity of that predicted by the GHE.

  14. scaddenp Thanks for replying. I believe you misunderstood me. The theory I spoke of was of a polar shift: https://www.scientificamerican.com/article/earth-s-impending-magnetic-flip/

    My question, not theory, not challenge to man made global warming, was, what is the effect of the magnetic field weakening http://www.livescience.com/46694-magnetic-field-weakens.html

    on global warming. Are there any studies coorelating the polar flip, weakening magnetic field and global warming. http://viewzone.com/magnetic.weather.html

    It seems logical that as the field weakens, more radiation get in and an increase in the surface temperature occurs. I am a firm believer that man made global warming exists, but so does natural global warming (there was a glacier here in Kentucky 20k years age), and I believe the magnetic field weakening is the trifecta that we are experiancing.

    Thanks for reading!

  15. ergodicity... Again, the earth's poles have flipped in the past, like during what's called the LaChamp anomaly, and there was zero response in the climate system.

  16. The Viewzone article is a rather breathless take on Knudsen et al 2009 but the it is correct in that the supposed effect is from effects of GCR on cloud formation (which still lacks supporting evidence - see IPCC AR4 for papers that have examined this in detail). The amount of extra radiation (GCR) reaching the surface could not directly cause any measurable temperature change.

  17. ergodicity @539, the direct contribution to the Earth's energy balance from cosmic rays is 0.0000032 W/m^2.  The direct contribution from the solar wind, in the absence of Van Allen belts would be 0.00035 W/m^2.  Both are so negligible that their presence or absence would make no discirnible difference to the Global Mean Surface Temperature in the event of a collapse of the Earth's magnetic field.

    The only way they could be significant would be through secondary effects, as proposed by Svensmark in "The Chilling Stars", but that hypothesis was always overblown given that it ignored the abundance of Cloud Condensation Nuclei generated by natural, earth bound events.  Worse for that hypothesis, the LaChamp anomaly all but kills the hypothesis, as noted by Rob Honeycutt above. 

     

    (Data and sources here.  I apologize for the formating.  Blogger has decided to mess with the column widths on the table, but it appears with proper formating in the editing window, so I do not know how to fix it.)

    Response:

    [PS] For context, doubling CO2 would add an extra 3.7W/m^2. Further discussions about GCR should take place on this article.

  18. Thanks all for the replies!

    Tom Curtis Where does your statistics of "the direct contribution to the Earth's energy balance from cosmic rays is 0.0000032 W/m^2." come from? Is that stat during a weakened magnetic field or at full strength.

    Rob: Thanks for mentioning the LaChamp anomaly: I have never heard of this and am excited to read about it more. But your statement "there was zero response in the climate system." is false.

    https://phys.org/news/2012-10-extremely-reversal-geomagnetic-field-climate.html Clearly states, The reversed field was 75% weaker whereas the strength dropped to only 5% of the current strength during the transition. This resulted in greater radiation reaching the Earth, causing greater production of beryllium 10 and higher levels of carbon 14.[2  Detectable higher levels of carbon, even if negligible as to global warming, is not zero response.

    Scaddenp: Knudsen et al 2009 is interesting. That also seems to indicate that the weakening magnetic field has some effect on climate: " In addition to supporting the notion that variations in the geomagnetic field may have influenced Earth's climate in the past, our study also provides some degree of support for the controversial link between GCR particles, cloud formation, and climate."

    Again, I am not at all trying to refute the man made global warming effect. I am only trying to determine what effect the weakening magnetic field has on global warming. All links to any studies/articles/information on this subjuct would be greatly appreciated!

    Thanks all for comments!

  19. Sorry Tom, missed your data and sources, reading now.

  20. You can have detectable change in C14 (formed from Nitrogen by cosmic ray interaction) with zero effect on climate. (well no effect that is discernable from noise). Can you see any discernible effect on climate by the Laschamp event?

    Knudsen et al discerned some possible correlation in precipitation which others have also looked at. However if you look at papers citing Knudsen you will see that effect is regional, statistically weak and open to other interpretations. Other major transitions have been studied without much success and it is hard to give the hypothesis much support if a climatic effect cannot be detected in major magnetic changes. Eg try here for Laschamp, also here and here which looked for changes in cloud cover.

  21. ergodicity - Conversion of one carbon isotope to another doesn't change the total amount of carbon dioxide present. And the data we have for the climate during the LaChamp anomaly indicates no discerneable change in temperatures. 

    To all intents and purposes, cosmic ray levels have negligible impact on the climate, and it's noteworthy that current variations in those cosmic rays are tiny compared to historic varations - variations that likewise had no effect. 

  22. KR @546, C14 is mostly formed from N14 (a stable isotope of Nitrogen) when a neutron strikes the nucleis of a Nitrogen atom, displacing a proton.  Therefore an increase in C14 does represent a real increase in atmospheric CO2.  Given that C14 represents just 0.0000000001% of all atmospheric carbon, any such increase would be negligible in terms of atmospheric forcing. 

    As a side note, about 1% of C14 is formed from C13, but 1% of 0.0000000001% not significant.

  23. Interestin discussion. looking at the full tread it started  with "this happened before" and eventually devolves to technical minutia. Yes this has happened before, CO2 concentrations have increased in the past resulting in heating, then cycled through to an eventual ice age or cooling period. What happened to cause the reduction in CO2 in those past cooling cycles? Carbon sequestration in coal beds, etc.?

    At present we are approximately 10,000 years past the last glacial maxus and have been in a long term warming cycle. Natural cycles are typically logarithmic in nature, not linear. Point is: is the current acceleration part of a normally occurring natural cycle following a logarithmic trend? Does the increase in man-made pollution simply create an overprint on what would have happened naturally. The real big picture question?

  24. Jim, "natural cycles" is a very vague term and you need to talk about scale. On short term (

    Climate changes when the energy balance changes. First law of thermodynamics stuff. You can do that by varying the incoming solar (either by changing distance to sun, solar output, screening with aerosols), albedo, or the GHG composition in that atmosphere. It is not at all clear to me why you are claiming "cycles are logarithmic".

    So what among those are changing and by how much? The milankovich cycles driving ice ages actually vary the distribution of solar energy on the surface but in practise vary climate through change in albedo - more or less ice on northern hemisphere continents. Those changes also trigger variations in CH4 and CO2 but over long time scales, turning NH effect into a global event. (Freezing or thawing of asiatic wetlands; vegatation changes; ocean CO2 by dependence of solubility on temperature). They are entirely reversable.

    Currently, the milankovich cycles should be inducing a very slow cooling. However, this cycle is slow and effect at 65N are changes of around 0.008W/m2/ century. Also worth noting that while we had milankovich cycles in the Pliocene when CO2 was over 400pm, we didnt have ice age cycle.

    By comparison, changes in GHG are inducing a warming of around 4W/m2/century globally, not just NH. (We can measure this increase in surface irradation from GHG directly).

    What about the sun? We can now measure output directly and see that this is not contributing.

    What about some hidden ocean cycle? Well it is very hard to claim increase in surface temperature is coming from ocean when the oceans are warming too. Violation of 1st law - where is the energy coming from.

    Something undiscovered by science? Can never be ruled out, but the current climate change is perfectly adequately explained by GHG changes. If it is something else, then why is the extra irradation of the surface NOT the cause?

  25. Look, the main issue in general is that no other solutions to today's climate change are ever discussed other than replacing fossil fuels with wind and power. Added to this are the uncertainties of today's climate science. Specifically:

    • Science cannot provide an average temperature change for the history of the world or accurately determine the rapidty of temperature changes during specific periods because these are all based on inferred evidence and those models could be incorrect.
    • Science cannot accurately measure the CO2 produced year-over-year from such things as ocean-atmospheric exchange, plant and animal respiration, soil respiration and decomposition or determine if these things are increasing or decreasing. These are solely an approximation based upon a model that could be incorrect. And, even a slightly incorrect approximation or model could have a tremendous effect considering that these natural processes produce orders of magnitude more CO2 than burning fossil fuels.

    It is incorrect of climate scientists to claim certainty with so much uncertainty present. It is also incorrect to only ever talk about replacing fossil fuels with wind and solar when a tremendous number of other, alternative solutions are available.

    https://theobjectiveobserverblog.wordpress.com/2017/06/10/one-trick-pony/

    Response:

    [PS] As pointed out earlier, the link is to misinformation not supporting evidence. Until you can figure out what is true, there is little point coming here to demonstrate poor critical thinking.

    When you make a claim (like scientist claim certainty), then provide a link to the science that supports that claim. Nothing is easier to disprove than a strawman. Further strawman arguments will be summarily deleted.

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