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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 401 to 425 out of 766:

  1. Here's a theoretical stab in the dark about a biological fingerprint that has adapted to a hotter climate in the past. Dinosaurs. Not sure if this view or even topic has been circulated but I thought I'd share it here considering people are trying to rely on the past to understand our present situation.

    Searched the net to no avail on the subject of dinosaurs and their cooling systems but this doesn't mean it's not out there. Looking over some graphs it was interesting to note that during the times of the dinosaurs it was much hotter on average than it is today. So I asked myself how dinosaurs kept cool to survive and arrived at some intersesting theories that I havn't seen around yet.

    1. Were the vascular bony structures protruding from some dinosaurs originally meant for cooling the dinosaur?

    2. Were dinosaur feathers originally filled with liquid instead of air?

    3. Were feathers on dinosaurs originally meant to act as air-conditioners to cool the dinosaurs instead of flight?

    The reason these questions are on my mind right now is that some people think we can survive dinosaur age global temperatures because life existed during these temperatures. My real questions are how did life at the time adapt to those temperatures?

    Think about this, dry cotton insulates but wet cotton cools a person faster than if they didn't have it on to begin with. What if feathers are the same way? What if when they're hollow with air they maintain heat and help flight for today's climate. But during hotter times they could have been filled with liquid switching them to air conditioning units instead of flying and insulating units.

    Not sure if this is true or even possible but it's really, really bugging me because I can imagine it as a truth and this points to the fact that not even birds will survive going back to those temperatures without them losing their ability to fly.

  2. JCMac1 I don't have my dinosaur books with me today, but I'll try and address some of the points you make:

    Firstly dinosaurs adapted to various climates, but they did so over the course of tends of thousands to millions of years.  It is the rate of climate change that is the problem rather than just the final temperature reached.

    Most dinosaurs, like most modern reptiles are unable ti directly regulate their body temperature (which is one of the reasons that the require so much less food endergy than us mammals - so there is an evolutionary advantage to this).  If heat were a serious problem, dinosaurs would not have evolved to be very large as this increases the volume to surface area ratio, which in turn makes gaining or loosing heat more difficult.

    The bony plates on e.g. stegosaurus were for cooling, ..., and heating.  This is fairly well known.  Apparently the bony plates could be flattened in the morning to raise body to operating temperature after loosing heat overnight, but could also be made vertical and pointed away from the sun, inwhich case they could be used to cool body temperatre.


    I am no aware of any suggestion that dinosar feathers were filled with liquid rather than air (or indeed that birds do this either), or how this would really help with cooling.

    I think it is quite likely that the feathere were for insulation, rather that cooling, helping to keep the body temperature approximately constant, rather than specifically for cooling.


    Life can certainly survive a return to the sort of temperatures seen in the Cretaceous.  The same is unlikely to be true of human civilisation as it exists today.  There are simply too many of us for us to be able to adapt to that sort of change in our agricultural environment.  I'm sure that we as a species would also survive, although there would be great hardship and loss of life along the way (mostly due to starvation).  I personally don't think that makes it a case of "well that's alright then". 

    More seriously, the "skeptics" have invented the concept of CAGW (catastrophic anthropogenic global warming) simply because the know that they can't defend the argument that there is no AGW, so a shift of the goalposts is required.  AGW doesn't need to be catastrophic for it to be worthwile taking steps to mitigate against in in order to maximise the quality of life for the current generation and for the next.

    The problem with the air-conditioner feather theory seems to me that the feathers themselves would insulate the skin from the area at the surface of the feathers that were actually evaporating the water.  It is also not clear why this would be any better than simply sweating through the skin.  Why don't birds, such as ostriches do this?

    Birds will continue to fly, should temperatures reach Cretaceous levels once more, this is demonstrated by the fact that there were flying birds in the cretaceous.

  3. JCMac1 and Dikran Marsupial, you will probably find this article on the thermal regulation of dinosaurs interesting.  It reviews the evidence that dinosaurs maintained fairly stable core body temperatures, but did so not by maintaining a high metabolism (as do birds and mammals), but by the use waste heat from normal muscular action to warm the body, coupled with various tricks to prevent the two rapid loss of heat.  Importantly, heat production by that means is one quarter to one tenth of that in animals with high metabolic rates.  Therefore dinosaurs needed to dispose of  only a quarter or less of the heat of a similarly sized mammal.

    High temperatures are only a potential problem to humans because they restrict the rate at which heat can be disposed of.  If only a quarter of the heat needs to be disposed of, a similarly sized animal can safely live with much higher external temperatures.

    Further, feathers are (from memory) a feature of small dinosaurs only.  There are large dinosaurs among the branch that developed feathers, but no evidence that they retained feathers into adulthood.  Even at high temperatures, for animals of low body mass to retain stable internal temperatures without high metabolisms, they need substantial insulation.  Hence feathers.  

  4. We can't argue about all the evidence shows change is occurring.

    To sugest human intervention in total is not playing a role, the degree an arguement only. The human intervention, in the last 100 years must be recognised.

    The speed of change being the only unknown factor, and on evidence accelaerating.

  5. I'm not an expert on global warming, so I won't attempt to argue with anyone about that. What I am wondering is why you are worried. Since from your article you obviously believe in an old earth ("hundred thousand year cycle", "last 700 thousand years", etc.), then I am assuming you also believe in evolution. If evolution is true, than life must be adaptable enough to survive global warming, or there is no way it would have survived in the past!

  6. Autumnleaves @405, first a technical point.  I do not believe in evolution.  Evolution is not something I put my trust in as though it were a deity.  Rather, I believe that modern living things have evolved from earlier forms through a process of random mutation and natural selection; and that all life currently on Earth share a common ancestor, which lived certainly more than a billion years ago, and possibly more than three billion years ago; and that all life share a first common ancestor that lived around four billion years ago.  These are scientific claims having very little religious implication.

    Second, your objection is specious because it does not take into account the pace at which evolution proceeds.  Fixing a new genotype takes thousands to billions of generations.  If there is little selective advantage, the time taken on average is the inverse of the size of the population (ie, currently 7 billion years for humans).  The greater the selective advantage the shorter the time, but that selective advantage is measured in reduction in population size.  For very rapid evolution, populations must teeter on the edge of extinction.

    That fact creates a major problem when many species must evolved rapidly at once.  Species are massively interdependent on each other in an ecological network.  The near extinction of a few species can create large risks of exinction in their own right.  The near extinction of many species simultaneiously means that very many of them will go extinct. 

    Further, the rapid rate of evolution I am describing depends on the existence of a large reservoir of genetic variability within a species that typically exists.  Rapid evolution reduces this variability.  After it is exhausted, evolution can proceed no faster than the rate of introduction of new, beneficial genes by random mutation, a much slower process.  Given that following a period of very large selection pressures for very many species, species will need to adapt not just to the new environmental conditions but to the new ecological conditions, that means recovery from such large selection pressures will be very slow and extinctions consequently more likely.  Indeed, it is worse than that.  Humans have placed other organisms under massive selection pressures due to ecological changes over the last century, which will have already greatly reduced genetic variability in most species, limiting their ability for further adaption.

    Finally, the impact of BAU global warming mirrors in impacts, but exceeds in pace, that of the End Permian mass extinction which saw the exinction of 90% of marine Genera.  We know, therefore, that living things cannot, in general adapt to the current rate of environmental change is sustained over the next 100 plus years.  The question among ecologists, SFAIK, is no longer whether or not the comming centuries will mark one of the greatest mass extinctions the Earth as seen, but only whether it will be comparable to that which killed of the dinosaurs, the Permian mass extinction, or something worse. 

  7. Autumnleaves's comment brings to mind Charles Darwin's observation that "ignorance more frequently begets confidence than does knowledge".

    The response by Tom Curtis was outstanding.

  8. Interesting website. A lot more data that most sites which just say something and offer no evidence to back it up.

     

    That said, I'm still somewhat skeptical of the overarching theory.

    Response:

    [TD] Your comment violates the Skeptical Science comment policy, by being devoid of substance and therefore being sloganeering.  Future content-free comments will be deleted without warning.  Please carefully read the original post above this comment thread, and if you want to complain about lack of evidence, then describe specifically what of that evidence in that post you believe is inadequate, and why.

  9. wcgulick @408, we can hardly help you with so vague a criticism.  I suggest that you start reading the basics, eg, here or here, and raise any specific concerns you have with that aspect the theory on those pages.  If you have no concerns at that level, we can then move on to other relevant pages and discuss them there. 

  10. Tom Curtis @ 409:

    What I mean is that I still see serious problems with "stating as a fact" that the planet is warming due to human activity. 

    To wit:

    The people doing this reseach are working with an incomplete data set. This is the largest kinetics problem anyone has ever tried to solve and it's not solveable unless you know the inputs and outputs of the overall system, which we don't.

    Related to this problem is a second one, where people are making a series of assumptions based on said incomplete data set. The real cold (no pun intended), hard truth is that no one really knows if things like water vapor in the atmosphere, which is present at levels orders of magnitude greater than carbon dioxide, create a positive or negative feedback loop or any feedback loop at all. It could produce a slow (or rapid) occilation. As I said, no one knows. 

    On top of that, with a system as complex as the climate, data on prior occurances is not necessarily predicitive of future occurances. Simply put, we don't know exactly what was happening to all inputs and outputs when this occured. 

    On top of that there is no way that anyone can argue in a situation like this that the researchers don't bring their own biases to their research. This isn't something you can prove or disprove rapidly in a chemistry lab, in fact there are no experiments being done at all (so it's not science, it's research). Therefore, unlike real hard science where experiments can prove or disprove a hypthoisis, we have no way of controlling for biases that may be introduced by the researchers themselves. 

    Long story short, my real issue here is that the data set is incomplete and there is absoultely no way to know how all of the variables (including researcher bias) are acting at any given time in the past. Therefore, any conclusions we draw from the data are, IMHO, unreliable at best. 

    Response:

    [TD] You need to post comments on relevant threads.  You must read the original post for the relevant thread, and if you comment, address that original post.  Many posts have Basic, Intermediate, and Advanced tabbed panes; read all three before commenting.

    For your "incomplete data" claim, see "Are Surface Temperature Records Reliable," among other posts.

    For water vapor, see "Explaining How the Water Vapor Greenhouse Effect Works."

    Projecting future climate is not done merely by extrapolating from past trends; climate models are physical models, not statistical ones.  In any case, the proof is in the pudding.  See "How Reliable Are Climate Models?"  That same post addresses your incorrect and inappropriate claim that researchers' biases cannot be controlled; the empirical data have and continue to validate the theoretical projections that have been made for over 150 years.

    All your claims are incorrect.  You are most welcome to make claims on Skeptical Science as long as you do so on the relevant threads, as long as you specifically address the original posts of those threads, and you respond to other commenters with specific data rather than merely your opinions.

  11. wcgulick @410, I'm sorry, you are simply boring and arrogant.  Arrogant because you assume that because you certainly do not know something, scientists do not know it either.  Boring because your position if adopted consistently would cause you to reject all science, not merely climate science.  There is no scientific subject about which we know everything.  Never-the-less there are many scientific subjects about which we know much, including climate science.  I am more than willing to work through the details of climate science with those who are willing to discuss it rationally.  I am, however, totally disinclined to waste time discussing it with those who hold their ignorance to them as a shield against learning.

  12. Explain, precisely, how the temperatures are measured.  I assume land-based temperatures have fixed locations.  If so, how many are there, both in the United States and worldwide.  How are temperatures obtained over the seas? 

  13. Google didn't work?

    http://en.wikipedia.org/wiki/Weather_buoy

  14. Also, for future reference, after doing a basic level of Googling yourself, it's "Could someone please <request>?" or "I need help understanding <thing> and would really appreciate..."

  15. schema, you can find general information on some of the methods of measuring global temperature anomalies here. Note that the 'fixed location' land-based temperature measurements you cite, and ocean buoy measurements for sea temperatures, have also been confirmed by weather balloon and satellite measurements. Numerous temperature proxies (e.g. tree rings, coral growth layers, ice cores, ocean sediment cores, et cetera) also match the various forms of direct measurement.

  16. It is amazing that so many experts think they know what happened millions, billions of years ago on the earth!  Let alone that the techonlogy that is used to record the earth's temperature has only existed since the latter 1800s.  Add to it in the 60s and 70s we endured a global cooling scare!  When you take and issue and use it for an agenda you will you scare tactics!  But in the end we dont even use the data attained during the 100 years of numbers collected but a brief 30 year period from 1950-1980 as our "average"!  Yet we all know that this is the period of global cooling!  Add to the alarmism of the 30/4,500,000,000,000 and science thinks it can answer every question with such a small sample, amazing!  But I digress, let us take this one little degree over the past 100 years... now let us remove the co2 created by the mere existance of man and the population explosion.... next let us remove the co2 produced by the livestock explosion needed to feed the world.... let us remove the co2 from any additional solar activities.... let us remove the co2 that is created from all sources apart from fossil fuel usage!  Next let us go to the remainder and break it down between countries in comparision to their production of co2 to run their economies... So now we in the usa are responsible for a fraction of one tenth of one degree!  Let us now see what we can do to affec that number but also remembering that as we punish those who produce co2 one of two things will happen.  They will increase the price of their goods or services and in the end punish the poorest among us or second they will move thier companies overseas where they will have fewer regulations and be able to increase production and co2 thereby eliminating any potential positives by attacking them in the first place... hmmmmmm!

    Response:

    [TD] Only your first two sentences are on the topic of this post.  If you want to discuss the topics of the other sentences, post them on the appropriate threads.  In each of those original posts, after you read the Basic tabbed pane, read the Intermediate and then the Advanced tabbed panes, if they exist:

     

  17. Wondering if there are better studies on the idea that CO2 preceded a temperature rise. Ex. More than one warming event. 

    Looking at that abstract are there any studies that support the idea of cause? There is a lot of correlation here but no causality.

    Also is there any additional data on the role on methane in the past events and its involvment in warming and extinction related activity?

    Response:

    [TD] In all of the following Skeptical Science posts, after you read the Basic tabbed pane, read the Intermediate and then Advanced if they exist:

    And "It's Methane."

    The Search field at the top left of every page is useful.

  18. ZMathblasterZ, the first and foremost thing to keep in mind is that CO2 absorbs/emits at various pressure-broadened bands within the thermal infrared range, the range within which the sun-warmed Earth emits.  The emission is in random direction, effectively half up half down.  The process lengthens--in space and time--the path of energy from surface to space.

    Downwelling thermal radiation has been directly measured from the surface.

    So an increase in CO2 is going to result in an increase in energy storage, regardless of the situation.  Beyond that, though, there are interesting questions about the timing of the CO2 amplification effect in the process of the Pleistocene glacial cycles.

  19. Re. [3], [5]: Hoyt's argument is flawed on a very basic level. It goes like this: "Water vapour has a atmospheric life time of about a week. So if there is more water vapor in the atmosphere, we must somehow lift that water from the ground up into the atmosphere every week. That requires energy, which must come from somewhere. There is not enough energy to do that. So water vapor cannot increase as much as claimed". What he ignores is the very simple fact that that energy required to lift the water is not lost to the system. Whenever water vapor leaves the atmosphere (via precipitation), those raindrops and snow flakes and hailstones fall back to the ground, and their potential energy is converted back to kinetic energy, and ultimately heat.

  20. Evidence about Earth's climate in the last few million years show rapid oscillations of the climate started about 3 million years ago when CO2 concentrations dropped from above today's level to about half today's level.
    http://upload.wikimedia.org/wikipedia/commons/1/1b/65_Myr_Climate_Change.png
    Milankovitch cycles and variations in solar activity can not alone account for these rapid changes because they have existed long before the rapid changes started. Variations of CO2 concentrations in the range of 100ppm can not explain it either, because such variations have also been present long before the period of rapid climatic changes started.
    It seems the only difference between the past 3 million year period of rapid climatic changes and relatively more stable climate before it is the average CO2 level. During the stable climate it was above today's level and during the unstable climate it was about half today's level.
    Is it possible that low CO2 levels lead to unstable climates due to the reduced green-house effect and therefore wouldn't we be shooting ourselves in the foot by trying to reduce the CO2 level to preindustrial levels and therefore risking a swing to a colder climate which would have a much more dire consequences than even the direst IPCC predictions?

    Response:

    [TD] I hotlinked your link.  In future please do that yourself.

  21. skeptic1223, I believe you are mistaking the higher temporal resolution of the temperature measurements the closer to recent your linked graph shows, for more rapid changes in temperature.

  22. @420, Who said we were trying to reduce CO2 to preindustrial levels?

    Sure it's an interesting thought that 'low CO2 levels' could lead to unstable climates, I'll grant you that, but it's irrelevant.

  23. skeptic1223, for example see Figure 5 in Zhang et al. (2013).  Note the circles that are the actual datapoints, and note how far apart they are in various time periods.

  24. @421, Surely the time resolution for more recent periods is better than for older ones, however statistics and probability theory tell us that even sparser sampling would still show large variations (the probability of the sparser samples all hitting similar values is miniscule). I hope you would agree that the trend in increasing climate instability in the last 3-5 million years is quite obvious
    LINK
    the graph is from wikipedia
    en.wikipedia.org/wiki/Geologic_temperature_record#
    Also, the current ice age epoch started only about 3 million years ago and the difference in temperatures between glacial and interglacial periods has been quite significant, for 260 million years before that there were no ice ages and CO2 levels were relatively high compared to today's
    LINK

    again from wikipedia
    en.wikipedia.org/wiki/Carbon_dioxide_in_Earth's_atmosphere#

    Response:

    [RH] Shortened links that were breaking page format.

  25. @422 If we reduce the emissions to preindustrial levels eventually the concentration would follow too. Also, we don't know exactly what is the threshold of CO2 concentration that leads to unstable climate. Before the current ice age epoch started CO2 levels were above today's.

    The climate instability is not irrelevant, I think it is actually the most relevant thing. Unstable climates are unpredictable and swings from interglacials to glacials can happen quite rapidly in a matter of a few decades, for example

    en.wikipedia.org/wiki/Younger_Dryas#

    So, to summarize, in an unstable climate we wouldn't be able to tell if a new ice age is waiting for us just a few decades down the road, we wouldn't be able to prepare for it, and an ice age would have absolutely catastrophic consequences for us. The catastrophic consequences of rapid climate changes has actually been discussed by skepticalscience

    www.skepticalscience.com/Rapid-climate-change-deadlier-than-asteroid-impacts.html

     

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