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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.

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Comments 301 to 325 out of 611:

  1. Solar activity doesn't just warm the earth, it can also COOL the Earth by solar winds, which are deflected by the Earth's magnetic field, and then divert cosmic rays. Normally, cosmic rays strike the oceans, increasing water-vapor in the atmosphere, and thus causing it to absorb more sunlight and warm slightly; but when solar-plasma ejection reduces, then the earth's water-vapor increases. And as your site says, solar activity has decreased. This means that the Earth is being subjected to greater cosmic winds, and this explains the increased water-vapor and temperatures.
    Response: [muoncounter] Your information on cosmic rays is severely incorrect. Please read the thread scaddenp references. Also please note that this is a science-based site; claims such as 'normally cosmic rays strike the oceans,' which run contrary to much that is known about cosmic rays, must be substantiated - on the appropriate thread.
  2. Except... please see Its Cosmic Rays. That is the appropriate thread for the discussion but please read the article first.
  3. Hi all, I've been directed to this great post by DB. Unfortunately, it doesn't quite counteract the argument that has been presented to me by a denialist. In the Pleistocene period, human beings first emerged http://en.wikipedia.org/wiki/Pleistocene yet the climate history is telling us that there were some severe spikes and troughs during that period. http://en.wikipedia.org/wiki/Paleoclimatology Is the current increase in global warming greater than those experienced during Pleistocene periods? Can you provide me with a peer-reviewed paper that would help me to combat this denialist rubbish.
  4. mace @303, humans emerged in the Pleistocene as hunter gatherers in small groups of from 50 to 200 individuals. There is nothing in the consequences currently predicted from global warming that would make the life of small groups of hunter gatherers particularly difficult. In contrast, humans did not invent agriculture, cities, or civilization until the Holocene. There is a reason for that based in climate. During the Pleistocene climate was very variable over short time spans, with variations in temperature of up to half a degree C occurring in the space of a few hundred years, a condition likely to be inimical to agriculture. And certainly there is much in the potential consequences that will make agriculture much more difficult. So the question for your denier friend is, does he want all 7 billion of us to go back to being hunter gatherers? And how many of us does he think would survive if we did? Because todate, only Holocene conditions have proved capable of supporting agriculture, and we are rapidly leaving those conditions.
  5. mace, This is always a sobering graphic to ponder. Two degrees C on a global average is a lot - especially if it occurs quickly. Just be sure to note the non-linear time scale.
  6. I would say the modern agriculture would have a very hard time coping with climate change that happened as fast as the YD. However, no agriculture existed. Human's even as hunter-gatherers apparently did indeed struggle Pleistocene changes, with populations surviving in enclaves, driving migration. Fortunately, rapid climate change events are associated with the ending of ice-ages not interglacials. As to deeper time, time resolution becomes a problem unless the event can be interpreted from a single sequence. Rates of change far less than present are associated with the great extinctions in the paleo record.
  7. muoncounter, could you provide the above graphic in a normal scale for reference?
  8. Dr T, Here's the stock version. --source I plotted Vostok temperature anomalies along with the Moberg et al 2005 reconstruction's low frequency (LF) component, a smoothed version (LOESS) and GISS temps: The time scale is calendar year rather than 'years BP.' So the jump up at circa 8000 BP appears at -6000 on the bottom graph. It certainly looks like 8000 years of relatively slow, small temperature variation is over. Anyone who says 'the modern warming is nothing new' clearly doesn't understand the concept of 'rate of change.' Exit Holocene, enter Anthropocene.
  9. Moon, it would be great if someone graphed the first derivative for illustration of the rate of change. Some people have really hard time with that concept. Thank you for the previous graphs.
  10. I read this somewhere in one of those textbooks on prehistoric life, but in the ages in and before the dinosaurs, there where no ice caps. And what about the switching of poles?
  11. The "myth" is Human belief of separation from nature. The first chlorophyllic plants 'industry' was tasked with the ( apparently disastrous Great Oxygenation Event) modification of the primitive atmosphere We need to remember that we are that original green slime. It did not pave the way for us - it did so for its own continuity, diversity and complexity.
  12. "Greenhouse gas increases have caused climate change many times in Earth’s history", in the article it is amazing to me to listen this news, could you tell me what are the cases of climate change caused by greenhouse gas increasing? what is the status of the greenhouse gas incrasing caused?
  13. #312: henanlkf, you'll find a lot of great information and good examples of CO2's role in palaeoclimate (geological, Quaternary and more recent) in a presentation given by the great Richard Alley at AGU in 2009: "The Biggest Control Knob: Carbon dioxide in Earth's climate history" He's a really engaging speaker too, so this is well worth your time to watch and listen!
  14. Also, look at Fig 3 here. Temperature compared to forcing from albedo and GHG alone. Also, look up PETM. The cause of the CO2 spike is debated but the effect on climate is remarkable.
  15. michaelcomaha - climate does not change without a cause - conservation of energy etc. The "normal" glacial/interglacial cycle is driven by orbital variations and as such are highly predictable. They vary the distribution and strength of incoming solar energy, something easily measured. On that basis, we should be slightly cooling but instead we are warming. The size of the solar forcing creating the glacial change is also quite predictable and measurable. The man-made GHG forcings though are an order of magnitude higher so little wonder climate is changing so much faster than it does during glaciation.
  16. 'This forcing doesn't necessarily have to come from CO2.' Yes, correct. As I understand it, CO2 comprises 0.057% of the Earth's total volume of atmospheric gases, so why all the hoopla about CO2? Presumably industrial CO2 output is a fraction of this tiny fraction (correct me if I'm wrong about this), so even if the atmosphere of the Earth is extremely sensitive to changes in overall composition it cannot be said with the certainty with which it is said that an increase as large as a 'doubling' of CO2 will have 'catastrophic consequences' (as so many in the media would like us to believe). 'However, our climate has feedbacks, both positive and negative. The strongest positive feedback is water vapour.' Yes, water vapour, we must not forget about that, and yet it is hardly (if ever) mentioned in, for example, the popular press. Why not? 'There are also negative feedbacks - more water vapour causes more clouds which can have both a cooling and warming effect.' In other words, we just don't know. The truth of the matter is that our atmosphere is such a highly complex and (inherently, over the long term) unpredictable system that we really, in spite of how much progress we have apparently made over the years towards understanding it, know next to nothing about it. Why are the words, 'I don't know' so difficult to utter?
    Response: [DB] You put forth an argument from personal ignorance. As DSL counsels below, perhaps you would be better served with a bit more study interspersed with questions to elucidate better understanding before pontificating.
  17. Peter, correct yourself. The answers to these questions are all over this website. Read a little before you make claims with the kind of tone that suggests you have access to the absolute Truth (and gosh aren't we all a bunch of idiots). I'll answer one part. You claim CO2 is a tiny fraction of the atmosphere. True. However, about 96-99% of the atmosphere is O2, N2, and Ar. These gases do not absorb/emit radiation in the thermal infrared range, the range within which the sun-warmed Earth emits. If nothing in the atmosphere prevents thermal radiation from escaping Earth, then the Earth should be about 33K cooler than it is. Fortunately, much of the remaining 1% of the atmosphere is composed of gases that do, in fact, absorb/emit within the thermal infrared range. Those gases intercept outgoing thermal radiation, and those gases then emit or conduct the energy in random directions (effectively half up/half down). Increasing the concentration of those gases effectively increases the energy temporarily stored in the system. Energy in temporarily does not equal energy out. The system warms. As for CO2 vs. H2O as a greenhouse gas, see Lacis et al. (2011).
  18. Okay, I'm sorry, I haven't read all of the comments here, I'm new to this site (just found it today). However, I did qualify my claims with statements like 'As I understand it' and 'correct me if I'm wrong'; this is not an 'argument from ignorance' or any other such pseudo-fallacy. The information I provided is accurate, 'DSL' (above) mentioning the fact that most of the gases within the Earth's atmosphere being non-absorbers of infrared radiation, which is true, but I also did mention the fact that water vapour is a greenhouse gas is never mentioned in the mainstream media. That was my point there, and all I did was question why this is the case. 'Peter, correct yourself. The answers to these questions are all over this website. Read a little before you make claims with the kind of tone that suggests you have access to the absolute Truth (and gosh aren't we all a bunch of idiots).' - DSL (-snip-). I never claimed to be the bearer of 'absolute truth'; don't quote what I did not write.
    Response:

    [DB] Please refer to this site's Comment Policy before lodging charges of ad hominem (snipped above). The link is adjacent to the comment box.

    Further, this site is devoted to advancing the understanding of climate science and exposing the myths and skeptic memes commonly found about it. As such, many thousands of threads and blog posts exist here, all with links to the primary, peer-reviewed literature.

    As such, if you differ with the commentary found in a blog post, it is then incumbent upon you to provide a link to the primary, peer-reviewed literature which supports your position. It is insufficient to merely offer up unsubstantiated assertions which run counter to the accepted literature.

    Lastly, it is presumed that when placing comments on comment threads here that you will have already read the OP and the previous comments made prior to yours. In many threads, much clarification is given in the comments threads that often subsumes and obviates the need to place a comment or question covering something already asked.

  19. @ PeterA #316: Using a scale of 0 to 10, with 0 being “know absolutely nothing” and 10 being “know absolutely everything”, how do you rate the scientific understanding of the Earth’s climate system?
  20. Peter, you said "The truth of the matter is" -- that never fails to ring the warning bells. You said you were new to the site, but then you made some claims about some really basic stuff. When you ask a basic question about the relative strength of water vapor and CO2, and then you make a broad claim about what climate science doesn't understand, your broad claim can't be taken seriously.
  21. Peter, on the other thread you suggested you had read the litterature. On comparing the radiative effects of water vapor and CO2, that would include Iacono and Clough (1995), the MODTRAN and LBLRTM works, numerous papers using the ERBE data, many more papers treating of absolute and relative humidity and why water vapor can't be considered a forcing. Statements that you made are considerably at odds with knowledge of all that litterature, hence the reactions you got.
  22. Peter A wrote: "...the fact that water vapour is a greenhouse gas is never mentioned in the mainstream media. That was my point there, and all I did was question why this is the case." It isn't the case. A simple Google News search proves that this 'fact' is fiction. So the real question is... why do you believe things which are plainly untrue? Did you never bother to check?
  23. As a scientifically ignorant person, and too old to be trained, could I ask if some of you clever chaps could tell me what is wrong with the picture contained on the following site. It purports to show that despite CO2 levels which were often very much higher than at present, temperatures on the planet stabilised around 22C for most of the planet's geological history, with only three 'relatively' short periods, including the present, when temperatures dropped to current levels. The table included on this site is entitled: "Global Temperature and Atmospheric CO2 over Geologic Time". It has a note following saying: "Late Carboniferous to Early Permian time (315 mya -- 270 mya) is the only time period in the last 600 million years when both atmospheric CO2 and temperatures were as low as they are today (Quaternary Period )". The table is allegedly sourced as follows: " Temperature after C.R. Scotese http://www.scotese.com/climate.htm CO2 after R.A. Berner, 2001 (GEOCARB III) " Please don't ask me to study every thread on every page to find out for myself. That would be a bit like saying no-one should make even an implied comment on a subject unless they know every little detail. From: http://www.geocraft.com/WVFossils/Carboniferous_climate.html ------------------------------------------------------- Similarities with our Present World ------------------------------------------------------- Average global temperatures in the Early Carboniferous Period were hot- approximately 20° C (68° F). However, cooling during the Middle Carboniferous reduced average global temperatures to about 12° C (54° F). As shown on the chart below, this is comparable to the average global temperature on Earth today! Similarly, atmospheric concentrations of carbon dioxide (CO2) in the Early Carboniferous Period were approximately 1500 ppm (parts per million), but by the Middle Carboniferous had declined to about 350 ppm -- comparable to average CO2 concentrations today! Earth's atmosphere today contains about 380 ppm CO2 (0.038%). Compared to former geologic times, our present atmosphere, like the Late Carboniferous atmosphere, is CO2- impoverished! In the last 600 million years of Earth's history only the Carboniferous Period and our present age, the Quaternary Period, have witnessed CO2 levels less than 400 ppm. Global Temperature and Atmospheric CO2 over Geologic Time Late Carboniferous to Early Permian time (315 mya -- 270 mya) is the only time period in the last 600 million years when both atmospheric CO2 and temperatures were as low as they are today (Quaternary Period ). Temperature after C.R. Scotese http://www.scotese.com/climate.htm CO2 after R.A. Berner, 2001 (GEOCARB III) Compare Oxygen (O2) concentrations. See an estimate of CO2 concentrations over the past 15,000 years, based on plant stomata. There has historically been much more CO2 in our atmosphere than exists today. For example, during the Jurassic Period (200 mya), average CO2 concentrations were about 1800 ppm or about 4.7 times higher than today. The highest concentrations of CO2 during all of the Paleozoic Era occurred during the Cambrian Period, nearly 7000 ppm -- about 18 times higher than today. The Carboniferous Period and the Ordovician Period were the only geological periods during the Paleozoic Era when global temperatures were as low as they are today. To the consternation of global warming proponents, the Late Ordovician Period was also an Ice Age while at the same time CO2 concentrations then were nearly 12 times higher than today-- 4400 ppm. According to greenhouse theory, Earth should have been exceedingly hot. Instead, global temperatures were no warmer than today. Clearly, other factors besides atmospheric carbon influence earth temperatures and global warming.
  24. Kiwi Bill, You need to be very careful comparing much earlier times to today because physics tells us that solar output was much less. In fact, the prediction by physics that the sun was much cooler before yet the geologic evidence showed liquid water on Earth was known as the "Faint Young Sun Paradox" which is resolved precisely by noting that CO2 levels were higher then. In other words, the Earth's temperature is a consequence of all forcings acting on it, and if the solar irradiance is less then higher levels of CO2 are required to maintain temperatures similar to today. The next question is what happy coincidence was it that caused the Earth to always seem to gravitate towards a certain range of temperatures, rather than e.g. stay stuck in a snowball Earth forever or boil away the oceans? Why were CO2 levels just the right amount higher then to maintain liquid water on Earth? The explanation is absolutely fascinating to me, and explained exceptionally well by Richard Alley in this thoroughly entertaining talk. Essentially, CO2 acts like a thermostat, automatically adjusting its levels to maintain "nice" conditions on Earth over geologic timescales. When temperatures are too low, ice sheets form, covering the rock -- the same rock that normally CO2 would react with to form carbonates. Since the rock is covered by ice, and CO2 is emitted by volcanos, CO2 levels rise until the greenhouse effect is strong enough to melt the ice, expose the rock, and start reacting with the rock like crazy, bringing CO2 levels down again. Conversely, if CO2 levels are too high, the rate of rock weathering increases because it is a temperature-dependent reaction. Since the injection of CO2 by volcanos is roughly constant over long periods of time but the removal of CO2 by rock weathering increases with temperature, this acts to bring CO2 levels down and therefore lowers the temperature. This means that our current massive increase in CO2 will ultimately be corrected by the system but unfortunately it operates on long timescales (compared to what's important for humans) so it doesn't help us with our current problem. Anyway, watch the video because Richard Alley explains it in much more detail and more entertainingly than I could!
    Clearly, other factors besides atmospheric carbon influence earth temperatures and global warming.
    Of course, climate science has always said that, which means your comment "According to greenhouse theory, Earth should have been exceedingly hot" is not correct because "greenhouse theory" is precisely what explains why the temperatures were what they were then despite the fainter sun. The reason for the focus on CO2 today (and to a lesser extent, other greenhouse gasses) is because they are the main factor influencing the change in Earth temperatures in the recent past. If you look at the official explanation for the early 20th century warming you'll see that much of that is put down to an increase in solar irradiance at that time. If there had been a further increase in solar irradiance in the last 50 years then the temperatures would be even higher and the IPCC reports would have apportioned a greater share of the recent temperature rise to that increase. The fact is, however, that over the last 50 years or so it just happens to have been CO2 that is responsible and, luckily, this is one forcing that we can do something about because it was us that lead to that increase in forcing -- without our CO2 emissions the temperature would have cooled slightly over the last 50 years.
  25. JasonB: An excellent explanation. That said, I would be careful in attributing too much of Kiwi Bill's comment as his own. As he notes, he is extracting a commentary from a site (in this case, geocraft) and unless I am mistaken, everything after the line "From: (website link)" is from geocraft. (Correct me if I am wrong, Kiwi Bill.)

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