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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 226 to 250 out of 890:

  1. 225, lancelot, 1. Yes. The term you want to search for is "hindcast" (e.g. "hindcast climate model"). But note that your specific parameters (+/- 0.5 deg C) may be too constrained. There are lots of parameters to be accurately modeled beyond global mean temperatures, and lots of ways in which a model may or may not be accurate, but in particular, models handle climate, not weather. It is nearly impossible to predict weather, and no model attempts to do so. In fact, models work using ensembles (multiple runs with the same starting conditions, which are averaged together). Also, we do not have accurate, precise measurements for starting conditions a thousand years ago. These must be inferred from proxies or just plain guesstimates. We also do not have accurate values for many inputs (solar irradiance, volcanism, etc.). These can only be guessed at. So getting things within any predetermined expectation ("a half a degree in June, 1980") is asking a whole lot, and in the end is not meaningful in evaluating the skill of the model. But yes, obviously one important way to evaluate the models is measure their ability "predict" what we already know has happened. Yes, scientists do this, and yes, models hindcast with enough accuracy to validate their ability to forecast as well. 2. Again, see the answer above. This isn't really a valid criterion. There is a lot more to climate than just temperature, and choosing a point in time is problematic (what if there's a real world ENSO event at that time, but not in the model run?). 3. I don't know that anyone has answered this question in exactly that way, because it's not really meaningful. Various events do affect climate to some degree (active volcanism, solar variations, etc.). There is no real "normal" temperature. Taking an average of the last 2000 years is of little value. Beyond this, proxies are difficult to calibrate and work with. While scientists have done a wonderful job filling in the blanks and trying to infer past temperatures, we just don't have enough global readings (proxies) with enough accuracy to compute anything like what you are asking.
  2. Sphaerica thanks. I was not expecting to find precise 'hindcasts', just quantification of trends. But I appreciate that setting precise start conditions are vital to run a non-linear model, hence the difficulties. My next question would have arisen from the last, but maybe is off topic. How confident are you that all natural forcings have been accurately estimated in the late 20th century? However I see from other parts of this site that this has been addressed. eg Hansen 1988 and subsequent studies seem to give broad agreement that since 1900, natural forcing is estimated as about 0.2 deg c, so the rest 'must be' CO2. I have an innate skeptical difficulty with accepting the simple logic of 'must be CO2', (there can be, and are, other suspects) but I do concede that the greenhouse effect of CO2 is the only mechanism proposed to date which is backed up by both evidence and modelled predictions. So by far the most likely. And the data since 1970 fits very well with the AGW predictions. If it is of any interest, I suppose I would be called a skeptic until a few days ago. In view of Richard Muller's release of the results of the Berkley Earth (BEST) study on October 22, I have significantly reviewed my attitude to the AGW debate. My skepticism of the 'dangerous AGW' theory was based on the following: 1. Distrust (whether right or wrong) of the quality and integrity of evidence of global temperature (GT) increases as presented by NASA/GISS, Hadley CRU, NOAA. 2 Belief, as evidenced by proxy records for last 1100 years, that the current GT was probably not much more than 0.5 deg C above the historic average. 3 Evidence from proxy records that 0.5 deg C was within the range of historic natural variations, and thus plausibly explained as part of natural forcings. 4 Suspicion that the extent of natural forcing may have been under-estimated, and thus the effect of CO2 forcing over-estimated, due to lack of accuracy or completeness of the models. However: 1 I have no reason to doubt that Richard Muller and his team have done a very thorough job. 2 The BEST study indicates a rise of 1.25 deg C from what seems to be the historic mean level over the last 1100 years, at around 1900 AD. 3 1.25 deg C is well above the historic range of proxy natural variation estimates of +/- 0.5 deg C, even at peaks. 4 The rise post-1970 correlates well with the predictions of the IPCC and of Hansen 1988. 5 Therefore, in the absence of any evidence for abnormally high solar forcings, galactic cosmic ray activity, or other natural events, it is no longer plausible to suggest that warming to date is wholly or mainly natural. 6 The greenhouse effect of CO2 is the only mechanism proposed to date which is backed up by evidence and modelled predictions. As a result, on the basis of that evidence, I am bound to say I now give much more credence to the IPCC presentations of the effects of CO2 as a significant greenhouse gas, creating warming well in excess of natural warming, and likely to increase with increasing levels of CO2. From what you say, my logic may be based on seeing too much 'precision' in the proxy records. But that is what it was based on. If you have any more comments I would of course be very pleased to hear them. Thanks for the help in understanding.
  3. lancelot#227: "If it is of any interest, I suppose I would be called a skeptic until a few days ago." There's nothing wrong with being a 'true skeptic;' I suspect most of us are. It is those who wear the cloak of skepticism to hide the mantle of denial that are a problem. As we see from the reaction to BEST, some of those folks are capable of holding mutually contradictory positions and refusing to see any problem with that. However, it appears you have carefully weighed the evidence and reached a reasonable, science-based solution. That's great news!
  4. 227, lancelot, An interesting comment, for sure. I think if I were to pick one aspect out that you should correct, it is this:
    I have an innate skeptical difficulty with accepting the simple logic of 'must be CO2'
    This is a repeated "skeptic" meme that is wholly untrue. Skeptics made it up to belittle the science. I would have the exact same problem if it were remotely true. No one ever said (or would have said) "Oh, shoot, all I can think of is CO2, so let's just assume that's the case and stop thinking there." You recognize this yourself to some degree when you list the other known potential culprits (solar activity, GCRs, etc.) and recognize that there is no evidence to support them. First, to know that there is no supporting evidence, obviously scientists have and continue to investigate them. Secondly, the science does recognize contributions to the system by a variety of forcings, both positive and negative. No one ever said that it was all CO2, and any statement along such lines is really completely missing the point. The scientists have constructed an understanding of the climate that recognizes the ongoing influence of a variety of factors, and has established to some degree of certainty the influence of all of those factors. There would really be no way to measure the expected influence of CO2 without doing so. Any implication to the contrary is just absurd. No one ever said "CO2 and only CO2" and no one ever would. To get to the core of why we do believe that CO2 is a major problem you have to start with the same basic physics from which Tyndal started in 1864, and progress from there. I would very, very, very strongly advise reading Spencer Weart's The Discovery of Global Warming. It is exactly what you need to see, a historical, step by step accounting of how we got from 1864 to today, what scientists really believed and pursued from then until now, and thus why we believe that CO2 is a big, big problem. Please take the time to read that, and good luck. I look forward to seeing more comments and questions by you here at SkS. I'll do my best to help answer anything you may throw out.
  5. 227, lancelot, As one final thought, that you'll discover from reading Spencer Weart's excellent writings... it would interest most skeptics to know that for a very, very long time greenhouse gas theory and its proposed effects on climate were not mainstream science. They were viewed as crackpot or at best unlikely. GHG was once, for a very long time (100 years!), the "skeptical" point of view. It is only since the 1980s that strong evidence had mounted enough to turn that tide and transition GHG theory to the accepted, mainstream belief. Since then, evidence has continued to mount and the pendulum has swayed even further. So when one argues with scientists who now believe in GHG and the influence of massive amounts of CO2 on climate, one is arguing with true skeptics!
  6. 227, lancelot, I'm sorry, one more final final thought, something I'd meant to say. In the end, CO2 is tagged as a culprit not because it's the only thing we can think of, but rather because by looking at the physics of the molecule and the system as a whole, it was predicted a priori, before any such effects could have been detected and entirely without doing any observations, that this would be the end result. When observations of all varieties -- current temperature, humidity, stratospheric temperature changes, paleoclimate studies of past climate change events (the topic of this thread), changing ecosystems, other planets, etc. -- all converge to confirm the abstractly and completely independently predicted outcome, it has to give one pause. Nobody ever looked at the observational evidence and then backed into CO2 as the cause. It was quite the opposite. With our eyes closed, we said "if I punch myself in the face, logic tells me it will hurt." When we opened our eyes and looked in the mirror there was bruising, a bloody nose, a split lip, and a whole lot of pain. Skeptics then went on to say "well, you can't think of anything else that might have hit you, so you immediately presume that it was your own fist."
  7. lancelot - you should probably make a start with Ch6 of AR4, WG1 report. In it, there is diagram (Fig 6.13) showing various reconstructions of temperature with hindcast modelling in grey, giving you a good idea on the uncertainties too.
  8. This paper in a recent edition of Climate Research by Svante Björck would appear to debunk the skeptics argument Current global warming appears anomalous in relation to the climate of the last 20000 years In fact it would seem global climate has NOT changed by as much as it has recently in 20,000 years. The abstract states To distinguish between natural and anthropogenic forcing, the supposedly ongoing global warming needs to be put in a longer, geological perspective. When the last ca. 20000 yr of climate development is reviewed, including the climatically dramatic period when the Last Ice Age ended, the Last Termination, it appears that the last centuries of globally rising temperatures should be regarded as an anomaly. Other, often synchronous climate events are not expressed in a globally consistent way, but rather are the expression of the complexities of the climate system. Due to the often poor precision in the dating of older proxy records, such a statement will obviously be met with some opposition. However, as long as no globally consistent climate event prior to today’s global warming has been clearly documented, and considering that climate trends during the last millennia in different parts of the world have, in the last century or so, changed direction into a globally warming trend, we ought to regard the ongoing changes as anomalies, triggered by anthropogenically forced alterations of the carbon cycle in the general global environment. and there's a pretty good explanation of it at Science Daily Would like to see skeptical science do a feature on this paper please?
    Response: [DB] I have invited Dr. Björck to participate via a guest blog on the paper.
  9. Sphaerica; Re 'must be CO2': I picked up that particular skeptic meme from the SkS intro - How Reliable are Climate Models? [my CAPS] "So all models are first tested in a process called Hindcasting. The models used to predict future global warming can accurately map past climate changes. If they get the past right, there is no reason to think their predictions would be wrong. Testing models against the existing instrumental record suggested CO2 MUST CAUSE global warming, because the models could not simulate what had already happened unless the extra CO2 was added to the model. NOTHING ELSE could account for the rise in temperatures over the last century." Maybe a bit of anti-meme spraying is needed here before those bugs propagate too far? :) Having said that, I know the history of CO2 greenhouse gas theory, Arrenhius et al, and that CO2 was not just plucked out of thin air to 'fill the gap' because noone could think of anything else. When I say myself use the phrase 'must be C02' I refer to the logic that there is a gap between observed warming and estimated natural warming, there are possible 'suspects', and CO2 is the by far the most likely one; that is fine, but I would want to be assured before a lynching that all other suspects have been fully checked out first! GCR/ cloud forcing theory seems to have some credibility in the scientific community, and certainly looked like a good suspect to me for a while. Even biological influence on cloud formation is a possibility. But while those theories are very appealing, the evidence is very slim to date. And the killer blow is : warming seems to be well beyond any natural variations in recent history. I will certainly look into the references given, thanks to all for those. One point which still is unclear to me: Crowley T J, in Science, 14 July 2000, Causes of Climate Change over past 1000 years: Abstract: "Comparisons of observations with simulations from an energy balance climate model indicate that as much [sic] as 41 to 64% of preanthropogenic (pre-1850) decadal-scale temperature variations was due to changes in solar irradiance and volcanism." [That seems to leaves 36 to 59% of variations pre 1850 unaccounted for by the models, which has really troubled me, seeming to cast doubts on the completeness or accuracy of the models] In view of sphaericas' comment (226) on the inability to precisely determine past temperature, how does Crowley arrive at his conclusion with such relative precision?
    Response: No, GCR / cloud forcing theory does not have "some credibility" in the scientific community. See It’s cosmic rays.
  10. scaddenp: CH6 is indeed a great source. Thanks for reminding me.
  11. 234, lancelot, Going bottom to top... Relative precision? He gave a range of 23% for known versus unknown forcings, leaving what I would consider to be pretty wide error bars in his estimates (even if the precision of the boundaries seems fine, the range between them is not -- scientists always use precise numbers to define embarrassingly imprecise estimates -- a science habit, I guess -- they like to be very precise in admitting to how much they don't know). What you term "unnaccounted for" by the models actually is accounted for, but it is not the result of ongoing forcings, but rather internal variability. No matter what we do, the system is still chaotic. Like weather, you are just never going to pin it down completely and make everything add up perfectly so that the model exactly matches reality on date X with temperature Y. It's just not feasible. What is more important to recognize, which is the core of your realization that today's CO2 truly represents a unique event in the planet's climate history, is that the temperature range being discussed for the 1,000 years prior to today's crisis covers a mere +/- 0.1˚C range on average and a +/- 0.2˚C range at extreme events. If close to half of that is attributable to known forcings, then in most periods only +/- 0.05˚C is "unaccounted for" and even in the extremes that number is only +/- 0.1˚C. And the greater swings are generally those that are accounted for by notable solar/volcanic events, which makes the real "natural variation" range very narrow indeed. I can't find a free version of the paper, but the figures and data area available here. Too, it is important to recognize the methodology and limitations that Crowley used. First, the inputs for solar and volcanic forcings for the past 1,000 years are themselves only estimates inferred from available proxies. Inputs for other variations, like ocean currents, land/albedo changes and such are non-existent, so what is left as "natural variability" could well have been accounted for with better inputs. Secondly, the temperatures against which he is comparing are also only proxy estimates, so the error range is by necessity broad. Who knows... it's possible that with precise solar and volcanic forcing inputs and temperature outputs, the accountability might be much greater. You have to consider the fog through which the scientists is attempting to squint to get to the inference. On to the last item, the "CO2 must cause" statement. The phrasing is unfortunate, but the take away message should not be "we can't think of anything else so the thought process stops here" but rather "everything we've tried does not account for the warming, and yet CO2 does." CO2 is not the culprit simply because someone saw it hanging around on the street corner the same night that today's Climate was murdered. His fingerprints are in fact all over the gun, and he was the only one with the key to the locked room in which Climate's body was found. What is unsaid on that page (and maybe it should be said to make things more clear) is that this conclusion was reached because models do incorporate every known forcing and none of them did the job. The use of every known forcing successfully models temperatures up to the current time. The dual implication of this is not only that CO2 appears to be the culprit, but also that if there were another, unknown forcing -- why did it never influence anything prior to the current dilemma? Where were similar dramatic changes in GCRs, solar activity and such for the last two thousand years? We know why there were no swings in CO2, but why no swings in any other magically hidden forcings? What's also left unsaid, as we have discussed, is that it is not like scientists then close their eyes to any other possible forcing. There are tens of thousands of scientists around the globe studying everything imaginable, from the Amazon to the Arctic to black carbon to land use changes to methane deposits to solar forces beyond our current understanding. No one is stopping their thought process and saying "let's focus on CO2 because it makes our job easier." Quite to the contrary, scientists would like to find, understand and pin down every forcing imaginable, because in so doing that 41% to 64% range that worries you drops to zero, all doubt vanishes, and the climate models become marvelously accurate. Such an accounting "benefits CO2 proponents" (as if anyone is a sports fan rooting for CO2 in a game) as much as anyone. By the way: Here at SkS it's considered bad form to use all caps (it's shouting). Instead use italics or bold for emphasis. You can click on the "Click for tips on posting images or hyperlinks" link just below the "Post a Comment" text box for instructions on how to do so (or just click this link here).
  12. lancelot#234: "... assured before a lynching that all other suspects have been fully checked out first!" Odd statement. What makes you think that climate science doesn't work that way? If there were other reasonable candidates for causal agents, who wouldn't be checking them out? "GCR/ cloud forcing theory seems to have some credibility in the scientific community, and certainly looked like a good suspect to me for a while." Now there's an example of the thinking process you seem to feel is out there. There are many whose first principle is 'it can't be CO2' and therefore climb on to any bandwagon that happens to pass by. GCR threads are plentiful here (use Search), so I won't go off topic any further than to say that this idea had neither a reliable mechanism nor a reliable history and yet it still gained traction among the 'skeptics' - because it wasn't AGW. A fine example of not fully checking things out before making a lot of extravagant claims.
  13. 234, lancelot, Forgive muoncounter for bristling. GCRs are a touchy subject. The fact is that the potential mechanism has always been taken seriously to some extent, but the mechanism is proposed, but still vague and unproven, some poor papers have used bad techniques to try to prove correlations that did not exist, there are no matching temperature changes to pair with identified major cosmic ray changes in the paleoclimate record, and most importantly there are no recent cosmic ray changes to parallel the temperature changes we've seen. This is not new knowledge. These things we've known for a decade or more. Still, the idea was never dismissed out of hand, and still isn't. It is being given its due, and researched as far as is worth the effort. But it has never been a serious "contender" to explain the warming trend because of the long known issues just described. But as muon said, there are other posts and threads to discuss that. We don't want to drag this thread any further off topic (which is another habit here at SkS... people like to keep threads focused, rather than wandering all over where ever the conversation decides to go).
  14. Sphaerica, Thanks again. My background is building and engineering, which tends to rely on fairly precise calculations, hence my expectations for precision. Your point is well taken that Climate is never going to oblige my personal wishes by making itself easy to quantify. As for 'must be Co2' I take all your points; the SkS intro on that might benefit from some re-phrasing, to avoid possible misunderstandings by others about the 'thought processes'. I do see that certain words such as G*R prompt emotive reactions. Not a reason to jump on the speaker please. I am not a climate scientist,I rely a lot on pre-digested reports. I see no reactions to the idea of micro-organisms having a role in cloud formation. Maybe grasping at straws with that, and surely way off-topic. But I am currently involved in supporting research into micro-organisms in the air and in precipitation, so I take a keen interest. Maybe we can breed some bugs to seed clouds and cool the planet? (Just half-serious on that one) Thanks for taking the time to respond in such detail.
  15. GCRs do not prompt emotive reactions. People who claim GCRs are the solution to all their climate change problems, without the benefit of legitimate analysis, prompt emotive reactions.
  16. @DB #233 - great DB - thanks - hopefully he will be interested and I look forward to a possible future article on SS :)
  17. Coming back on natural causes: Sunshine records in S. England apparently show increasing hours of daily sunshine since 1970. I don't see any mention in IPCC reports of sunshine hour records, analysed globally. (If I have missed a mention, could you point me to one.) Regardless whether any mechanism other than GCR to influence cloud cover is conceivable, is sunshine hours not a relatively simple check on possible natural forcing which should be considered, separately from solar irradiance?

    [DB] "Sunshine records in S. England apparently show increasing hours of daily sunshine since 1970."

    Assertions without source citations tend to get ignored.  Unless you can provide a source for your claim?

  18. lancelot @242, it is true that sunshine hours have increased by about 7% in the UK over the period from 1974 to the present: It is certainly not clear that this is a forcing, ie, an phenomenon independent to temperature which drives temperature change, rather than a feedback, ie, a phenomenon largely controlled by temperature which in turn drives temperature change. More importantly, it is far from clear that this is a global phenomenon. Certainly in China the trend has been in the opposite direction towards less sunlight hours. A similar reduction of sunshine has been found in Switzerland, so the observed increase in the UK is not even a Europe wide phenomenon, let alone a world wide phenomenon. That the trends are opposite in different locations around the globe strongly suggests the cause is not astronomical (GCR) but rather global or regional climate conditions. That is, the phenomenon is likely a climate feedback or a response to major oceanic variations. Certainly the UK data shows a correlation to the Atlantic Multi-decadal Oscillation. But the AMO may itself just be an artifact of the pattern or Northern Hemisphere warming and cooling due to variations in CO2 and aerosol levels during the 20th century.
    Response: [Sph] Typo corrected.
  19. Ok, following on from the data Tom cited I found that this is based on something called a 'Campbell-Stokes recorder' which basically focuses sunshine to burn holes through a card. There is some subjectivity in determining whether a hole was burned through by 15 minutes or 30 minutes of bright sunshine, but overall it is a measure of sunlight reaching the ground at that location. Thus, a reduction in smog could result in an increase in sunlight hours... as could local changes in cloud cover. I'm not sure this measurement can really tell us much about global temperatures... even if an extensive network exists. It only has two 'intensity levels' for sunlight... either there is enough to burn through the card or there is not. Yet the 'not' could be anything from complete blackness to just barely not enough while the 'enough' could be anything from just barely enough to two, three, four, et cetera times as much as needed. Basically, I'm saying that the 'resolution' of the data doesn't seem sufficient to make any determination of the total sunlight experienced.
  20. 242, lancelot, To clarify/summarize Tom's points: 1) How do you know the change is not a result of GHG warming (i.e. one form of positive cloud feedback) rather than the cause? 2) How do you know the change does not have to do with decreased aerosols (due to less pollution or changed atmospheric patterns)? 3) How do you know that a single regional effect is global? To answer your direct question: scientists do measure the albedo of the earth (which is sort of the exact opposite of sunlight hours) in a variety of ways. I'm not sure if anyone has tried to globally measure sunlight with ground based stations, but you can see that this effort would no doubt suffer from the same issues that "plague" ground based temperature observations ("the Urban Smog Effect is artificially reducing solar irradiance measurements! It's all a farce!"). I don't know if the spat of lost (or warehoused!) satellites would have helped here, either. But it's not like scientists are stupid, or aren't trying.
  21. Sphaerica @245, thanks for the correction of my typo, and the summation. Your point (2), however, though perfectly valid, is not in my original post. In essence, it is known that increased aerosol numbers increases cloudiness by increasing the duration of clouds. If clouds take longer to dissipate, but form at the same rate, the net result will be more clouds in the sky at any give time - and hence less sunlight hours. Conversely, if aerosols are reduced, as happened in the UK after the passage of clean air acts in the 1970's, clouds will dissipate quicker, resulting in less cloudiness, and more sunlight hours. If this phenomenon is a major driver of changes in sunlight hours, that would explain why sunlight hours have increased in the UK (with reduced aerosol emissions) but decreased in China (which has increasing aerosol emissions). Of course, if that is the major driver, than this is a subject climate scientists are actively studying very closely. It is know as the Cloud Life Time effect, and is one of several indirect aerosol effects.
  22. 246, Tom, Sorry, I added 2 myself after I'd written the response and forgot about my own lead in statement.
  23. Change in sunshine hours is surely just a measure of changing cloudiness. Satellite records would provide far better global measure of this rather than station records. IPCC reports certainly mention cloud cover.
  24. The full term you are interested in is "bright sunshine hours", and it has a formal WMO definition (CIMO Guide chapter 8) based on direct beam solar radiation - you accumulate bright sunshine when direct solar exceeds 120 W/m². The Campbell-Stokes instrument is the classic, but has its limitations. The Campbell-Stokes instrument has been around much longer than the formal WMO definition. There are several other manufacturers of more modern instruments (e.g., Kipp and Zonen). There are long records of Campbell-Stokes data around the world, but the data is of quite limited value compared to actual measurements of solar radiation. I think sunshine hours was part of the whole "Global Dimming" craze a few years ago. Using the SkS search tool gives a few hits for "global dimming", and RealClimate had at least one discussion of it. A good place for global surface radiation data is the Baseline Surface Radiation Network. A fairly recent paper that looks at some of this is Wild 2009.
  25. Scaddemp: IPCC reports do mention cloud cover: AR4: "Significant uncertainties, in particular, are associated with the representation of clouds, and in the resulting cloud responses to climate change. " "Cloud feedbacks remain the largest source of uncertainty in climate sensitivity estimates." Not very definitive! Bob Lodlaw 129: That BSRN is really interesting work. Sadly I am not equipped to analyse the BSRN data, and I dont have access to the Wild 2009 paper right now. But I will keep looking into it and would like to come back to you on that subject. However my point was not really about 'bright sunshine' but about the net energy received at earth surface. Scaddenp 248: If it useful to know the energy actually received at earth surface, as a possible contribution to forcing, I cannot see point of measuring incoming solar irradiance by satellites at the exosphere, then making more satellite observations of 'cloudiness' to make an estimate how much actually gets through to earth surface,instead of simply measuring the actual energy received at the surface in the first place. That is in essence, and crudely, what sunshine hour records tend to do. It is not hard to measure the surface energy received per day - a PowerMonkey and a calorimeter I would imagine. I have not found data on surface energy receipts in the IPPC reports, other than the simplistic 'energy balance' diagrams. How do climatologists know the trends or variations of net energy received globally at the earth surface? How can you account for that or eliminate it as a possible forcing in models if you don't have the data? Tom Curtis, I take your point that England is not the world. Surely all the more reason to have a global measuring system such as BSNR. Sphaerica, you mention aerosols etc, but my point was about cloud forcing. Are you certain that there are no possible mechanisms driving cloud formation, other than those considered in AR4? GCR's may be out of the picture; I have mentioned micro-organisms a number of times in this forum, but the response has been a stony silence. Is that because readers are much too kind to lambast me for my stupidity in thinking that the airborne biosphere might(repeat might!) have a part to play in climate?

    [DB] An open copy of Wild 2009 can be found here.

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