tag:blogger.com,1999:blog-5337555368793819627.post5335505156721965658..comments2023-06-07T09:04:36.390-04:00Comments on More Grumbine Science: Results on deciding trendsRobert Grumbinehttp://www.blogger.com/profile/10783453972811796911noreply@blogger.comBlogger26125tag:blogger.com,1999:blog-5337555368793819627.post-66884897505749659612013-08-17T18:43:25.314-04:002013-08-17T18:43:25.314-04:00I humbly recommend:
http://bartonpaullevenson.com...I humbly recommend:<br /><br />http://bartonpaullevenson.com/30Years.html<br /><br />and<br /><br />http://bartonpaullevenson.com/NoWarming15Years.htmlBarton Paul Levensonhttp://bartonpaullevenson.comnoreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-53636264173228384742011-10-13T19:48:16.761-04:002011-10-13T19:48:16.761-04:00Another cross-reference (there's no convincing...Another cross-reference (there's no convincing people about statistics, as this seems to prove again): <br /><br />http://www.skepticalscience.com/news.php?p=2&t=58&&n=1025#65219<br />(Pielke Senior in the comments)Hank Robertshttps://www.blogger.com/profile/07521410755553979665noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-87886149900071352762011-07-16T20:29:46.074-04:002011-07-16T20:29:46.074-04:00A cross-reference:
http://tamino.wordpress.com/20...A cross-reference:<br /><br />http://tamino.wordpress.com/2011/07/16/trend-and-noise/Hank Robertshttps://www.blogger.com/profile/07521410755553979665noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-57577017936607233482010-11-29T09:57:14.707-05:002010-11-29T09:57:14.707-05:00Hank:
Never too late. Science is a long-term disc...Hank:<br />Never too late. Science is a long-term discussion. That's why comments are still open.<br /><br />For once, I might already have addressed one of your questions. The post is aimed at the average, rather than trend, but the basic mathematics involved are the same:<br />http://moregrumbinescience.blogspot.com/2010/03/how-can-annual-average-temperatures-be.htmlRobert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-9348090479485976142010-11-28T16:50:56.718-05:002010-11-28T16:50:56.718-05:00Rather belatedly, but as I keep pointing to this t...Rather belatedly, but as I keep pointing to this thread -- I wonder if you can explain with the same basic approach how taking more instrument observations is also a way to reduce uncertainty or increase precision well beyond the precision available from the individual instrument on one observation?<br /><br />I'm thinking of the obvious -- temperature weather stations -- but also thinking of another issue, satellite GPS for elevation in looking at glacial rebound and sea level rise -- where I've seen people claim (a "professional surveyor" was cited) that because one instrument making one measurement is is only accurate to plus or minus X, the science can't claim any better precision using multiple instruments in multiple passes.<br /><br />I'm going from my vague recollection from Stat 101 in the 1970s, thinking about things<br />like soil loss on a mountainside, taking a few observations over decades was comparable to taking many more observations over a few years, to have a reasonable chance of detecting a trend.Hank Robertshttps://www.blogger.com/profile/07521410755553979665noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-23875135868690280992010-03-17T16:10:31.539-04:002010-03-17T16:10:31.539-04:00Another perhaps useful example of how this is figu...Another perhaps useful example of how this is figured out for a particular data set -- and how it requires first accumulating enough data to have an idea of natural variability, when it's a new instrument:<br /><br />http://www.biogeosciences.net/7/621/2010/bg-7-621-2010.pdf<br /><br />"... over ten years of satellite-derived chlorophyll and productivity data have accumulated, can we begin to detect and attribute climate change-driven trends in productivity? Here we compare recent trends in satellite ocean colour data to longer-term time series from three biogeochemical models (GFDL, IPSL and NCAR). We find that detection of climate change-driven trends in the satellite data is confounded by the relatively short time series and large interannual and decadal variability in productivity. Thus, recent observed changes in chlorophyll, primary production and the size of the oligotrophic gyres cannot be unequivocally attributed to the impact of global climate change. Instead, our analyses suggest that a time series of ∼40 years length is needed to distinguish a global warming trend from natural variability. In some regions, notably equatorial regions, detection times are predicted to be shorter (∼20−30 years). Analysis of modelled chlorophyll and primary production from 2001–2100 suggests that, on average, the climate change-driven trend will not be unambiguously separable from decadal variability until ~2055. Because the magnitude of natural variability in chlorophyll and primary production is larger than, or similar to, the global warming trend, a consistent, decades-long data record must be established if the impact of climate change on ocean productivity is to be definitively detected."Hank Robertshttps://www.blogger.com/profile/07521410755553979665noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-3095822447284003192010-02-14T13:10:55.594-05:002010-02-14T13:10:55.594-05:00Bob,
Thanks for the link to the code. I was look...Bob,<br /><br />Thanks for the link to the code. I was looking at this for data in a different realm, and coded up something similar in R. I haven't generalized it much, but it looks like a similarly sized window applies to my realm as well.<br /><br />I tried it with non-overlapping regions, and it didn't see a significant difference, but the trend in my data is fairly stable.<br /><br />Re. John Mashey's point: Numerically, the backwards looking trend from t=today is the same as the middle-centered trend around t=today-window/2. They are just different names for the same quantity. For any decision made at t=today, you'd have exact same information available no matter which name you give it.Dave Fnoreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-49237847502083096492010-02-13T09:34:36.529-05:002010-02-13T09:34:36.529-05:00Dave:
I've updated the article with the link, ...Dave:<br />I've updated the article with the link, and will repeat it here -- http://www.radix.net/~bobg/blogsupport/trend_results.tar<br /><br />The trend computations are repeated every 12 months. When you're looking at a 25 year period for the trend, it's a long time before you have independent trend estimates. Actually somewhat more than the 25 year period itself -- the weather is autocorrelated, so we need a bit of time to elapse between 25 year spans before they are truly independent.<br /><br />Optimal, of course, depends on what you want the trend estimates for. My concern here was the simple business of 'how long do you need in order for your trend estimate to not be sensitive to how long a period you chose'. That's clearly in the 20-30 year range.Robert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-59307581960953496822010-02-13T02:02:22.865-05:002010-02-13T02:02:22.865-05:00Thanks for this.
Two questions:
1) Where is yo...Thanks for this. <br /><br />Two questions:<br /><br />1) Where is your website with the programs?<br /><br />2) Do the trend-windows overlap in the analyses for plots 1 & 2, or are they disjoint? <br /><br />It seems like successive slopes would become more significantly autocorrelated as the trend length window increases, which would also lower the variance of the slopes.<br /><br />Alternately, if you estimated the slopes over independent chunks of data, you'd have a smaller number of slope observations which might increase your variance estimate. Perhaps that would give guidance on an optimal window length.Dave Fnoreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-13402862806315610302009-06-14T18:17:30.811-04:002009-06-14T18:17:30.811-04:00Yes. My issue is the pedagogy, and good articulat...Yes. My issue is the pedagogy, and good articulations & especially good visualizations of:<br /><br />a) At any point, what do we know, and how much uncertainty is there?<br />(both in measurements, and in computed trends).<br /><br />b) Now that we have more data, what more do we know *was* happening?<br /><br />The goal is explaining this better to the general audience.<br /><br />Your trends posts are helpful.John Masheyhttps://www.blogger.com/profile/17786354229618237133noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-33108879645694114322009-06-14T18:17:23.564-04:002009-06-14T18:17:23.564-04:00Yes. My issue is the pedagogy, and good articulat...Yes. My issue is the pedagogy, and good articulations & especially good visualizations of:<br /><br />a) At any point, what do we know, and how much uncertainty is there?<br />(both in measurements, and in computed trends).<br /><br />b) Now that we have more data, what more do we know *was* happening?<br /><br />The goal is explaining this better to the general audience.<br /><br />Your trends posts are helpful.John Masheyhttps://www.blogger.com/profile/17786354229618237133noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-80910648103340708652009-06-13T22:54:38.746-04:002009-06-13T22:54:38.746-04:00Hank: I've been running around a lot lately, a...Hank: I've been running around a lot lately, and will be for another week, so catching up is a challenge. The comment/question you link to is a good one, though, and worth a more direct response than can be had from the notes I've already written. I've put a placeholder comment over at realclimate.<br /><br />His question, I think, is different from what you think. Namely, I used only 120 (or 150, depends on which climate note) years of data. And, those 120 years are special -- they're the period where for at least part of it, we argue that human activity has affected the climate system in significant ways. So a) do you get the same answers if you use 120 years that are definitely not affected this way, b) do you get the same answers if you use 1000 years to look at the question (maybe from 1500-1620 30 year averaging doesn't have the properties I've demanded). I've at least eyeballed enough graphs of annually-resolved data to be confident myself that the 30ish year period will hold up. But that's my eyeballs. A post with directly relevant illustrations is much better.<br /><br />Your suggestion of a post which collects links and provides an overview to my assorted notes on trying to figure out what climate is is a good one. I'll be getting to that, perhaps after one more individual post which will look at setting the upper bound on a reasonable climate period.<br /><br />John:<br />As a matter of pragmatics, I agree that for the recent period, we're stuck with using trends computed with data ending today rather than centered here. I think we do want to wave a flag that this is something we do with reluctance. If we don't, or can't, use trends centered on the date of interest we're subject to the same mistake as people in the late 1960s to mid 1970s could have made in thinking that they were in a cooling trend. Time centered shows that they weren't. You can't know what the next 15 years will be (though we're working on it of course :-). <br /><br />I agree that in reading history, you can't blame people for not knowing what was going to happen in the next 15 years. Depending on situation, though, I might be critical of them for not allowing for the fact that they <i>don't</i> know what the next 15 years will look like. <br /><br />The backward trends, since they're computable all the way to the present day, I agree, have their pedagogical uses. So does, for example, my <a href="http://moregrumbinescience.blogspot.com/2008/09/summary1-of-simplest-climate-model.html" rel="nofollow">Simplest climate model</a>. Both just need to be used with caution, which I realize you do.Robert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-21784804475037606692009-06-09T11:17:09.029-04:002009-06-09T11:17:09.029-04:00As per discussions at Deltoid, #56-58, let us talk...As per discussions at <a href="http://scienceblogs.com/deltoid/2009/06/always_click_on_the_links.php#comment-1688982" rel="nofollow">Deltoid, #56-58</a>, let us talk about centered vs backwards trends.<br /><br />Of course, I agree 100% that centered trends are the most accurate description of what was happening in some year.<br /><br />Unfortunately, since "data from the future is unavailable at this time", I've found this awkward to explain to people, and sometiems not very effective.<br /><br />It is unsatisfying to many people to be told: "we really can't say much about what's happening now until 10-15 years from now", especially in competition with those who think 7 years is a definite trend :-) and say so.<br /><br />But, it seems to me that there is a motivational & pedagological role for backward trends (n addition to centered), as I said at Deltoid:<br /><br />"Consider the question: at the end of any year in the 20th century, if we do the linear regression from year-29 to year, what would we have thought was happening?"<br /><br />Especially in looking at history (of science or anything else), people generally resonate to "With the information available at the time, the belief was... or the decision was...", whereas centered/forward is taking advantage of hindsight, and for many people, that just doesn't "feel good". <br /><br />Among other things, using <a href="http://i41.tinypic.com/2i88s2.jpg" rel="nofollow">my Fig 1.</a>, since it has the actual years on it, shows it was plausible for people in the 1960s to have been thinking that a clear (but modest) cooling trend had been established.John Masheyhttps://www.blogger.com/profile/17786354229618237133noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-17289626978844774972009-06-06T20:29:45.341-04:002009-06-06T20:29:45.341-04:00Three notes -- Gavin found the site I was remember...Three notes -- Gavin found the site I was remembering (above), and it's been withdrawn, they'd oversimplified their advice for field workers, apparently. <br /><br />On the question of how to make sure you have a big enough sample that it doesn't affect the result -- I think this question over at RC is for you; it's so far down in a huge thread on a different subject, you may not see it; it arose from my pointing the fellow here and he's read some of your Trends posts, so http://www.realclimate.org/index.php/archives/2009/05/moncktons-deliberate-manipulation/langswitch_lang/sp#comment-126244<br /><br />I think he wants to know how you can be sure 30 years is enough, and why won't 100 or 1000 change the result. But I wasn't understanding his questions well, so over to you if you like.<br /><br />Third loose end -- would you list your several Trends posts in one place with a pointer to the samples you posted on the web for people to practice on? I'm not sure I've found everything and don't know how to best point someone to the whole.Hank Robertshttps://www.blogger.com/profile/07521410755553979665noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-43113084052127881492009-01-10T16:59:00.000-05:002009-01-10T16:59:00.000-05:00I will argue with Eli to save 'trend' and 'change'...I will argue with Eli to save 'trend' and 'change' as simple words understood by nonscientists.<BR/><BR/>Google define:trend<BR/>and define:change<BR/><BR/>Back on topic -- long ago I had found a FTP site somewhere that made available a way for budding new field workers to figure out what statistical value they could get from their work.<BR/><BR/>Things like -- what will you measure, how often will you measure it, how many different places on your site will you measure it -- and then help deciding whether to choose to detect "up or not" or "down or not" or "up, no change, or down" --- and then told them how many years, or how many site visits, or how many sample observations, they'd have to add to their research plan to have a hope of detecting a change.<BR/><BR/>I wonder if there's still something like this around. I haven't found one recently.<BR/><BR/>It was humbling. I'd started a forest fire revegetation by doing a baseline botany survey, and then making some changes on some of the grid squares. The site told me to come back in 200 years to have some clue if anything I'd done would help. This confirmed what the several good biologists I knew told me -- and really emphasized the value of doing a solid baseline and recording it somewhere someone could find it later and be able to use it.Hank Robertshttps://www.blogger.com/profile/07521410755553979665noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-36744313581883001932009-01-10T09:19:00.000-05:002009-01-10T09:19:00.000-05:00Hank: Thanks for the positive mentions. I don't ...Hank: Thanks for the positive mentions. I don't see anything novel in the referral logs or google. Google only shows your comments at realclimate, Jules' Klimaatblog, and one other blog. Mostly just looks like people are coming back from holidays.<BR/><BR/>Still, a different hope I have is to write things that people can use elsewhere.Robert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-52930048522254055972009-01-10T08:08:00.000-05:002009-01-10T08:08:00.000-05:00dko, anon(P. Lewis) on tipping points.One of the t...dko, anon(P. Lewis) on tipping points.<BR/><BR/>One of the things I don't like about the term is that, as the comments by the two of you illustrate, it seems to mean rather different things to different people. On the more minimal side, which it looks like dko is taking, I agree that the Arctic ice changes illustrate a tipping point. On the more major side (commitment to systemic global change), I'd agree with P. Lewis that it doesn't.<BR/><BR/>2007 alone doesn't, I think, show a tipping point. The year was meteorologically unusual in the Arctic, with multiple parts (clouds, temperatures, winds) all aiming towards destroying the ice cover. And it did so with great abandon. The mark of a tipping point is that when other parts of the system return to more normal (as 2008 was in the Arctic melt season), your part of interest does <I>not</I> show the formerly normal response. In 2008, with more or less typical weather, there was not a more or less typical ice minimum. We repeated the wildly unusual minimum of 2007.<BR/><BR/>As dko suggested, you can't draw your lines through different sides of a tipping point. We have a clear change in summer 2007. Extents at other times of year haven't (yet) shown such a dramatic change. But for August-September, we're now in a new regime.Robert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-12482210871440681862009-01-10T07:38:00.000-05:002009-01-10T07:38:00.000-05:00Hank, Eli: I see there's a language issue involved...Hank, Eli: I see there's a language issue involved. The distinction we're all drawing is between a number that you can compute (and, Eli, I've seen slopes presented based on zero data points -- young earth creationists and other such people), and a number that means something about the physical system.<BR/><BR/>I think trying to use change or slope for the merely computable, and trend for the meaningful number runs in to the problem that many of these distinctions do of people forgetting which is which (accuracy vs. precision anyone?), or actively disagreeing about which is which. <BR/><BR/>What I think I'll do is go for 'meaningful trend', rather than merely trend, for the one that is, and slope or change for the merely computable. The distinction is important. But I'm not sure how to make it clearly without adding words (like meaningful).Robert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-68067719667211681362009-01-09T19:31:00.000-05:002009-01-09T19:31:00.000-05:00Regarding dko's comment.Isn't use of "tipping poin...Regarding dko's comment.<BR/><BR/>Isn't use of "tipping point" a step too far at this juncture?<BR/><BR/>I thought the major reason for the 2007 losses was put down to wind and sea conditions being favourable for forcing the ice out of the Arctic basin.<BR/><BR/>Of course, higher temperatures played a part in making the ice thinner and more amenable to being compressed and "transported" by the winds. But I thought that one reason 2008 wasn't another record low year was precisely because the synoptic conditions weren't (as) favourable this last summer as they were in 2007.<BR/><BR/>P. LewisAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-54211573134831104942009-01-09T19:18:00.000-05:002009-01-09T19:18:00.000-05:00PS, for early warning, google your own topic title...PS, for early warning, google your own topic title; far more pointers to it are out there than the few I posted, many from blogs I've never heard of. Apres moi, le deluge.Hank Robertshttps://www.blogger.com/profile/07521410755553979665noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-86050323965392389422009-01-09T19:11:00.000-05:002009-01-09T19:11:00.000-05:00Having pointed a lot of innocents to this thread, ...Having pointed a lot of innocents to this thread, I feel it might help to elaborate on Eli's point. <BR/><BR/>Would it be clearer to say if we have annual temperature numbers, for two years (any two, not necessarily contiguous) we have a change. Because we don't have a series, we don't know what's going on.<BR/><BR/>If we had daily measurements of temperature, and took two of them, we would have a change, but not a trend. Again we don't know what's going on, without measuring at many intervening points -- how many? Depends, which is the point of your excellent topic.<BR/><BR/>And by contrast, <BR/><BR/>If we have, say, the height of an oak sapling for two years, we have a trend; we know that there is an ordered time series -- trees never grow smaller -- although we haven't made a measurement every day or every year.Hank Robertshttps://www.blogger.com/profile/07521410755553979665noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-31643219513015018592009-01-09T13:28:00.000-05:002009-01-09T13:28:00.000-05:00In response to quasarpulse, you wrote:"In the case...In response to quasarpulse, you wrote:<BR/><BR/>"In the case of the Arctic sea ice, the 2007 ice was something like 6 standard deviations away from the mean. In the case of global mean temperatures, we're not looking at such extraordinarily large jumps."<BR/><BR/>I think that is a good example of a tipping point. Temperature a little warmer produced a sudden, non-linear result -- the stunning summer 2007 breakup.<BR/><BR/>Around tipping points statistical measures (trend, deviation) become useless. Tread carefully, ever mindful of thin ice....Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-76123129761736659072009-01-08T10:16:00.000-05:002009-01-08T10:16:00.000-05:00You can always compute a slope given two points (o...You can always compute a slope given two points (otoh if you have every taught you know some students make do with one), however the slope is not a trend. It only becomes a trend if there is a time series with a significant number of points. You have actually shown this <BR/><BR/>The reason for being picky (and right) is that if you stick with two points being able to define a trend you are going to be run over by the pseudos.EliRabetthttps://www.blogger.com/profile/07957002964638398767noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-53501596714193717532009-01-06T09:15:00.000-05:002009-01-06T09:15:00.000-05:00Eli: Not sure where you're seeing the conflict be...Eli: Not sure where you're seeing the conflict between me and the definition you cite. One can always compute the slope and call it a trend. Given, however, the presence of cyclic and irregular contributions, that computation (as I illustrated) will be unreliable. In the case of global mean temperature, we need 20-30 years to define a trend.<BR/><BR/>Quasarpulse: If things happen that are very far from the climatology of the weather, then, sure. In the case of the Arctic sea ice, the 2007 ice was something like 6 standard deviations away from the mean. In the case of global mean temperatures, we're not looking at such extraordinarily large jumps.<BR/><BR/>In the vein of extraordinarily large deviations was (a few years ago now) the first thunderstorm on the north slope of Alaska, and the first butterfly. Both were so extraordinary that the Eskimo had no words for them.Robert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-26098335710957236822009-01-06T05:27:00.000-05:002009-01-06T05:27:00.000-05:00Isn't it possible, though, to label a particularly...Isn't it possible, though, to label a particularly large change (e.g. the record Arctic melt in 2007) as significant even when it's too early to calculate a 20-year trend around the year? It should be possible to identify a "weather" element as a leading indicator of a "climate" change by comparing it to the variance over some sufficiently large timescale, shouldn't it?<BR/><BR/>If, for instance, the average temperature in Portland this January was 45 below, the Columbia froze solid, and we were buried in seven feet of snow, shouldn't it be possible to say "That's not normal!" with some degree of scientific certainty?quasarpulsehttps://www.blogger.com/profile/08762550806982089851noreply@blogger.com