Two decades ago, the so-called “Hockey Stick” curve, published in 1999 by me and my co-authors (Mann, Bradley and Hughes, 1999), was featured in the all-important “Summary for Policy Makers” (SPM) of the 2001 IPCC Third Assessment report. The curve, which depicted temperature variations over the past 1000 years estimated from “proxy data such as tree rings, corals, ice cores, and lake sediments”, showed the upward spiking of modern temperatures (the “blade”) as it dramatically ascends, during the industrial era, upward from the “handle” that describes the modest, slightly downward steady trend that preceded it.
The Hockey Stick became an icon in the case for human-caused climate change, and I found myself at the center of the contentious climate debate (I’ve described my experiences in “The Hockey Stick and the Climate Wars”).
Featured two decades later now in the AR6 SPM is a longer Hockey Stick with an even sharper blade. And no longer just for the Northern Hemisphere, it now covers the whole globe. The recent warming is seen not only to be unprecedented over the past millennium, but tentatively, the past hundred millennia.
The relevant statements in the SPM and Technical Summary are:
A.2.2 Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years (high confidence). Temperatures during the most recent decade (2011–2020) exceed those of the most recent multi-century warm period, around 6500 years ago13 [0.2°C to 1°C relative to 1850– 1900] (medium confidence). Prior to that, the next most recent warm period was about 125,000 years ago when the multi-century temperature [0.5°C to 1.5°C relative to 1850–1900] overlaps the observations of the most recent decade (medium confidence). {Cross-Chapter Box 2.1, 2.3, Cross-Section Box TS.1}
SPM AR6
Global surface temperature has increased by 1.09 [0.95 to 1.20] °C from 1850–1900 to 2011–2020, and the last decade was more likely than not warmer than any multi-centennial period after the Last Interglacial, roughly 125,000 years ago.
Cross Section Box TS.1
As the new IPCC report lays bare (you can find my full commentary about the new report at Time Magazine), we are engaged in a truly unprecedented and fundamentally dangerous experiment with our planet.
References
- M.E. Mann, R.S. Bradley, and M.K. Hughes, "Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations", Geophysical Research Letters, vol. 26, pp. 759-762, 1999. http://dx.doi.org/10.1029/1999GL900070
It’s warmed by about .2 degree in those twenty years since MBH98-99… So AR6 blade is slightly higher… This is probably you mean Mike, by calling the hockey stick “sharper”. Otherwise the overlapping parts of the two sticks look very similar, MBH98-99 is rock solid.
Chris – Totally agree. It is a little sad that anyone seriously disputes the findings of the original multiply-confirmed stick studies, however 20 years later I find I still need my file of arguments (The PCA myth, the Bristlecone myth, the Red noise myth etc) to deploy in online debates with self-described sceptics. I guess they find it hard to accept a chart with such a strikingly clear visual message.
Obviously, it is great that Dr Mann and his co-authors have been (repeatedly) vindicated, but as with the climate predictions now being fulfilled on a regular basis (read the news) it is something of a bittersweet victory.
AR6 concludes that rising global surface temperatures are reaching levels unseen since at least the Last Interglacial, around 125,000 years ago. This compares to AR5, which concluded that the Northern Hemisphere had warmed to levels unseen in 1400 years. What’s changed?
First, it’s warmer now. Estimated global surface temperature over the last decade is nearly 0.3°C higher than reported by AR5, eight years ago. (That’s 0.19°C of further warming since AR5 plus 0.1°C due to better methods and new data).
Second, there’s no going back, at least not for centuries. Even the low emissions scenario shows that temperatures at least as high 1°C warmer than 1850-1900 will likely persist for centuries. Changes in the ocean will likely continue for thousands of years. Looking both backward and forward enables a like-to-like comparison with multi-century-scale changes reconstructed from the paleoclimate data.
Third, there’s more paleoclimate data available now. The global coverage of proxy records has increased, and their temporal resolution and age control has improved. Major new global data compilations have benefited from internationally coordinated campaigns and from better practices for data reuse.
…Looking more and more like a scythe, appropriately.
Yes, we say Mann with the scythe (ljå) in Norway, and that is what is so funny because obviously so threatening.
Thus what makes people so anxious and desperate.
Just think of that, to fight the man with the scythe.
In his original publications of the stick, Mann purported to use a standard method known as principal component analysis, or PCA, to find the dominant features in a set of more than 70 different climate records.
But it wasnt so. McIntyre and McKitrick obtained part of the program that Mann used, and they found serious problems. Not only does the program not do conventional PCA, but it handles data normalization in a way that can only be described as mistaken.
Now comes the real shocker. This improper normalization procedure tends to emphasize any data that do have the hockey stick shape, and to suppress all data that do not. To demonstrate this effect, McIntyre and McKitrick created some meaningless test data that had, on average, no trends. This method of generating random data is called Monte Carlo analysis, after the famous casino, and it is widely used in statistical analysis to test procedures. When McIntyre and McKitrick fed these random data into the Mann procedure, out popped a hockey stick shape!
https://www.technologyreview.com/2004/10/15/274740/global-warming-bombshell/
And of course the part you leave out. When the minor errors (and they were relatively minor as it turned out) in the original paper are corrected, the trend is still there, and just as large as before.
Now why would you leave out an important detail like that?
Ray Ladbury says; “Now why would you leave out an important detail like that?” Ray Ladbury needs to address that question to the author of the article that I cited, Richard Muller. Now the question for Ray Ladbury to try to answer is, why didn’t you supply that information that you think is of such importance? Here is some of the factual information that actual scientist have discovered by doing some actual scientific exploration into the MWP.
“Dendroecological studies indicate enhanced conifer recruitment during the twentieth century. However, conifers have not yet recolonized many areas where trees were present during the Medieval Warm period (ca AD 800–1300) or the Holocene Thermal Maximum (HTM; ca 10 000–3000 years ago)”
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2606780/
It now appears that my reply to Ray Ladbury must have contained too much verifiable scientific information arrived at by research done by actual scientist to explain my position regarding this issue that the ‘moderators’ felt that it should not be presented so their loyal followers could view it. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2606780/
And, as I recall, it turned out that although M & M’s synthetic data exercise turned out equal numbers of ‘upward’ and ‘downward’ hockeystick shapes, they ‘just happened’ to select all upward ones in their discussion.
JDS: This improper normalization procedure tends to emphasize any data that do have the hockey stick shape, and to suppress all data that do not.
BPL: And with random data, like M&M used, you get a hockey stick about 1/16 the magnitude of the one Mann et al. got. So it didn’t really affect their conclusion, did it?
And yet I still still see denialists in the media claiming Manns hockey stick was proven wrong. You explain to them that numerous other studies have confirmed his findings, and they ignore you and go on saying Manns Hockey stick was proven wrong, like bleating sheep. Baa baa.
Hr Mann
I never had to fight or to defend that “hockeystick” because I find fighting hockeysticks too vulgar and leave that to others.
But your worki of that sort has supported and sustained my own interests in dendrochronology and soil science, history of Nature and other things, and my most recent discovery in that area is Corallinales.
They live everywhere in the seas, in the polar ocean and in the tropics, and here in the fjord.
On swimming and diving you ignore them because they are too obvious and common. They look like lichen everywhere on the rocks on land, and not worthy of any further attension than that.
But when it comes to it, they are probably the most important carbon sink throughout all times at any time, making ut the very Alps and Himalaya, the very Dolomites and Marbles of all sorts in the world.
Taken for serious by the Danes north of Grønland to pick into climate history there.
“Marlboro.. is that same. Fossile Mergel and Corallinales on land.
Thanks Carbo. Very kind of you to say.
What I love about these plots is the ludicrously confident error range.
You say you have reconstructed the global average temperature anomaly 2000 years ago to within a half of a degree. Where we know that within most years, or decades the global anomaly varies by more than this amount. This applies whether you look at the late 19th century when the world was perfect, or the code red 21st century
KW: You say you have reconstructed the global average temperature anomaly 2000 years ago to within a half of a degree. Where we know that within most years, or decades the global anomaly varies by more than this amount.
BPL: Keith, why is the error on ten readings different from the error on one reading? When you can understand that, you’ll be able to answer your own question.
I doubt the Woollard fellow would get it. Obviously he hasn’t heard about the difference between the std. deviation of ind. measurements, and the std. error of means, (SE = SD/SQRT(n)). A formula taught in the first few classes of any intro statistics course. Hence confirming the positive relationship between ignorance and arrogance:
“What I love about these plots is the ludicrously confident error range.”
Keith Woollard
But that’s the same statistical genius who self-confidently dismissed influence of global climate change on Australia – by saying that … in some town in Australia there was … no correlation between local temperature and local precipitation, Keith Woollard, everyone…. ;-)
You put something as the first graph of the summary for policy makers and I would guarantee 98% +/-2% of policy makers thinks that is how accurately we know the temperature and that is absolute rubbish.
I have no problem with a technical paper having a figure like that but we do not know the average temperature anomaly of the globe to within half a degree 2000 years ago, and policy makers think we do
Calling something “absolute rubbish” is only as credible as the person behind the claim. In this case – Keith Woollard. who derisively commented:
“What I love about these plots is the ludicrously confident error range […] where we know that within most years, or decades the global anomaly varies by more than (+/- 0.5C)”.
Which proves that arrogance correlates with ignorance – Mr Woollard does not know the difference between the std. deviation of IND. measurements (within a year, or years within a decades), and the standard error of MEANS
(respectively. annual or decadal).
And this is the same person who not long ago was “disproving” the effect of global warming on Australia by saying that …. in some town in West Australia …. there was no correlation between local temperature and local precipitation.
And that’s the level of knowledge and statistical argument from which Keith Woollard dismisses an IPCC graph as “absolute rubbish” and “ludicrous”.
Mike, as I have high confidence in your medium confidence about decadal temperature data being as confidence worthy as the palaeoclimate record, I will as likely as not look through AR6 when a hard copy comes to hand
Mike, Thanks for this article. No surprises here. I stood with you back in 2001 and remember well the attacks then and later that you suffered.
Thanks David!
It’s a pity that the subject has brought out the worst in climate deniers; they just can’t accept reality, so they feel compelled to dispute it.
The real story here is that the most recent efforts (esp. the PAGES2K project) have extended, strengthened, and confirmed the “hockey stick,” Cast all the aspersions you like, it really is hotter now than it has been in 2000 years, and maybe in 125,000 years. It’s time for us to prepare for the pain that’s here, and the more that’s coming.
Expect deniers to push the “won’t really hurt” story more and more as the “didn’t really happen” story gets harder and harder to sell.
Tamino,
“reality” – you have got to be kidding.
It’s amazing isn’t it how a regional warm period that only occurred in Europe……. and Antarctica and the Arctic and SW North America, some of the tropics, South America, Australia and New Zealand just managed to be negated by a few selected tree proxies to make such a beautifully flat record. It’s almost like someone wanted to deal it a mortal blow
KW: It’s amazing isn’t it how a regional warm period that only occurred in Europe……. and Antarctica and the Arctic and SW North America, some of the tropics, South America, Australia and New Zealand just managed to be negated by a few selected tree proxies to make such a beautifully flat record. It’s almost like someone wanted to deal it a mortal blow
BPL: Appeal to motive = ad hominem. If Mann et al. were wrong, prove it. Show your work.
“The global coverage of proxy records has increased, and their temporal resolution and age control has improved. Major new global data compilations have benefited from internationally coordinated campaigns and from better practices for data reuse.”
While the above might be true, and certainly the PAGES project is a tremendous effort from a great deal of scientists all across the globe, it does seem like Darrell is downplaying uncertainty here, and trying to instate a warm fuzzy feeling with the reader that he/she should not be too worried about all the methodological limitations and pitfalls involved in multiproxy studies.
There are certainly papers out there that 1) do not fully support the PAGES reconstructions, e.g. Buntgen et al., 2020, 2) question the multi-proxy global approach from a methodological point of view, 3) find it uncomforting that the divergence issue in TRW is still not resolved.
e.g. https://www.sciencedirect.com/science/article/pii/S1125786520300965
Unfortunately, none of the above KEY points are discussed in IPCC AR6.
Would be great if the realclimate scientists here could speak a few words to the above.
The Buntgen et al. 2020 paper made several mistakes in their comparison figures:
– They compare JJA Northern Hemisphere reconstructions (of different regions within the NH) with the PAGES 2k annual mean, global mean, ensemble mean reconstruction (they also misidentify the PAGES 2k reconstruction as JJA). A fair comparison, particularly for analyzing variance, would be one over the same regions, the same seasons, and for each reconstruction method, not the average of many.
– To make the comparisons between all the reconstructions, even though they represent different regions, they ‘re-scaled’ all the tree-ring based reconstructions (including their own which is based on a composite plus scaling approach) by the target observational data, while failing to do the same processing step for the multiproxy PAGES 2k reconstruction. They then claim that the variance differences are due to the proxies without acknowledging that the inconsistent re-scaling makes the comparisons meaningless. (This of course sets aside the issue of whether ‘rescaling’ for comparing reconstructions of different regions and seasons makes any sense at all.)
These problems make their comparison figures (Fig 4-6) just wrong and misleading.
It’s amazing how denialists don’t look at the actual dates for the different regions when they make such claims.