A new study by British and Canadian researchers shows that the global temperature rise of the past 15 years has been greatly underestimated. The reason is the data gaps in the weather station network, especially in the Arctic. If you fill these data gaps using satellite measurements, the warming trend is more than doubled in the widely used HadCRUT4 data, and the much-discussed “warming pause” has virtually disappeared. More »
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Readers will be aware of the paper by Shaun Marcott and colleagues, that they published a couple weeks ago in the journal Science. That paper sought to extend the global temperature record back over the entire Holocene period, i.e. just over 11 kyr back time, something that had not really been attempted before. The paper got a fair amount of media coverage (see e.g. this article by Justin Gillis in the New York Times). Since then, a number of accusations from the usual suspects have been leveled against the authors and their study, and most of it is characteristically misleading. We are pleased to provide the authors’ response, below. Our view is that the results of the paper will stand the test of time, particularly regarding the small global temperature variations in the Holocene. If anything, early Holocene warmth might be overestimated in this study.
Update: Tamino has three excellent posts in which he shows why the Holocene reconstruction is very unlikely to be affected by possible discrepancies in the most recent (20th century) part of the record. The figure showing Holocene changes by latitude is particularly informative.
Time for the 2012 updates!
As has become a habit (2009, 2010, 2011), here is a brief overview and update of some of the most discussed model/observation comparisons, updated to include 2012. I include comparisons of surface temperatures, sea ice and ocean heat content to the CMIP3 and Hansen et al (1988) simulations.
Houston, we have a problem.
Admittedly, not a huge problem and not one that most people, or even most climatologists, are particularly fascinated by, but one which threads together many topics (climate models, tree rings, paleo-climate) which have been highlighted here in the past. The problem is that we have good evidence in the ice core records for very large tropical eruptions over the last 1000 years – in particular the eruptions in 1258/1259, 1452 and 1809 to 1815 – but for which many paleo-reconstructions barely show a blip in temperature. Models, in attempting to simulate this period, show varied but generally larger (and sometimes much larger) responses. The differences are significant enough to have prompted a few people to try and look into why this mismatch is occurring.
Whenever there is a mismatch between model and observation, there are, roughly speaking, at least three (non-exclusive) possibilities: the model is wrong, the observational data are wrong or the comparison is not like-with-like. There have been many examples of resolved mismatches in each category so all possibilities need to be looked at.
As described in a previous post earlier this year, Mann et al., 2012 (pdf), postulated that for extreme volcanoes, the cooling would be sufficient to saturate the growth response, and that some trees might `skip´ a ring for that year leading to a slight slippage in tree-ring dating, a potential smearing of the composite chronologies, and a further underestimate of the cooling in tree-ring based large-scale reconstructions.
This hypothesis has now been challenged by a group of authors in a comment (Anchukaitis et al.) (pdf, SI, code), who focus on the appropriateness of the tree ring growth model and the spatial pattern the volcanic climate responses. The Mann et al. response (pdf, SI) presents some further modeling and 19th century observational data in support of the original hypothesis.
Of course, there are still two other possibilities to consider. First, the models may have an excessive response. This could be due to either models responding excessively to the correct forcing, or could be related to an excessive forcing itself. There are indeed some important uncertainties in estimating the history of volcanic forcing – which involves inferring a stratospheric aerosol load (and effective radius of the particles and their distribution) from a network of sulphate peaks in ice cores in Greenland and Antarctica. For example, the forcing for the big eruption around 1453 differs by a factor of 2 in the inferred forcing (-12 W/m2 and -5.4 W/m2) in the two estimates proposed for the recent model-intercomparison (Schmidt et al., 2012). Note too that the details of how aerosols are implemented in any specific model can also make a difference to the forcing, and there are many (as yet untested) assumptions built into the forcing reconstructions.
It is also conceivable that climate models overreact to volcanic forcing – however, excellent matches to the Pinatubo response in temperature, radiative anomalies, water vapour and dynamic responses, where we know the volcanic aerosol load well, make that tricky to support (Hansen et al, 2007) (pdf, SI). (As an aside, the suggestion in this paper that the response to Krakatoa (1883) was underpredicted by the historical SST fields was partially vindicated by the results from HadSST3 which showed substantially more cooling).
The third possibility is that some tree-ring reconstructions can’t be easily compared to simple temperature averages from the models. As both the original paper and the comment suggests, there are important effects from memory from previous years in ring widths and, potentially, increases in diffuse light post-eruption promoting growth spurts. This needs to be assessed using more sophisticated forward models for tree ring growth applied to the models’ output – a feature in both the Mann et al, and Anchukaitis et al. approaches. More work is likely needed on this, and using the wider variety of model experiments coming out of CMIP5/PMIP3.
There is clearly potential for these competing hypotheses to get sorted out. Information from newly-digitised old instrumental records in the early 19th Century such as shipping records for the East India Company (Brohan et al, 2012), doesn’t support the largest modelled responses to Tambora (1815), but does suggest a response larger and more defined than that seen in some reconstructions. However, other 19th Century temperature compilations such Berkeley Earth show larger responses to Tambora – though there are spatial sampling issues there as well. There is also the potential for non-tree ring based reconstructions to provide independent confirmation of the magnitude of the response.
So while neither of the latest comments and responses provide a definitive answer to the principal problem, there is certainly lots of scope for extended and (hopefully) productive discussions.
- M.E. Mann, J.D. Fuentes, and S. Rutherford, "Underestimation of volcanic cooling in tree-ring-based reconstructions of hemispheric temperatures", Nature Geoscience, vol. 5, pp. 202-205, 2012. http://dx.doi.org/10.1038/ngeo1394
- K.J. Anchukaitis, P. Breitenmoser, K.R. Briffa, A. Buchwal, U. Büntgen, E.R. Cook, R.D. D'Arrigo, J. Esper, M.N. Evans, D. Frank, H. Grudd, B.E. Gunnarson, M.K. Hughes, A.V. Kirdyanov, C. Körner, P.J. Krusic, B. Luckman, T.M. Melvin, M.W. Salzer, A.V. Shashkin, C. Timmreck, E.A. Vaganov, and R.J.S. Wilson, "Tree rings and volcanic cooling", Nature Geoscience, vol. 5, pp. 836-837, 2012. http://dx.doi.org/10.1038/ngeo1645
- M.E. Mann, J.D. Fuentes, and S. Rutherford, "Reply to 'Tree rings and volcanic cooling'", Nature Geoscience, vol. 5, pp. 837-838, 2012. http://dx.doi.org/10.1038/ngeo1646
- G.A. Schmidt, J.H. Jungclaus, C.M. Ammann, E. Bard, P. Braconnot, T.J. Crowley, G. Delaygue, F. Joos, N.A. Krivova, R. Muscheler, B.L. Otto-Bliesner, J. Pongratz, D.T. Shindell, S.K. Solanki, F. Steinhilber, and L.E.A. Vieira, "Climate forcing reconstructions for use in PMIP simulations of the Last Millennium (v1.1)", Geoscientific Model Development, vol. 5, pp. 185-191, 2012. http://dx.doi.org/10.5194/gmd-5-185-2012
- J. Hansen, M. Sato, R. Ruedy, P. Kharecha, A. Lacis, R. Miller, L. Nazarenko, K. Lo, G.A. Schmidt, G. Russell, I. Aleinov, S. Bauer, E. Baum, B. Cairns, V. Canuto, M. Chandler, Y. Cheng, A. Cohen, A. Del Genio, G. Faluvegi, E. Fleming, A. Friend, T. Hall, C. Jackman, J. Jonas, M. Kelley, N.Y. Kiang, D. Koch, G. Labow, J. Lerner, S. Menon, T. Novakov, V. Oinas, J. Perlwitz, J. Perlwitz, D. Rind, A. Romanou, R. Schmunk, D. Shindell, P. Stone, S. Sun, D. Streets, N. Tausnev, D. Thresher, N. Unger, M. Yao, and S. Zhang, "Climate simulations for 1880–2003 with GISS modelE", Climate Dynamics, vol. 29, pp. 661-696, 2007. http://dx.doi.org/10.1007/s00382-007-0255-8
- P. Brohan, R. Allan, E. Freeman, D. Wheeler, C. Wilkinson, and F. Williamson, "Constraining the temperature history of the past millennium using early instrumental observations", Climate of the Past, vol. 8, pp. 1551-1563, 2012. http://dx.doi.org/10.5194/cp-8-1551-2012
As many will have already heard, our colleague, RC co-founder and friend Michael Mann will receive the Oeschger medal from the European Geosciences Union this week in Vienna. We are delighted to announce this and to congratulate Mike.
Hans Oeschger was a Swiss scientist originally trained as a nuclear physicist. His name is well known in climate science, especially because of his discovery, with Willi Dansgaard, of the Dansgaard-Oeschger events (the rapid climate changes during the last glacial period, first observed in Greenland ice cores). He was even better known in the radiocarbon research community as famously having developed one of the first instruments (the “Oeschger counter”) for measuring carbon-14. This paved the way for determining the age of very small organic materials, including samples from deep-sea sediment cores, which eventually led to the validation of the Milankovitch theory of ice ages. Oeschger and his colleagues in Bern were the first to measure the glacial-interglacial change of atmospheric CO2 in ice cores, showing that atmospheric concentrations of CO2 during the glacial period was 50% lower than the pre-industrial concentration, a result predicted by Arrhenius nearly a century earlier. Oeschger may thus be credited with work that was critical to validating two of the most important theories in science: the role of CO2 in climate change, and the role of changes in the earth’s orbit. Oeschger was also an accomplished musician, and was known to join colleagues in playing chamber music at the International Conference on Radiocarbon.
Oeschger left rather large shoes to fill, and it is a great honor for Mike Mann to win an award bearing Oeschger’s name. Most everyone will probably assume that the award is for Mike’s well known “hockey stick” work. No doubt this is part of it, but the Oeschger award has never been given simply for the publication of one study, but rather for a career’s-worth of outstanding achievements. Most of the previous medalists are a good deal more senior than Mike Mann, and include paleoceanographer Laurent Labeyrie, limnologist Francoise Gasse, ice core pioneers Dominique Raynaud and Sigfus Johnsen and number of other major names in the climate and paleoclimate research, including RC’s own Ray Bradley.
Mike’s work, like that of previous award winners, is diverse, and includes pioneering and highly cited work in time series analysis (an elegant use of Thomson’s multitaper spectral analysis approach to detect spatiotemporal oscillations in the climate record and methods for smoothing temporal data), decadal climate variability (the term “Atlantic Multidecadal Oscillation” or “AMO” was coined by Mike in an interview with Science’s Richard Kerr about a paper he had published with Tom Delworth of GFDL showing evidence in both climate model simulations and observational data for a 50-70 year oscillation in the climate system; significantly Mike also published work with Kerry Emanuel in 2006 showing that the AMO concept has been overstated as regards its role in 20th century tropical Atlantic SST changes, a finding recently reaffirmed by a study published in Nature), in showing how changes in radiative forcing from volcanoes can affect ENSO, in examining the role of solar variations in explaining the pattern of the Medieval Climate Anomaly and Little Ice Age, the relationship between the climate changes of past centuries and phenomena such as Atlantic tropical cyclones and global sea level, and even a bit of work in atmospheric chemistry (an analysis of beryllium-7 measurements). Mike’s earliest work, as a physicist, involved studying the behavior of liquids and solids, and trying to understand phenomena such as the structural ordering of high temperature superconductors. In the earth sciences, he has published on topics as varied as the recovery from the KT-boundary mass extinction event and the factors driving long-term changes in the volume of the Great Salt Lake. He has studied and published on the impacts of historical and projected climate change on everything from the behavior of the Asian Summer Monsoon, to Atlantic Hurricanes, to rainfall patterns in the U.S. And for those interested in the hard-nosed statistics by which a scientist’s productivity gets measured, a quick check on the ISI web site will tell you that he has an “H Index” of 40 (that means that 40 of his papers have been cited at least 40 times), more than twenty of his papers have over 100 citations each, and two have over 700. Those are high numbers by any comparison.
But back to the hockey stick. Mike has weathered some rather intense scrutiny and criticism over the years, mostly over the details of a paper nearly 15 years old. Yet the basic conclusions of the “hockey stick” remain, and indeed have been strengthened by subsequent work. Most will be aware, for example, that the conclusion that the past few decades are likely the warmest of the past millennium — i.e. the conclusion of the best-known of Mike’s papers in Nature and Geophysical Research Letters –has never been seriously challenged. But well beyond the simple fact of having been right, Mike’s work was seminal, like Oeschger’s, in playing a pivotal role in launching an entirely new field of study. Although some earlier work along similar lines had been done by other paleoclimate researchers (Ed Cook, Phil Jones, Keith Briffa, Ray Bradley, Malcolm Hughes, and Henry Diaz being just a few examples), before Mike, no one had seriously attempted to use all the available paleoclimate data together, to try to reconstruct the global patterns of climate back in time before the start of direct instrumental observations of climate, or to estimate the underlying statistical uncertainties in reconstructing past temperature changes. Since Mike’s pioneering work (starting in 1995), hundreds of papers have adopted the basic approach he pioneered, and numerous PHD projects have been launched to try to improve upon it. Methods have improved of course, and no doubt will improve further (paleoclimate reconstruction using weather forecast data assimilation methods is the latest and most promising recent development). That Mike is a co-author on many of the latest and most innovative publications in this area — with dozens of different people — attests to the groundbreaking nature of his work.
We look forward to seeing Mike’s award lecture in Vienna, and we offer our heartfelt congratulations to a well-deserved honor. And while we are at it, we should congratulate Mike in advance for his election as a Fellow of the American Geophysical Union; that honor will be bestowed this fall in San Francisco.
Finally, we would be remiss to not mention that Mike has spent much of the past few months touring and lecturing on his experiences as an accidental and reluctant public figure in the debate over human-caused climate change, as detailed in his recent book The Hockey Stick and the Climate Wars: Dispatches from the Front Lines.
P.S. For those at EGU, you should also check out glaciologist Ian Joughin’s award lecture (Wednesday evening) for the Agassiz medal, for his important work in documenting and understanding the acceleration of Antarctica and Greenland’s glaciers.