Guest commentary from Tim Osborn, Tom Melvin and Keith Briffa, Climatic Research Unit, UEA
Records of tree-ring characteristics such as their width (TRW) and density (usually the maximum density of the wood formed towards the end of the growing season – the “maximum latewood density” – MXD) are widely used to infer past variations in climate over recent centuries and even millennia. Chronologies developed from sites near to the elevational or latitudinal tree lines often show sensitivity to summer temperature and, because of their annual resolution, absolute dating and relatively widespread nature, they have contributed to many local, continental and hemispheric temperature reconstructions. However, tree growth is a complex biological process that is subject to a range of changing environmental influences, not just summer temperature, and so replication, coherence and consistency across records and other proxies are an important check on the results.
Tree-ring records have greater replication (both within a site and between nearby sites) than other types of climate proxy. Good replication helps to minimise the influence of random localised factors when extracting the common signal, and it also allows the comparison of information obtained from different independent sets or sub-sets of data. If independent sets of data – perhaps trees with different mean growth rates or from different sites – show similar variations, then we can have greater confidence that those variations are linked to real variations in climate.
In a new QSR paper (Briffa et al., 2013), (BEA13) we have used these approaches to re-assess the combined tree-ring evidence from the Yamal and Polar Urals region (Yamalia) of northern Siberia, considering the common signal in tree-growth changes at different sites and in subsets of data defined in other ways. Together with our Russian colleagues and co-authors, we have incorporated many new tree-ring data, to increase the replication and to update the chronology to 2005 and have reassessed the inferences about summer temperature change that can be drawn from these data. The paper is published as an open-access paper (no paywall) and supplementary information including the raw tree-ring and instrumental temperature data are available from our website.
K.R. Briffa, T.M. Melvin, T.J. Osborn, R.M. Hantemirov, A.V. Kirdyanov, V.S. Mazepa, S.G. Shiyatov, and J. Esper, "Reassessing the evidence for tree-growth and inferred temperature change during the Common Era in Yamalia, northwest Siberia", Quaternary Science Reviews, vol. 72, pp. 83-107, 2013. http://dx.doi.org/10.1016/j.quascirev.2013.04.008
Guest commentary by Darrell Kaufman (N. Arizona U.)
In a major step forward in proxy data synthesis, the PAst Global Changes (PAGES) 2k Consortium has just published a suite of continental scale reconstructions of temperature for the past two millennia in Nature Geoscience. More information about the study and its implications are available at the FAQ on the PAGES website and the datasets themselves are available at NOAA Paleoclimate.
The main conclusion of the study is that the most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the 19th century, and which was followed by a warming trend in the 20th C. The 20th century in the reconstructions ranks as the warmest or nearly the warmest century in all regions except Antarctica. During the last 30-year period in the reconstructions (1971-2000 CE), the average reconstructed temperature among all of the regions was likely higher than anytime in at least ~1400 years. Interestingly, temperatures did not fluctuate uniformly among all regions at multi-decadal to centennial scales. For example, there were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age. Cool 30-year periods between the years 830 and 1910 CE were particularly pronounced during times of weak solar activity and strong tropical volcanic eruptions and especially if both phenomena often occurred simultaneously.
M. Ahmed, K.J. Anchukaitis, A. Asrat, H.P. Borgaonkar, M. Braida, B.M. Buckley, U. Büntgen, B.M. Chase, D.A. Christie, E.R. Cook, M.A.J. Curran, H.F. Diaz, J. Esper, Z. Fan, N.P. Gaire, Q. Ge, J. Gergis, J.F. González-Rouco, H. Goosse, S.W. Grab, N. Graham, R. Graham, M. Grosjean, S.T. Hanhijärvi, D.S. Kaufman, T. Kiefer, K. Kimura, A.A. Korhola, P.J. Krusic, A. Lara, A. Lézine, F.C. Ljungqvist, A.M. Lorrey, J. Luterbacher, V. Masson-Delmotte, D. McCarroll, J.R. McConnell, N.P. McKay, M.S. Morales, A.D. Moy, R. Mulvaney, I.A. Mundo, T. Nakatsuka, D.J. Nash, R. Neukom, S.E. Nicholson, H. Oerter, J.G. Palmer, S.J. Phipps, M.R. Prieto, A. Rivera, M. Sano, M. Severi, T.M. Shanahan, X. Shao, F. Shi, M. Sigl, J.E. Smerdon, O.N. Solomina, E.J. Steig, B. Stenni, M. Thamban, V. Trouet, C.S. Turney, M. Umer, T. van Ommen, D. Verschuren, A.E. Viau, R. Villalba, B.M. Vinther, L. von Gunten, S. Wagner, E.R. Wahl, H. Wanner, J.P. Werner, J.W. White, K. Yasue, and E. Zorita, "Continental-scale temperature variability during the past two millennia", Nature Geosci, vol. 6, pp. 339-346, 2013. http://dx.doi.org/10.1038/ngeo1797
It is well known that ice shelves on the Antarctic Peninsula have collapsed on several occasions in the last couple of decades, that ice shelves in West Antarctica are thinning rapidly, and that the large outlet glaciers that drain the West Antarctic ice sheet (WAIS) are accelerating. The rapid drainage of the WAIS into the ocean is a major contributor to sea level rise (around 10% of the total, at the moment).
All of these observations match the response, predicted in the late 1970s by glaciologist John Mercer, of the Antarctic to anthropogenic global warming. As such, they are frequently taken as harbingers of greater future sea level rise to come. Are they?
Two papers published this week in Nature Geoscience provide new information that helps to address this question. One of the studies (led by me) says “probably”, while another (Abram et al.) gives a more definitive “yes”. More »
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.
Last July (2012), I heard from a colleagues working at the edge of the Greenland ice sheet, and from another colleague working up at the Summit. Both were independently writing to report the exceptional conditions they were witnessing. The first was that the bridge over the Watson river by the town of Kangerlussuaq, on the west coast of Greenland, was being breached by the high volumes of meltwater coming down from the ice sheet. The second was that there was a new melt layer forming at the highest point of the ice sheet, where it very rarely melts.
A front loader being swept off a bridge into the Watson River, Kangerlussuaq, Greenland, in July 2012. Fortunately, nobody was in it at the time. Photo: K. Choquette
I’ve been remiss in not writing about these observations until now. I’m prompted to do so by the publication in Nature today (January 23, 2013) of another new finding about Greenland melt. This paper isn’t about the modern climate, but about the climate of the last interglacial period. It has relevance to the modern situation though, a point to which I’ll return at the end of this post.
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