[asa] The Non-Smoking Gun

Much has been made about the cooking the data for example "hide the decline" in the following Nature paper:

Nature 393, 450-455 (4 June 1998) | doi:10.1038/30943; Received 13 October 1997; Accepted 31 March 1998

Influence of volcanic eruptions on Northern Hemisphere summer temperature over the past 600 years

K. R. Briffa1, P. D. Jones1, F. H. Schweingruber2 & T. J. Osborn1

Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, UK
Swiss Federal Institute of Forest, Snow and Landscape Research, Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland
Correspondence to: K. R. Briffa1 Correspondence and requests for materials should be addressed to K.R.B. (e-mail: Email: k.briffa@uea.ac.uk).

Topof pageAbstract

But note the graph here shows a decline. Nothing hidden here.

http://www.nature.com/nature/journal/v393/n6684/fig_tab/393450a0_F1.html

Another paper in Nature from the same year talks specifically about the decline itself and speculates why the tree rings diverged from the instrumental record:

Nature 391, 678-682 (12 February 1998) | doi:10.1038/35596; Received 14 May 1997; Accepted 11 November 1997

Reduced sensitivity of recent tree-growth to temperature at high northern latitudes

K. R. Briffa1, F. H. Schweingruber2, P. D. Jones1, T. J. Osborn1, S. G. Shiyatov3 & E. A. Vaganov4

Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, UK
Swiss Federal Institute of Forest, Snow and Landscape Research, Zürcherstrasse 111, CH-8903, Birmensdorf, Switzerland
Institute of Plant and Animal Ecology, Ural Branch of the Russian Academy of Sciences, 8 Marta Street, Ekaterinburg 620219, Russia
Institute of Forest, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk, Russia
Correspondence to: K. R. Briffa1 Correspondence and requests for materials should be addressed to K.R.B. (e-mail: Email: k.briffa@uea.ac.uk).

Topof pageAbstract
Tree-ring chronologies that represent annual changes in the density of wood formed during the late summer can provide a proxy for local summertime air temperature1. Here we undertake an examination of large-regional-scale wood-density/air-temperature relationships using measurements from hundreds of sites at high latitudes in the Northern Hemisphere. When averaged over large areas of northern America and Eurasia, tree-ring density series display a strong coherence with summer temperature measurements averaged over the same areas, demonstrating the ability of this proxy to portray mean temperature changes over sub-continents and even the whole Northern Hemisphere. During the second half of the twentieth century, the decadal-scale trends in wood density and summer temperatures have increasingly diverged as wood density has progressively fallen. The cause of this increasing insensitivity of wood density to temperature changes is not known, but if it is not taken into account in dendroclimatic reconstructions, past temperatures could be overestimated. Moreover, the recent reduction in the response of trees to air-temperature changes would mean that estimates of future atmospheric CO2 concentrations, based on carbon-cycle models that are uniformly sensitive to high-latitude warming, could be too low.

Whatever the cause, this change in tree-growth response has important implications for studies of past and future climate change. The common use of least-squares regression for developing dendroclimatic transfer function equations to estimate past climates, imposes an equality of means in both the predictand and predictor time series over the fitting or 'calibration' period10. Any bias in mean tree growth will, therefore, be 'corrected' during calibration, with the consequence that the derived regression coefficients will be biased. Our results imply that this might increasingly result in systematic overestimation of past temperatures, [RDB Note: this means that the MWP is potentially overestimated by the tree data] particularly in regions where the loss of low-frequency temperature sensitivity in tree growth is greatest (eastern Siberia and eastern North America: see Fig. 1), where tree-ring 'standardization' is designed to preserve maximum long-timescale chronology variability4, and where transfer functions are calibrated over the most recent decades.

NOAA's paleoclimatic web site mentions this from a page that dates from August 20 2008:

http://www.ncdc.noaa.gov/paleo/globalwarming/briffa.html

A team of scientists has developed a circum-northern Hemisphere network of temperature sensitive tree-ring density data that was used to generate a 600 year record of Northern Hemisphere summer temperatures in this study. The calibration of tree-ring data to instrumental temperature records yielded a high-quality quantitative record of past hemispheric summer temperatures.

The data used in this study are distinct from the data used in other long temperature reconstructions, none of which have been based solely on tree-ring data. However, as with the other available state-of-the-art long temperature reconstructions, the results of Briffa et al. indicate that the 20th century is the warmest of the last 400-600 years. Many of the short (about 1-5 year) cold events during the last six centuries can be explained by the cooling effects of large volcanic eruptions.

The authors were able to compare (or calibrate) their density records directly against instrumental data; note that the tree-ring density records become de-coupled from temperature after 1950, possibly due to some large-scale human influence that caused wood densities to decline. Thus, the reconstructed temperature record after 1960 is considered unreliable. [emphasis mine]

As for the "trick" note the figure of Mann here:

http://www.meteo.psu.edu/~mann/shared/research/INTERNAL/MILLENNIUM/nhem-millennium.pdf

The instrumental data is clearly labeled in red and the reconstruction in blue and stops in 1980 (because that was when the data ended) before the real rapid increase happened.

The NAS looked into the divergence problem in Surface Temperature Reconstructions for the Last 2000 Years (2006).

http://books.nap.edu/openbook.php?record_id=11676

Here's their discussion starting on page 48:

All paleoclimatic reconstructions rely on the “uniformity principle” (Camardi 1999), which assumes that modern natural processes have acted similarly in the past, and is also discussed as the “stationarity” assumption in Chapter 9. Although limiting factors controlled tree ring parameters in the past just as they do today, it is possible that the role of different factors at a single location or over an entire region could change over time. This possibility has been raised to explain the “divergence” (i.e., reduced correlation) between temperature and ring parameters (width and maximum latewood density) during the late 20th century (Jacoby and D’Arrigo 1995, Briffa et al. 1998). In Alaska, it appears that increasing air temperature over the past decades is not reflected in increasing tree ring records because water (i.e., drought stress) has become the limiting factor (Barber et al. 2000, Lloyd and Fastie 2002, Wilmking and Juday 2005). In Siberia, on the other hand, reduced correlation of tree ring chronologies with summer temperature has been attributed to increasing winter precipitation, which leads to delayed snowmelt in permafrost environments, thus shortening the tree growing season (Vaganov et al. 1999). Other hypotheses have been formulated for the reduced correlation between temperature and tree ring chronologies, such as a negative effect on tree growth due to greater ultraviolet radiation reaching the ground as a result of thinning stratospheric ozone (Briffa et al. 2004), or the possibility that surface instrumental temperatures are affected by an upward bias (Hoyt 2006). Elevational treeline sites in Mongolia (D’Arrigo et al. 2001) and the European Alps (Büntgen et al. 2005) are not affected by “divergence.” This geographic separation was confirmed by Cook et al. (2004), who subdivided long tree ring records for the Northern Hemisphere into latitudinal bands and found not only that “divergence” is unique to areas north of 55°N but also that the difference between northern and southern sites found after about 1950 is unprecedented since at least A.D. 900.
An especially suitable strategy to minimize confounding effects is to sample sites along ecological gradients, such as elevation or latitude (Fritts and Swetnam 1989, Bugmann 1996). For example, Naurzbaev et al. (2004) selected sites along latitudinal (from 55 to 72°N) and elevational (from 1120 to 2350 meters above sea level) transects and used the parameters of the Regional Curve Standardization to infer climatic influences and past temperature variability. Other strategies are available to improve tree ring reconstructions of surface temperature. Some of these strategies involve using maximum temperature instead of mean temperature (Luckman and Wilson 2005), combining multiple tree ring parameters related to temperature (Helle and Schleser 2004), sampling species with opposing responses to temperature (Biondi et al. 1999), and applying mechanistic models to tree ring records (Anchukaitis et al. 2006).
The possibility that increasing tree ring widths in modern times might be driven by increasing atmospheric carbon dioxide (CO2) concentrations, rather than increasing temperatures, was first proposed by LaMarche et al. (1984) for bristlecone pines (Pinus longaeva) in the White Mountains of California. In old age these trees can assume a “strip-bark” form, characterized by a band of trunk that remains alive and continues to grow after the rest of the stem has died. Such trees are sensitive to higher atmospheric CO2 concentrations (Graybill and Idso 1993), possibly because of greater water-use efficiency (Knapp et al. 2001, Bunn et al. 2003) or different carbon partitioning among tree parts (Tang et al. 1999). Support for a direct CO2 influence on tree ring records extracted from “full-bark” trees is less conclusive. Increasing mean ring width was reported for Pinus cembra from the central Alps growing well below treeline (Nicolussi et al. 1995). Free-Air CO2Enrichment (FACE) data for conifer plantations in the Duke Forest (Hamilton et al. 2002) and at the alpine treeline (Hättenschwiler et al. 2002) also showed increased tree growth after exposure to atmospheric CO2 concentrations about 50 percent greater than present. On the other hand, no convincing evidence for such effect was found in conifer tree ring records from the Sierra Nevada in California (Graumlich 1991) or the Rocky Mountains in Colorado (Kienast and Luxmoore 1988). Further evidence comes from a recent review of data for mature trees in four climatic zones, which concluded that pine growth at the treeline is limited by factors other than carbon (Körner 2003). While “strip-bark” samples should be avoided for temperature reconstructions, attention should also be paid to the confounding effects of anthropogenic nitrogen deposition (Vitousek et al. 1997), since the nutrient conditions of the soil determine wood growth response to increased atmospheric CO2 (Kostiainen et al. 2004). However, in forest areas below the treeline where modern nitrogen input could be expected to influence dendroclimatic records, such as Scotland (Hughes et al. 1984) and Maine (Conkey 1986), the relationship between temperature and tree ring parameters was stable over time.
I'm pretty shocked how quickly people scream data fixing. This needs to be reserved when there is a lot of evidence because the scientists are charged with something that's not only immoral but it's potentially criminal. All this merely from e-mails and not even checking the papers. I'm reminded of when I was gently corrected on this list when I accidentally conflated the Jan Hendrick Schoen incident with Pons and Fleischmann implying that the cold fusion failure was a fraud when I had no evidence that was the case. At the very least they should look at the papers in question and the NAS survey (which I highly recommend BTW as a good introduction on paleoclimatology). The Mann paper may have kicked things off but paleoclimatology has progressed very far and has expanded greatly from a small cadre of researchers. Even if Mann et al 1998/9 was as worthless as the Soon and Baliunas paper (which was the subject of the other major controversy) there are so many other players now it wouldn't matter.

Rich Blinne
Member ASA

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Received on Fri Nov 27 00:18:26 2009