A few days ago, we discussed the unresponsive answers provided to climate scientist Jeff Severinghaus in February 2003 when he inquired about the validity of tree ring widths as proxies due to the inconsistency (divergence) between temperature and ring widths, answers characterized by Severinghaus here as not being a “straight answer”.

In first quarter 2003 (almost exactly the same time as Severinghaus’ inquiry), Soon et al raised almost precisely the same question in Soon et al (EE 2003). The answer of Mann and a long list of coauthors (Ammann, Bradley, Hughes, Rutherford, Jones, Briffa, Osborn, Crowley, Oppenheimer, Overpeck, Trenberth and Wigley), which is the topic of today’s post, took hide the decline to new levels.

As noted above, Soon et al (EE2003) clearly articulated the impact of the divergence problem on the validity of temperature reconstructions:

Strong evidence has been accumulating that tree growth has been disturbed in many Northern Hemisphere regions in recent decades (Graybill and Idso 1993; Jacoby and D’Arrigo 1995; Briffa et al. 1998; Feng 1999; Barber et al. 2000; Jacoby et al. 2000; Knapp et al. 2001) so that after 1960-1970 or so, the usual, strong positive correlation between the tree ring width or tree ring maximum latewood density indices and summer temperatures have weakened (referred to as “anomalous reduction in growth performance” by Esper et al. 2002a). The calibration period of Mann et al. (1998, 1999, 2000a) ended at 1980, while 20 more years of climate data post-1980 (compared to the 80 years length of their calibration interval, 1902-1980) exist. If the failure of inter-calibration of instrumental and tree growth records over last two to three decades suggests evidence for anthropogenic influences (i.e., from CO2, nitrogen fertilization or land-use and land-cover changes or through changes in the length of growing seasons and changes in water and nutrient utilization efficiencies and so on), then no reliable quantitative inter-calibration can connect the past to the future (Idso 1989). Briffa and Osborn (1999) have also criticized the impact of unusual tree growth on the calibration procedure of tree-ring climate proxies (see additional discussions in Jacoby and D’Arrigo 1995; Briffa et al. 1998; Barber et al. 2000; Briffa 2000; Jacoby et al. 2000). This matter has largely been unresolved, which means that global or Northern Hemisphere-averaged thermometer records of surface temperature cannot be simply attached to reconstructed temperature records of Mann et al, based mainly on tree-ring width, which cannot yet be reliably calibrated, to the latter half of the 20th century.

These issues remain unresolved eight years later, even though Mann et al (2003) was hailed at the time as “discrediting” Soon et al 2003. In fact, Mann et al (2003) did not directly respond to the divergence issue.

Instead, on this point, they relied on the rhetorical effect of an expanded spaghetti graph, showing six reconstructions (MBH99, Jones et al 1998, Mann and Jones 2003, Crowley and Lowery 2000, Esper et al 2002, Mann and Jones 2003), none of which, in the diagram, showed evidence of a convergence problem, reinforcing the argument of Mann et al on the supposed unimportance of the divergence problem: (Note Dec 1: LucySkyWalker reminds me of a prior discussion of this graphic at CA here in which Tim Lambert pointed out that the (yellow) “Crowley and Lowery” series shown here wasn’t actually the Crowley and Lowery series, but MBH99 plus 0.5 sigma.)



Fig. 1.Comparison of proxy-based NH temperature reconstructions [Jones et al., 1998; Mann et al., 1999; Crowley and Lowery, 2000] with model simulations of NH mean temperature changes

over the past millennium based on estimated radiative forcing histories [Crowley, 2000; Gerber et al., 2002—results shown for both a 1.5°C/2*CO2 and 2.5°C/2*CO2 sensitivity; Bauer et al., 2003]. Also shown are two independent reconstructions of warm season extra-tropical continental NH temperatures [Briffa et al., 2001; Esper et al., 2002] and an extension back through the past 2000 years based on eight long reconstructions [Mann and Jones,2003].All reconstructions have been scaled to the annual,full northern hemisphere mean, over an overlapping period (1856–1980), using the NH instrumental record [Jones et al., 1999] for comparison, and have been smoothed on time scales of > 40 years to highlight the long-term variations.The smoothed instrumental record (1856–2000) is also shown. The gray/red shading indicates estimated two-standard error uncertainties in the Mann et al. [1999] and Mann and Jones [2003] reconstructions.Also shown are reconstructions of ground surface temperatures (GST) based on appropriately areally-averaged [Briffa and Osborn, 2002; Mann et al., 2003] continental borehole data [Huang et al., 2000], and hemispheric surface air temperature trends, determined by optimal regression [Mann et al., 2003] from the GST estimates.All series are shown with respect to the 1961–1990 base period

At the time, no one knew about “hide the decline”. Mann et al do not mention anything about deleting adverse data. The Briffa reconstruction labeled in the legend as “Briffa et al scaled 1856-1980”, giving no clue to readers of hide-the-decline. Let’s now look at a magnified version of this graphic, blown up so that we can see how they handled the Briffa (orange) reconstruction. As sharp-eyed CA readers FergalR and haroldw observed, if you squint closely, you can see that the Briffa reconstruction was chopped off before its end.

Indeed, they did not simply “hide the decline”, their “hide the decline” was worse than we thought. Mann et al did not merely delete data after 1960, they deleted data from 1940 on, You can see the last point of the Briffa reconstruction (located at ~1940) peeking from behind the spaghetti in the graphic below:



Detail from Mann et al (EOS 2003) Figure 1. Arrow points to Briffa series peeking out from behind the spaghetti

Had Mann et al used the actual values, the decline would have been as shown in the accompanying graphic:



Figure 3. Re-stated Mann et al (EOS 2003) Figure 1 showing the decline.

Had Mann and his 13 co-authors shown the Briffa reconstruction, without hiding the decline, one feels that von Storch (and others) might have given more consideration to Soon et al’s criticism of the serious problem arising from the large-population failure of tree ring widths and density to track temperature.



