I said that I post the graphic from Loso et al if someone sent it to me today. In fact, Loso et al is online here and interested parties can consult it for themselves. I don’t have time to comment on this study other than very briefly, but here are some of the key graphics.
Loso is studying a proglacial lake and, prior to showing the Loso graphics, I’d like to remind people that we’ve looked at some proglacial lakes in other locations e.g. Venezuela, Peru, Kilimanjaro and I think that interesting information can be derived from them. For example, here is a graphic from Polissar et al on Venezuela (discussed here) showing a difference in magnetic susceptibility between MWP and LIA sediments in Venezuela, which Polissar et al interpreted as showing an absence of glaciers in this location in the MWP.
Venezuela Proglacial Lakes – see http://www.climateaudit.org/?p=704
So Loso adds to this class of evidence. Loso examined 12 cross-sections at Iceberg Lake in a glacial lake, providing the following interesting diagram.
Original Caption: Figure 3. Stratigraphic sections from twelve sites at Iceberg Lake. Marked depths are distances, in cm, below the top of the section. All sites except I” were documented in outcrops exposed subaerially by stream incision in the exposed lakebed. Site I is above the most recent stable lake level and was collected with a Livingstone-type Corer. Using a combination of varve-counting, cross-dating, and radiogenic dating, we tied the sections shaded in red to a master chronology. Dating on other sections is uncertain. Note that Sites H and L do not exist—these two breaks in the lettering system divide sites visited in 2001, 2002, and 2003.
Some of these exposures could be dated. From my reading, Sites C, D, E, F, and G could not be dated. Thus, the set of cores was reduced by 5, yielding the following set of core measurements.
Original Caption: Figure 5. Well-dated varve thickness chronologies from seven sites at Iceberg Lake. Each chronology includes measurements from all laminae confidently dated through a combination of varve counting, cross-dating of visually distinctive laminae, and radiogenic techniques. For nine macrofossils found within these sections, thick bars show comparison of calibrated radiocarbon age estimates (2-sigma range given by vertical bars) with varve ages (horizontal bars). Radiocarbon data from Table 3.
Varve thicknesses were measured and averaged, from which a master chronology was obtained as shown below, with elevated 20th century values. I’ve rotated the original Loso Figure 6 below and added in a horizontal red line at the maximum from the smoothed 40-year filter version during the 1000-1200 period, for reasons discussed below.
Original Caption: Figure 6. Master varve thickness chronology from Iceberg Lake. Values are averages of raw measurements from overlapping, stratigraphically correlated sections providing continuous coverage from A.D. 442 to 1998. Across all years, mean thickness (dashed vertical line) is 3.8 mm. Thick line shows the same record smoothed with a 40-yr lowpass Butterworth filter. Gray bars on right side of panel indicate numbers of laminae used in construction of master chronology, and opposite them the number of laminae excluded from each year in the plotted chronology due to evidence of turbidites, resuspended sediment, or ice-rafted debris
If one looks in detail at a blowup of the 20th century, we see what seems to be a frequent pattern in proxies: elevated values in the 1930s, with lower recent values during what is supposed to be higher recent temperatures. The Divergence Problem – Varve Style. The values at the end of the 20th century were significantly lower than the values 30 years ago.
It is certainly not correct to characterize the Loso results as presented as evidence supporting higher medieval values as compared to 20th century values. They don’t. On the other hand, it’s not a magic bullet the other way either. I’d like to see some other varve chronologies before concluding anything very much from this series. There are many assumptions and exclusions in the methodology which may or may not have biased the results. I don’t see much point in analyzing their procedures much until some other comparanda are available. The Divergence Problem, Varve Style, is very real. However plausible the rationale may be for varve thicknesses as a proxy for temperature, the apparent divergence is disquieting. In addition, and this is just an impression from the first diagram illustrated here, the appearance of the medieval section in Core J has a “quieter appearance” than the LIA sections of other cores, or for that matter, the modern cores. In the Venezuela cores, the medieval sections were also “quieter” than the modern section. This is nothing more than an impression. It would take a lot of work to really analyze what’s being done here.
I don’t exclude the possibility that well-conceived proxy studies could show to a reasonable person that the modern warm period is warmer than the MWP. However, that doesn’t mean that the Hockey Team studies have done so or , even if some future study was done correctly, that that would vindicate the Team, any more than subsequent evidence has vindicated the Piltdown Mann, using a comparison that I’ve made previously. Is Loso et al such a study? It doesn’t look like a magic bullet to me.
Updated Text – Lee has challenged the apparent contradiction between the abstract of the Loso article and the characterization of the Loso article by the Idsos. Like realclimate, the Idsos review many articles and their reviews are of unequal quality. I haven’t relied on their characterization of this or other articles. I don’t rely on realclimate’s characterization of articles.
The abstract to the article said:
Varve thickness is positively correlated with Northern Hemisphere temperature trends, and more strongly with a local, ‘ˆ¼600 yr long tree ring width chronology. Varve thickness increases in warm summers because of higher melt, runoff, and sediment transport (as expected), but also because shrinkage of the glacier dam allows shoreline regression that concentrates sediment in the smaller lake. Varve thickness provides a sensitive record of relative changes in warm season temperatures. Relative to the entire record, temperatures implied by this chronology were lowest around A.D. 600, warm between A.D. 1000 and A.D. 1300, cooler between A.D. 1500 and A.D. 1850, and have increased dramatically since then. Combined with stratigraphic evidence that contemporary jökulhlaups (which began in 1999) are unprecedented since at least A.D. 442, this record suggests that 20th century warming is more intense, and accompanied by more extensive glacier retreat, than the Medieval Warm Period or any other time in the last 1500 yr.
The “Northern Hemisphere temperature trend” used as a comparandum through linear correlation is Mann and Jones 2003, a reconstruction which I do not regard as gold standard. The tree ring chronology is Davi et al QR 2003 from the Wrangell Mountains, Alaska, a result which I’ve read and will review again in this context. Here is an excerpt from Loso’s comparandum graphic for the 1400-2000 period with the Mann and Jones 2003 hockey stick in solid, while the Davi et al tree ring chronology declines in the latter part of the 20th century, similarly to the Loso varve chronology – both showing the Divergence Problem disguised by the Mann bristlecone PC1 in Mann and Jones 2003.
As noted by Willis, the Idsos’ characterization of the graphic focused on the closing 20th century value rather than peak 20th century values, stating:
This work revealed “a clear manifestation of the Medieval Warm Period” between AD 1000 and 1250, the peak warm-season temperature of which (”smoothed with a 40-year lowpass Butterworth filter”) was clearly higher than it was at the end of the 20th century.
In my opinion, neither Loso nor the Idsos adequately report on the Divergence Problem, Varve Style and both are too quick to claim the result as vindication for one view or another. Various CA commenters with geological backgrounds have identified many problems with interpreting the Loso results as temperature proxies.
Reference: Loso online here http://polar.alaskapacific.edu/mloso/manuscripts/losoetal2006.pdf
Update: Data ftp://ftp.ncdc.noaa.gov/pub/data/paleo/paleolimnology/northamerica/usa/alaska/iceberg2008.txt