Steve McIntyre has a new analysis up, one that has a strong headline.
Though as he says, “not in so many words”, but more about techniques and exclusions. He writes:
Briffa Condemns Mann Reconstructions
Not in so many words, of course. However, Briffa et al 2013 took a position on the use of radially deformed tree ring cores that would prohibit the use of strip bark bristlecones in temperature reconstructions, thereby emasculating Mann’s reconstructions. And not just the Mann reconstructions, but the majority of the IPCC reconstructions used by Briffa in AR4.
I’ll report on this issue in today’s post. I’ve been looking closely at Briffa et al 2013 over the past 10 days and unsurprisingly there is issue after issue. According to CRU, they’ve been working on this article for over seven years and, needless to say, it is impossible to fully observe the pea in only a few days, especially when the adjustments have become so baroque that the chronology style is most aptly described as East Anglia Rococo, making the weary reader long for the classic simplicity of earlier CRU illusions like the Briffa Bodge and Hide the Decline. But more on this on another occasion.
full post here: http://climateaudit.org/2013/06/16/briffa-condemns-mann-reconstructions/
Completely agree.
His Yamal hockey stick was just as egregious as Mann’s, and he defended it vigorously.
If he publicly denounces his previous, obviously erroneous, work, then maybe he deserves to be cut some slack.
Tilo Reber says:
June 17, 2013 at 1:06 pm
This is a good time to remember what Briffa said in the freedom of information emails:
What’s more important to remember is that he wrote that in “private”, and has not stated it publicly. That’s his “sin”.
Briffa’s selective use of tree ring data just shows “When you’ve seen one, you’ve seen Yamal.”
climatereason asks at June 17, 2013 ,12:54 am why “Tree rings are believed to be reliable regional thermometers”.
The answer is in the principle known as Growing Degree Days. All else held equal, there is a very well understood and quite well quantified relationship between the length of time that a plant’s environment is above a threshold temperature and its growth. For most temperate plants, the threshold is about 50F. So if you take the minimum and maximum daily temperatures for the day, subtract 50 from each (if negative, go to zero) and average the result, you get the degrees for the day. For example, a day with a high of 70°F and a low of 54°F would contribute 12 GDDs.
Keep a cumulative total for the season and you get the Growing Degree Days so far. Daffodils bloom at about 110 GDD and dandelions at 140 GDD. Apple and cherry trees start to blossom at 300 GDD but goldenrod doesn’t bloom out until about 2200 GDD. Other lifecycle events including the weight that a tree puts on during the year are well correlated to GDD.
Okay, a couple of caveats to the algorithm above. Plants also have an upper threshold above which you stop getting incremental benefit. For most temperate plants, 86F is about right. So the real equation is:
sum ((Tmax -50 +Tmin – 50)/2)
where Tmax is [86F, measured max, 50F] and Tmin is the similarly constrained minimum daily temperature.
(I’m doing a poor job of explaining this. BeeCulture magazine did a much better job back in Feb 2002. You can find more technical sources but theirs was very well written.)
Second, yes this is a loose heuristic. Some days, the temperature will spike and a cumumlative instantaneous record would be very different from the two-point average. For agricultural purposes, we’re trying to get within a day or two, not predict the hour of blooming. The two-point average has proven very reliable.
So back to the original point. Yes, GDD is a good predictor of tree growth. All else held equal, yf average temperatures for a year are above average, you’ll get more GDD and wider tree rings. In colder years, you get narrower tree rings.
It’s that first assumption that presents the real challenge to paleo-reconstructions. I can make very reliable statements about GDD and growth over population averages of plants that I can observe today. I can control for or at least adjust for variables like precipitation changes or shading. We don’t have that prior knowledge about the conditions for fossil tree rings. Properly done, we could assume that precipitation and shade should average out over a large enough area but there just aren’t that many usable tree fossils to make those kinds of statistical statements.
To me, that means that tree ring trends are a clue about possible temperature trend but they are far from sufficient to make definitive statements.