Bottom line: The global warming debate is over tree rings.
Scientists have counted on tree rings to give them a record of global temperatures, particularly for the 20th century, but also for earlier centuries for which tree samples can be found. Generally, the wider and denser the tree rings, the warmer the year was.
But that works only for the years up to 1960, according to Penn State's Michael Mann, a climatologist and global warming believer. Then, for some not-completely-explained reason, Mann says, from 1961 on, tree ring density has not been a reliable indicator of temperatures.
Explanation needed. Mann says pollution, rainfall and other factors seem to have rendered tree-ring analysis useless since 1960. Steve McIntyre, the mathematician and global warming skeptic, says Mann and other climatologists have not explained why tree rings suddenly didn't work, and until they can explain that, scientists cannot blindly accept the premise that tree-ring temperature studies were ever valid. This question affects most major studies on global warming.
The "hide the decline" in Phil Jones' Nov. 16, 1999, e-mail referred to a temperature chart that hid the declining reliability of tree-rings after 1960. The chart, prepared by Jones using numbers from Keith Briffa, apparently shows temperatures from 1402 to 1960, as reflected in tree ring studies, and then tacks on actual thermometer readings for the years 1961 to 1998.
Both Mann and McIntyre confirmed this yesterday. According to skeptic McIntyre, the "hiding" may not have been intentional deception, but it was misleading all the same. McIntyre also said Jones has not yet fully explained why tree rings were a good measure until 1960, but a bad measure after 1960.
Hockeystick blade. This matters because 20th century temperatures are the "blade" of Mann's famous "hockeystick" chart. The "hockey stick" is different from the diagram that was discussed in the 1999 "hide the decline" e-mail, but the two charts have a similar shape -- showing the 20th century to be the warmest in at least 1,000 years.
In the case of the "hockey stick," the tree ring data and data from ice core and coral studies showed 20th century temperatures rising to 1980, when the studies end. But for other reasons, skeptics say Mann's chart is wrong. Skeptics say his "hockey stick" relies on tree-ring data that are calculated incorrectly to show that "blade" going up from 1900 to 1980. (After 1980, the chart uses thermometer readings, which continue the upward course. That's the only direct similarity between Mann's chart and Jones' diagram.)
Mann says the National Academy of Sciences reviewed his "hockey stick" chart and vindicated his work in 2006. However, it's hard to tell exactly what the National Academy of Sciences thought of his calculations. It found the chart "plausible," but said:
"substantial uncertainties currently present in the quantitative assessment of large-scale surface temperature changes prior to about A.D. 1600 lower our confidence in this conclusion compared to the high level of confidence we place in the Little Ice Age cooling and 20th century warming. Even less confidence can be placed in the original conclusions by Mann et al. (1999) that 'the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium.'"
Test and re-test. I have to admit, I had no idea until yesterday how central this tree-ring question was to the global warming debate. And I don't know who it will take to sort out whether the planet is warmer now or in the year 1200. (If it was warmer in 1200 and humans survived, the warming might not be so serious now, unless the atmosphere is being primed recklessly for even higher temperatures.)
Every study can be tested, and the pressure is on to make sure we get these studies right.
Frank Warner
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