Over on Climate Audit, Steve McIntyre has been making a series of posts that have been putting the final nails in the coffin for Michael Mann’s MBH98 paper. This paper was responsible for the famous hockey stick graph which is based on tree ring data from Bristlecone Pine trees. Mann’s work implies them to be excellent proxy indicators of temperature, and due to their age, a profound record of temperature. Problem is, it looks like most of the results is Mann’s paper have been thoroughly discredited by the work of McKittrick and McIntyre in 2005, plus McIntyre’s more recent work.
At 4600-4800 years old for some of the oldest trees, Bristlecone Pines (BCP) certainly have seen most if not more than all of human recorded history, so it seems logical to look to them for answers about our temperature history.
One of the graphs Steve McIntyre recently produced was this one:
About this graph he notes:
Here’s the MBH98 PC1 (bristlecones) again marking 1934. Given that bristlecone ring width are allegedly responding positively to temperature, it is notable that the notoriously hot 1934 is a down spike.
Since 1934 is generally accepted now to be the hottest year on record in 20th century it is indeed curious that 1934 in Mann’s data shows up as a down spike.
But seeing what happened with 1934, one has to wonder what do these trees really record in their tree ring growths? Is it temperature as Mann speculates? Or is it any number of other things related to plant growth in various combinations?
I was curious about what others had to say about these ancient pines. One of the first articles I came across was by NOVA, the PBS science program. They had an in-depth article on the “Methuselah grove” where the most ancient trees reside in the White Mountains of California’s Inyo National Forest.
What caught my eye in the NOVA article were these passages:
It turns out that the bristlecone pine has evolved survival strategies that might make other, less hardy plants, well, green with envy. These strategies help it cope with one of the most flora-unfriendly environments on the planet.
But the really interesting one is this:
Bristlecone rings, which vary in width year to year, reveal that the trees have an innate ability to endure times of stress, such as a string of drought years. In such periods, the species can go almost dormant. “There is something a little fantastic,” wrote Edmund Schulman in the March 1958 National Geographic, “in the persistent ability of a 4,000-year-old tree to shut up shop almost everywhere throughout its stem in a very dry year, and faithfully to reawaken to add many new cells in a favorable year.”
No where in the NOVA article does it link temperature and tree ring growth for Bristlecone pines, but it seems clear that water is a major factor in BCP growth.
Another article I found on NASA’s Earth Observatory website initially talks briefly about temperature proxies, but then focuses on precipitation for the remainder of the article with a review of early work with BCP tree rings by Andrew Ellicott Douglass.
“Through long-past ages and with unbroken regularity, trees have jotted down a record at the close of each fading year—a memorandum as to how they passed the time; whether enriched by added rainfall or injured by lightning and fire…. So, in the rings of the talkative pines we find lean years and fat years recorded. The same succession of drought and plenty appears throughout the forest.”
The NASA article goes on to say:
In the 1950s one of Douglass’ former students and a respected tree researcher in his own right, Edmund Schulman, headed into the White Mountains to look at the trees rumored to be very old. He discovered Methuselah and the old bristlecone pines surrounding it. Around the trees, even older dead trees remained on the ground. Together, they gave a climate record of the Southwest United States that extends back 9,000 years, the longest record for a single tree species.
Douglas’ rings tell about rainfall in the southwestern United States, but trees also respond to changes in sunlight, temperature, and wind, as well as non-climate factors like the amount of nutrients in the soil and disease. By observing how these factors combine to affect tree rings in a region today, scientists can guess how they worked in the past. For example, rainfall in the southwestern United States is the factor that affects tree growth most, but in places where water is plentiful, like the Pacific Northwest, the key factor affecting tree ring growth may be temperature.
So in the case of the BCP in the USA desert southwest, it appears that since water is it’s scarcest resource, it has the most effect on it’s growth rather than temperature.
This situation fits well with Liebig’s Law, from Wikipedia:
Liebig’s Law of the Minimum, often simply called Liebig’s Law or the Law of the Minimum, is a principle developed in agricultural science by Carl Sprengel (1828) and later popularized by Justus von Liebig. It states that growth is controlled not by the total of resources available, but by the scarcest resource. This concept was originally applied to plant or crop growth, where it was found that increasing the amount of plentiful nutrients did not increase plant growth. Only by increasing the amount of the limiting nutrient (the one most scarce in relation to “need”) was the growth of a plant or crop improved.
Liebig used the image of a barrel—now called Liebig’s barrel—to explain his law. Just as the capacity of a barrel with staves of unequal length is limited by the shortest stave, so a plant’s growth is limited by the nutrient in shortest supply.
So when looking at trees that grow in a desert environment, you could naturally conclude that water, and not temperature is the shortest barrel stave.
No wonder 1934 is a negative on Man’s graph above, it was hot and dry that year. It’s been said that even grasshoppers were starving during that drought.
Above: USA Palmer drought index for 1934 see original source here
The White Mountains where Mann and other researchers took core samples from BCP’s is marked with the arrow above. Notice that the location is in the “extreme drought” area. Below is the map of all tree samples for the MBH98 paper:
It looks like, at least for 1934, BCP’s in the USA desert southwest are better at being rain gauges than “treemometers”. Given “Liebig’s barrel”, it makes one wonder whether BCP’s are a good proxy for temperature at all. Perhaps “Mann’s rain barrel” would be a better name for the MBH98 paper.