Tree Rings & Michael Mann’s Hockey Stick

By Paul Homewood

 

The Spectator reviews a new book about tree rings:

 

History is only as good as its sources. It is limited largely to what has survived of written records, and in prehistory to random fragments unearthed by archaeologists and paleontologists. Climate history is no different. As the effects of global warming accelerate, it becomes ever more urgent to reassemble what we can of the atmospheric conditions of the past to gather evidence from wherever it may be.

Glacial ice cores are one place, with their frozen snapshots of long-ago air and traces of ash and pollen and greenhouse gases such as carbon dioxide or methane. Other climate proxies include the annual accretion of stalagmites, the growth of corals and the incremental layers of bone in the ears of fish. But in recent decades, tree rings have emerged as one of the most precise and revealing of all paleoclimatic indicators.

Each growing season, a tree adds an outer layer of cambium to its core of dead wood. How much is added is a function of thermal and pluvial conditions. The exact width of the ring and its microscopic structure give a very clear picture of the weather during a period of a few warm months, like a postcard from a distant summer.

Valerie Trouet is a leading tree-ring scientist and her fascinating book recounts, among much else, how she and her fellow dendrochronologists are building a data base of astonishing accuracy and detail. In some areas — where deadwood is preserved in alluvium or peat — the record can go back millennia. The German oak-pine tree-ring series gives an annual record of more than 12,500 years.

Unlike carbon14-dating, which can only offer a temporal range, tree rings pinpoint the conditions for a precise year, even the beginning or end of the season. Cross-referencing with known events has helped add a missing climate element to history. The disappearance, for instance, of the pioneering English colony at Roanoke in North Carolina had long baffled historians; after three years, in 1590, a relief ship arrived to find everyone gone. Tree-ring analysis has confirmed not only the intervening years as ones of drought but as the most extreme dry period on the eastern seaboard in the last eight centuries.

Dendrochronology has also helped explain such curiosities as a dip in Caribbean piracy in the 17th century (a spate of hurricanes), riots in Ptolemaic Egypt (rain failure), the Ottoman crisis of the early 17th century and the rise and fall of the Mayan, Mongolian and Uyghur empires. When, a few years ago, Stradivari’s famous Messiah violin was deemed a copy, it was analysis of the rings in its wood that confirmed its authenticity.

But it is the study of historical climate change that has benefited most from this sylvan corner of the scientific world. Using multiple cores from trees in an area, and multiple areas, a comprehensive picture of wider climatic variation can be assembled. Such analysis provided key evidence for the 1998 Hockey Stick graph, which stirred up a hornet’s nest of opposition from deniers, but has since persuaded all but the lunatic fringe that the spike in global temperatures is something seriously abnormal. Oscillating within a given range for 1,000 years or so, global temperatures suddenly started rising beyond that range in the 20th century, to reach skywards in 1998 — and that year is now only the tenth hottest.

https://www.spectator.co.uk/article/tree-ring-analysis-has-solved-many-historical-mysteries

  

Somehow what starts as a perfectly sensible review morphs into Michael Mann and his discredited hockey stick!

But, as the review itself admits, tree rings tell you more about rainfall than temperature, Indeed, in a much better review in Newsweek, we read how the book reveals in detail the effect that a long period of drought had on the declining Roman Empire in the 4thC.

In fact Mann’s Hockey Stick was hopelessly flawed in many ways. (I would recommend Andrew Montford’s book, “The Hockey Stick Illusion”, for anyone interested.

For a start, the Hockey Stick was based on shonky statistics, which were guaranteed to produce a hockey stick curve regardless of the data fed into it. This was because of the way Mann used Principal Component analysis. In simple terms, Mann’s statistics blew out of all proportion any data which showed a hockey stick effect and ignored all other data.

Secondly, as far as tree rings were concerned, it was heavily dependent on bristlecone pines. It has long been known that the marked increase in bristlecone growth in the 19th and 20thC is due to CO2 fertilization, not temperature. When bristlecones are taken out of Mann’s analysis. the hockey stick disappears.

Thirdly, when tree ring and other proxy data diverged from rising temperature data in the late 20thC, Mann ignored the proxies and spliced the temperature data onto his graph.

There are also a whole host of other major flaws in the Hockey Stick, not related to tree rings.

 

The Spectator review is written by Philip Marsden, who apparently is an English travel writer and novelist. Perhaps next time they might get someone who actually knows about the subject to write their reviews.

Far from Marsden’s brainless assertions about lunatic fringes and unprecedented temperatures, there is a wealth of data which proves that today’s climate is not unusual, and that the world has been warmer than now for most of the last 10,000 years or so.

 

For more technical detail from Steve McIntyre and Ross McKittrick, see here and here.