Cook explains that the correlation between tree rings and thermometers is not reliable, so Mann should selectively throw out the data for any years which don’t support the hypothesis he is determined to prove.
What does “hide the decline” refer to?
Phil Jones’ email is often cited as evidence of an attempt to “hide the decline in global temperatures”. This is incorrect. The decline actually refers to a decline in tree-ring density at certain high-latitude locations since 1960. However, Muller doesn’t make this error – he clearly understands that global temperatures have been rising in recent decades as indicated by the instrumental record.
Tree-ring growth has been found to match well with temperature, and hence tree-ring width and density is used to plot temperature going back hundreds of years. However, tree-rings in some high-latitude locations diverge from modern instrumental temperature records after 1960. This is known as the “divergence problem”. Consequently, tree-ring data in these high-latitude locations are not considered reliable after 1960 and should not be used to represent temperature in recent decades.
h/t to Marc Morano