---
title: "Climatic Research Unit documents"
chunk: 3/8
source: "https://en.wikipedia.org/wiki/Climatic_Research_Unit_documents"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:22:39.055334+00:00"
instance: "kb-cron"
---

I've just completed Mike's Nature trick of adding in the real temps to each series for the last 20 years (ie, from 1981 onwards) and from 1961 for Keith's to hide the decline. The "trick" was a technique to combine instrumental temperature record data with long term reconstructions based on proxy data, and "the decline" was a well known issue with Keith Briffa's reconstruction using certain tree ring proxies which appeared to decline after 1950, when measured temperatures were rising. The email was widely misquoted as a "trick" to "hide the decline" as though it referred to a decline in measured global temperatures, but this was obviously untrue as when the email was written temperatures were far from declining: 1998 had been the warmest year recorded. On 9 December 2009, Sarah Palin said the truncated phrase showed a "highly politicised scientific circle" had "manipulated data to 'hide the decline' in global temperatures", and at the 2009 United Nations Climate Change Conference in Copenhagen, senator Senator Jim Inhofe quoted Jones, and said "Of course he means hide the decline in temperatures". The graph showed three series of paleoclimate reconstructions, based on records of tree rings, corals, ice cores, lake sediments, etc., along with historical and instrumental records. "Mike's Nature trick" referred to Michael E. Mann's paper on temperature trends published by Michael Mann in Nature in 1998, which combined various proxy records and related them to actual temperature records: it included a figure later dubbed the hockey stick graph, which clearly distinguished between the proxy and instrumental data. Mann described the "trick" as simply a concise way of showing the two kinds of data together while still clearly indicating which was which. He said that there was nothing "hidden or inappropriate" about it, and that his method of combining proxy data had been corroborated by numerous statistical tests and matched thermometer readings taken over the past 150 years. A press release by the University of East Anglia said that the "trick" was using instrumental data to meet a requirement of showing temperatures more recent than those covered by the proxy based temperature reconstructions, and that the use of the word "trick" was not intended to imply any deception. An editorial in Nature said that 'trick' was slang for a clever (and legitimate) technique. The phrase "hide the decline" referred specifically to the divergence problem in which some post 1960 tree ring proxy data indicates a decline while measured temperatures rise. The reconstruction by Briffa et al. was based solely on tree ring data, which shows a strong correlation with temperature from the 19th century to the mid 20th century. They had published a statement on the divergence problem in 1998, and had recommended that the post 1960 part of their reconstruction should not be used. Jones stated that the email was "written in haste" and that, far from seeking to hide the decline, CRU had published a number of articles on the problem. The implications of the decline are discussed in Chapter 2 of the IPCC Third Assessment Report, and in Chapter 6 of the 2007 IPCC Fourth Assessment Report (AR4) which describes discussion of various possible reasons for the divergence which does not affect all the trees, and says that there is no consensus about the cause. It notes that Briffa et al. specifically excluded the post 1960 data, which is therefore not shown in the graph of their tree ring reconstruction in the AR4 report. John Tierney wrote in The New York Times that "the graph adorned the cover of a report intended for policy makers and journalists. The nonexperts wouldn't have realized that the scariest part of that graph – the recent temperatures soaring far above anything in the previous millennium – was based on a completely different measurement from the earlier portion. It looked like one smooth, continuous line leading straight upward to certain doom." Tierney quotes Michael E. Mann replying to a denier who raised this very issue about tree-ring data at the RealClimate blog, "No researchers in this field have ever, to our knowledge, 'grafted the thermometer record onto' any reconstruction. It is somewhat disappointing to find this specious claim (which we usually find originating from industry-funded climate disinformation Web sites) appearing in this forum." Tierney continued, "Dr. Mann now tells me that he was unaware, when he wrote the response, that such grafting had in fact been done in the earlier cover chart, and I take him at his word. But I don't see why the question was dismissed so readily, with the implication that only a tool of the fossil-fuel industry would raise it."
In December 2009, most scientists interviewed by The Philadelphia Inquirer considered there was ample other evidence supporting the original graph and they had not changed their views on the issues. In an exception, Richard Lindzen of the Massachusetts Institute of Technology alleged that the emails "explicitly refer to falsification and rigging of data" in covering up the divergence problem of tree ring proxies for the 1960s, which he said called earlier periods into question, and said "The trick here is replacing the kind of data you're using with something to make it look different." Other climatologists disputed Lindzen's accusations. Thomas Peterson of the National Climatic Data Center said he had seen nothing in the emails that called the fundamental science into question, and Andrew Solow of Woods Hole Oceanographic Institution agreed that there was no trickery, saying he would use the word trick to describe some methodological step, but expressed the view that the basis of reconstructions had been unclear. Several scientific sources state that the decline being referred to is a decline in tree ring climate proxy metrics, not temperature. Andrew Watson, Royal Society Research Professor at the UEA, said that the scientists had drawn "the line to follow the tree-ring reconstruction up to 1960 and the measured temperature after that."
Before the incident, Mann and others had presented reconstructions based on more proxies, and found similar results with or without the tree ring records. The United States Environmental Protection Agency (EPA) had issued an "endangerment finding" in 2009 in preparation for climate regulations on excessive greenhouse gases. Petitions to reconsider this citing the email were raised by the state of Texas and conservative activists and business groups the Coalition for Responsible Regulation, the Southeastern Legal Foundation, the Competitive Enterprise Institute and the coal company Peabody Energy. They said the email "evidences CRU staff's effort to deliberately manipulate data to yield desired results", and highlighted the word "trick". The Southeastern Legal Foundation said it showed "an attempt to cook the books to conceal the fact that the famous 'hockey stick' is a manipulated, misleading barrel of scientific nonsense." The EPA rejected these allegations as both irrelevant and inaccurate, noting that the WMO cover illustration by Jones bore no relationship to the IPCC assessment reports, and used different methods. The issues with tree rings had not been hidden, but were extensively discussed in scientific literature and in IPCC reports.