The Hockey Stick Controversy

The Climate Change Hockey Stick
Was it AGW or not?

If ever there was a single “trigger” to galvanise the average citizens of the world to accept the theory on humans causing dangerous global warming, then it would have to be the Mann, Bradley and Hughes – MBH “hockey stick” schematic diagram.

The Hockey Stick Diagram

MBH constructed their first diagram from AD 1200 to 1900 by looking at 183 tree ring records from across the Northern Hemisphere on the assumption that the width of each ring was related to temperature as the tree grew.  There was some scientific fact to this. They then added actual thermometer records to project temperatures from 1900 to 2100.  Mann et al later reconstructed the hockey stick extending it back to AD 1000.

Figure 1: Michael Mann’s hockey stick schematic diagram

The diagram was originally published in Nature in 1998, revised and published again in 1999 in Geophysical Research Letters.  Mann went on to become one of the Lead Authors of the IPCC Third Assessment Report in 2001 in which an updated hockey stick diagram featured prominently. It also appeared in subsequent IPCC Assessment Reports in some form.

The IPCC Third Assessment Report of 2001 also included a comment that the 1990s was the warmest period during the preceding 1000 years, even though there was abundant evidence to the contrary.  Statements like these and the use of the hockey stick diagram, which by now was being severely questioned, became central to the promotion of AGW by the alarmist cause that humans were causing dangerous climate change.

Figure 2: The Hockey Stick as published in the IPCC First Assessment Report – Summary for Policy Makers 2001

This hockey stick diagram shown at Fig. 2 was constructed using tree rings, coral, ice cores and historical records.  The red line is a “smoothed” 50-year average. The grey lines indicates the degree of uncertainties i.e. a fudge factor which is up to 5% – give or take.  From 1860 to 2000 the line is averaged annually because of more certainty in the data and now including actual thermometer records. From 2000 to 2100 shows 6 computer model “baseline” scenarios which allegedly do not take into account current or future greenhouse emissions.

Understandably the hockey stick immediately attracted attention on both sides of the debate. At least 14 separate reconstructions were done by other scientists. They could use whatever collection of proxy measurement data sets they desired, but apparently were required to only use the proxy data sets provided by the IPCC. Similar results to Mann’s et al were produced leading to an assertion that burning of fossil fuels was causing immediate and drastic heating of the planet.

Climate sceptics proceeded to analyse it to death, from every conceivable angle. Some problems were immediately noticed:

1. That the temperatures on the “shaft” showed a fairly even temperature range from AD 1000 to 1900 (prior to industrialisation), unlike “normal” climate fluctuations e.g. compare Fig 1 & 2 over 1,000 years against Fig 3 over 2000 years, both of which represent primarily tree ring calculations.

Figure 3: Southern Hemisphere tree ring records with an even temperature range and no dramatic changes.

2. That the “blade” showing temperatures rising at such an alarming fast rate just begged to be double checked, especially since other records such as quality spelotherms (mineral deposits in caves) and tree ring data from the Southern Hemisphere hadn’t shown any similar temperature rise at in the late 20th century. This appeared to negate any global warming allegations.

3. Statistician trained scientists noted a basic flaw in statistical compilation in that assumed measurements from proxy data e.g. tree rings had been combined with actual thermometer records in the same diagram and used to draw an alleged scientific conclusion. Additionally there was doubt about the conditions under which those thermometer records may have been taken and may not be standardised.

Despite the claims from non-aligned IPCC scientists, other computer modelled hockey stick versions started appearing, not a few of which were highly exaggerated such as used by Al Gore in his “An Inconvenient Truth”.

Thus the dogma of AGW – Anthropogenic Global Warming started becoming widely accepted by the lay public. Anyone who did not believe in AGW were dubbed as “denialist” or “sceptic” and world opinion turned against those seeking better answers.

Any scientist labelled as such began to be stonewalled when trying to gain access to raw data from which to do further research, including as I understand it data about the Medieval Warm Period and Little Ice Age. If it could be proved that the temperature during the MWP was the same or higher than today it would negate the statement that the 1990s where the warmest in the last 1000 years.

I will explore this further in a later post.


Climate: The Counter Consensus, Professor Robert M. Carter 2010.