The rise and fall of the Hockey Stick and Mann-made global warming alarm

A Sceptical Mind:

The rise of the so called Hockey Stick graph is pivotal to the story of the rise of the alarm about man made global warming.

The fall of the Hockey Stick graph is pivotal to the rise of scepticism about man made global warming.

Here is the story of the rise and fall of the Hockey Stick.

The Background

A central and critical plank of the alarmist global warming case is that the current phase of warming that started in the late 19th century is unprecedented.

Why is this claim so important?

Because if a similar or greater warming phase has occurred in the very recent past, before human CO2 emissions had caused CO2 levels to rise, then clearly any such recent warming must have been natural and was not caused by CO2. And if any recent similar warming phase was natural then clearly the current phase of warming could also be a natural phenomena.

If the current phase of warming could be natural then those arguing that it was primarily caused  by human CO2 emissions would have to prove their hypothesis. And this is something they cannot do.

The only “proof” that CO2 is currently forcing up global temperatures is the claim that the current warming is somehow unusual, unique and unnatural. That’s the total argument for CO2 forcing. Something unprecedented is happening to the climate and CO2 is the only candidate for what is causing this unique phenomena.

Its certainly true that the well understood physics of CO2 in the atmosphere demonstrates (see “CO2 the basic facts“)that CO2 is indeed a greenhouse gas and will have a warming impact. No one disputes that. The issue is what is the scale of impact that this CO2 warming is having on the overall climate system. Is the effect of the CO2 so big that it can drive the temperature of the whole planet up in a way that is big enough to actually alter the climate?

This is a much harder question to answer because no one has a model of the total climate system that actually works and which verifiably produces even remotely accurate forecasts about climate trends.

So without a working model of the total climate system the only way to “prove” that CO2 is driving climate change is to prove that something truly unique is happening to the climate, that there is unprecedented warming occurring, and and then propose man made CO2 change as the only candidate as the cause of this ‘unprecedented’ warming.

The “problem” of the Medieval Warm Period

Until the 1990s there were many, many references in scientific and historical literature to a period labelled the Medieval Warm Period (MWP) lasting from about AD 800–1300. It was followed by a much cooler period termed the Little Ice Age. Based on both temperature reconstructions using proxy measures and voluminous historical references it was accepted that the Medieval Warm Period had been a period when global temperatures were a bit hotter than today’s temperatures. Until about the mid-1990s the Medieval Warm Period was for climate researchers an undisputed fact. The existence of the Medieval Warm Period was accepted without question and noted in the first progress report of the IPCC from 1990. On page 202 of that 1990 IPCC report there was the graphic 7c (see below), in which the Medieval Warm Period was portrayed as clearly warmer than the present.

By the time of the second IPCC report in 1995 where for the first time CO2 forcing began to be proposed more prominently as a cause of serious alarm, the Medieval Warm Period was sidelined in the text and narrative. An important way that this was done in the report was to alter the diagram of recent climate history by simply shortening the time period it covered so that it now started after the Medieval Warm Period. All that was shown was the long slow recovery from the Little Ice Age to today’s temperatures, i.e. a long period of increasing temperatures. But clearly this was only a short term solution. The way that the Medieval Warm Period dominated the recent climate graph challenged the basic argument for CO2 forcing which was that the late 20th century climate was some how unique. As Jay Overpeck, an IPCC participant said in his email to Professor Deming, “We have to get rid of the Medieval Warm Period”.

In order to prove CO2 forcing the Medieval Warm Period had to be eliminated.

The Rise of the Hockey Stick

Between the 1995 second IPCC report and the 2001 third IPCC report there was a complete revision in the way that recent climate history was portrayed. The supporters of the theory that CO2 changes were driving temperatures up had succeeded in their goal of eliminating the Medieval Warm Period. This rewriting of climate history and the elimination of the Medieval Warm Period was achieved through the famous Hockey Stick graph.

To understand the scale of the revision that had taken place compare the two graphs below. The one on the left is diagram 7c from page 202 of the 1990 IPCC report in which the Medieval Warm Period was portrayed as clearly warmer than the present. On the right is the Hockey Stick graph from the 2001 IPCC report in which the Medieval Warm Period and the Little Ice Age have all but disappeared and the recent climate history is dominated by a rapid temperature rise in the last 20th century.

The first blow against the accepted understanding of climate history came in 1995 when the English climatologist Keith Briffa (based at the Climate Research Unit at East Anglia) published in the journal Nature a study with sensational results. According to his studies of tree rings in the Siberian Polar-Ural, there had never been a Medieval Warm Period and the 20th century suddenly appeared as the warmest of the last 1000 years. The most recent part of this study is known as the Yamal study, because of the name of the region it was done in, and it has recently been discredited – see here.

Briffa’s work boldly proposed that the 20th Century had experienced the warmest climate of the millennium and this claim was now the central battlefield for the scientific argument about CO2 forcing. This of course ignored the Climatic Optimum (see Happy Holocene) between 5000 and 9000 years ago when temperatures were significantly higher than today but most people (and certainly the media and politicians) actually think that 5000 years is a long time ago so there was no need to undermine the Climatic Optimum in order to win wide public support for the CO2 forcing hypothesis. Hottest in the last 1000 years would do.

Briffa’s work had an impact and laid the ground work but the real knock out blow that finally succeeded in eliminating the Medieval Warm Period was a paper published in 1998 in Nature by Mann, Bradley and Hughes entitled, “Global-scale temperature patterns and climate forcing over the past six centuries” (you can download it here). This was the original peer reviewed hockey stick article.

Michael Mann of the Department of Geosciences, University of Massachusetts, who was the primary author of the paper, had in one scientific coup overturned the whole of climate history. Using tree rings as a basis for assessing past temperature changes back to the year 1,000 AD, supplemented by other proxies from more recent centuries, Mann completely redrew climate history, turning the Medieval Warm Period and Little Ice Age into non-events. In the new Hockey Stick diagram the Medieval Warm Period and Little Ice Age have disappeared, to be replaced by a largely benign and slightly cooling linear trend in climate until 1900 AD after which the Mann’s new graph showed the temperature shooting up in the 20th century in an apparently anomalous and accelerating fashion.

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