Michael E. Mann, assistant professor of Climatology in the Department of Environmental Science at the University of Virginia is famous for the historical temperature graph published in the Intergovernmental Panel on Climate Change (IPCC) draft report of Jan 2000 and the actual report of 2001. In contrast with the bulk of previous research on climate over the last 1000 years, Mann’s graph, as published by the IPCC, shows a very slight cooling trend for 900 years with minor variations, followed by a sharp up-tick in the 20th century. That distinctive shape led to his graph being described as a “hockey stick”.
This new picture of climate over the last 1000 years stood in sharp contrast to the graph published by the IPCC in 1995. That graph clearly shows the Medieval Warming Period (MWP) that lasted from 1000 AD to 1450 AD followed by the Little Ice Age (LIA) that lasted from 1450 AD until 1900 AD.
Our results suggest that the latter 20th century is anomalous in the context of at least the past millennium. The 1990’s were the warmest decade, and 1998 the warmest year, at moderately high levels of confidence.
This claim has become received wisdom, even within the scientific community. Certainly, the environmental movement takes it as gospel and opinion makers and politicians have followed suit. The debate has moved from whether or not global warming is actually happening or even what is causing the observed changes (other than the assumption that it is CO2) to a discussion about what can be done to ameliorate the impact. Still, not everyone has bought into the “hockey stick”. Mann’s research has been criticized on a number of grounds. The 20th century temperatures were based on land based temperature records while the temperatures for earlier centuries were based on proxy measures of temperature, primarily tree ring cores. This is an apples-to-oranges comparison that should never have passed peer review. The 20th century ground based records do not correlate very well with satellite based readings of the upper atmosphere, which show far less warming. Moreover, the satellite readings are corroborated by readings from weather balloons. These show negligible warming over the last few decades. Had Mann appended these readings onto his tree-ring readings, his graph would have been known as the billiard cue. It is also amazing that Mann would have the effrontery to claim “moderately high levels of confidence”. In reality, Mann’s margins of error are so large they could accommodate any number of wild swings in temperature over the last 1000 years. Mann’s claim that 1998 was the warmest year on record ignored that fact that that was the year of an exceptional El Nino event and not even Mann can claim that slight increases in CO2 precipitated El Nino events. Here’s the hockey stick again with the error ranges shown in yellow.
Longer term analyses of temperature based on ice cores, for example, show that temperatures frequently increased and decreased in very short time spans. The graph shows temperatures estimated from ice-core samples going back into the last ice-age. This graph really is scary.
There are also problems with Mann’s reliance on tree ring cores, particularly when they are not geographically dispersed, as David Legates points out in his TCS column:
But do proxy records really represent air temperature fluctuations? Most of the analyses on which the “hockey stick” relies are taken from tree-ring cores. Trees, however, respond not only to temperature fluctuations but also to species competition, fire episodes, pest infestations, and droughts. For example, if rainfall is limited, as often is the case in western North America (where the preponderance of data for Mann’s pre-A.D. 1400 analysis is located), tree growth is severely restricted, regardless of the temperature conditions. It is impossible under such conditions to discriminate between a cold period and a dry period — which is why Soon and Baliunas correctly characterized their assessments as “climate anomalies” rather than boldly assert they reflect air temperature fluctuations, as Mann does. Moreover, Dr. Jan Esper of the Swiss Federal Research Institute and colleagues demonstrated that their careful analysis of tree-ring chronologies yields an annual temperature curve for a large portion of the Northern Hemisphere that, unlike the “hockey stick,” clearly shows the existence of the Medieval Warm Period and that temperatures during the early years of the millennium were commensurate with those of the 1900s.
Soon and Baliunas followed Mann’s limited quantitative study with a qualitative study that reviewed 240 recent research papers that analyzed climate over the past 1000 years. They looked for climate anomalies rather than attempt to assign a global average temperature to each period. Their research clearly showed that the MWP and LIA were real events; the scientific evidence corroborates the historical evidence. The reaction to their work has been hostile and ad hominem. Stephen McIntyre and Ross McKitrick obtained Mann’s original data and tried to reproduce his work. According to the abstract of their paper CORRECTIONS TO THE MANN et. al. (1998)PROXY DATA BASE AND NORTHERN HEMISPHERIC AVERAGE TEMPERATURE SERIES:
The data set of proxies of past climate used in Mann, Bradley and Hughes (1998, “MBH98” hereafter) for the estimation of temperatures from 1400 to 1980 contains collation errors, unjustifiable truncation or extrapolation of source data, obsolete data, geographical location errors, incorrect calculation of principal components and other quality control defects. We detail these errors and defects. We then apply MBH98 methodology to the construction of a Northern Hemisphere average temperature index for the 1400-1980 period, using corrected and updated source data. The major finding is that the values in the early 15th century exceed any values in the 20th century. The particular “hockey stick” shape derived in the MBH98 proxy construction – a temperature index that decreases slightly between the early 15th century and early 20th century and then increases dramatically up to 1980 — is primarily an artefact of poor data handling, obsolete data and incorrect calculation of principal components.
Mann and company responded to McIntyre and McKitrick in a series of exchanges that suggest Mann was having trouble supplying his opponents with his original data files and then blasting them for using the wrong data. It also reveals that Mann’ evidence against the MWP rests on just three data series. That is very slight evidence to cite against the preponderance of scientific and historical evidence for the MWP. The graph shows Mann’s original graph on top and the revised version that McIntyre and McKitrick derived from Mann’s datasets. Note how many datapoints lie above the base line in the revised version.
There are valid reasons to be concerned about adverse human impact on the environment. But the current hysteria about global warming is diverting attention and resources away from addressing real environmental problems. Poor research that fits the climate activists’ agenda makes headline news while good news is ignored. I recently read about a study that claimed global warming was going to destroy 95% of the coral in the Great Barrier Reef by 2050. The author’s of the study do not address how the coral made it through the Medieval Warming Period, presumably because they assume it never happened – it isn’t on Mann’s IPCC sanctioned graph. They are simply assuming global warming of the order of 2°C to 6°C will occur over this century. Such is the power of Mann’s hockey stick. There are threats to the reef that are under human control; man can cut the flow of sediment, fertilizers and pesticides on to the reef. But, as in the past, the climate will vary and there won’t be much humanity can do about it. This will become abundantly clear when the current interglacial vacation ends. Climate activists claim that human induced greenhouse gas emissions will lead to a catastrophic increase in global temperatures. In their view, recent warming trends over the last century are solely due to human influence and mans’ burning of fossil fuels has disturbed a natural equilibrium that goes back at least a thousand years. Mann’s hockey stick is their proof and they deny that there even was a MWP or LIA. Mann himself denies that there ever was a warmer period than the 1990s over the last 1000 years. But the weight of scientific evidence will eventually break Mann’s hockey stick and the consensus view based on it, just as scholars eventually demolished the work of Foster and Bellesiles.