If you spend a little time talking to global warming skeptics, you’ll run across some variation of this argument: the Mann Hockey Stick graph, which shows global temperatures trending dramatically upward, is flawed/faked/broken.
The so called “hockey stick” is a graph from a paper written in 1998 by Michael Mann and others (often referred to as MBH 98). It shows temperatures for the past 1000 years, with a dramatic upward swing in the 20th century. The graph attained a fair bit of notoriety when the International Panel on Climate Change featured it in its Third Assessment Report in 2001. Since then there have been several scientific papers published about it, endless websites and blog posts, and a congressional hearing, all on this single paper.
The general thrust of all this study, criticism, accusations of fraud, reworking, reexamining etc. is that there were indeed some flaws in the statistical methods used in MBH 98. However, multiple papers since then have indicated that the conclusions are valid.
Climate scientists have compared multiple datasets (called proxies) – ice samples, treerings, boreholes, historical documents, marine sediments – to get an idea of past climate variations. I’ll quote from the executive summary of the National Academy of Sciences “Surface Temperature Reconstructions for the Last 2000 Years”:
It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries….less confidence can be placed in large-scale surface temperature reconstructions for the period from AD 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since AD 900….Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about AD 900….
The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on icecaps and the retreat of glaciers around the world, which in many cases appear to be unprecedented during at least the last 2000 years. Not all individual proxy records indicate that the recent warmth is unprecedented, although a larger fraction of geographically diverse sites experienced exceptional warmth during the late 20th century than during any other extended period from AD 900 onward.
The Wegman report, presented to Congress in 2005, was a detailed study of the statistical methods used in MBH 1998. Although it was widely hailed by climate change skeptics as a thorough debunking of Mann’s work, it is limited in scope. It’s worth some study if you want to understand the issue more fully, and a PDF can be found here. Commentary on the Wegman report here.
Other scientists have studied Mann’s conclusions and methods and found them to be robust. Some of these papers are summarized in the Wegman report.
Also take a look here.
I draw two conclusions from this:
1. There are lots of unknowns in the study of climate – both future predictions and past climate reconstructions.
2. There is substantial evidence that the current warming is unusual and goes beyond the ranges seen in the past 2000 years.
Also note that, whatever your opinion of the hockey stick graph itself, there is still strong evidence that the earth is warming and that humans are the cause.