Since 2001, there have been repeated claims that the reconstruction is at best seriously flawed and at worst a fraud, no more than an artefact of the statistical methods used to create it (see
The great hockey stick debate).
Details of the claims and counterclaims involve lengthy and arcane statistical arguments, so let’s skip straight to the
2006 report of the US National Academy of Science (pdf). The academy was asked by Congress to assess the validity of temperature reconstructions, including the hockey stick.
“Array of evidence”
The report states: “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 1000 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 ice caps and the retreat of glaciers around the world”.
Most researchers would agree that while the original hockey stick can – and has – been improved in a number of ways, it was not far off the mark. Most later temperature reconstructions fall within the error bars of the original hockey stick. Some show far more variability leading up to the 20th century than the hockey stick, but none suggest that it has been warmer at any time in the past 1000 years than in the last part of the 20th century.
It is true that there are big uncertainties about the accuracy of all past temperature reconstructions, and that these uncertainties have sometimes been ignored or glossed over by those who have presented the hockey stick as evidence for global warming.
The problems
Climate scientists, however, are only too aware of the problems (see
Climate myths: It was warmer during the Medieval period), and the uncertainties were both highlighted by Mann’s original paper and
by others at the time it was published.
Update: as suggested by the academy in its 2006 report, Michael Mann and his colleagues have reconstructed northern hemisphere temperatures for the past 2000 years using a broader set of proxies than was available for the original study and updated measurements from the recent past.
The new reconstruction has been generated using two statistical methods, both different to that used in the original study. Like other temperature reconstructions done since 2001 (see graph), it shows greater variability than the original hockey stick. Yet again, though,
the key conclusion is the same: it’s hotter now than it has been for at least 1000 years.
In fact, independent evidence, from ice cores and sea sediments for instance, suggest the last time the planet approached this degree of warmth was during the interglacial period preceding the last ice age over 100,000 years ago. It might even
be hotter now than it has been for at least a million years.
Further back in the past, though, it certainly
has been hotter – and the world has been a very different place. The crucial point is that our modern civilisation has been built on the basis of the prevailing climate and sea levels. As these change, it will cause major problems.
