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What If … the “Hockey Stick” Were Wrong? Et si …. la “Crosse de Hockey” était fausse ?

The “hockey stick” reconstruction of temperatures of the past millennium has attracted much attention – partly as it was high-lighted in the 2001 IPCC report as one of the important new results since the previous IPCC report of 1995, and partly as it has become the focus of a number of challenges. Discussion about the “hockey stick” is conducted with considerable fervor in the public media, where this curve is often presented as if it were a proof, or even the most important proof, of anthropogenic influence on climate.

As someone who has not worked on the past millennium, I do not want to discuss the merits of the often rather technical challenges (which have been dealt with elsewhere on this site). Rather, I want to discuss the “what if…” question: what if really some serious flaw was discovered in the “hockey stick” curve? What would that mean?

Par Stefan Rahmstorf (traduit par Claire Rollion-Bard)

La reconstruction “crosse de hockey” des températures du dernier millénaire a beaucoup attiré l’attention – en partie car elle a été mise en avant dans le rapport IPCC 2001 comme l’un des nouveaux résultats importants depuis le précédent rapport IPCC de 1995, et en partie car elle est devenue le point de mire d’un certain nombre de défis. La discussion sur la “crosse de hockey” est transmise avec une ferveur considérable dans les médias, où cette courbe est souvent présentée comme une preuve ou même la preuve la plus importante de l’influence anthropogénique sur le climat.

En tant que personne n’ayant pas travaillé sur le dernier millénaire, je ne veux pas discuter des mérites des défis techniques (qui sont discutés dans une autre section du site). Je veux plutôt discuter de la question “et si…” : et si de sérieux défauts étaient trouvés dans la courbe “crosse de hockey” ? Qu’est ce que cela signifierait ?


So let’s assume for argument’s sake that Mann, Bradley and Hughes made some terrible mistake in their statistical analysis, so we need to discard their results altogether. This wouldn’t change our picture of the last millennium (or anything else) very much: independent groups, with different analysis methods, have arrived at similar results for the last millennium. The details differ (mostly within the uncertainty bounds given by Mann et al, so the difference is not significant), but all published reconstructions share the same basic features: they show relatively warm medieval times, a cooling by a few tenths of a degree Celsius after that, and a rapid warming since the 19th Century. Even without Mann et al, we’d still be stuck with a “hockey stick” type of curve – quite boring.

So let’s try some more exciting “what ifs”. In mid-20th Century, medieval temperatures are exceeded in all the reconstructions, hence recent (last 10-15 years, say) temperatures appear to be unprecedented for at least a millennium (that even holds for the alternative histories presented by the “hockey stick” critics). Now what if that were wrong – if all proxy reconstructions as well as model simulations of the past millennium were fundamentally in error?

Let us assume that medieval temperatures after all had been warmer than the present. Even that would tell us nothing about anthropogenic climate change. The famous conclusion of the IPCC, “The balance of evidence suggests that there is a discernible human influence on global climate”, does not depend on any reconstruction for the past millennium. It depends on a detailed analysis of 20th Century data. In fact, this conclusion is from the 1995 IPCC report, and thus predates the existence of quantitative proxy reconstructions like the “hockey stick”.

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