Et si …. la "Crosse de Hockey" était fausse ? " />

What If … the “Hockey Stick” Were Wrong?

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?

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”.

Climate changes can have several different reasons, and the cause of any particular climate change needs to be investigated on a case by case basis. It cannot be found by looking at one temperature curve. Had medieval climate been warmer than the present, this would probably have been due to some natural cause – perhaps a peak in solar output. That would only tell us that in principle, natural causes can cause warming larger than what we’ve seen in the past decades. But we know that already – one need only go back far enough in time (e.g., fifty million years) to find examples of unquestionably warmer climates than today. However, it would be naive to conclude that the observed strong 20th Century warming therefore also must have a natural cause.

Investigating the cause of 20th Century warming is done in so-called detection and attribution studies, which analyze the various forcings (e.g., solar variations, greenhouse gases or volcanic activity) and the observed time and space patterns of climate change in detail. These studies, with a range of different techniques, have invariably concluded that the dominant cause of 20th Century warming is man-made greenhouse gases.

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