Nic Lewis's discovery of a statistical alteration applied
by the IPCC lends strong support to lukwarming
As most people know, I am a lukewarmer -- somebody who accepts
carbon dioxide's full greenhouse potential, but does not accept the
much more dubious evidence for net positive feedbacks on top, and
who therefore thinks that a temperatuire rise of more than 2C in
this century is unlikely.
This view just got a strong boost. Nic Lewis, the indefatigable mathematical
sleuth who helped expose the mistakes in a paper about Antarctic
temperature trends has been looking at how the IPCC estimates
climate sensitivity -- that is, the warming expected for a doubling
of CO2. He finds that the one study that estimated sensitivity
entirely from experimental data -- Forster and Gregory 2006 -- was
distoted by the IPCC when it came to present their results. The
distortion was the imposition of a Bayesian "uniform prior" in a
way that statisticians say is wholly inappropriate, because it
effectively assumes a priori that strong warming is more probable
than it is. Yet you don't even have to know that the use is
inappropriate to know that it's inappropriate to take a published
result and alter the graph from it, adding an obscure footnote to
say you have done so. A published result is a published result.
The effect was to fatten the tail of the graph, making a warming
of more than 2C look much more probable.
I defy you to look at that graph -- the green one -- and tell me
that a temperature rise oif more than 2C is not "unlikely"
according to that study. I defy you to look at the graph -- the
blue one -- and not conclude that whoever drew it had better have a
very good argument for fattening the tail compared with what the
authors had originally published.
NIc has found that the IPCC did much the same to most of the
other estimates of climate sensitivity, which rely mostly on
models. This mistake is central to the IPCC's case, not peripheral.
It undermines the credibility of the case for urgent action against
climate change and strongly supports the argument that, other
things being equal, CO2 doubling will not cause more than a mild
and net beneficial warming.
Here's Nic's first paragraph:
The IPCC Fourth Assessment Report of 2007
(AR4) contained various errors, including the well publicised
overestimate of the speed at which Himalayan glaciers would melt.
However, the IPCC's defenders point out that such errors were
inadvertent and inconsequential: they did not undermine the
scientific basis of AR4. Here I demonstrate an error in the core
scientific report (WGI) that came about through the IPCC's
alteration of a peer-reviewed result. This error is highly
consequential, since it involves the only instrumental evidence
that is climate-model independent cited by the IPCC as to the
probability distribution of climate sensitivity, and it
substantially increases the apparent risk of high warming from
increases in CO2 concentration.
