One of the most common arguments against anthropogenic climate change, is that our current warming is nothing other than a post-ice-age reheat.  This is natural, so no need to cut engine capacity or build windmills.  Top Gear can go to air with a clear conscience.

One of the key arguments against the "this is normal, why panic" brigade is the kind of graph on the right, commonly referred to as the "hockey stick graph".  The right hand side of the stick, the part for hitting the puck, is the worry.  It's the sharp rise in temperature that has been happening since the industrial revolution.  It is this kind of graph that the IPCC featured prominently in its 2001 report on global warming, and the BBC most recently featured in its pre-Copenhagen guide to global warming.

Climate sceptics have been quick to jump on the hockey stick.  Michael Mann's original 1998 version had error bars as thick as the tree trunks where the data had come from, and his computer models would have produced hockey sticks, they claimed, even if they had been fed the numbers from a telephone directory (so called "red noise").

What sceptics like Christopher Booker fail to mention in their gleeful criticism is that there are newer, better, more robust hockey sticks out there. 

When he produced the 1998 stick (so called MBH98)  Mann was clear about the fragility of his starting data ("it's hard to imagine how much more explicit we could have been") but he has since been hard at work with more diverse sources of data, and more refined algorithms.  Mann and numerous other colleagues produced a new stick in 2008, and the head was still as pronounced.  We're getting too hot for this just to be about the end of an ice age.  Something else is going on.  This is the hockey stick that BBC use in this graph here, and the article here.

Mann's most trenchant critics remain unimpressed.  Analyst Steve McIntyre, often favourably quoted by Christopher Booker, has given up years of his life to breaking the hockey stick.  He blogs at Climateaudit.org and I am happy to agree that though he has a background in "mineral exploration financing", his work should still be regarded on its merits.

The problem for the lay person is how to judge between the "warmist" Mann and the "sceptic" McIntyre.

When McIntyre writes

Their non-dendro network uses some data with the axes upside down, e.g., Korttajarvi sediments, which are also compromised by agricultural impact (M. Tiljander, personal communication), and uses data not qualified as temperature proxies (e.g., speleothem δ¹³C).

And Mann counters

The claim that “upside down” data were used is bizarre. Multivariate regression methods are insensitive to the sign of predictors. Screening, when used, employed one-sided tests only when a definite sign could be a priori reasoned on physical grounds. Potential nonclimatic influences on the Tiljander and other proxies were discussed in the SI, which showed that none of our central conclusions relied on their use.

The lay person is at a loss to judge (although Booker fancies himself perfectly qualified).  I just want to know if my central heating should be switched off, if I should take me lead from Jonathan Porritt or Jeremy Clarkson?  I haven't got time to find out about Korttajarvi sediments.

So who to believe?

I think we've to go with Mann.

Firstly, the more proof must be demanded of the sceptics.  If there is even a 30% chance that we are killing the planet, that is enough to take global action.  If a chemical has a 30% of giving you cancer you stay clear.  The same applies here.

Secondly, for all McIntyre's indepence, this may also be isolationism.  Even critics of Mann's 1998 hockey stick, and supporters of McIntyre's right to be heard state this:

McIntyre has not published a regular review paper on this issue in a peer-reviewed journal. We have advised Steve McIntyre several times that he should write a paper just on this issue, without blending many other aspects into such a paper. He has not done so, it seems.

and add

We would also like to emphasize that we consider Steve McIntyre often unfairly treated by the scientific community. He has, as everybody else, the right to be heard and to participate in the debate as long he is contributing scientific arguments. His GRL paper has demonstrated that he is qualified to participate. We have supported Steve McIntyre in his quest to be heard; that does not mean that we agree with all his views and knowledge claims. Indeed, we mostly do not.

I want to come back at Booker on this point "Why do you mention none of this?  You are sharp, pugilistic and combative, but underneath the dazzling rhetoric, I fail to find the truth."