Of the 1001 nit-pickings of the climate change skeptics, their key and most persistent claim is that "The Hockey Stick is Broken!!". This is indeed understandable, because their whole case melts like an ice cream in the sun if the account of the scientists is anything like correct.
So the controversy deserves a minute or two to understand.
The climate skeptics were bitterly disappointed with Mann's graph, because up to then they had this little chap stuck up on their walls:
As far as I can see, this graph is based on data from the 1995 IPCC report - which is now 15 years old. The end point of the graph may be 1990. Science moves on. Unlike climate change denial.
Michael Mann in 1998 put together a record that joined up present day temperature observations with past temperature reconstructions. It was republished in expanded form in Nature in 1999, and republished in the 2001 IPCC report. (see the upper of the two graphs). Mann later published a small corrigendum which corrected the descriptions of supplementary information that accompanied the Mann et al article detailing precisely what data were used.
This minor error was seized on by the
Because of the controversy, the problem was considered by the US National Academies of Science, who concluded:
"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".
Note that "array of evidence". Mann's central finding has been confirmed by up to a dozen other groups of scientists. Mann relied strongly on tree ring data, but data from coral, ice cores and other proxies. This is how science works: start with one observation, and then run and re-run over the field to make sure that it is not invalidated by other data. In this case it is not.
As I said before, the current position is a lousy hockey stick for sure, since hockey sticks made of cooked spaghetti lack the structural integrity to deek, let alone deliver a slap shot, but it does very convincing picture of a world that is clearly undergoing warming in recent history.
The conclusion is that the skeptics have through generalisation, obsessive persistence and voluminous internet bloggery, thrown up a cloud of dust around the inescapable conclusion of climate science:
it is hotter now than it has been for at least 1000 years, possibly for 100,000 or even more. The only explanation that we have for this is the industrially enhanced greenhouse effect.
There is a full discussion of this topic here.