http://www.realclimate.org/index.php?p=11


Numerous myths regarding the so-called "hockey stick" reconstruction
of past temperatures, can be found on various non-peer reviewed
websites, internet newsgroups and other non-scientific venues. The
most widespread of these myths are debunked below:


MYTH #0: Evidence for modern human influence on climate rests entirely
upon the "Hockey Stick" Reconstruction of Northern Hemisphere mean
temperatures indicating anomalous late 20th century warmth.


This peculiar suggestion is sometimes found in op-ed pieces and other
dubious propaganda, despite its transparant absurdity. Paleoclimate
evidence is simply one in a number of independent lines of evidence
indicating the strong likelihood that human influences on climate play
a dominant role in the observed 20th century warming of the earth's
surface. Perhaps the strongest piece of evidence in support of this
conclusion is the evidence from so-called "Detection and Attribution
Studies". Such studies demonstrate that the pattern of 20th century
climate change closely matches that predicted by state-of-the-art
models of the climate system in response to 20th century anthropogenic
forcing (due to the combined influence of anthropogenic greenhouse gas
concentrations and industrial aerosol increases).

MYTH #1: The "Hockey Stick" Reconstruction is based solely on two
publications by climate scientist Michael Mann and colleagues (Mann et
al, 1998;1999).


This is patently false. Nearly a dozen model-based and proxy-based
reconstructions of Northern Hemisphere mean temperature by different
groups all suggest that late 20th century warmth is anomalous in a
long-term (multi-century to millennial) context (see Figures 1 and 2
in "Temperature Variations in Past Centuries and The So-Called 'Hockey
Stick'").

Some proxy-based reconstructions suggest greater variability than
others. This greater variability may be attributable to different
emphases in seasonal and spatial emphasis (see Jones and Mann, 2004;
Rutherford et al, 2004; Cook et al, 2004). However, even for those
reconstructions which suggest a colder "Little Ice Age" and greater
variability in general in past centuries, such as that of Esper et al
(2002), late 20th century hemispheric warmth is still found to be
anomalous in the context of the reconstruction (see Cook et al, 2004).


( more available in the link posted above )