The thin line is the "hockey stick" curve allegedly showing recent temperatures (the handle of the stick at right) as the highest since 1400. Authors of the curve, Mann, Bradley et al. (see Reference 21), claimed that "temperatures in the latter half of the 20th century were unprecedented," that "even the warmer intervals in the reconstruction pale in comparison with mid-to late 20th-century temperatures," and that the 1990s was "likely the warmest decade." The IPCC adopted the Mann et al. analysis, calling 1998 the "warmest year" of the millennium.
The thick line is the corrected curve, which is derived from the same data set, showing the 20th Century temperatures to be colder than those of the 15th Century, and actually emerging from the Little Ice Age around the turn of the 20th Century.
Source Adapted from S. McIntyre and R. McKitrick, 2003. "Corrections to the Mann et al. (1998) Proxy Data Base and Northern Hemispheric Average Temperature Series" Energy & Environment, Vol. 14, No. 6, pp. 751-771