FIGURE O-4 Multiproxy reconstruction of Northern Hemisphere surface temperature variations over the past millennium (blue), along with 50-year average (black), a measure of the statistical uncertainty associated with the reconstruction (gray), and instrumental surface temperature data for the last 150 years (red), based on the work by Mann et al. (1999). This figure has sometimes been referred to as the “hockey stick.” SOURCE: IPCC (2001). Reprinted with permission; copyright 2001, IPCC.

depicted as being on the order of 1°C. The pronounced warming trend that began around 1975 was not indicated in the graphic.

IPCC (2001) featured the multiproxy Northern Hemisphere surface temperature reconstruction reproduced in Figure O-4, which includes error bars. In comparison to the previous figure, the reconstructed surface temperature variations prior to the 20th century were less pronounced, and the 20th century warming was rendered more dramatic by the inclusion of data after 1975. On the basis of the results summarized in this figure, the IPCC concluded that “the increase in temperature in the 20th century is likely4 to have been the largest of any century during the last 1,000 years. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year.


The IPCC defines “likely” as having an estimated confidence of 66–90 percent, or better than 2 to 1 odds. Note that this falls well short of the high confidence level (>95%) considered standard for strong quantitative arguments.

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement