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Figure 3.20 of the Contribution of Work Group I to the Second Assessment Report of the IPCC^[1] with description:

Decadal summer temperature index for the Northern Hemisphere, from Bradley and Jones (1993), up to 1970-1979. The record is based on the average of 16 proxy summer temperature records from North America, Europe and east Asia. The smooth line was created using an approximately 50-year Gaussian filter. Recent instrumental data for Northern Hemisphere summer temperature anomalies (over land and ocean) are also plotted (thick line). The instrumental record is probably biased high in the mid-19th century, because of exposures differing from current techniques (e.g., Parker, 1994b).

1. ↑ Intergovernmental Panel on Climate Change (IPCC) (1996) Second Assessment Report^[1], Press Syndicate of the University of Cambridge, page 175