FIGURE 63.10 Temperature changes in the Northern Hemisphere for the past 2,000 years. By combining indirect measures of temperatures with direct measures after 1880, we can gain a longer-range perspective on how current global warming compares with past global warming. The average temperature from 1901 to 2000 serves as a baseline against which all other years are compared. As a baseline, the average temperature from 1901 to 2000 is set at “zero temperature change.”

(Data from Moberg, A., et al. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433:613–617.)