Similarly, the Twentieth Century increase in U.S. GCRI is consistent with the expected sign of change due to an enhanced greenhouse effect. Still, the increase of the GCRI is not large enough to unequivocally reject the possibility that the increase in the GCRI may have resulted from other factors including natural climate variability, and the similarity in the sign of the change of the GCRI and model projections say little about the sensitivity of the climate system to the greenhouse effect. Both indices increased rather abruptly during the 1970s, at a time of major circulation changes over the Pacific Ocean and North America. The smooth curve on this and subsequent plots is generated from a nine-point binomial filter of the annual values. The data ends in 1994 for this and subsequent figures.
Red circles represent increases and blue circles decreases.
Figure 3 Same as Figure 1 except for temperature. Closed circles represent warming, open circles cooling.