The atmospheric δ¹³C record as derived from 56 pinyon trees at 14 sites in the southwestern United States

Regressions of first differences of ring-width indices and δ¹³C values from each site were used to "correct' individual δ¹³C chronologies for climate effects which appear primarily related to high-frequency δ¹³C fluctuations. These climate-corrected chronologies were normalized as deviations from their respective 1800-1849 δ¹³C means, and these normalized chronologies were averaged into the master. The overall δ¹³C drop from 1600 to the present is c1.2-1.4‰, consistent with recent ice-core data showing a drop of 1.14±0.15‰ from 1740 to present. However, the δ¹³C decline in the late 19th and early 20th centuries is greater in the pinyon chronology than that of the ice cores. -from Authors