Reply to Comment on ‘‘Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981–1999’’ by J. R. Ahlbeck

[1] Ahlbeck [2002] raises an important issue: Is the increase in the atmospheric concentration of CO2 [Keeling and Whorf, 2001] partially responsible for the increase in the normalized difference vegetation index (NDVI), which we report in the work of Zhou et al. [2001]? To demonstrate its effect, Ahlbeck [2002] adds the atmospheric concentration of CO2 (hereinafter referred to as CO2) to equation (11) in the Zhou et al. [2001] article. Ahlbeck [2002] finds that CO2 is correlated with NDVI and concludes that ‘‘fertilizing due to increased atmospheric carbon dioxide concentration also may increase greenness.’’ As described below, this conclusion, and the results on which it is based, is a statistical artifact. When the regression equation is specified correctly, we find that there is no relation between the NDVI and the atmospheric concentration of CO2. [2] To confirm Ahlbeck’s results, we estimate the following equation: