FIG. 1 c,d. Time variations in normalized difference vegetation index (NDVI) compared with changes in amplitude of the seasonal cycle of atmospheric CO₂ for the period from July, 1981 through June, 1991. The data in Fig. 1a and 1b have been smoothed by a 3 month running mean. Zonal total NDVI and its anomaly were calculated from pixels having a 10-year monthly average NDVI greater than 0.1 and within 3F of the monthly average. The first condition guaranteed that bare or sparsely vegetated pixels were not included in spatial averages, while the second condition removed most of the influence of snow and bad scan-lines. (c) Seasonal amplitude of NDVI averaged over selected latitudinal bands. The amplitude, defined as the July and August average, is a good approximation because, at the northern latitudes shown, the winter-time NDVI value is close to zero. Spatial averaging was for July and August data combined over pixels with 10-year averages of NDVI greater than 0.1, in order to exclude bare areas, such as the great deserts of Asia. Results from both the Pathfinder (left ordinate) and GIMMS (right ordinate) NDVI data sets are shown together with the corresponding rates of increase. The higher rates of increase inferred from GIMMS data may be due to the lack of desert correction for the version of GIMMS data used in this analysis. (d), Seasonal amplitude of atmospheric CO₂ relative to a base-period of 1961-1967 as registered at Point Barrow, Alaska (71°N, 157°W).¹ Linear trend estimates of the increase in seasonal amplitudes of NDVI and CO₂ are statistically significant (10% level) for all latitudinal bands shown. However, the limitations of regression analysis on short samples, i.e., the determination of trend in the presence of low frequency variations, must be noted.