FIG. 1 a,b. 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. (a) Monthly average NDVI for selected latitudinal bands and the whole globe. The zonal total NDVI was normalized by the total vegetated land area in the month of August to obtain a zonal average that exhibited seasonality.¹³ (b), Monthly total anomalies of the above, expressed as departures from the 10-year record averages of monthly NDVI, summed over each latitudinal band¹³ for each month. The vertical scale of the global plot is twice that of individual latitudinal bands.