FIG. 3a. Geographic distribution of the change from 1982 to 1990 in normalized difference vegetation index (NDVI) of land areas north of 27.75° N, expressed as the average over the northern active growing season of May through September. (a), NDVI increase in percentage over 9 years, determined by linear regression of year to year northern growing season averaged NDVI, aggregated to 0.25 x 0.25 degree pixels from original 8 km resolution data. Only estimates with a positive slope and a statistically significant (10% level) regression coefficient were contoured. There were no pixels with statistically significant negative slopes. Again, we emphasize the limitations of regression analysis on short term samples. [NOTE: For an updated version Figure 3a (not in the Nature paper) please see Update 2]