FIG. 2 a,b. Interannual changes in seasonality of NDVI and in surface temperature, averaged north of 45°N for selected pairs of years from 1982 through 1990. (a), NDVI averaged from 10-day maxima in NDVI for 1982-3, '85-6, '87-8, and '89-90. Data from 1984 were not included because the number of years was odd. Spatial averaging was similar to that described in Fig. 1a caption. Changes in the timing of the active growing season over this 9-year record period were estimated from differences between the first and last bi-yearly average profiles at 6 threshold values of NDVI (from 0.1 to 0.35 in 0.05 increments). These values occurred during intervals of about 60 days each in spring and autumn when, respectively, NDVI was increasing and decreasing, at almost a constant rate. The 6 estimates each of the timing of rise and fall of NDVI may actually be correlated because of low frequency variations (e.g., soil moisture and/or equatorial sea surface temperature oscillations), and therefore, the standard errors given in the text must be interpreted in light of this limitation. We also inferred similar changes in the active growing season duration from an alternate pixel-by-pixel and year-by-year analysis.¹³ (b), Changes in the annual cycle of near surface air temperature from 1982 to 1990. Daily thermometer observations of maximum and minimum temperature were averaged in order to approximate daily mean temperatures and interpolated on a 1x1 degree grid.¹⁹ The daily data were further averaged over three separate approximately 10-day periods per month to obtain 36 observations per year. These 10-day average temperatures were then linearly regressed on the year (from 1982 through 1990) to obtain the slopes shown here. [NOTE: For an updated version of Figure 2b (not in the Nature paper) see Update 1]