Variations in the amplitude and timing of the seasonal cycle of atmospheric CO2 have shown an association with surface air temperature consistent with the hypothesis that warmer temperatures have promoted increases in plant growth in the northern high latitudes.1 We present evidence from satellite data that the photosynthetic activity of terrestrial vegetation increased from 1981 to 1991 in a manner suggesting an increase in plant growth associated with an increase in the duration of the active growing season. The regions of greatest increase lie between 45°N and 70°N where marked warming has occurred in the spring time2 due to an early disappearance of snow.3 The satellite data are concordant with an increase in amplitude of the seasonal cycle of atmospheric carbon dioxide exceeding 20% since the early 1970s, and an advance in the timing of the drawdown of CO2 in spring and early summer of up to 7 days.1 Thus, both the satellite data and the CO2 record indicate that the global carbon cycle has responded to interannual fluctuations in temperature which, although small at the global scale, are regionally highly significant.
We have made use of data from the Advanced Very High Resolution Radiometers (AVHRR) on board the National Oceanic and Atmospheric Administration (NOAA) series of meteorological satellites (NOAA-7, -9 and -11). From daily observations of channel 1 (approx 0.58-0.68 µm) and channel 2 (approx 0.72-1.1 µm) reflectances, global land data sets of normalized difference vegetation index (NDVI) were produced.4,5 The NDVI is expressed on a scale from -1 to +1 . It is between -0.2 to 0.05 for snow, inland water bodies, deserts, and exposed soils, and increases from about 0.05 to 0.7 for progressively increasing amounts of green vegetation.6 NDVI data are strongly correlated to the fraction of photosynthetically active radiation (0.4-0.7 µm) absorbed by vegetation,7 i.e., to the photosynthetic activity of vegetation canopies.8 Two global data sets of NDVI were analysed: (1) the land segment of the joint NOAA/NASA Earth Observing System AVHRR Pathfinder data set at 8 km spatial resolution and 10 day intervals, for the period July 1981 through June 1991,5 and (2) the Global Inventory Monitoring and Modelling Studies (GIMMS) AVHRR NDVI data set at a similar spatial resolution, but at 15 day intervals, for the period January 1982 through December 1990.9 The Pathfinder data were calibrated to correct for post-launch degradation from estimates of the relative annual degradation rates of the two channels (in %): 3.6 and 4.3 (NOAA-7), 5.9 and 3.5 (NOAA-9), and 1.2 and 2.0 (NOAA-11).10 NOAA-9 data were used for inter-satellite normalization. The GIMMS data were independently calibrated;11 shown here to illustrate how a different calibrating scheme affects derived trends in the AVHRR data.
For each equal-area pixel and at either 10- or 15-day intervals, depending upon which of the two satellite data sets was used, the maximum of NDVI with minimal atmospheric effects was retained.12 The NDVI data from high northern latitudes ( > 40o N) did not show El Chichon related anomalies during the mid-1982 to 1983 time period. These effects in the low latitude data were not corrected for in either of the two satellite data sets.
The calibrated Pathfinder NDVI data still showed residual non-vegetation related variations.13 We revised them by adjusting the NDVI for a hyper-arid portion of the Sahara desert ( 1.42 x 106 km2 ) which has been found to be invariant as viewed by all three satellites.9 An alternate correction scheme based on desert pixels from 10o N- 50o N yielded nearly identical results. Importantly, when this desert correction was applied to the NDVI anomaly time series of desert pixels from five-degree latitude bands between 10°N- 50 o N, the residuals resembled noise.
Time series of spatially averaged monthly NDVI, evaluated as the mean of three 10-day maximum value NDVI composites, comprising a 10-year record, are plotted, first directly for reference (Fig. 1a), and then as anomalies to display interannual variability (Fig. 1b). Averaged for regions north of 45°N (uppermost curve in Fig. 1b ) the NDVI anomaly shows evidence of increasing amplitude, summer values being low early in the record, high near the end. The NDVI anomaly in the tropics shows a large increase starting from November 1988, which also coincided with the change in satellites from NOAA-9 to NOAA-11. A somewhat smaller increase is seen during the switch from NOAA-7 to NOAA-9 in January 1985, although this increase began in the last months of the NOAA-7 record (and the anomaly north of 45 o N actually shows a decrease). This raises a question regarding anomalous variations in NDVI from sensor changes. Although efforts have been made to establish proper inter-sensor calibration linkages, 10,11 some residual effects cannot be ruled out, especially between NOAA-9 and NOAA-11.13 This situation, for example, confounds proper interpretation of the tropical NDVI anomaly time series. For instance, intense sea surface temperature (SST) oscillatory events in the tropical Pacific and Atlantic oceans from 1982 to early 1989 have been linked to decreased vegetation growth in large regions of the semi-arid tropics.14 The increase in tropical and global NDVI anomaly starting from late 1988 also coincided with an unprecented decline in atmospheric CO2 anomaly, from a peak value in late 1988 to a minimum in late 1993.15 Nevertheless, these interpretations, as they involve the NDVI data, are limited by possible sensor change effects.
Changes in the amplitude of the seasonal cycle of NDVI at northerly latitudes greater than 35 o N are plotted in Fig. 1c , as characterized by changes in the July and August average NDVI. This broad measure of the seasonal maximum approximates the seasonal amplitude because winter-time NDVI at these northern latitudes is close to zero (cf. Fig. 1a). The seasonal amplitude, by this definition, increased by 7 to 14%, depending on the latitude and data set, from 1981 or 1982 through 1990 (Fig. 1c ). Because NDVI is a measure of photosynthetic activity of vegetation as noted above,7,8 this increase indicates a substantial change in photosynthetic activity of plants at higher northern latitudes. A similar increase (14%) is indicated in the amplitude of the seasonal cycle of atmospheric CO2 measured at Point Barrow, Alaska1 ( Fig. 1d ). This CO2 cycle, although observed in the arctic (71 o N), registers changes in CO2 gas exchanges, and hence in the biotic activity of plants and soil over all northern temperate and polar latitudes.16 Together, the NDVI and CO2 data indicate increased biospheric activity north of about 35 o N. Two recent studies have also reported increased photosynthetic activity in the northern high latitudes as increased biomass from deposition in European forests17 and from tree-ring analysis in Mongolia,18 respectively.
Timing of the seasonal rise and fall in NDVI suggests possible changes in the length of the active growing season, i.e., the period during which photosynthesis actually occurs (as opposed to the concept of growing season, measured for example in degree days). As shown in Fig. 2a in Pathfinder data, the rise in NDVI, spatially averaged from 45 o N to the northern limit of the data, came progressively earlier in the season between 1982 and 1990, as shown by successive 10-day averages, where each plot shows an average over two years for clarity. Because spatially averaged NDVI rose each year at nearly a constant rate from early April (about day 110) to late June (about day 170) the advance in the active growing season is apparent, notwithstanding the relatively coarse time resolution (10 day) afforded by the NDVI data. From six estimates of the time advance at six successive thresholds of NDVI, we estimate an advance of 8 ± 3 days ( Fig. 2a).
An advance of about 7 days in the seasonal cycle was previously inferred from atmospheric CO2 data as having taken place between the 1960s and early 1990s, with most of the increase occurring after 1980 (Fig. 1 of ref. 1). The NDVI data suggest that this increase occurred over an extensive region of the extratropical northern hemisphere. The NDVI data in Fig. 2a further indicate a prolongation of the declining phase of the active growing season, estimated at 4 ± 2 days between 1982-3 and 1989-90. Therefore, the active growing season north of 45 o N, appears to have lengthened by 12 ± 4 days over the 1980s. These estimates must be interpreted as suggestive of a longer active growing season, rather than in an absolute sense, in view of the coarse temporal resolution (10 days) and residual atmospheric effects in NDVI data. The associated standard errors given here are not rigorous, for low frequency variations in NDVI data invalidate the assumption of statistical independence required of the successive threshold values.
Variations in the amplitude and timing of the seasonal cycle of atmospheric CO2 have shown an association with surface air temperature consistent with the hypothesis that warmer temperatures have promoted increases in biospheric activity outside the tropics. A likely cause is an increase in the length of the active growing season brought about by warmer temperatures.1 As shown in Fig. 2b, a pronounced increase in late winter and early spring temperatures took place over the period of NDVI changes, especially during March.
Because of their high spatial resolution, relative to ground-based meteorological measurements, NDVI data provide spatial detail of where the average changes in amplitude and timing of the active growing season occurred. To address regional variations in NDVI, we show in Fig. 3a a map related to the time plots shown in Fig. 1 together with a map of the 9-year average of NDVI for comparision.
The linear rate of change in NDVI, averaged over the 9 years of seasonal NDVI data in northern latitudes, from 1982 through 1990, are mapped in Fig. 3a. Data were averaged from May through September, to approximate the main active growing season of land vegetation in the northern hemisphere.
In Eurasia, a band of increasing NDVI extends from Spain in a northeasterly direction across Asia to the western Pacific ocean. In this band, central Europe, southern Russia, and a broad region near Lake Baikal in Siberia are most affected. Outside this band, northern Scandinavia, northern China, and northeastern Siberia are also strongly affected. In North America, a band of increasing NDVI extends from Alaska in a southeasterly direction to the Great Lakes, thence northeasterly to Labrador. In this band, northwestern Canada is most strongly affected. Outside this band, the continental United States, exclusive of Alaska, and the area around the Hudson Bay show little change in NDVI.
In general, the regions of greatest increase in NDVI are inland from the oceans, except in the arctic, and are north of 50 o N. The prominent bands of increased NDVI referred to above in both Eurasia and North America, correspond generally with areas of high NDVI (Fig. 3b). Thus, most of the areas where changes in NDVI amplitude and seasonality were observed are also regions of significant vegetation density. Notable exceptions are several arctic regions in Eurasia where NDVI rose sharply from low initial values.
We believe that the increasing trend in photosynthetic activity of the northern high latitudes, inferred from satellite observations of NDVI amplitude and phase, to be robust despite varying satellite overpass times and the lack of an explicit atmospheric correction. These effects, however, could modify the magnitudes of NDVI amplitude and estimates of the active growing season duration.
Analyses of station temperature trends during 1961-90 indicate pronounced warming over substantial areas in Alaska, northwestern Canada and northern Eurasia.2 The greatest warming, up to 4 oC, has occurred in winter. Only slightly lesser warming has occurred in the same regions in spring, but considerably lesser warming in summer and even less in autumn.2 Associated with warming at high latitudes is an approximate 10% reduction in annual snow cover from 1973 through 1992, especially an earlier disappearence of snow in spring (Table 1 of ref. 3). Where snow-lines have retreated earlier due to enhanced warming, we expect an early start of the active growing season.
The winter and spring warming in the interior of the continents of Asia and North America in the 1980s may be a result of natural causes not yet explained, but its timing is consistent with an enhanced greenhouse effect caused by build-up of infrared-absorbing gases in the atmosphere.20 The unusual warming which peaked near 1990 was of global extent. Although it amounted to only a few tenths of a degree departure from previous record temperatures,21 it was associated with far greater warming in the spring months at high northern latitudes. Biospheric activity there, based on our analysis, increased remarkably as a result of this warming, suggesting that small changes in global temperature may reflect disproportionate responses at the regional level, and may be accompanied by positive feedbacks which can markedly influence processes such as photosynthesis and litter decomposition.
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