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Earth is Becoming a Greener Greenhouse Greening in the North: An index of persistence of the greening trend was defined as follows. Linear trends in growing season average NDVI were calculated for the periods 1982-87, 82-89, 82-91, 82-93, 82-95, 82-97 and 82-99. We denote these trends as t(i), i = 1, 2, ..., 7. A score of 1 is given if t(i+1) is greater than 80% of t(i); otherwise the score is zero. The sum of these scores is calculated as an index of persistence. The maximum possible score is 6. This index can identify regions where NDVI has increased consistently, as opposed to a trend estimate, which in the case of a small sample, can be biased by outliers. A persistence index of 5 or more, which shows a pixel where NDVI has increased in five or six of the six periods, is termed high persistence (red and purple colors in the figure below), while low persistence refers to a pixel with persistence index between 1 and 4. This analysis is restriced to vegetated areas north of 30N, which experienced unprecedented warming during the past 25 years. The pattern of high persistence in NDVI is especially noteworthy in boreal Eurasia, along a broad swath of land east of 25E and north of 50N. This region includes the grasslands and croplands of the south central Russian uplands and extends northeast through the unmanaged mixed and needle forests to the Bolshezemalskaya Tundra. East of the Urals, there is a contiguous region of high persistence over the west Siberian plain and the central Siberian plateau. East of lake Baikal, there is a band of pixels which displays high persistence between 50N-55N, that extends east to the Aldan plateau. These regions in Siberia and eastern Russia consist mostly of natural forests with arctic grasses and tundra to the north. Outside of this broad swath, large regions of densely vegetated areas in central Europe and Sweden also are notable. About 78% (9.8 million square km) of the vegetation in these regions between 40N-70N is unmanaged, and almost 58% (7.3 million square km) is forests and woodlands, an area equivalent to about 78% of the USA. The degree of spatial coherence seen in Eurasia, i.e., large areas of high persistence circumscribed by regions of low persistence, is absent in North America. There, pixels with high persistence are relatively fragmented and the densely vegetated temperate and boreal forest regions in North America do not show a noteworthy pattern. Pixels with high persistence in North America are located mainly in the needle forests of the east and grasslands of the upper Midwest. In total, only about 30% of the vegetated pixels between 40N-70N in North America display high persistence, compared to more than 61% of the vegetated area in Eurasia.
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Sep-04-2001 |
