杏吧原创

THE northern hemisphere is blooming, with spring coming earlier and trees
growing faster. A new analysis of billions of satellite images collected daily
by NASA over the past 20 years shows that the land between 40掳 and 70掳
north is markedly greener than it was in the early 1980s鈥攁nd global
warming is almost certainly the cause.

The study shows that the area, which includes most of Europe, Russia and many
central Asian republics is 12 per cent greener, while Canada and the northern
half of the US are 8 per cent greener. Most of the land studied is covered in
natural vegetation, mainly forest.

The greening coincides with a rise in average summer temperatures of around
0.8 掳C since the early 1970s with Europe and northern Asia feeling the heat
more than North America. Lead author Liming Zhou of Boston University says the
link to temperature is reinforced by the discovery that 鈥測ear-to-year changes in
growth are tightly linked to year-to-year changes in temperature鈥�
(see Graph).
Cold years such as 1987, 1992 and 1996 all correspond to a temporary dip in
greenness, for instance.

Overall, Zhou says, the vegetation hasn鈥檛 spread, it鈥檚 simply increased in
density. And the plant-growing season has increased, by 12 days in North America
and 18 days in Europe and Siberia.

The study uses a measure known as the normalised difference vegetation index
(NVDI) which analyses red and near-infrared sunlight reflected from the Earth鈥檚
surface. This shows if the vegetation below is getting greener or browner. The
only overall browning appears in parts of northern Canada, Alaska and the
Russian far east. Here, drought could be the cause.

The study reinforces recent research which shows a reduction in snow cover in
the same regions, and birds and butterflies migrating north. It also reaffirms
suggestions that vegetation in the middle and higher latitudes will benefit from
global warming. 鈥淭he possible implications of the study for the global carbon
cycle are important,鈥� says co-author Ranga Myneni, also of Boston
University.

It confirms that the forests in Europe, Siberia and North America may all be
partially damping down global warming by soaking up carbon from the atmosphere.
The recent agreement on the Kyoto Protocol, which allows countries to claim
credit for absorbing more carbon in their forests, makes this politically
important, too.

Peter Cox of Britain鈥檚 Meteorological Office says the findings are just what
models of global warming predict. Plant growth in these regions is limited by
the growing season and temperature. But the same is not true in the tropics,
where plants already have optimum temperatures and anything hotter, especially
when combined with drying, will damage growth.