42 The new study shows that natural weather events, such as the brief warming caused by El Nifio, have a much more dramatic effect than reviousl believed on how much carbon dioxide is absorbed by plants and how much of the as is ex lled by the soil.
Atmospheric carbon dioxide, or CO2, has been increasing steadily for decades. This is thought to be caused by an expanded use of fossil fuels and by toppling of tropical forests. Scientists have linked the CO2 rise to global warming, a phenomenon known as the greenhouse effect. 43 Alarmed, nations of the world now are drawing up new conservation policies to reduce fossil fuel burning,in hopes of reducing greenhouse gases in the atmosphere.
But David Schimel of the National Center for Atmospheric Research, a co-author of the new study, says that before determining how much to reduce fossil fuel burning we should consider the effects of natural climate variations on the ability o f plants to absorb CO2.
Schimel said satellite measurements of C02, plant growth and temperature show that natural warming events such as E1 Nifio at first cause more CO2 to be released into the atmosphere, probably as the result of accelerated decay of dead plant matter in the soil. But later, within two years, there is an explosion of growth in forests and grasslands, which means plants suck more carbon dioxide out of the atmosphere.
"We think that there is a delayed response in vegetation and soil to the warming effects of such phenomena as E1 Nifio, and this leads to increased plant growth," said Schimel.
44 However, he said, it is not clear whether the warming by E1 Nifio causes a net decrease in the build-up of CO2 over the long haul. "We don''t really know that yet,"said Schimel. What the study does show, however, is that the rise and fall of CO2 in the atmosphere is strongly influenced by natural changes in global temperature, said B. H.Braswell of the University of New Hampshire, another co-author of the study.
Braswell said that in years when the global weather is cooler than normal, there is a decrease in both the decay of dead plants and in new plant growth. This causes an effect that is the opposite of El Nifio warming: CO2 atmosphere levels first decline and later increase.
45 "I think we have demonstrated that the ecosystem has a lot more to do with climate chan e than was reviousl believed," said Braswell, "Focusing on the role of human activity in climate change is important, but manmade factors are not the only factors."
……
購物車/收銀台(0) | 在線留言板
| 付款方式
| 聯絡我們
| 運費計算
| 幫助中心 |
加入書簽
HOME
新書上架
