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全球变暖:植物没有过错

最近,科学家们报告说,大气中三分之一的甲烷是由植物排放的,而甲烷是一种强有力的温室气体。在发表报告八天后(1月18日),这些科学家们发表了一份声明,指出气候变化不应该由植物承担责任。(参见控制气候变化的手段可能对环境造成危害)

这些作者们表示,他们对自己研究引起的广泛的媒体报道感到吃惊,因此急着发表这份声明。声明指出,因为这些以前从未受到怀疑的甲烷排放者来自自然环境,所以它们不是造成当前气候变化的元凶。这个研究小组写道,“它们存在于人类开始影响气候之前很长时间。根本的问题还是全球大规模地燃烧化石燃料。”

在媒体报道之后,德国海德堡的马普学会核物理研究所的Frank Keppler及其同事收到了来自科学家和对此感兴趣的公众的大量电子邮件。一些人向研究组成员询问,站在植物旁,或者访问亚马逊雨林是否安全。

在接受本网络记者采访时,Keppler说他想向公众清楚地传达三点内容。

第一,他们的研究并不意味着植树造林计划应该被责难。树木吸收二氧化碳,这是最重要的温室气体,因此种植树木仍然是有益的。当考虑到树木排放的甲烷时,种树吸收二氧化碳的收益只会减少百分之一到四,这是一个可以忽略不计的效应。

第二,植物排放的甲烷总量的变化——包括那些可能让全球变暖更严重的变化——可能是由诸如森林砍伐等人类活动引起的。

最后,他们说需要进行更多研究来揭示植物中甲烷的释放量如何依据物种、温度、湿度、日照和其它因素的变化而变化。同时,也要研究这些释放量如何可能随着环境因素发生改变。

Keppler说:“从科学的观点看,这是一件令人着迷的事情。我们想要与其他人分享这一发现。这可能是对未来至关重要的难题中的一环。”

对于那些捉摸着是否要开始砍伐树木的人,Keppler说他们应该想想一个没有树的世界是怎样的,Keppler问道:“那时候我们还有什么呢?”

阅读本网站关于碳沉降(carbon sink)问题的新闻聚焦

Keppler等人声明的全文(英文)

Global warming - the blame is not with the plants

In a recent study (Nature, 12 January 2006), scientists from the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, Utrecht University in the Netherlands, and the Department of Agriculture and Rural Development for Northern Ireland, United Kingdom, revealed that plants produce the greenhouse gas methane. First estimates indicated that this could account for a significant proportion of methane in the atmosphere. There has been extended media coverage of this work with unfortunately, in many instances, a misinterpretation of the findings. Furthermore, the discovery led to intense speculations on the potential relevance of the findings for reforestation programmes in the framework of the Kyoto protocol. These issues need to be put in the right perspective.

The most frequent misinterpretation we find in the media is that emissions of methane from plants are responsible for global warming. As those emissions from plants are a natural source, they have existed long before man’s influence started to impact upon the composition of the atmosphere. It is the anthropogenic emissions, which are responsible for the well-documented increasing atmospheric concentrations of methane since pre-industrial times. Emissions from plants thus contribute to the natural greenhouse effect and not to the recent temperature increase known as “global warming”. Even if land use practices have altered plant methane emissions, which we did not demonstrate, this would also count as an anthropogenic source, and the plants themselves cannot be deemed responsible.

Furthermore, our discovery led to intense speculation that methane emissions by plants could diminish or even outweigh the carbon storage effect of reforestation programs with important implications for the Kyoto protocol, where such programs are to be used in national carbon dioxide (CO2) reduction strategies. We first stress that our findings are preliminary with regard to the methane emission strength. Emissions most certainly depend on plant type and environmental conditions and more experiments are certainly necessary to quantify the process under natural conditions. As a first rough estimate of the order of magnitude we have taken the global average methane emissions as representative to provide a rough estimate of its potential effect on climate. These estimates (for details, see below) show that methane emissions by plants may slightly diminish the effect of reforestation programs. However, the climatic benefits gained through carbon sequestration by reforestation far exceed the relatively small negative effect, which may reduce the carbon uptake effect by up to 4 per cent. Thus, the potential for reduction of global warming by planting trees is most definitely positive. The fundamental problem still remaining is the global large-scale anthropogenic burning of fossil fuels.

Details of calculations used:

In our study, we have linked global methane emission estimates to plant growth, which is generally quantified as net primary productivity (NPP). On a global basis NPP amounts to ~62´(10^15) g of carbon/yr, which corresponds to an uptake of 227´(10^15)g of CO2/yr. On the emission side, our study suggests annual global methane emissions by plants of 62-236 ´(10^12)g/yr methane. Thus, for each kg of CO2 assimilated by a plant roughly 0.25 to 1g of methane is released. During growth of a new forest, up to 50 per cent of plant tissue is lost again in the short term through decomposition of plant litter of leaves and roots [1]. This then doubles the estimate to 0.5 to 2 g methane emitted per kg of CO2 assimilated and stored in plants for longer periods. Over a 100-year horizon, the global warming potential of methane is ~20 times higher than that of carbon dioxide. Thus, for climate, the benefits gained by reforestation programs would be lessened by between 1 and 4 per cent due to methane emissions from the plants themselves.

Thomas Rãckmann (1,2), Jack Hamilton (3), Frank Keppler (2) and Marc Brass (1,2)

(1) Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlands
(2) Max Planck Institute for Nuclear Physics, Saupfercheckweg 1, 69117 Heidelberg, Germany
(3) Department of Agriculture and Rural Development, Agriculture, Food and Environmental Science Division, Newforge Lane, Belfast BT9 5PX, UK

Reference:
[1] Schulze, Beck, Muller-Hohenstein; Plant Ecology (Springer Verlag, 2005)

Original work:
Frank Keppler, John T. G. Hamilton, Marc Brass and Thomas Rãckmann
Methane emissions from terrestrial plants under aerobic conditions
Nature, 12 January 2006