Indirect feedbacks to rising CO_2

摘要

甲烷和一氧化二氮是变暖潜力比二氧化碳高rn很多倍的温室气体。以前的研究工作表明，rn使环境中二氯化碳水平增加到当前大气浓度rn之上，可以刺激前两者从土壤中排放，但相rn关证据一直是零散的，整体情况并不清楚。rn现在，Kees vall Groer~’lgerl等人所做的元分析rn(meta-arlalysis)证实，整体上，增加二氧化碳rn浓度，可以刺激一氧化二氮从陆地生态系统的rn土壤中排放，刺激甲烷从稻田和自然湿地中排rn放。本文作者们提出，这些响应将使仅根据二rn氧化碳浓度增加预计的陆地“碳汇”的增加所rn产生的减缓效应(至本世纪下半叶)降低至少rn17％，这意味着陆地生态系统减缓气候变化的rn自然容量被高估了。%ALEXANDER KNOHL & EDZO VELDKAMP Human activities have caused atmospheric concentrations of carbon dioxide, a major greenhouse gas, to increase at an accelerating pace. Starting at around 280 parts per million (p.p.m.) in pre-industrial times, they have now exceeded 390 p.p.m., and are expected to reach 600-800 p.p.m. by the end of the century1. On page 214 of this issue2, van Groenigen and colleagues add to our awareness of the complex consequences of this trend, in terms of the effect that it will have on emissions of other greenhouse gases from various ecosystems. In producing global warming, CO_2 is responsible for the largest part of the anthropogenic impact on Earth's energy balance. It is, of course, also an essential nutrient for plant metabolism. Numerous CO_2-enrichment experiments over the past two decades have demonstrated the positive effect of elevated CO_2 on plant growth - increased biomass and increased carbon storage in soils~3. The vegetation response to elevated CO_2 might be constrained by various interactions with water and nutrients such as nitrogen4'5. However, experiments and model projections suggest that accelerated plant growth due to CO_2 fertilization could draw down some of this gas from the atmosphere, and hence could weaken future rates of CO_2 increase and lessen the severity of climate change~6.