鲁达 Can the world find solutions to the nitrogen pollution crisis?
目前，全球氮肥的使用效率越来越低，流入河流海洋的比例也比以往更高。 科学家认为，一场环境灾难正在逼近，国际社会迫切需要制定预防策略。 The world is using nitrogen fertiliser less and less efficiently. A greater proportion than ever before is washing into rivers and oceans. An environmental catastrophe looms, nitrogen scientists say, and the world urgently needs to develop strategies to prevent it.
二战之后的物理学家们曾担心核灾难将敲响人类末日的警钟。到了二十世纪八十年代，在关注生态崩溃的生物学家眼中，“生物多样性”的损失成了衡量危机的标志。此后不久，气候学家重新提出了对全球变暖的担忧，并警告称一个世纪内全球温度将上升到有史以来的最高值。 Post-war physicists fearing nuclear apocalypse came up with the Doomsday Clock. In the 1980s, biologists contemplating ecological meltdown began talking about “biodiversity” loss as a way to tag and measure the crisis. Soon after, climate scientists recast concern over global warming with a warning that within a century it would lead to temperatures greater than any in human history.
现在，氮污染正在成为下一个危机。 Now, it is nitrogen’s turn.
上个月，来自世界各地的20位专家齐聚在纽约大学的一间会议室内，就未来严重的氮污染给地球带来的危害展开讨论，并提出了一些简单的指标来鼓励全球共同努力，扭转这一危机。 Last month, in a seminar room at New York University, a score of nitrogen experts from around the world began drawing up scenarios of what a future nitrogen-soaked planet might look like – and to devise simple metrics for encouraging a global effort to head off disaster.
本次研讨会是“国际氮管理系统”的一部分。该研究项目耗资6000万美元，是联合国环境署和全球环境基金共同开展的一项为期5年的研究项目。项目负责人马克·萨顿表示，这个系统类似于关于氮污染的“政府间气候变化专门委员会”。 They met as part of the International Nitrogen Management System, a five-year, US$60-million (380 million yuan) research project from the UN Environment Programme and the Global Environment Facility, that is intended, says its chief Mark Sutton, as nitrogen’s equivalent to the Intergovernmental Panel on Climate Change.
不少与会专家认为，我们必须在本世纪中叶将排放到环境中的氮减半，这是治理氮污染的底线，否则整个生态系统就将陷入有毒潮汐和死河死海肆虐的危机之中。 实现这一点需要做很多工作，包括将全球农业的氮利用率提高一倍。 The bottom line, many there concluded, was that we must halve the amount of nitrogen we dump into the environment by mid-century or our ecosystems will face epidemics of toxic tides, lifeless rivers, and dead oceans. And that to do that will require, among other things, almost doubling the efficiency of nitrogen use on the world’s farms.
过去的半个世纪里，人类排放到环境中的氮总量超过了任何其他主要元素。导致氮流量翻倍的原因包括污水、畜禽粪便和化石燃料燃烧，尤其是合成肥料的施用。 如今，全球一半左右作物的生长都要依靠氮肥，而氮肥是通过固化空气中的惰性氮制成的。 In the past half-century, humans have increased the amount of nitrogen in the environment more than any other major element. Sewage, livestock waste, fossil-fuel burning, and especially our use of synthetic fertiliser have all contributed to a doubling of nitrogen flows. Half the world’s crops today are grown with the aid of fertiliser made by capturing inert nitrogen from the air.
地球系统科学家说，氮是造成生物地球化学污染的主要因素，也是已经被突破的四个“地球界限”之一，可能造成“不可逆转的、突然的环境变化”。所谓“地球界限”，指的是地球在某个方面的最大承载力。目前，全球正努力解决另外三个被突破的“地球界限”，包括气候变化、森林砍伐以及生物多样性丧失。 但是，爱丁堡生态与水文中心的英国研究员萨顿表示，氮污染是一个被严重忽视的环境问题，目前还没有任何国际协议或联合国机构来推动相关治理行动。 Earth system scientists say nitrogen is the major factor in biogeochemical pollution, one of four “planetary boundaries” that we have exceeded, risking “irreversible and abrupt environmental change.” The world is attempting to address the other three: climate change, deforestation, and biodiversity loss. But, says Sutton, a British researcher at the Centre for Ecology and Hydrology in Edinburgh, nitrogen pollution is a largely ignored environmental story, with no international agreement or UN agency to galvanize action.
本次纽约会议是人们为改变这一现状所做的一次努力。会议关注的重点是农业，因为农业贡献了全球氮污染总量的三分之二。 萨顿说：“我们在全球食品供应方面已经做得很好了，但是现在我们必须解决由此产生的氮污染问题。” The New York meeting was part of an effort to change that. The focus was on agriculture, which is responsible for about two-thirds of global nitrogen pollution. “We’ve done pretty well feeding the world,” says Sutton. “But now we have to address the fallout from that.”
氮污染比比皆是。 比如，在美国，氮污染随处可见 ：从俄亥俄州到加利福尼亚州的克拉马斯河水体都爆发了藻华; 加利福尼亚州的地下水受到了污染; 切萨皮克湾的鱼群严重缺氧; 佛罗里达州的近海水域遭遇了有毒“赤潮”的侵袭。 That fallout is all around us. In the United States, it is being felt in virtually all parts of the country – unleashing algal blooms in rivers from the Ohio to the Klamath in California; poisoning underground water reserves in California; leaving fish gasping for oxygen in Chesapeake Bay; and creating toxic “red tides” off the shores of Florida.
墨西哥湾水域已经形成了一个常态化的“死亡区”，原因是过量的氮刺激了水生植物的生长，而这些植物最终会腐烂，将水中的氧气消耗殆尽，从而导致大多数其他海洋生物窒息。该水域的氮主要来自美国中西部地区密西西比河沿线的粮田。每年夏季，这个死亡区的总面积可达5300平方英里，去年夏季更是史无前例地达到了8800平方英里。 The Gulf of Mexico has a regular “dead zone,” where excess nitrogen stimulates so much aquatic plant growth that its eventual rotting consumes all the available oxygen, suffocating most other marine life. The zone arises from nitrogen pouring down the Mississippi from the grain fields of the Midwest. It typically extends each summer for 5,300 square miles. Last summer’s reached 8,800 square miles, the largest ever.
据统计，全球海洋中目前共有400多个死亡区，面积是1950年的四倍，其中最大的死亡区位于部分封闭的波罗的海，面积经常超过20000平方英里。 All told, there are more than 400 dead zones in the world’s oceans, covering an area four times as great as in 1950. The largest, in the partially enclosed Baltic Sea in Europe, often covers more than 20,000 square miles.
造成这种生态混乱的最大原因是合成氮肥 。全球每年的农用合成氮肥施用量高达1.2亿吨，是有机肥（如动物粪便、作物肥料和固氮豆科植物）的含氮量的两倍。 The largest cause of this ecological mayhem is the 120 million tonnes of synthetic nitrogen used globally in agriculture each year. That is twice the amount of nitrogenreaching fields from organic sources such as animal manure, crop waste, and leguminous plants that fix their own nitrogen.
但最令人震惊的统计数字是，氮肥的作物吸收率已经越来越低，半数以上都经由灌溉流入了河流。 马里兰大学环境科学家张鑫表示，全球氮肥利用率已 经从1961年的50％以上下降到今天的42％左右。 But the most shocking statistic is that less and less of the nitrogen poured onto fields is being incorporated into crops; more than half is washing from fields into rivers. The nitrogen-use efficiency (NUE) of the world’s farmers has slipped from more than 50％ in 1961 to about 42％ today, according to Xin Zhang, an environmental scientist at the University of Maryland.
世界上大部分自然资源的使用效率都越来越高，但化肥恰恰相反。 在过去的30年里，全球生产的合成肥的一半以上都浪费在了地里。 While most of the world’s natural resources are being used with increasing efficiency, fertilizer is being used with evermore abandon. As a result, more than half of all the synthetic fertilizer ever produced has been applied to farmland in the past 30 years.
其中，亚洲国家做得最差。 张鑫说，印度的化肥施用量在20年间翻了一番，氮肥利用率从40％下滑到30％。 但情况最糟糕的还是中国，平均氮肥利用率已经从1961年的60％一路下滑到了到如今的25％。 Asian countries are doing the worst. In India, where fertilizer application has doubled in 20 years, NUE has slipped from 40％ to 30％, Zhang said. But the absolute worst case is China, which has gone from an average NUE of more than 60％ in 1961 to just 25％ today.
相比之下，包括美国在内的许多发达国家的氮肥利用率却有所提高，达到了68％。 半个世纪以前，中国的氮肥利用率与美国相似，如今却只有美国的三分之一多。 This compares with modest efficiency improvements seen in many developed nations, including the US, which is at 68％. Half a century ago, China’s NUE was similar to that of the US; now it is little more than a third as good.
中国的生态系统因此陷入困境。无论是在北部的黄河还是南部的珠江，都出现了氮过量导致大量鱼群死亡的事件。 据报道，中国有三分之一的湖泊都出现了藻华，而含有大量有毒藻类的赤潮在东海各个入海口水域四处扩散。 Chinese ecosystems are under siege as a result. Nitrogen kills fish in huge numbers from the Yellow River in the north to the Pearl River in the south. Algal blooms are reported in a third of the country’s lakes. Massive “red tides” of toxic algae spread from river estuaries across the East China Sea.
亚洲氮肥使用量激增有两个主要原因：一是化肥价格低；此外，拼命加大农用化学品的投入，正是半个世纪前，为解决全球人口快速增长、粮食供应不足的所谓“绿色革命”的核心理念。 There are two main reasons for the dramatic decline in NUE in Asia: cheap fertilizer prices and the genetic makeup of the “green revolution” crops developed half a century ago to feed fast-rising populations in a hungry world.
二十世纪中叶，包括诺贝尔奖获得者诺曼·博洛格在内的多位植物育种专家培育出了多个玉米、水稻等粮食作物新品种，这些品种对肥料的吸收效果非常好。 要想增产，农民只需要加大施肥量就可以。 In the mid-20th century, plant breeders, such as Nobel Prize winner Norman Borlaug, produced varieties of grain crops such as corn and rice that responded exceptionally well to additional fertilizer. To grow more, farmers simply have to pour on more fertilizer.
而不断加大化肥施用量的同时，带来的收益却在不断递减，换句话说，其增产效应在不断降低。 农民通常是根据经济效益而不是生态影响来决定施肥量的多少。如果肥料很便宜，那么农民就会不断加大施肥量。 因此，中国农民的氮肥施用量通常是欧洲农民的两倍。 At high applications, there is a law of diminishing returns. Extra nitrogen has a diminishing effect on yield. Farmers make a judgment about how much more is worth pouring on – usually based on economic rather than ecological reasons. When fertiliser is cheap, it makes sense to keep on pouring. As a consequence, Chinese farmers typically put twice as much nitrogen onto their fields as their European counterparts.
在张鑫教授的分析结果中，最可怕的莫过于非洲有可能走中国的老路。目前，由于非洲化肥供应不稳定、农民普遍贫困，所以大多数非洲农民的化肥施用量还非常少。因此，非洲农田平均单产不过每公顷1吨左右，而亚洲大部分地区为3吨，欧洲和北美为7吨。 但非洲农民使用的肥料很容易被营养匮乏的作物吸收，所以目前非洲的平均氮肥利用率全球最高，达到72％。 The scariest result from Zhang’s analysis is the prospect of Africa following the path taken by China. At present, most African farmers apply only small amounts of fertilizer. Supplies are sporadic and many farmers poor. As a result, Africa’s average grain yields are not much more than one ton per hectare, compared to three tons in most of Asia and seven tons in Europe and North America. But what fertilizer African farmers do use is readily taken up by nutrient-starved crops. So Africa’s average NUE is currently the highest in the world, at 72％.
包括世界银行和盖茨基金会在内的多家机构都在积极呼吁非洲进行绿色革命，而随着非洲自给作物种植量逐步加大，这里也将难逃收益递减的规律。 张鑫表示，未来几十年里整个非洲大陆的化肥流失量预计会迅速增加。 But as it tries to grow more to feed itself – the African green revolution called for by everyone from the World Bank to the Gates Foundation – the law of diminishing returns will kick in here too. In future decades, we can expect a rapid rise in the amount of fertilizer run-off across the continent, says Zhang.
张鑫的研究数据恰好符合环境库兹涅茨曲线的走势。 俄裔美国人、诺贝尔经济学奖获得者西蒙·库兹涅茨认为，随着各国工业化程度的提高和财富的积累，自然资源利用效率也会呈现出一个共同的模式。 首先，各国会采用“低价格+重污染”的资源利用模式，利用效率堪忧。但随着这种低效资源利用模式带来的污染和其他负面影响的增加，各国会加大投资改变这一现状。最终，至少对于某些材料而言，效率的增益将超过产品需求的增长。 就像洛克菲勒大学未来学家杰西·奥苏贝尔所说的那样，这时经济就可以开始“去物质化”。 Her data show a clear sign of what economists call the environmental Kuznets curve. The Russian-American, Nobel Prize-winning economist Simon Kuznets argued that as countries industrialise and grow wealthy, the efficiency with which they use natural resources shows a common pattern. They begin in a cheap-and-dirty way, with terrible resource efficiency. But gradually, as pollution and other downsides of this inefficiency increase, they invest in doing things better. Eventually, at least for some materials, efficiency gains exceed rising demand for the products being produced. At that point, economies can start to “dematerialise,” as Rockefeller University futurologist Jesse Ausubel has put it.
在大多数富裕国家，农用氮肥的使用规律就符合这一曲线。 上世纪七十年代之前，由于农民化肥用量不断增加，美国和欧洲的农用氮肥利用率一直在下降。但是之后情况就开始逐渐好转。 自2001年以来，尽管化肥用量减少了，但美国的农田产量却一直在提高。 In most rich countries, use of agricultural nitrogen shows this curve. The NUE of American and European farms deteriorated until around 1970, as farmers poured on more fertilizer. But after that, it began to improve. Since 2001, the US has been getting higher yields despite putting on less fertilizer, says Zhang.
但发展中国家目前还没有到达这个临界点的任何迹象。绿色革命中所培育的作物实际上最大限度地利用了“低价格+重污染”的资源利用模式。由于中国和印度等国的化肥得到了大额补贴，农民完全没有动力削减化肥用量。张鑫表示，这也就是为什么这么多国家（实际上可以说是全世界的农民作为一个整体）还没有达到库兹涅茨曲线中的拐点的原因。 But in developing countries, there is so far little sign of similar tipping points. The crops bred by the green revolution in effect optimize the cheap-and-dirty approach. With heavily subsidized fertilizer prices in countries like China and India, there are no incentives for farmers to use less. That is why so many nations — and the world’s farmers as a whole — remain stuck on the wrong side of the Kuznets curve, says Zhang.
那我们该怎么办？ 各国之间在氮肥利用率上的巨大差异意味着我们可以通过重新安排作物种植区来直接削减氮流失。哈佛大学生态学家纳撒尼尔·穆勒近日指出，如果将作物种植在氮肥利用率较高的地区，全球氮排放量将减少69％。但是，要想实现这种调整，似乎不可能。 So what to do? The huge NUE discrepancies between countries mean that the world could cut nitrogen losses simply by rearranging where crops are grown. Ecologist Nathaniel Mueller, of Harvard University, reported recently that the world emits 69％ more nitrogen from fields than it would if crops were grown in places with optimum nitrogen-use efficiency. But such a global rearrangement of crops sounds unlikely.
那么到本世纪中叶，我们如何在保证全球90亿人口粮食供给的同时解决氮肥使用难题呢？ So how can we reconcile feeding a world of nine billion people by mid-century with slaying the nitrogen dragon?
张鑫建议，我们应该在2050年前将全球农田氮流失从目前的约1亿吨削减到5000万吨。也就是说，需要将农用氮肥的平均利用率从目前的42％提高到70％左右。 要实现这个目标，欧洲和北美地区的氮肥利用率可能就要提高到75％，中国和亚洲其他地区提高到60％，同时让非洲保持在70％以上。 Zhang suggests the world should aim to reduce nitrogen runoff from crop fields from the current roughly 100 million tons to 50 million tonnes by 2050. That, she says, will likely require raising average agricultural NUE from the current 42％ to about 70％. To achieve that might involve getting Europe and North America to 75％, and China and the rest of Asia to 60％, while finding ways to keep Africa from dropping below 70％.
如何做到这一点是个大挑战。 经济学的观点是，当增产效益出现边际递减效应时，通过大幅提高化肥价格可以抑制过度施肥现象。但优化氮肥生产是否应该以牺牲饥饿人口（尤其是非洲民众）的利益为代价呢？ 所以，我们必须找到一个更好的办法。也许技术方案可以使亚洲国家尽快达到库兹涅茨曲线的拐点，并帮助非洲跨越“低价格+重污染”的阶段。 How to achieve that is the big question. Economics suggests that a big hike in the price of fertilizer would help, by discouraging over-fertilizing when the yield benefits are marginal. But should the hungry, especially in Africa, be sacrificed to optimize nitrogen production? A better way is needed. Perhaps technical solutions can accelerate Asian countries on the upside of the Kuznets curve, and help Africa leapfrog the cheap-and-dirty phase.
植物育种专家也可能会研制出可以自己从空气中固化氮的高产谷物。 但是，无论这种作物是否能够出现，明智的投资者如今已经着眼于寻找更好的方法，确保在真正需要的时间和地点施用肥料，使其到达作物根部。实际上有一种简单方式是把肥料颗粒直接放置到接近作物根部的土壤中，这种施肥方式虽然劳动强度比较高，但已经在孟加拉国得到了应用。 Plant breeders may come up with high-yielding grains that fix their own nitrogen from the air. But, with or without such a boon, the smart money now is on finding better ways to make sure fertilizer is only applied when and where it will actually get to plant roots. Low-tech ideas include fertilizer granules that can be planted in soils close to plant roots. This is labour-intensive, but is already being tried in Bangladesh
还有一种被称为“精准农业”的高科技方法，具体来说，就是通过计算来分析植物健康状况、当地土壤和气候条件，进而通过定制程序确定具体的施肥时间和用量，然后利用GPS定位等手段进行精准施肥。 A high-tech approach would involve what is becoming known as precision agriculture. This involves using algorithms that analyse plant health and local soil and climate conditions to provide a bespoke program for the amount and timing of fertilizer applications, which are then carried out with pinpoint accuracy, often using GPS-guided equipment.
当然，农用化肥并不是氮污染的唯一来源。 国际氮肥管理体系将在2021年底提交最终报告，其中可能提出多种全球性措施，包括推动畜禽粪肥回收、将更多人类污水转化为肥料、减少食物浪费、甚至鼓励人们改变饮食习惯。 Fixing the fertilizer failings of modern farming is only part of the solution to the nitrogen problem, of course. When it produces its final report in late 2021, the International Nitrogen Management System is also likely to push for global efforts to recycle livestock manure, to turn more treated human sewage into fertilizer, to reduce food waste, and even to encourage changes to our diets.
该报告可能还会建议有环保意识的公民像检查自己的碳足迹一样检查自己的氮足迹。（来自新罕布什尔大学的足迹研究先驱阿利森·利奇介绍说，一个普通美国人每年的氮足迹约为41公斤，而荷兰人的这一数字仅为24公斤。） The group’s report may also suggest that environmentally aware citizens start checking their nitrogen footprint with as much concern as their carbon footprint. (An average American has a nitrogen footprint of about 41 kilograms a year, compared to 24 kilograms for an average person in the Netherlands, says Allison Leach of the University of New Hampshire, a footprint pioneer.)
在这个过程中可能需要综合考虑各种环境问题。 比如，上个月参加纽约会议的氮专家们担心，未来生物燃料的发展将对氮排放产生重大影响。为了解决气候变化问题，很多人正积极推动生物燃料的发展， 到本世纪中叶，生物燃料可能成为环境中氮的最大来源。 There may be environmental trade-offs along the way. Those gathered in New York last month were spooked by the impacts of the push to grow biofuels as a solution to climate change. By mid-century, biofuels could become the biggest source of nitrogen in the environment.
但是，归根结底，作为环境中过量氮的最大来源，氮肥使用率低下的问题必须解决。每个国家是否应该设定一个氮使用效率目标？或者像萨顿建议的那样，各国像达成《巴黎协定》中“2摄氏度”的温度控制目标那样，达成在2050年前将全球氮废弃量削减一半的目标？但即便如此，参与纽约会谈的氮专家们依旧认为，这还不足以将氮排放总量削减到“地球界限”之下。不过，就像“2摄氏度”的气候目标一样，这样做至少可以帮助我们避免最坏情况发生。But the bottom line is that the scandalously inefficient use of nitrogen fertiliser, the biggest source of surplus nitrogen in the environment, has to be tackled head on. Should every nation be given a nitrogen use efficiency target? Or should, as Sutton suggested, the world agree to adopt a target of halving nitrogen waste by 2050 – the nitrogen equivalent of the 2-degree temperature target set by the Paris Accord on climate change? Even that, those at the New York meeting thought, might not bring total emissions below the planetary boundary for nitrogen. But, like the 2-degree target for climate, it might avoid the worst.