If geoengineering goes rogue
如果地球工程胡作非为
Reaching for the sunshade: July 2030
打造遮阳篷：2030年7月
Efforts to cut emissions may fall short. What if some countries try to fix things a different way? Animagined scenario from 2030
减排的努力可能还不够。要是一些国家想以另一种方式解决问题呢？2030 年的想象场景
The paris climate deal commits its signatories to cuts in climate-changing greenhouse-gas emissions over the coming decades. But even if countries stick to their promises (and some may not), that may not be enough to avert catastrophe. Imagine that by 2030 global temperatures are still creeping up, and sea levels are tens of centimetres higher—significantly worsening the impact of storm surges that push seawater over low-lying areas and corrode coastal infrastructures. In Europe and America, summer heatwaves and winter flooding have become more severe. In America’s southern states, the Caribbean and South-East Asia, coastlines are battered by stronger tropical cyclones. The global South suffers worse droughts and more irregular monsoons, undermining fragile agricultural systems and causing famines and civil unrest. The reality of global climate change becomes apparent to rich and poor countries alike.
《巴黎气候协议》要签署国承诺在未来几十年内减少引发气候变化的温室气体排放。但哪怕各国信守承诺（有些国家可能不会），这可能还不足以避免灾难。想象一下，到2030 年，全球气温仍在攀升，海平面上升了数十厘米，显著加剧了风暴潮将海水推向低洼地区并腐蚀沿海基础设施的影响。在欧洲和美国，夏季热浪和冬季洪涝愈发严重。在美国南部各州、加勒比海地区和东南亚地区，更强烈的热带气旋不断冲击海岸线。南半球遭受更严重的干旱和更不规则的季风，破坏了脆弱的农业系统，造成饥荒和内乱。不论穷国富国，全球气候变化的现实都已显而易见。
Under these conditions, it seems likely that some countries will propose the use of a technique called “solar geoengineering” to cool the planet or slow its warming. One way to do this involves injecting tiny reflective particles into the stratosphere, where they would act as a sunshade by bouncing part of the sun’s energy back out into space. Something similar also happens naturally: big volcanic eruptions have, in the past, thrown large amounts of material into the atmosphere, cooling the planet for months or years. The eruption of Mount Pinatubo in the Philippines in 1991, for example, reduced temperatures in the northern hemisphere by as much as 0.5ºC for four years. Solar geoengineering would, its advocates say, do the same thing in a more controlled manner.
在这种情况下，很可能有些国家会提议使用一种名为“太阳地球工程”的技术来冷却地球或减缓其变暖。具体方法之一是将微小的反射粒子注入平流层充当遮阳篷，将太阳的一部分能量反射回太空。类似的事情也会自然发生：过去，巨大的火山喷发将大量物质抛入大气层，使地球持续降温数月或数年。例如，1991 年菲律宾皮纳图博火山爆发，使北半球的气温连续四年降低了多达0.5 摄氏度。其倡导者说，太阳地球工程将以更加可控的方式达到同样的效果。
Imagine that the idea starts to gain political support. The first detailed international discussions of the options, starting in the mid-2020s, are fraught. Developing countries, more exposed but less well equipped to cope with the impacts of climate change, call for discussions at the United Nations. A motion is proposed by a group of “least developed countries”, led by Bangladesh, a medium-sized economy with a strong voice in international climate talks. Eventually, the issue makes it onto the agenda in the General Assembly. But, as with negotiations to cut global emissions, years of discussions and resolutions lead to little concrete action. Few see a planetary sunshade as a desirable solution. Supporters observe that a sunshade would buy more time to reduce greenhouse-gas emissions, given that cuts are not happening fast enough. But opponents say it will reduce the urgency of cutting emissions.
让我们想象这个想法开始获得政治支持。关于这些方案的详细的国际讨论从2020 年代中期开始出现，充满了焦虑。发展中国家受气候变化的影响更大但缺乏应对的能力，要求在联合国展开讨论。孟加拉国是一个在国际气候谈判中态度坚定的中型经济体，由它领导的一些“最不发达国家”提出了一项动议。最终，这个问题进入了大会议程。但是，与减少全球排放量的谈判一样，多年的讨论和决议并没有带来多少具体行动。很少人觉得行星遮阳篷是个理想的解决方案。支持者认为，考虑到减排的速度不够快，遮阳篷会赢得更多时间来减少温室气体排放。但反对者表示，这将降低减排的紧迫性。
A further objection is the risk of unintended consequences, given that the technique has never been tried before, and academic studies and small-scale field trials have been underfunded, for fear of giving countries an easy way to avoid the difficult choices emissions cuts require. There is also the danger of “termination shock”: if a geoengineering project is launched, and it successfully cools the planet, then any failure of the sunshade (due to technical problems, say, or sabotage) could cause a sudden increase in temperatures in just a few years. For all these reasons, there are calls for international rules to govern the use of the technology—because without them, there is nothing to stop one or more countries launching a “rogue geoengineering” scheme on their own.
另一个反对意见是它有可能产生意想不到的后果，因为该技术以前从未被尝试过，而且学术研究和小规模实地试验一向资金不足——因为担心这会给各国提供一种简单的方法来避开减排涉及的困难选择。此外还存在“终端激波”的危险：如果一个地球工程项目启动，并且成功地冷却了地球，那么遮阳篷的任何故障（比如技术问题或蓄意破坏）都可能导致温度在短短几年突然升高。鉴于上述所有原因，有人要求制定国际规则来管理这项技术的使用——因为如果没有规则的话，就没有什么能够阻止一个或多个国家自己发起“流氓地球工程”计划。
But that is exactly what some countries might start to consider, perhaps in 2030, after the un debate fails to reach any agreement. Fed up with yet more inaction, a small group of developing countries might choose to engage in “minilateral” discussions over whether to “go it alone” with a sunshade scheme that would, if it worked, both cool the planet and provide a proof of principle that might persuade other countries to back the idea.
但这恰恰是联合国辩论未达成任何协议后，一些国家（可能在2030 年）开始考虑的问题。一小部分发展中国家受够了长期的不作为，可能会选择就是否“自行”使用遮阳方案来展开“少边”讨论。如果该方案奏效了，它就能在给地球降温的同时验证其原理，也许就能说服其他国家支持这一方案。
The fastest way to do this would be to build a fleet of specialised planes. An analysis published in 2018 by Wake Smith, at Yale University, and Gernot Wagner, at New York University, maps out how to do it. The planes need to fly at altitudes of 20km (66,000ft) or higher, ruling out the possibility of using existing commercial aircraft for the purpose. Instead, a custom fleet of several dozen aircraft would be needed, with four jet engines mounted on two huge, glider-like wings, which would allow them to stay aloft in the thin air of the stratosphere. In the first year, eight aircraft could carry out 4,000 five-hour flights (four spent in ascent and descent and one in the stratosphere). By year five, this would be ramped up to 34 aircraft making 20,000 flights a year; by year ten, 71 aircraft would be making 44,000 flights year. After 15 years the fleet would be 100 strong.
最快的实施方法是建立一支专用机队。耶鲁大学的韦克·史密斯（Wake Smith）和纽约大学的格诺·瓦格纳（Gernot Wagner）于2018 年发表的一项分析描绘了如何做到这一点。飞机需要在2 万米或更高的高度飞行，这就排除了使用现有商用飞机的可能性。相反，它需要一个由几十架飞机组成的定制机队，四个喷气发动机安装在两个巨大的滑翔机式机翼上，让它们可以在平流层的稀薄空气中飞行。在第一年，八架飞机可以进行4000 次每次持续五小时的飞行（四小时用于爬升和下降，一小时在平流层飞行）。到第五年将增加到34 架飞机，每年飞行20,000次；到第十年，71 架飞机每年飞行44,000 次。15 年后，飞机总数达到100 架。
Throwing shade
投掷阴影
This first-generation sunshade would probably be made from dispersed sulphur dioxide (SO2), which is one of the chemicals produced during volcanic eruptions. Dr Wagner suggests that the most efficient way to deliver it would be for geoengineering aircraft to be loaded with solid sulphur, which they would burn at altitude in their engines to produce SO2. All this would cost around $3.5bn a year (at today’s prices) to deploy. A parallel research programme would also be needed to monitor the dispersal of the particles, determine their interaction with other molecules in the atmosphere and model the climate impacts. This would cost about the same again. According to Janos Pasztor, executive director of the Carnegie Climate Governance Initiative, the only existing monitoring network capable of carrying this out at the required level of detail is the World Meteorological Organisation’s satellite and ground-based global atmosphere monitoring system.
这种第一代遮阳篷可能用分散的二氧化硫（SO2）制成，二氧化硫是火山爆发过程中产生的化学物质之一。瓦格纳认为，最有效的输送方式是由地球工程飞机装载固体硫，到达高空后在发动机中燃烧产生二氧化硫。所有这些将耗资约每年35 亿美元（按当前价格计算）。还需要一个并行的研究计划来监测颗粒的扩散，确定它们与大气中其他分子的相互作用并对气候影响建模。这要再花掉差不多同样多的钱。据卡内基气候治理倡议执行主任杰诺斯·帕兹托（Janos Pasztor）说，世界气象组织的卫星和地面全球大气监测系统是唯一能够以所需精度实现这一目标的现存监测网络。
Bangladesh on its own seems unlikely to foot that bill. As well as financial help, it would almost certainly want safety in numbers, which is why a coalition of developing countries seems more likely. Such a coalition might also want the security provided by the support of a larger power, such as India or China, both of which have large populations at risk from considerable climate impacts. Handily, China is also a big producer and exporter of sulphur.
孟加拉国本身似乎不太可能为此买单。除了经济上的帮助之外，它几乎肯定寻求“人多势众”，这就是发展中国家看起来更有可能结盟的原因。这样一个联盟还可能希望得到更强大的国家（如印度或中国）提供的安全感，这两个国家都有大量人口面对重大气候影响的威胁。中国恰好也是硫磺的主要生产国和出口国。
It would be both smart and efficient to start such a programme slowly. Drs Smith and Wagner calculate that the fleet could scatter 200,000 tonnes of SO2 in the stratosphere in the first year, causing an unremarkable 0.02°C of cooling. By the fifth year, those figures would rise to 1 megatonne of SO2 and 0.1°C. The cooling would reach 0.2°C in year ten and 0.3°C in year 15. At these levels, there should be a real impact on the rate of warming.
逐步缓慢地启动这样的项目既明智又高效。史密斯和瓦格纳计算出，第一年机队可能在平流层中散布20 万吨二氧化硫，带来并不明显的0.02°C 的冷却效果。到第五年，这些数字将升至100 万吨和0.1°C。冷却幅度将在第10 年达到0.2°C，在第15 年达到0.3°C。到了这样的水平，就应该能对变暖的速度产生实际影响。
But there is a catch. Regional geoengineering is impractical (stratospheric winds disperse particles across whatever hemisphere they are deposited in) so a solar sunshade would have to be either hemispheric or global. The former could be catastrophic, because models suggest it could shift the balance of energy in the upper atmosphere in a way that causes large-scale disruptions to tropical monsoons.
但这里还有个问题。区域性地球工程不切实际（粒子投在哪个半球，平流层风就会将它散布到整个半球上），因此遮阳篷要么是半球形，要么是全球形。前者可能是灾难性的，因为模型表明它可能改变高层大气中的能量平衡，导致热带季风的大规模失调。
To avoid the disastrous geopolitical fallout of such a scenario, the coalition would therefore seek to deploy a global sunshade that would offer equal or comparable cooling to all regions. Some studies suggest that this may be possible, though research is still very much in its early days. Simone Tilmes at the us National Centre for Atmospheric Research calculates that injections of SO2 at 15° and 30° north and south of the equator would produce a reasonably uniform global cooling.
为了避免这种情况造成灾难性的地缘政治后果，联盟将寻求部署一个全球遮阳篷，为所有地区提供相同或相当的降温。一些研究表明这或许可行，尽管研究还基本处于起步阶段。美国国家大气研究中心的西蒙妮·迪尔姆斯（Simone Tilmes）计算得出，在赤道北部和南部15°和30°注入二氧化硫将产生相当均匀的全球冷却。
Even so, a coalition of states acting unilaterally to cool the whole planet would still risk military reprisals. To avoid conflict, flights would have to remain within participating countries’ own airspace, so a coalition would need to span those latitudes. China, India and Bangladesh could take care of the northern latitudes, but the southern hemisphere would require collaborators with the right capabilities in Africa, South America or Australasia.
即便如此，一个单方面行动冷却整个地球的国家联盟仍将面临军事报复的风险。为避免冲突，飞机必须保持在参与国自己的领空内，因此整个联盟就需要能覆盖上述纬度。中国、印度和孟加拉国可以顾到北纬地区，但南半球需要在非洲、南美洲或澳大拉西亚找到具有合适能力的合作者。
There is another possibility. America has the money to build a fleet, the research capacity to track its impact and military bases around the world from which to launch planes. As for motives, look no further than hurricanes. Modelling published earlier this year suggested that sunshades might reduce the intensity of hurricanes compared with a warmer world. And it is possible, if hard, to support solar geoengineering without taking a position on the causes of global warming. A sceptical American administration could still insist that climate change was not man-made; it need only concede that temperatures are rising.
还有另一种可能性。美国有资金打造一支机队，有追踪其影响的研究能力，以及在世界各地起飞的军事基地。至于动机，只看飓风的影响就够了。今年早些时候发布的模型表明，遮阳篷带来的降温可能会降低飓风的强度。而即便不就全球变暖的原因表明立场，支持太阳地球工程就算难也依然是可能的。持怀疑态度的美国政府仍然可以坚持认为气候变化不是人为造成的；它只需承认气温上升就行了。
It is difficult to predict what the international response to a unilateral American sunshade programme would be. It would, of course, depend on how the sunshade was deployed and how the climate responded. America, the Soviet Union, Britain, France and China were all rebuked for carrying out atmospheric nuclear testing in the 20th century, but suffered little actual diplomatic cost. Unilateral geoengineering might provoke condemnation, but not war.
如果美国单方面实施遮阳计划，很难预测国际反应会如何。当然，这取决于如何部署遮阳篷，以及给气候带来了什么变化。美国、前苏联、英国、法国和中国都因在20 世纪进行大气层核试验而受到指责，但实际付出的外交成本却很低。单边地球工程可能会招致谴责，但不会引发战争。
Countries opposed to the idea might respond by developing counter-geoengineering programmes. They could either shoot down geoengineering planes or, more tactfully, build a second fleet to deliver a separate stratospheric payload to neutralise the sunshade (either by reacting with the SO2 to break it down, or by making the sulphate particles clump together and rain out faster). The development of counter-geoengineering tools might provide a deterrent against the unilateral deployment of a sunshade.
反对这种方案的国家可能会发展反地球工程计划作为回应。它们可以击落地球工程飞机，或者更巧妙地做法是建造另一支机队，散步另一种平流层载荷来中和遮阳篷（要么是和SO2 反应而将其分解，或者让硫酸盐颗粒聚集在一起并更快地被雨水冲走）。反地球工程工具的研发可能会阻碍单方面部署遮阳篷。
With or without counter-geoengineering, the global climate blame-game would undoubtedly become even more heated if a sunshade were deployed. One problem would be distinguishing its cooling effect amid natural variability. The cooling would take effect gradually, and global average temperatures would continue to rise in the early years of its deployment. Eventually, a slowdown in the rate of warming would become apparent in global data sets. But because temperatures vary naturally from one year to the next, reliably identifying a sunshade signal within the data would be tricky. And even before its effect became apparent, the sunshade would start to influence the weather, and the frequency of droughts, floods and tropical cyclones. Teasing apart the relative influences of global warming and natural variability on an extreme weather event is devilishly complicated today. Add a sunshade into the mix and fingers will be pointing in all directions, which will make international climate talks even more difficult.
无论有没有反地球工程，如果部署了遮阳篷，全球气候的相互指责游戏无疑会变得更加激烈。一个问题是如何在自然波动中辨别冷却效果。冷却将逐步生效，全球平均温度将在部署初期继续上升。最终，变暖速度的放缓将在全球数据集里变得明显起来。但由于温度年年都会自然变化，因此在数据中可靠地识别遮阳信号并非易事。甚至在其影响清晰显现之前，遮阳篷就会开始影响天气，以及干旱、洪水和热带气旋的频率。目前，对于极端天气事件，要将全球变暖和自然变化的相对影响区分开来是极度复杂的。再把遮阳篷掺和进来，矛头肯定指哪里的都有，这将使国际气候谈判更加困难。
But if a sulphate-based sunshade was successfully deployed and was shown to work, it might then be time for phase two. David Keith, who runs Harvard University’s solar-geoengineering research programme, has suggested that it might be possible, perhaps even preferable, to design synthetic particles that are more efficient at reflecting the sun’s radiation, or can stay suspended in the stratosphere for longer than sulphate particles can. And perhaps, having failed to reach international agreement on geoengineering in the 2020s, the United Nations might try again, with a new treaty being signed in Kyoto in 2047, 50 years after the original Kyoto Protocol. Kyoto 2 would concede that efforts to tackle climate change had fallen short, and would endorse the use of a sunshade as a way to give countries more time to reduce their emissions. In the worst case, the unilateral deployment of a sunshade could lead to conflict. But in the best case, it might provide a pathway to a lasting solution to the climate problem.
但是，如果成功部署硫酸盐遮阳篷并且证明有效，也许就该开始第二阶段了。负责哈佛大学太阳地球工程研究项目的大卫·基思（David Keith）表示，设计出能够更高效地反射太阳辐射，或者可在平流层中比硫酸盐留存更久的合成粒子可能行得通，甚至更好。也许，由于未能在2020 年代就地球工程问题达成国际协议，联合国可能会再次尝试，在最初的《京都议定书》问世50 年后的2047 年在京都签署了一项新条约。《京都议定书2》将承认应对气候变化的努力不足，并认可使用遮阳篷作为让各国有更多时间减少排放的手段。在最坏的情况下，单方面部署遮阳篷可能导致冲突。但在最好的情况下，它可能给出了一条通往持久性解决气候问题的道路。
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